Politecnico di Milano Scuola di Ingegneria Industriale e dell’Informazione

Corso di Laurea Magistrale in Ingegneria Energetica

MSc in Energy For Development

MICROBIOENERGY® : HEAT AND BIOGAS BIOMASS-FUELED GENERATION PLANT FOR DOMESTIC USE

Design, construction and monitoring of Microbioenergy® prototype

Relatore: Prof.ssa Emanuela COLOMBO

Correlatore: Ing. Andrea SCHIEVANO

Tesi di laurea di: Mara ZANTEDESCHI

Matricola 864213

Anno Accademico 2017–2018

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Summary

Introduction

In the context of today energy challenges it is even more clear the necessity to look beyond the current

"take, make and dispose” extraction industrial model and move towards the circular economy restorative

and regenerative approach [6]. Sustainable and renewable energy sources, as biomass waste, are taken

more and more importance and applied also in small-scale plants of heat and electric power generation

as an alternative or supplements to traditional centralized grid-connected power. In Italy, where every

building that is constructed or renovated from January 1st 2014 must use renewable energy sources to

cover a share of its energy needs [8], biomass waste represents an applicable energy vector and also an

effective solution for waste management.

Two valuable solutions for combined sustainable energy production and waste management are

Composting Heat Recovery System (CHRS) and the Anaerobic Digester (AD), two different technologies

for biomass valorization, occurring respectively under aerobic and anaerobic conditions, to obtain

compost, heat and methane. While AD technology diffuse use worldwide is largely described in the

literature, especially for small-scale application [30,31], literature regarding heat generation based on

composting is disjointed and incomplete due to the fact that many CHRS advances have been made

through independent projects.

From the analysis of the aerobic biodegradation and the anaerobic digestion processes and technologies

it is evident that both systems present a limiting condition: environmental conditions and in particular

temperature for the AD [25,30]; relatively short operating life, methane undesirable production for the

CHRS [41]. On the other hand, CHRS constant yearly heat production can compensate constant

temperature demand of the AD and an External feeding system can compensate the short operating life

of the CHRS suppling new organic substance to the biochemical process bacteria.

The challenge of Microbioenergy® here presented, is the integration of these technologies in way to

compensate the disadvantages of each reactor type with the advantages of the other. Microbioenergy®

based system has been developed from an upgrade of the Composting Heat Recovery System (CHRS)

with the introduction of the Aerobic Digester and an External Feeding System specifically designed for

the project.

Microbioenergy® system for domestic use is ideal for inhabitants of rural areas with high availability of

green waste from tree pruning and biomass waste (as farmers and countryside restaurants holders) or

public and private companies for OFMSW (Organic Fraction of Municipal Solid Wastes) collection in

rural or urban areas. It can represent an alternative heating system or the integration to an existent one,

depending on selected size, biomass cost and biomass availability.

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Materials and methods

I. Dimensioning

The first prototype construction have been studied basing on the energy demand of the holder of a

community wood waste collection point located in Lozzo Atestino (Padua), who has already

experimented the use of a Thermocompost CHRS (which did not include the anaerobic digester neither

the use of external feeding). The system design and appropriate modifications implementation have been

studied to satisfy the energy demand for space heating and domestic hot water (DHW) production taking

into account the existing heating system consisting of a pellet boiler and a tank-in-tank hot water inertial

buffer with a solar coil. Moreover the additional methane production has been designed on methane

demand for cooking.

1 Microbioenergy® main body material is woodchip which available characteristics allow the use

known-data from on field experiments and literature revision. Dimensioning phase resulted in the

construction of a cylindrical pile of woodchip of 3 m diameter, volume 16 m3 and estimated average yearly

of 18921.6 MJ/year.

2 Anaerobic digestion (AD) chamber and external feeding tank represent the two reactors of the same

two-stage anaerobic digestion plant : in the external tank the first steps of the digestion process takes

place (acidification reactor) while in the AD chamber located in the bottoming part of Microbioenergy®

main body the methanogenesis takes place(methane reactor).

The AD is a fixed portion of Microbioenergy® main body and is obtained from its more internal part and

lower part (almost a quarter of woodchip pile overall volume), while the external feeding system

PELLET BOILER RETURN

PELLET BOILER DELIVERY

HOT

WATER

BUFFER

DHW TANK

SOLAR COIL

DHW

DELIVERY

SPACE HEATING DELIVERY

SPACE HEATING RETURN

BIOGAS COLLECTING SYSTEM

EXTERNAL FEEDING PUMP

TO AEROBIC REACTOR

TO ANAEROBIC REACTOR

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Figure 1 Plant Scheme

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dimensions is calculated in way to maintain the continuous energy supply even after the end on the CHRS

operating life.

3 The objective of the external feeding, so, is to maintain the yearly energy output previously obtained

with woodchip, using a different organic material. For the project the external tank is filled with a mixture

of OFMSW and biomass rich in fruit and vegetables waste, with a Biochemical Methane Potential of

almost 350 Sm3/tonDM. To maintain the energy output of 16 m3 of woodchip with this material it results

a biomass waste volume of 9.45 m3.

Different control parameters are taken into account for the external tank and the AD chamber:

• Waste to Water (WtW) ratio of 1:1 for the external tank correct management and a higher, and

of almost 1:3, for the AD chamber;

• Solid Retention Time (SRT) of 60 days for external feeding tank,

Hydraulic Retention Time (HRT) of few days for the AD chamber;

• Organic Loading Rate (OLR) not exceed a 4 kgVS/m3 day in the AD chamber;

• Gas Volume Ratio (GVR) of 1:3 for the AD chamber

II. Process simulation and lab-scale measurements

A laboratory phase is performed to evaluate Microbioenergy® performance by process simulation. It

includes three Activities conduct at “e-bio center” laboratory at the Department of Environmental

Science and Policy of university Università Degli Studi Di Milano in a period of 60 days and analyzes

the CHRS material: mixed woodchip from sycamore wood (30%), bay tree wood (30%), undergrowth

wood (30%), linden wood (10%).

TEST1 (conducted by e-bio center laboratory professionals):

Anaerobic Biogasification Potential (ABP) on dry basis = 287.367 Sm3 BIOGAS/ton DM

Methane percentage on biogas = 55%

TEST2 (conducted by the author):

Biomass Characterization determines the Dry Mass (DM) percentage = 38.145 %

BMP (elaborated from the values above):

Biochemical Methane Potential (BMP) on wet basis = 60.289 Sm3 CH4/ton WM

III. Construction

Paralleling to process simulation phase and the construction of the monitoring system the construction

of the plant takes place in Lozzo Atestino. Construction lasts two days and includes Thermocompost

demolition December 15th and Microbioenergy® construction December 16th 2018 with the addition of

monitoring system set-up.

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IV. Monitoring

To understand the real behavior of Microbioenergy® a monitoring system including a heat meter data,

a pH probe and eight temperature probes has been included in the plant.

• The heat meter is positioned closed to the inertial hot water buffer in order to estimate the real power

exchanged between the system and the final user. Data have been collected four times a day, one day

a week, in the month of February 2018. The final average thermal power value is the geometric mean

obtained considering the frequency (fi) of every value of power (Pi) Mean thermal power output

= ∑Pi*fi / total number of measurements

• The temperature probes are positioned according to schematic Figure 2, two Arduino systems

respectively positioned on the North and the South side of Microbioenergy® has allowed the

temperature data continuous measurement and collection. No temperature probe has been inserted

in the lower part of the system (corresponding to the anaerobic reactor), in order to avoid any oxygen

inlet. To control the anaerobic reactor water temperature, probe T4_S has been positioned in a T

valve on the pipe connecting the reactor itself and the external tank. After a phase of electronic system

construction and informatic code creation to combine the temperature probes and the Arduino

system, four probes per side have been positioned on February 16th 2018.

Results and discussion

I. Energy output

Energy output values from Dimensioning, Process simulation and Monitoring are reported in Table 2.

Their comparison is possible under the following preconditions:

Energy from Process simulation is calculated as 𝐸𝑛𝑒𝑟𝑔𝑦𝑌𝐸𝐴𝑅 = 𝐵𝑀𝑃 ∗ 𝐿𝐻𝑉_𝐶𝐻4 ∗ 𝑚𝑎𝑠𝑠 where the mass

quantity is only the part of Microbioenergy® working under aerobic conditions;

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Energy from Heat meter data is calculated considering a constant operative regime during the year; Since

laboratory BMP result represents the maximum value developed under optimal conditions it is likely to

suppose a lower value for daily biogas on-field production and a much longer time to achieve the

complete degradation. For this reason the energy produced in 60 days of laboratory tests has been

considered as the correspondent energy produced in one year of on-field application.

Energy output estimated with the first approach represents the activity of composting microorganisms

which are able to extract energy from biomass cellulose, hemicellulose and lignin, while energy value

estimated with the second approach represents anaerobic digestion process in which only the most

degradable substrates are elaborated from anaerobic bacteria. So, at is was expected, energy from aerobic

biodegradation is slightly higher than energy from anaerobic digestion. On the other hand, real energy

output measured with the heat meter shows a better performance with respect to the estimated one.

II. Temperature distribution

Figure 3 shows the south side temperature distribution from February 16th to March 16th with particular

focus on data collected by temperature probes placed inside Microbioenergy® reactor. The general trend

of internal temperatures seems to follow the external minimum temperature decrease and increase with

Approach / Result Yearly energy output [MJ/y]

Dimensioning 18921.6

Process Simulation (BMP) 18318.21

Heat meter data 19710

Table 2 Energy output

Figure 3 South side temperature distribution, weather station data source: “Stazione Ospedaletto Euganeo”, 10 km

from Lozzo Atestino, http://www.arpa.veneto.it website

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a short delay: for example, minimum and maximum internal temperatures values of T2_S (respectively

on March 4th and March 15th) follows the minimum and maximum external minimum temperature

(respectively on February 27th and March 10th) with a delay of some days. The system actually reacts to

environmental change as an inertial thermal buffer.

Moreover, temperature distribution in space indicate the presence of three zones inside the woodchip

pile: the upper zone is affected by thermal dispersion with the external air so that T1_S shows a the

intermediate values; the more internal zone suffers of oxygen lack and exothermic bio-oxidation process

stop so that T3_S shows the lower values; the intermediate zone shows a correct behavior with

consequently higher temperatures T2_S. This behavior also described in “Investigation on Biomeiler

Operating Behavior” [41] is the expected in case of correct operation and favorable conditions.

Figure 4 showing the north side temperature distribution from February 16th to February 21nd, presents

two main highlights:

- There is a deep temperature decrease every day at the same hour (around 12 AM). This behavior

during these three days is linked to pump improper management since in these particular days the

pump was stopped during the central hours of the day. In this way the water contained in the pipes

connecting Microbioenergy® and the house remains exposed to outside temperature for a longer

time and so, enters in the system after a longer time of cooling.

- There is an evident change in highest temperature zone during time since T1_N and T2_N

alternately show the highest values. This inversion of highest temperature zone is connected to

surface higher sensitivity to environmental conditions change. “Temperature changes in an initially

frozen wood chip pile” [50] demonstrates that internal temperature distribution can vary during

time

Figure 4 North side temperature distribution

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The different behavior on North side and South side could be linked to a slightly different relative

position between water coils inside the pile and temperature sensors on the two sides: north temperature

probes in fact probably suffer of lower penetration, in addition to a much lower solar radiation on north

side. Despite highest temperature zone changes during time, latest data collection on March 22nd show a

similar trend for north and south side temperature distribution with highest temperature values

collected by the sensor placed at intermediate height (T2_N) as Figure 5 shows.

Finally, Figure 6 shows water temperature values collected by sensor T4_S located in a “T” component

on the pipe connecting Microbioenergy® and external feeding system. The sensor measure the

temperature of the water flow passing through the pipe every time Microbioenergy® is fueled with new

water enriched with the organic substances dissolved during dark fermentation process in the external

tank. Temperature T4_S highest values represent the internal water conditions exiting

Figure 6 AD temperature

Figure 5 March 22 temperature distribution

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Microbioenergy® while the lowest values represent air condition when no water is flowing in the pipe.

Since it is not possible to directly measure the temperature inside the anaerobic reactor neither inside

the external tank, to avoid oxygen inlet, these values can be used to evaluate the anaerobic process

condition and correct operation. Since T4_S highest values reach temperatures of 20-30°C the

mesophilic condition of anaerobic microorganisms can be correctly considered.

III. Profitability analysis

Economic Analysis

Microbioenergy® can represent an alternative heating system or the integration to an existent one,

depending on selected size, biomass cost and biomass availability. Due to the high number of variables,

a series of applications and base cases have been considered: 1 Microbioenergy® prototype as an

alternative to Thermocompost; 2 Microbioenergy® prototype as an integration to an existing thermal

system including pellet boiler and inertial buffer with solar coil or to an existing thermal system including

gas condensing boiler; 3 Microbioenergy® of bigger size

In the evaluation of investments, costs refer to components, biomass, installation and maintenance,

while benefits come from savings for space heating, thanks to the hot water coming from the system, and

cooking, related to the biogas estimated production. The investments have been compared in terms of

PBT and NPV on a 15-year period. Unless otherwise specified, the pellet price has been set at 0,35 €/kg

and the gas price at 0,7 €/Sm3.

1. Microbioenergy® prototype vs Thermocompost (with existing pellet boiler)

The first case (Figure 7) is a comparison between a traditional Thermocompost and Microbioenergy®, as

integrations to an existing pellet boiler with inertial tank. The analysis shows how Microbioenergy® results

Figure7 Microbioenergy vs Thermocompost (pellet boiler base case)

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in a higher NPV (around 4000€ instead of 2500€), despite the higher initial cost, thanks to the lower

operating and maintenance costs.

2. Microbioenergy® prototype vs pellet boiler / gas boiler

In this second case (Figure 8), the profitability of the investment on Microbioenergy® is evaluated with

respect to the base cases of having a pellet boiler with inertial buffer and having a gas condensing boiler. It

is possible to see how the initial cost for the inertial buffer heavily influences the returns (NPV around 2500

€) and PBT (7 years) in the case of gas boiler.

The sensitivity analysis (Figure 9) shows how the gas price affects the profitability of the investment in case

of integration with the gas boiler.

Figure 0 Microbioenergy vs pellet boiler / gas boiler base case

Figure 9 Sensitivity analysis

Figure 10 Scenario analysis

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The scenario analysis (Figure 10) investigates the impact of both gas price and pellet price on the

profitability of the investment in case of integration with the pellet boiler.

3. Microbioenergy® of bigger size

Here the investment on a Microbioenergy® with 79 m3 of woodchip (instead of 16 m3) is investigated.

As previously explained, there is not a direct proportion between size and power output, so a 5 times

greater woodchip volume does not deliver 5 times the energy production. In the same way, there is not a

linear proportion between costs and size due to economies of scale. The main result of this analysis is the

increase of NPV from almost 4000 € to almost 8000 €. PBT on the contrary shows no substantial

difference but a slight increase for greater dimension system (4 years instead of 3) due to the installation

cost impact.

Conclusion

Microbioenergy® plant monitoring indicates a correct operation of the system, with a maximum

temperature of over 60°C inside the woodchip main body, allowing the delivery of hot water of

temperature up to 55°C and the capability to cover the user yearly energy demand for heating for 33%.

Performances measurements demonstrate an energy overproduction with respect to the output expected

by the selected dimensioning approach. This result suggests the need to include, in CHRS performance

assessments, the system sensitivity to environmental conditions and compost material. The temperature

distribution inside the main reactor highlights the connection between the system operation and weather

conditions since the internal temperatures trend follows the external temperatures trend with a short

delay (around 5 days). Moreover, temperature monitoring shows the correct division of the system into

three levels of temperatures (the internal is the coldest part, the intermediate is the hottest one, the upper

part has intermediate temperatures). This condition has been always maintained by the south side of the

system while north side one shows an inconstant behavior explained by heat exchange dynamics inside

the woodchip matrix. In addition, differences between south and north measurements are related to

probe location (maximum temperature on south side is 60°C, maximum temperature on north side

around 40°C). In these first stages of the prototype life, it was possible to evaluate the performance on

heat power output, while biogas output has been estimated. The profitability analysis shows good results

in terms of NPV and PBT for the prototype application with respect to the pre-existent CHRS and a

relevant variability of these indicators depending on the fuel type considered for the base case: better

financial performance of Microbioenergy® in case of pellet use (NPV around 4000 €) with respect to the

case of gas use (NPV around 2500 €). Finally, thanks to little time of construction (only one day) and

minimum maintenance requirements (external feeding refilling), Microbioenergy represents an easy

solution applicable as integration to existent heating systems and an effective method to manage high

quantity of biomass in rural areas.

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Table of contents

Summary ..................................................................................................................................... 3

Table of contents ....................................................................................................................... 13

Abstract ...................................................................................................................................... 17

Sommario .................................................................................................................................. 19

1 Introduction ....................................................................................................................... 21

1.1 Sustainability .............................................................................................................. 22

1.2 Greening ..................................................................................................................... 22

1.3 Circular Economy ....................................................................................................... 23

1.4 Small-scale decentralized power generation .............................................................. 24

1.5 Energy from Biomass ................................................................................................. 25

1.6 EU and Italian regulations on waste and biomass ..................................................... 27

1.7 Objectives of the thesis ............................................................................................... 28

2 Background ........................................................................................................................ 29

2.1 Biomass aerobic composting ...................................................................................... 29

2.1.1 Process phases ........................................................................................................ 29

2.1.2 Main process parameters .................................................................................... 31

2.1.3 Bio-assays ............................................................................................................ 34

2.2 Anaerobic digestion and dark fermentation .............................................................. 35

2.2.1 Phases of AD processes ....................................................................................... 36

2.2.2 Main process parameters .................................................................................... 37

2.2.3 Biogas .................................................................................................................. 41

2.2.4 Dark fermentation (DF) and two-stages AD ....................................................... 42

3 State of the art .................................................................................................................... 45

3.1 Composting Heat Recovery System (CHRS) .............................................................. 45

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3.1.1 Jean Pain’s Method 1972 ........................................................................................ 46

3.1.2 CHRS developments ........................................................................................... 46

3.1.3 CHRS Modeling ................................................................................................... 48

3.2 Small-scale anaerobic digestion technologies ............................................................ 50

3.2.1 Fixed dome digester ............................................................................................ 50

3.2.2 Floating drum digester ........................................................................................ 51

3.2.3 Tubular digester .................................................................................................. 52

4 Microbioenergy®: an integrated solution ........................................................................ 55

4.1 Prototype Plant concept ............................................................................................. 55

4.2 Dimensioning ............................................................................................................. 57

4.3 Process simulation and lab-scale measurements ....................................................... 60

4.4 AD-chamber dimensions and external feeding tank .................................................. 65

4.4.1 External feeding PVC tank .................................................................................. 66

4.4.2 Portable tank ....................................................................................................... 67

5 Prototype construction at Lozzo Atestino .......................................................................... 69

6 Prototype operation and monitoring ................................................................................. 77

6.1 Heat Meter Data Collection ........................................................................................ 78

6.2 Temperature ............................................................................................................... 80

7 Economic analysis and utilization models......................................................................... 88

7.1 Utilization models ...................................................................................................... 88

7.2 Parameters .................................................................................................................. 88

7.3 Investment evaluation ................................................................................................ 93

8 Conclusions ........................................................................................................................ 98

9 Annex 1 – “Construction procedure” ............................................................................... 100

9.1 Materials ................................................................................................................... 100

9.2 Procedure .................................................................................................................. 103

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10 Annex 2 – “Native Power procedure” .......................................................................... 110

11 Annex 3 – Arduino codes .................................................................................................. 114

List of Figures .......................................................................................................................... 120

List of Tables ........................................................................................................................... 123

12 Bibliography ..................................................................................................................... 125

Ringraziamenti ........................................................................................................................ 129

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Abstract This paper presents a small-scale heat and biogas generator, which aim is to promote the use

of biomass as mean of sustainable energy production and waste management in areas with high

availability of ligneous and cellulosic biomass waste. The proposed system called

Microbioenergy® integrates two complementary technologies of biomass valorization,

respectively Compost Heat Recovery System (CHRS) and Aerobic Digestion (AD), and includes

an external feeding system based on the dark fermentation process. The plant is thought for

decentralized energy generation by biomass valorization and promotes greening, as well as EU

policies on circular economy.

In this project the first on-field Microbioenergy® prototype has been constructed and tested in

the rural area of Lozzo Atestino (Padua), in collaboration with the Italian association TERRE

(for the construction phase) and the department of Environmental Science and Policy -

Università di Milano (for the design phase).

The thesis includes a detailed literature research, to explain the two biomass valorization

processes, and Microbioenergy® combined solution development, including the design and

construction procedures and the simulation and monitoring results. Some relevant results, here

reported, indicate a system maximum temperature of over 60°C allowing the production of hot

water of temperature up to 55°C. The application of Microbioenergy® prototype as an

integration to a pellet boiler shows a 33 % estimated value of savings on pellet consumption to

cover the energy request, for both space heating and domestic hot water production, of the case

study and a 100% estimated value of savings on gas consumption for cooking in the case study.

KEY WORDS

Compost Heat Recovery System (CHRS), Aerated Static Pile (ASP), Anaerobic Digestion (AD),

biomass waste, biogas

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Sommario

Questo elaborato presenta un generatore di calore e biogas su piccola scala, che mira a

promuovere l'uso della biomassa come mezzo sostenibile di produzione di energia e di gestione

dei rifiuti in aree con elevata disponibilità di rifiuti di biomassa lignea e cellulosica. Il sistema

proposto chiamato Microbioenergy® integra due tecnologie complementari di valorizzazione

della biomassa, rispettivamente Sistema di Recupero di Calore da Compostaggio (acronimo

inglese CHRS) e Digestore Anaerobico (acronimo inglese AD), e include un sistema di

alimentazione esterno basato sul processo di fermentazione scura. L’impianto è pensato per la

generazione di energia decentralizzata attraverso la valorizzazione della biomassa e promuove

l'ecocompatibilità, così come le politiche dell'UE sull'economia circolare.

In questo progetto il primo prototipo di Microbioenergy® è stato costruito e testato nell'area

rurale di Lozzo Atestino (Padova), in collaborazione con l'associazione italiana TERRE (per la

fase di costruzione) e il dipartimento di Scienze e politiche ambientali - Università di Milano

(per la fase di progettazione).

Alcuni risultati rilevanti, qui riportati, indicano una temperatura massima del sistema di oltre

60 ° C che consente la produzione di acqua calda di temperatura fino a 55 ° C. L'applicazione

del prototipo Microbioenergy® come integrazione a una caldaia a pellet mostra un valore

stimato del 33% di risparmio sul consumo di pellet per coprire la richiesta di energia, sia per il

riscaldamento che per la produzione di acqua calda sanitaria, del caso in esame e un valore

stimato del 100% di risparmio sul consumo di gas per cucinare nel caso di studio.

PAROLE CHIAVE

Sistema di recupero di calore dal compostaggio (CHRS), pila statica areata (ASP), digestione

anaerobica (AD), rifiuti dalla biomassa, biogas

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1 Introduction

Energy is essential to many aspects of human development; it does not directly represent a vital

human requirement per se, but it is crucial to the fulfillment of many basic human needs and,

so, it is often assumed to be a prerequisite to all aspects of economic and social development.

Moreover, awareness on correct energy production and energy use has grown, leading to a more

conscious management of energy resources. The challenge lies in finding ways to reconcile the

necessity and demand for modern and sustainable energy services with its impact on the

environment and the global natural resource base in order to ensure that sustainable

development goals are realized. That’s why the UN General Assembly has included in the

program “United Nations Decade of Sustainable Energy for All” (2014) a specific goal dedicated

to energy among the so called Sustainable Development Goals (SDGs)[1].

Key concepts to keep in mind when facing today world energy challenges are sustainability

and circular economy which have been applied in this project in the construction of a

small-scale power generator fueled with biomass. This so called Microbioenergy®

promotes the use of a renewable energy source as mean of sustainable energy production and

waste management.

Figure 1.1 Microbioenergy

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1.1 Sustainability

There is no universally agreed definition on what sustainability is because it takes into

consideration multiple aspects: quality of life; fairness and equity; participation and

partnership; care for environment and respect for ecological constraints. It is "A process of

change in which the exploitation of resources, the direction of investments, the orientation of

technological development and institutional change are all in harmony and enhance both

current and future potential to meet human needs and aspirations"[2], the resulting definition

of sustainable development is "Development that meets the needs of the present without

compromising the ability of future generations to meet their own needs"[3]. It can be finally

summarized as the interconnection between three parameter: economics, environment and

society [4] (see the Figure 1.1). The translation of sustainability in matter of energy is: to

support economics, in way that financial investments in the energy sector are repaid in

acceptable time, to take environment into consideration, in way not to exhaust natural

resources, and to care for society, in way to have positive impacts on people standards of living.

1.2 Greening

As part of the 2014-2020 Common Agricultural Policy reform [5], European Union's farm

policy to support the sustainable development of the agri-food sector, greening is a major

innovation in the incentivation framework. It consists of a new and more environmental

friendly system of direct payments to incentivize the adoption of farming practices helping

Figure 1.2 Sustainability

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environment and climate goals, as market prices are not high enough to remunerate the effort

of providing such public goods.

Green direct payments account for 30% of the national expenditure on direct payments of Eu

countries. In order to access such area-based payments, farmers have to comply with three

requirements:

- Diversify crops, to make soil more resilient

- Maintain permanent grassland, to conserve soil carbon

- Dedicate 5% of land to Ecological Focus Areas (EFA), to protect water and habitats

These directives are translated into technical specification which depend on different factors,

such as the arable land surface, the geographic location and the kind of crops. National

governments can allow farmers to meet these requirements through equivalent practices,

aimed at support rural development or agri-food certification.

1.3 Circular Economy

Looking beyond the current "take, make and dispose” extraction industrial model, the circular

economy is restorative and regenerative by design. In a circular economy the value of products

and materials is maintained for as long as possible: waste and resource use are minimized, and

when a product reaches the end of its life, it is used again to create further value. This can bring

major economic benefits, contributing to innovation, growth and job creation[6]. Circular

economy offers an opportunity to boost our economy, making it more sustainable and

competitive in the long run: preserving resources, some of which are increasingly scarce,

subject to mounting environmental pressure or volatile prices, and saving costs for industries.

Bio-based materials can easily enter in a circular economy model since they are renewable,

biodegradable and compostable.

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Thanks to the promotion of a cascading use of biomass and the support to innovation in the

bio-economy, it is possible to minimize life-cycle environmental impact of these materials,

making sustainable sourcing an important priority. According to this principle, on 2 December

2015, the European Commission put forward a Circular Economy Package, to support the EU’s

transition to a circular economy, which includes revised legislative proposals on waste with

measures covering the whole cycle: from production and consumption to waste management

and the market for secondary raw materials[7]. Also, on November 2016 it proposed the revised

Renewable Energy Directive including updated sustainability criteria for bio-fuels used in

transport and for solid and gaseous biomass fuels used for heat and power[8].

1.4 Small-scale decentralized power generation

Micro-generation is the small-scale generation of heat and electric power by individuals, small

businesses and communities to meet their own needs, as alternatives or supplements to

traditional centralized grid-connected power. Although it is often related to power generation

in case of unreliable grid power or long distance from the electrical grid, the term is currently

mainly used for environmentally conscious approaches that aspire to zero or low-carbon

footprints or cost reduction (as in the case of autonomous buildings). A change of mindsets and

incentives are bringing micro-generation into the mainstream. According to the EU’s 20 20 20

goals for fossil fuels usage reduction, in Italy every building that is constructed or renovated

from January 1st 2014, must use renewable energy sources to cover a share of its energy needs,

even thermal energy (heating) or electric one or both[9].

Figure 1.3 Circular Economy

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1.5 Energy from Biomass

Biomass is the biodegradable fraction of products, waste and residues from biological origin. It

is considered a renewable energy, although releasing carbon dioxide during its exploitation for

energy production, because the final products are always produced in a close carbon cycle [10];

moreover it provides immediate benefits in the form of green-house gas (GHG) savings, energy

security and diversity, as well as complementary socio-economic benefits. Among all the energy

vectors Bio-fuel and waste represents 10% of total energy production in the World and 71%

among the only RES (data IEA 2015), moreover, in EU28 (European Union including 28

Countries) it represents 18% of total energy production and 66% among the only RES (data IEA

2015). It is worth to notice how the RES are increasing their role in the energy sector with

respect to coal, crude oil and natural gas (green in Figure 1.4).

Biomass role in energy production in Italy is strong and has seen a great increase in the last

decade, as Figure 1.5 demonstrates, energy production linked to bio-fuel and waste (green)

shows the highest growth between 2001 and 2015, increasing its level both in absolute value

Figure 1.4 EU28 Energy Production by fuel (source: IEA)

26

and relative terms with respect to the other energy vectors. Its role has increasing importance

since biomass represents an unconventional energy vector.

The “Piano di Azione Nazionale” (National Action Plan) for Renewable Energy provides for

biomass, compared to the total RES, to cover by 2020, 19% of total electricity consumption

(18.780 GWh), 54% of the total heating and cooling energy (5,670 ktoe) and 87% of the

transport sector energy (2,530 ktoe) [11]. To reach this objective for the 2020 a range of

incentive have been established and applied by the GSE (Energy Services Manager). According

to the GSE, public administrations are eligible to receive incentives for both energy efficiency

and renewable energy projects, and private individuals are eligible only for renewable projects.

Projects eligible include energy efficiency improvements in existing building envelopes

(thermal insulation of walls, roofs and floors, replacement of doors, windows and shutters,

installation of solar screens); replacement of existing systems for winter heating with more

efficient or renewable ones (condensing boilers or renewable such as biomass, solar thermal

and heat pumps); and audits and energy certification associated with these projects.

ENEA (National Authority for Energy and Environment) is promoting activities concerning the

optimal use of biomass resources at territorial level and the development, demonstration and

qualification of processes, technologies and innovative components for the combined

Figure 1.5 Energy production by fuel in Italy (source: IEA)

27

production of electricity, heat and / or biofuels at local level. Specifically, ongoing initiatives

have as their object: the production of biogas, to be used for the electricity generation or for the

introduction into the network (in the form of biomethane, a gas biofuel), by co-digestion of

OFMSW (Organic Fraction of Municipal Solid Wastes, Italian FORSU), agricultural waste and

/ or crops rich in sugars; the exploitation of the digestate as an energy vector by drying and

pyrogasification (even after mixing with lignocellulosic biomass) instead of use the digestate as

a traditional soil improver in agriculture; the development and demonstration of new

technologies for the final use of the produced biogas, such as the production of electricity by

combining with fuel cells, with a significant improvement in the final energy conversion yields

and the greenhouse gas balance [12].

1.6 EU and Italian regulations on waste and biomass

In addition to the directives about renewable energy generation and use previously mentioned,

regulation about waste management is now considered. EU Landfill Directive 99/31/CE (1999)

indicates recycling and composting as preferential strategies for waste management, with the

objectives of reducing landfill bio-waste of 35% in 2020 with respect to 1995 levels and reduce

related greenhouse gases emissions. However, incineration has been often preferred because

of its relative simplicity and cheapness.

Italian composting regulation can be framed by:

- Legislative Decree 152/2006, which designates Regional and Municipal

administrations as entities entitled to create the incentive schemes supporting

composting activities (integrated with in 2008 by Legislative Decree 4/08)

- Legislative Decree 75/2010, which defines and classifies fertilizers, regulating their

production, commercialization and use

- EU directive 91/676/CEE, which defines practices to avoid poisoning of water and

habitats due to nitrate spillages.

- EU directive 2008/98/CE, which encourages waste recycling and composting to reduce

environmental impact.

- DDL 1328-B 6/6/16: enlarge the list of biomass waste excluded from the OFMSW;

among them, green waste from maintenance activities becomes a resource with the

advantage of an enormous reduction of bureaucratic requirements for companies and

operators in the sector, as well as considerable savings for municipalities and public

administration

28

1.7 Objectives of the thesis

As previously explain, in the context of today energy challenges it is even more clear the

necessity to look beyond the current "take, make and dispose” extraction industrial model and

move towards the circular economy restorative and regenerative approach [6]. Sustainable and

renewable energy sources, as biomass waste, are taken more and more importance and applied

also in small-scale plants of heat and electric power generation as an alternative or supplements

to traditional centralized grid-connected power. In Italy, where every building that is

constructed or renovated from January 1st 2014 must use renewable energy sources to cover a

share of its energy needs [8], biomass waste represents an applicable energy vector and also an

effective solution for waste management.

Two valuable solutions for combined sustainable energy production and waste management

are Composting Heat Recovery System (CHRS) and the Anaerobic Digester (AD), two different

technologies for biomass valorization, occurring respectively under aerobic and anaerobic

conditions, to obtain compost, heat and methane.

From the analysis of the aerobic biodegradation and the anaerobic digestion processes and

technologies it is evident that both systems present a limiting condition: environmental

conditions and in particular temperature for the AD [25,30]; relatively short operating life,

methane undesirable production for the CHRS [41]. On the other hand, CHRS constant yearly

heat production can compensate constant temperature demand of the AD and an External

feeding system can compensate the short operating life of the CHRS suppling new organic

substance to the biochemical process bacteria.

The project first objective is the development of a win-win solution with the transformation of

a waste in a resource, lowering costs and environmental impact of waste management, allowing

sustainable production of heat and biogas for domestic use and promoting the production of

compost, a natural soil improver.

Moreover, it represents the up-grade of an existent CHRS, so the second object of the project is

the improvement of an existent system in way to make it more competitive among the energy

production technologies. The challenge of Microbioenergy® here presented, is the integration

of these technologies in way to compensate the disadvantages of each reactor type with the

advantages of the other.

29

2 Background 2.1 Biomass aerobic composting

Biomass aerobic biodegradation is an exothermic natural process in which different

microorganisms degrade the organic substrate into stabilized organic matter; remove

pathogens; allow mineralization and release CO2, water and heat according to the overall

reaction[13] (see the schematic diagram of Figure 3.1) :

organic substance [CH2O] + O2 ⟶ Compost + CO2 + H2O + NO3 −  + SO42 −  + HEAT

Figure 2.1 Schematic diagram of the aerobic bio-degradation process

2.1.1 Process phases

Bacteria different activities previously mentioned, happen simultaneously with decreasing

intensity during consecutive steps, these steps can be divided into two main bio-degradation

phases [14]:

Bio-oxidation phase

also called Active Composting Time (ACT) or high rate phase, is a thermophilic stage of quick

decomposition of the organic matter with high oxygen consumption. Bio-oxidation involves a

great quantity of easy molecules as sugars and lipids (ready to be metabolized and transformed

into intermediate composite as volatile fatty acids) and a high equivalent consumption of

oxygen. Heat production and temperature increase is as fast as there is a high oxygen

availability and a high fermentability composting substrate; aeration, so, is a fundamental

30

condition for the microbial process. The final product of this stage is fresh compost, a sanitized

material sufficiently stabilized by the bacteria activity, sanitation of the material is the most

important effect of bio-oxidation, with the elimination of pathogens.

Maturing phase

is a mesophilic stage, also called curing phase, of much slower decomposition and lower oxygen

consumption. With the disappearance of the most easily biodegradable compounds, the

metabolic transformations of decomposition involve the most complex organic molecules and

they are implemented with slower processes, also because of the death of a large part of the

microbial population due to lack of nourishment. This is the second phase, also called the

maturation phase, during which the metabolic processes decrease in intensity and fungi and

actinomycetes appears next to bacteria. These new microbial groups degrade starch, cellulose

and lignine, essential compounds of humus. In this phase the temperatures are lowered to

values of 40-45 ° C and then gradually go down, stabilizing around the environmental

temperature. During the process, the mass is also colonized by organisms belonging to the

micro-fauna, which enhance the composting process through its activity of shredding and

mixing the organic and mineral compounds. The final product is the mature compost, a stable

dark material with texture similar to that one of a well-structured soil, rich in humic

compounds. This material containing N, P, K nutrients is an organic soil improver useful in

mulching, it allows a lower use of chemical fertilizers in open field agriculture and a lower use

of peat for the preparation of growing substrates in floriculture.

Bio-oxidation phase Maturing phase

Degrading

organic substances

Simple molecules, ready to be

metabolized: sugars, organic acids,

aminoacids

Complex molecules, recalcitrant

substances remained in the matrix :

lignin, amide and cellulose

Active

microorganisms

Thermophilic microorganisms :

bacteria, fungi

Mesophilic microorganisms :

bacteria, fungi, actinomycetes,

eumycetes

Biodegradation

effects

H2O, CO2, NH3 production;

phytotoxic intermediates; odorous

substances : volatile fatty acids, sulfur

compounds

H2O, CO2, NH3 limited production

31

Temperature > 60°C ~ 45°C

Final product Fresh compost Mature compost

Table 2.1 Main features of Bio-oxidation and Maturing phases

2.1.2 Main process parameters

Temperature

Temperature is the parameter that gives information on the trend of the process and the

intensity of the reactions. Through the high rate phase temperature increase it is possible to

achieve the reduction of materials humidity; the sanitation of the product through the

abatement of the pathogenic charge present in the original matrix and the inactivation of the

seeds of weeds and plant pests [15].

Oxygen availability

As composting is an aerobic process oxygen is fundamental for active micro-organisms. The

amount of required oxygen is different depending on the phases of the process. The greatest

oxygen demands are found in the first stage of the process when the presence of readily

degradable materials favors multiplication and microbial activity with temperature increase

(between 40 ° C and 70 ° C) and carbon dioxide (CO2) production. The level of oxygen inside

the degradating mass must remain above 10% - 12%; if it falls below 5% anaerobic

microorganisms take over leading to the establishment of putrefactive type processes. In this

Figure 2.2 Forced and Passive aeration technologies (top and bottom respectively)

source [17]

32

case reduced compounds (such as volatile fatty acids , hydrogen, sulphide, mercaptans, etc.)

accumulate and producing a decidedly aggressive odor and high phytotoxicity [15]. In order to

enhance oxygen feeding inside a reactor a mechanism of forced aeration or passive aeration can

be applied (see Figure 2.2). Passive aeration, in particular, allows a sufficient oxygen flow inside

the reactor and a longer bio-oxidation time and, so, a longer reactor life [16].

Porosity, structure, texture and particle dimensions of the material within the heap

Porosity, structure and texture are correlated with physical properties of materials such as size,

shape and mechanical strength, and they affect aeration and, so, the composting process.

Porosity is a measure of the empty spaces in the biomass in composting and determines the

resistance to air circulation. It depends on the size of the particles, from the granulometric

distribution of the materials and the continuity of the interstices between the particles.

Obviously, larger and more uniform particles increase porosity. On the other hand, the

structure indicates the stiffness of the particles and, so, the resistance of the particles to collapse

and shrink and, therefore, a good degree of structure prevents the loss of porosity of the wet

substrate. Texture is the characteristic that describes the part of the substrate surface area

which is available for the aerobic microbial activity: the reactions of degradation predominantly

occur at the composting particle surface because oxygen easily diffuses through the empty

spaces delimited by the particles, but much slower through the liquid phase or the solid

materials. Thus, aerobic microorganisms are concentrated in the thin aqueous layer that

surrounds the substrate particles, using oxygen at the interface between the liquid phase and

the gaseous phase of the interstices (see Figure 2.3). As the extension of the surface area

increases with the reduction of the size, the rate of aerobic decomposition rises in an organic

matrix as much as possible the dimensions of the particles are small. Particles too small,

however, risk compromising porosity and it is therefore necessary to find a compromise

situation. Results satisfactory are normally obtained when the average particle diameter of the

matrix composted oscillates between 0.5 and 5 cm [15].

33

Figure 2.3 Composting particles of the organic substrate, source [15]

Moisture

Water plays a fundamental role in survival of micro-organisms as it represents a food, a means

for atmospheric oxygen dissolution and for the spread of nutrients and an important factor for

the thermo-regulation of the system. For these reasons, the compost heaps must be sufficiently

humid to allow adequate microbial activity without however preventing the oxygenation of the

mass. At humidity value lower than 20% aerobic microorganisms almost completely cease their

activity, if the water content exceeds 60% the oxygen flux is compromised becoming the

limiting factor[18][19]. Optimal humidity values (compatible with an aerobic condition) are in

the range from 40% to 55%[14][15].

C/N ratio between carbon (energy) and nitrogen (metabolism):

For the micro-organisms activity in the composting process carbon represent the energy source

and nitrogen the metabolism since it is necessary to synthesize proteins. An excess of carbon

causes a slowdown in the microbial activity and therefore in the decomposition, while an excess

of nitrogen involves losses of ammonia through volatization, above all with high pH and

temperature. Mixing green residues, rich in carbon, with mud of purification or organic waste,

rich in nitrogen, a good balance is ensured. Optimal C/N values are in the range 20-30. With

C/N >30 the substrate is nitrogen starved, with C/N

34

pH

During the composting process the pH value is extremely variable, even if the bacteria prefer

values close to neutrality. The initial phase presents an acidic pH with the development of

carbon dioxide and formation of organic acids; later, with aeration, the pH rises to values

between 8 and 9. At the end of the process however, the pH tends to values close to neutrality

(7.5-8).

2.1.3 Bio-assays

In the course of compost maturation the humic substances, which represent the most evolved

form of the organic substance, increase both in quantitative and qualitative terms [20]

(increase of humic acids with higher molecular weight with respect to fulvic acids). In addition

to the chemical-physical parameters above, the development of the composting process and the

quality of the final material depend on some bio-assay as:

• Phytotoxicity test: indicating the presence of mineral and organic substances which

are growth inhibitors and whose presence shows an insufficient maturation of the

compost (incorrect or incomplete process) [21].

• Nitrogen mineralization assay: which indicates the existing relationship between

organic nitrogen and mineral nitrogen and achieves a stability situation with the

product maturity [20].

• Potential dynamic respiratory index: IRDP evaluates the intensity of the

microbial activity through its consumption of oxygen and gradually decreases during

the process, when its value lows 500 [mgO2/(kgVS*h)] the material can be considered

as stable [22].

• Pathogens: which are often present in the initial matrix and are gradually eliminated

through the composting process and sanitation [23] (see Table 2.2).

Pathogens Time and temperature of elimination

Salmonella typhosa No development above 46 °C and die in 30’

at 55-60 °C or in 20’ at 60 °C

Salmonella sp. Die in 60’ at 55 °C or in 15-20’ at 60 °C

Shigella sp. Die in 60’ at 55 °C

Escherichia coli Die in 60’ at 55 °C or in 15-20’ at 60 °C

35

Eutamoeba

histolytica cyste

Die in few minutes at 45 °C or few seconds at

55 °C

Taenia saginata Die in few minutes at 55 °C

Trichinella spiralis

larva

Istantaneous die at 60 °C

Brucella abortus o

suis

Die in 3’ at 62 °C or in 60’ at 55 °C

Mycrococcus

pygenes var. aureus

Die in 10’ at 50 °C

Streptococcus

pyogenes

Die in 10’ at 50 °C

Mycobacterium

tubercolosis var.

hominis

Die in 15-20’ at 66 °C or instantaneously at

67 °C

Corynebabacterium

diphteriae

Die in 45’ at 55 °C

Ascaris lumbricoides

(eggs)

Die in 50-55’ at 50 °C

Table 2.2 Pathogens mortality depending on temperature, source [24]

2.2 Anaerobic digestion and dark fermentation

The Anaerobic Digestion is a microbiological process where different types of bacteria

decompose organic substrate, in absence of oxygen and with specific conditions of temperature

and pH.

Anaerobic Digestion overall reaction is:

CaHbOcNd⟶nCwHxOyNz + mCH4 + sCO2 + rH2O + (d − nx)NH3

With: s = a - nw - m and r = c - ny - 2s

As the reaction shows, the partial decomposition of complex organic substrate implies the

formation of methane, carbon dioxide, water and ammonia.

The schematic diagram of Figure 4.2 includes the main input materials and output products.

36

Figure 2.4 Schematic diagram of Anaerobic Digestion process

2.2.1 Phases of AD processes

The partial degradation does not happen in a single step, but needs different transformations.

In this process four different phases are present, with different groups of bacteria acting in

order to decompose the substrates organic portion in simpler matter, running in parallel time

and space inside the reactor[24]:

Hydrolysis

is the transformation of complex organic material, such as proteins, carbohydrates and lipids,

into liquefied monomer and polymers (respectively amino acids, monosaccharides and fatty

acids). Acting bacteria could directly decompose the organic material or produce extracellular

enzyme which are able to divide the organic complex molecules into oligomers and monomers,

available for the next acidogenic step. The hydrolytic process could be hindered by the

accumulation of amino acids and sugar because they generate interference and inhibition of

hydrolytic enzyme[24].

Acidogenesis

is the conversion of hydrolytic step products into methanogenic substrate by acidogenic

bacteria[10]. The bacteria transform the soluble organic monomers of sugars and amino acids

into ethanol, long chain acids (propionic and butyric) and short chain acids (volatile fatty

acids), acetate, hydrogen and carbon dioxide[25]. Moreover, from the degradation of amino

acids ammonia is produced[24].

37

Acetogenesis

is essential to convert, into methanogenic substrate, all the products from acidogenesis that

cannot be directly transformed into methane. Starting from the products of hydrolysis and

acidogenesis, the acetogenic bacteria produce acetic acids, formic acid, H2 and CO2. The

mechanism of degradation strictly depends by starting product, if they are long chain fatty acids

or short chain fatty acids [25]. During this reaction, the BOD (biological oxygen demand) and

the COD (chemical oxygen demand) are both reduced and the pH decreased. Hydrogen plays

an important intermediary role in this process, as the reaction will only occur if the partial

pressure is low enough to thermodynamically allow the conversion of all the acids.

Methanogenesis

is the production of methane, the final product of anaerobic digestion. Methanogenesis in

microbes is a form of anaerobic respiration [26] where methanogens do not use oxygen (in fact,

oxygen inhibits the growth of methanogens) but another electron acceptor: carbon. The two

main methanogenesis process pathways are the acetoclastic methanogenesis (which account

for 70% of methane production) and the reduction of carbon dioxide (which account for 30%

of methane production[27]):

• the methane production takes place from the acetic acid degradation:

CH3COOOH⟶CH4 + CO2

• the hydrogenotrophic bacteria produces methane oxidizing the hydrogen, using the

carbon dioxide as an electrons taker, following the reaction:

4H2 + CO2⟶CH4 + 2H2O

It’s worth to notice that, considering

α = hydrolysis/acidogenesis velocity, β = acetogenesis velocity, γ = methanogenesis

velocity

in order not to have intermediate products accumulation it should be α 

38

A low value of pH show an accumulation of volatile fatty acids, generally due to an overload of

the reactor that could cause the bacteria inhibition. A high value of pH, on the other hand, is

an index of accumulation of ammonia in the reactor that could inhibit the acidogenic and

methanogenic bacteria. It is possible to notice if the reactor is working in a wrong range of pH

because the gas production becomes irregular, within a rising of carbon dioxide percentage. In

order to adjust the pH value, in case it became more acid, one can add lime or sodium

bicarbonate. On one hand, lime is cheaper but generates solid precipitation that could obstruct

the piping system if used in large quantities. On the other hand, sodium bicarbonate is

completely soluble and can adjust the pH value very rapidly, but presents a high cost [25].

Temperature

Temperature is extremely important for bacteria operation and survival, outside the range that

goes from 4°C to 80°C anaerobic bacteria cannot survive[28]. Inside this interval it is possible

to identify three different bacteria categories each operating in a different temperature range

and able of a different methane productivity:

• Thermophilic bacteria (35-80°C): most productive, leading to a higher pathogen

destruction, but suffering of several problems of stability and adaptability of

temperature change

(-/+ 1°C/h) [10]

• Mesophilic bacteria (10-45°C): able to resist at higher temperature fluctuation (-/+

3°C/h), they don’t need a strict temperature control and can be implemented in tropical

and temperate region with no energy added cost; moreover, thanks to a lower

temperature there is less production of ammonia, which could inhibit the process [28]

• Psychrophilic bacteria (4-20°C): less productive due to their particularly stiffer cellular

membrane. Since the nutrient, useful for the bacteria activity and methane production,

are transported in and out of the cell throughout the cellular membrane, a stiffer

membrane makes it difficult to transport the nutrient [29].

39

Hydraulic Retention Time (HRT)

The HRT is the average time that the inlet flow rate needs in order to cross the digester. It

strictly depends by the type of bacteria and therefore by the temperature as the Table 3.3 shows

[30].

Bacteria type Range of temperature HRT

Thermophilic 55-60°C ~10 days

Mesophilic 27-35°C 20-25 days

Psychrophilic

40

It is evident that, in order to choose the right HRT value, multiple aspects must be take into

account: the operating temperature, the substrate availability and volume, and finally the cost

of the digester and the correspondent biogas productivity both depending on the digester size.

The final choice of the HRT value is a trade-off solution[30].

Solid Retention Time (SRT)

The SRT is the HRT equivalent parameter for biomass with high waste-to-water ratio; it

represents the average time the activated-sludge solids are in the system, meaning the average

time available for microorganisms to reproduce. It is an important design and operating

parameter for the activated-sludge processes and it is usually expressed in days.

Organic Loading Rate (OLR)

This parameter indicates the quantity of digestible substrate that enter in the digester per unit

of time and volume and is defined as:

OLR = m̊*VS ⁄ VD [kgVS/(m3*days)]

Where:

VD =volume of the digester [m3]

m̊ = mass of substrate fed by time unit [kg3/day]

VS = quantity of volatile solid per quantity of substrate [kgVS/kg]

The OLR must be chosen according to the reactor ability to decompose the substrate:

If the quantity of organic matter entering in the reactor is too high, the acetogenic bacteria

produces acetate faster than the methanogens bacteria can utilize, leading to a reduction in

methane production. It is also possible to notice an increment of volatile fatty acids (VFA).

If the quantity of organic substrate is too low, the bacteria does not have enough feeding matter

and this lead to a decrease of biogas production. For what concern an industrial reactor, the

lower limit of OLR is 4-8 kgVS/m3 day. Due to the fact that the biodigesters for rural area do

not present temperature control system nor mixing system, the value of OLR must be lower,

because the decomposition ability of the bacteria is lower. A recommended OLR value for these

technologies is 2 kgVS/m3 day [28].

41

Inhibitors

The most dangerous substances for the anaerobic digestion bacteria are: oxygen, hydrogen

sulphide (H2S), organic acids, free ammonia, heavy metals and other hazardous substances as

disinfectants, herbicides, insecticides and antibiotics [25].

Among them ammonia is the most common, especially in the substrates with animal origin,

and could be produced inside the digester starting from the proteins [10]. The inhibition

generates an accumulation of intermediate products, such as volatile fatty acids, and leads to

an acidification of the reactor. Due to the fact that the inhibition mainly affects the

methanogenic bacteria, the presence of ammonia generates a decreasing in methane

production.

Moreover, the quantity of lignin (with respect to cellulose and hemicellulose one) in the

degradating substrate must be considered because, even if is not a toxic substance, it slows

down the hydrolysis phase, leading to a reduction of gas production.

2.2.3 Biogas

Biogas is the main product of the anaerobic digestion and can be used in different applications

such as heating, power production and cooking fuel. The biogas is a mix of different gasses that

comes from the anaerobic digestion of organic substrate. In Table 2.4 is reported the common

composition of biogas, that could present some variations according to the variation in

temperature and substrate.[30]

Component Symbol Volume Concentration

[%]

Methane CH4 55-70

Carbon dioxide CO2 35-40

Water H2O 2 (20°C) – 7 (40°C)

Hydrogen Sulphide H2S 20-20000 ppm (2%)

Nitrogen N2

42

Among all components methane is the most relevant one and also it’s the compound greater

interest because it has a high lower calorific value. Methane is present in biogas in quantities

ranging from 40 to 70% (by volume). The production of this compound depends strongly on

many operating parameters, such as the temperature in the digester or the pH, as well as the

quality of the substrate, its handling and the water content.

2.2.4 Dark fermentation (DF) and two-stages AD

Dark fermentation is the simplest hydrolysis and primary fermentation to extract, from raw

biomass, gaseous and soluble mixtures of compounds, especially H2 and CO2, which are

precursors of secondary bio-transformations [31]. Dark fermentation is composed of the first

two phases of anaerobic digestion, hydrolysis and acidogenesis [32]. Its objective is to produce

hydrogen, hence the growth of methanogens, which consume hydrogen, is inhibited by

operating under suitable conditions: low hydraulic residence time, inoculum pretreatment and

pH lower than 7 [33].

Utilization of wastewater, green waste and agricultural residues as potential substrate of dark

fermentation has raised interest in the last years, thanks to their favourable characteristics in

term of cost, availability and biodegradability. In literature, dark fermentation studies have

focused mostly on maximizing H2 production, while integrated chain hydrolysis–fermentation

has been less studied as-a-whole.

Since dark fermentation represents the first steps of anaerobic digestion and it is exploited in

the acidification reactor of two-stage anaerobic digestion plants in order to divide the process

phases according to the following description.

1. Single stage plant consists in a single reactor in which all the anaerobic digestion

process steps take place with a change of internal environmental conditions and

bacteria populations during the process. Generally, it’s simpler than two-stage systems

and cheaper to construct but presents some limitations due to the fact that different

anaerobic trophic groups perform better under different environmental conditions.

2. To provide optimal environmental conditions for each group of microorganisms the

two stages system approach proposes to spatially separate the acidogenic stage from

the methanogenic stage by the use of the two reactors (Acidification and Methane

43

Figure 2.6 Single Stage System vs Two Stage System

reactor of Figure 2.6) [34]. In this way the first stage features the formation of volatile

fatty acids, while the conversion of volatile fatty acids to methane and carbon dioxide

takes place in the second phase [35]. Compared to the traditional single-stage AD

process, the two-stage approach has been proposed by several authors as a possible

solution to improve the overall process efficiency, in terms of biodegradation

rates/yields and overall energy productivity. Some authors reported that splitting and

separately optimizing hydrolysis/acidogenesis and methanogenesis could enhance the

overall reaction rate, maximize biogas yields, and make the process easier to control,

both in meso- and thermophilic conditions [36] [37]. Many authors relied on the

possibility of enhancing methane production rates in the second stage, taking

advantage of the fact that hydrolyzed and pre-fermented organic matter (OM) is more

available to methanogens, as compared to the untreated substrate [38] [39].

Furthermore different studies reported up to 7% higher methane yields in two-stage

leach-bed systems in comparison to single-stage systems with the digestion of

grass silage [40].

Advantages of Multi Stage Systems Disadvantages of Multi Stage Systems

Greater biological stability More complex control and operational

requirements

Potentially higher throughputs due to optimal

conditions

Potentially higher capital costs

Greater ability to cope with fluctuating

feedstock volume and quality

Table 2.5 Pros and Cons of Multi State Systems

44

45

3 State of the art 3.1 Composting Heat Recovery System (CHRS)

Even though there is a rich literature about composting process, literature regarding heat

generation based on composting is disjointed and incomplete. The main cause is that many

CHRS advances have been made through independent projects, carried out by individuals

enterprising seeking inexpensive energy for their homes or farms. While it is unknown when

humans first began to utilize the heat from decomposing organic matter, it is known that rural

farmers in northern China were capturing this renewable heat source over 2000 years ago, with

the use of hotbeds [41]. This CHRS was constructed by digging a 1 m trench, filling it with

manure, and covering it with a layer of topsoil for crop production (See Figure 3.1).

When planted above the decomposing manure, plants benefited from the microbial heat

production generated below, allowing for season extension of 1–2 months in the spring and fall.

Extracting heat from compost was further refined in France, starting in the 1600s, where

hectares of glass-enclosed hotbeds were utilized for winter cultivation and season extension

[43]. During this period, the most commonly used feed stock was horse manure. The glass-

enclosed French hotbeds also required less manure than the hotbeds used in China. Aquatias

[44] described using only a depth of 25.4 cm of compost when compacted, or 30.5 cm– 33 cm

when using loose manure. This heat recovery method was suitable for growing winter crops

capable of handling soil temperatures below 10°C–13°C. Large-scale use of French hotbeds

came to an end in the 1920s, as the horse was replaced with the automobile. With the primary

Figure 3.1 Hotbed scheme, source [42]

46

composting feed stock (horse manure) not being as abundant, large-scale use of this CHRS

disappeared [43].

3.1.1 Jean Pain’s Method 1972

CHRSs made a significant leap forward, with the publication of Pain and Pain (1972)[45]. This

book described the work of Jean Pain and his combined heat and power composting system in

France (see Figure 3.6). Prior to this work, the thermal energy from composting was primarily

recovered passively via convection of heat to the root zone of crops. Jean Pain’s system, called

a Pain mound, was very different, utilizing a 50 ton heap of chipped brushwood with hundreds

of meters of water-filled tubing embedded in the compost for heat exchange. The decomposing

brushwood warmed water within the tubing via conduction. The ability to warm water

significantly increased the user of the CHRS, as it could be used for more than just agricultural

purposes. Pain and Pain [45] reported that a 50 ton heap of brushwood warmed well water

from 10°C to 60°C at a rate of 4 liters per minute for 6 months, without interfering with the

composting process. The system supplied domestic hot water and heating to a 100 m2

farmhouse for 6 months, by circulating hot water from within the composting mass to a cast

iron heater. When looking at heat recovery, the system was capable of extracting 50,115 kJ/hr

or 4330 kJ/kgDM (DM:dry matter) over a 6-month period. In addition to using compost heat

for the farmstead, the authors described using a mound with 16,800 kg of feed stock to heat a

105 m2 (211 m3) high tunnel. The authors reported that they were capable of growing fruits and

vegetables in spring-like conditions during the winter season.

3.1.2 CHRS developments

During 1981-1990 there has been an increased interest in extracting heat from composting due

to the volatile energy prices that occurred during the previous decade, with the oil crises of 1973

and 1979 [42]. Small-scale lab experiments were conducted to determine the most effective

methods of extracting heat from the composting process and also in 1981, the Biothermal

Energy Center (BTEC) formed in Portland, Maine, USA, with the mission of developing small-

scale composting greenhouses[43].

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During centuries pilot demonstration projects have been scaled up to obtain commercial level

systems and parallel increased activity of peer-reviewed literature has helped confirming many

findings reported from independent organizations and individuals. Moreover student theses

have been added to practitioner-oriented sources and peer-reviewed publications.

The CHRS on which Microbioenergy® is based on is the Aerated Static Pile (ASP) composting

method, combined with a hydronic system for heat transport. This method has been developed

on a commercial level and particularly diffused in USA (Agrilab Technologies; Compost Power

Network), Canada (GORE TM), UK (Alpheco Composting), Germany (Native Power),

Netherlands (Biomeiler). The pioneer in CHRS application and diffusion in Italy is Andrea

Brugnolli, who has coined the name Thermocompost for the compost ASP with hydronic

system.

Figure 3.2 Jean Pain Method, source [42]

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Operative life

It must be underline that biomass actual amount of heat produced during composting and

process duration are determined by many variables such as feed stock energy content, feed

stock biodegradability, plant management and environmental conditions (e.g. moisture and

temperature). Literature research presented by Smith M. [42] indicates a variable CHRS

operative life ranging from 6 month to 12-18 months [41]; Native Power organization too

indicate.

3.1.3 CHRS Modeling

There is no linear proportion between size and thermal power, this is mainly due to two

processes:

1. Thermodynamic process: for two ASP with the same volume, the one with higher

external area (the rectangular-base one) suffers higher thermal dispersions through

convective heat transfer.

2. Chemical process: the ASP most internal part lacks of oxygen and, so, it is not able to

operate the bio-oxidation. That’s why it is mandatory for larger plants to include an

aeration system to reach a higher thermal power.

In order to understand temperature and oxygen distribution inside the ASP the figures below

have been reported from “Investigation on Biomeiler Operating Behavior” [41] (where

Biomeiler is the German name for the wood-chip ASP with hydronic system). Every figure

Figure 3.3 Temperature distribution inside the woodchip ASP

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shows three images of the same round wood-chip ASP middle section at different times (the

initial, the intermediate and the final condition), x and y axes show the wood-chip ASP

dimensions (the internal diameter on x-axes, from the external surface 0 m to the center 2 m;

the height on y-axes ) and the colors show the internal distribution of Temperature (Figure 3.3),

Oxygen, Methane and Carbon dioxide (Figure 3.4).

Figure 3.4 Oxygen, Methane and Carbon Dioxide distribution inside the wood-chip ASP

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Due to the thermodynamic and the chemical processes previously mentioned, different zones

can be identified inside the reactor:

• the more external and upper zone is affected by the convective heat transfer with the

external air, with the consequently decrease of external reactor layer temperature;

• the more internal zone suffers of oxygen lack and anaerobic conditions formation, this

effect causes the exothermic bio-oxidation process stop while methane and carbon

dioxide undesirable production starts;

• the final intermediate zone shows a correct behavior with consequently higher

temperatures.

3.2 Small-scale anaerobic digestion technologies

The anaerobic digester is a mature technology exploited all around the world for the production

of biogas and implemented both into thermal power plants and electricity generation plants.

For the application of the AD into the Microbioenergy® system, only the low-cost AD

technologies have been analyzed in this chapter, these technologies are usually applied in rural

areas context where they supply to the energy needs of single families. These technologies are

the most suitable for Microbioenergy® project necessities since they represent simple

solutions, able to fit preexistent plants minimizing costs and times of construction and

management. Generally, in all biogas systems, it is possible to identify three macro

components: the reactor (or digester) which is the central part of the plant; the digestate post

treatment system consisting in a piping and tank system that guarantees the correct digestate

treatment and use; the distribution system, composed by the piping system for gas distribution,

filter system, valves and storage tanks.

3.2.1 Fixed dome digester

The plant is composed of a dome-shaped reactor, a gasometer, and another displacement tank,

also called a compensation tank. When the biogas starts to form, there is a lowering of the level

of the gas - mud interface: at the same time part of the digestate goes up a tube that leads it to

the compensation tank and the biogas flows through a second pipe to be then stored in the

gasometer. This plant type construction requires specialized technicians supervision due to its

complexity.

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The plant is built under the ground level to provide thermal insulation, avoid temperature

fluctuations between day and night which negatively affect the process, and to protect it from

atmospheric events. Moreover, this location allows the reactor to be combined with heating

sources (for more complex digesters). The materials used in construction are generally bricks,

cement and reinforced concrete; these materials are not impermeable to the biogas created at

the top of the digester, for this reason it is necessary to treat the internal and external dome

surface with synthetic paints or to cover it with latex. The choice of materials depends on the

costs, the local availability, the ability to work, the efficiency and finally the duration.[30]

Advantages Disadvantages

Long life span Specific technical skills are needed

Not damageable due to underground

construction

Difficult to repair once constructed

Relative low construction cost Specific sealant is required for the inside

plastering

Table 3.1 Fixed dome advantages vs disadvantages

3.2.2 Floating drum digester

The plant is composed of a cylindrical reactor and a mobile gasometer, called drum, which

floats above the sludge or in an impermeable bag connected to a guide framed to the digester

walls which gives it stability and keeps it in the vertical position.

Figure 3.5 Fixed-dome digester work principle

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When the biogas production process begins, the drum rises and when biogas is consumed it

returns to the starting position. The materials used in the construction are bricks, concrete or

stone quarries previously plastered. The gasometer can be a tank of thin steel plates or a

cheaper and less robust plastic balloon. Since the floating drum is usually placed outside and

exposed to solar radiation, biogas production is influenced to solar radiation itself and

consequently is influenced to the gasometer color ( greater production for black and red colored

gasometer, lower production for blue or white ones).[30]

Advantages Disadvantages

Simple and easy operation Specific technical skills are needed