energy balance and exergy analysis of large scale algal biomass production

7/27/2019 Energy Balance and Exergy Analysis of Large Scale Algal Biomass Production

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The 2nd

Korea - Indonesia Workshop & International Symposium on Bioenergy from Biomass

DRN Building, Puspiptek, Serpong-BSD City, Indonesia, 13 15 June 2012

Page 66 of 153

ISSN: 2302-1454 (online)

Energy Balance and Exergy analysis of large scale algal biomass production

K. Sudhakar1, M. Premalatha2K.Sudharshan3

1Energy department, Maulana Azad National Institute of Technology, Bhopal, India

2 CEESAT, National Institute of Technology, Tiruchirapalli, India

* Corresponding author; e-mail: [email protected]; tel.: +62-21-7560929; fax: +62-21-756-0549.

Abstract:Microalgae Technology for the production of biochemical and bio fuels is emerging rapidly. Large-scale production facilities

are necessary to fulfil the expected future demand for biodiesel and biochemical produced with algae. The present study assesses the

sustainability of biofuel produced from microalgae and examines the environmental feasibility of a large-scale production through the

use of energy balance and Exergy analysis adopting Life Cycle concept.Estimates of energy requirements for cultivation, harvesting, and

oil extraction for algae biodiesel production are developed. Energy output in the form of algal biodiesel and the total energy content of

algal biomass are compared to energy inputs required for algal cultivation & processing. Exergy analysis of algal-biodiesel cycle shows

the overall process to be renewable. The results obtained indicate the energy content of the algae produced exceeds the energy required

for cultivating algae.

Keywords: Biodiesel, Large open pond, Life Cycle Analysis, Microalgae, Net energy Ratio

1. Introduction

Without a doubt, the world is now dependent on

alternate sources of energy. Eukaryotic microalgae

represent a promising alternative renewable source of

feedstock for biofuel production. With over 40,000

identified species; microalgae are one of the more diverse

groups of organisms on Earth. Algae has seemed like a

great renewable energy source because it's extremely

efficient at creating energy from sunlight and it could

potentially form closed loops for power plants - absorbing

exhaust while creating new fuel. (Sheehan, 1998). hey

naturally produce large quantities of biomass and many

biomaterials, including lipids/oil.

The theoretical maximum biomass and oil production

of from microalgae has been calculated at 240 T ha-1

yr-1

and 57, 000 L ha-1

yr-1

for Indian

Conditions.(Sudhakar.et.al .,2012),

Algal biomass residues derived from the oil extraction

process can be used to produce ethanol, and methane, and

high-value biomaterials, such as biopolymers, carotenoids,

and very long-chain polyunsaturated fatty acids.(Raja.R.et

al.,2008)

Microalgae naturally remove and recycle nutrients

(such as nitrogen and phosphorous) from water and

wastewater and carbon dioxide from flue-gases emitted

from fossil fuel-fired power plants, providing an added

environmental benefit (Li.Y.et al.,2008)

The integration of wastewater bioremediation and

carbon sequestration with biofuel production has not been

demonstrated on Industrial scale. The micro algal research

and development effort couples the use of microalgae for

biofuels production with environmental bioremediation.

However there are many challenges to be addressed which

include refinement of the cultivation process, downstream

processing of biomass and development of an economic

feasibility model for commercialization of algae-basedbiofuels and biomaterials.Algae may be grown in large

open raceway ponds or closed photo bioreactors. A

number of closed photo bioreactors are being investigated,

for cost-effective production of the algae. These include

horizontal tubular, vertical tubular, thin film and

helical/inclined systems. Productivity is higher in the case

of algae cultivated in a photo bioreactor, but capitals and

operating expenses are significantly higher than for open

systems.(Chisti.Y.,2007)

Fig. 1, Shows algae cultivation systems.

(a)Open Pond

(b) Tubular Photo bioreactor

Fig 1: Algae Cultivation System

Over the past two decades, algal-based biofuel

research has progressed from outdoor large open pond to

large-scale photobioreactor design and optimization, and

downstream processing (i.e., harvesting, dewatering, and

drying), to algal oil extraction. At the moment, algal

biodiesels are not commercially produced and no

economically viable production processes exist. Hence

there exist many unknowns associated with the algae

biodiesel production .Based on the state of the technology,there exists a need to quantify the energy and

environmental sustainability effects of microalgae-to-

biofuel process.

PAPER CODE: OP-027


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Hence the key objective of this study is to perform a LCA

of the large scale microalgae biodiesel process and to

identify the specific research efforts to make this process

environmentally sustainable. This consequential life cycle

analysis of algae biodiesel is based on estimated

production potential of large scale algae biomass

cultivation, process thermodynamics, and academicliterature data. This LCA follows algal biodiesel

production through five stages: cultivation, harvesting,

lipid extraction, transesterification, and reuse of the

leftover biomass. The carbon and energy balance for each

stage of production considering energy required, the

energy produced, and the environmental impact are

quantified.

2. Materials and Method

Large Scale open raceway pond cultivation system is

considered in this study. For this energy input at each stage

of the cycle during cultivation, harvesting, dewatering, oilextraction, transesterification is estimated. Total of energy

input in all the process gives the total energy demand of

the algae biodiesel cycle, which is energy input to the

system. This energy input is called as indirect energy. The

energy contained in the micro-algal bio-diesel (Eout) is

compared with the total energy input (Ein).

Fig.2. Flow diagram of Algal Biodiesel production process.

Fig. 2 shows a flowchart listing main stages and inputs

of microalgae biodiesel production process.Fig.3 shows

step involved in LCA process.

1. Goal Definition

2. Life Cycle Inventory

3. Impact Assessment

4. Interpretation

1. Goal Definition

2. Life Cycle Inventory

3. Impact Assessment

4. Interpretation

Fig.3. Life Cycle Assessment framework.

Life cycle assessment (LCA) is a decision making tool

to identify environmental burdens and evaluate the

environmental consequences of a product, process or

service over its life cycle from cradle to grave .The

functional unit for the LCA analysis is 1 hectare of land.

To evaluate the energy consumption during the life cycle

of the micro-algae bio-diesel, a cradle to grave analysis is

carried out. The data were organised in a MS-Excel

spreadsheet, for transparency and easier calculations.

Considered in the interaction during the lifecycle is

indirect energy given to the system and not the materials,

direct energy inputs like solar energy taken by the plant

(micro-algae).

2.1 Exergy Analysis and Energy balance

When converting energy from one form to another

more useful form the second law of thermodynamics

dictates that some energy will be lost. Net energy gain

(NEG), the difference between the total energy outputs and

total energy inputs, is one of the accepted indices for

analysing biofuels.

Net Energy Gain = Energy output Energy Input.

2.2. Carbon balance

CO2 Reduction = CO2 Sequestered by microalgae

CO2 produced during each Input stage.

2.3 Net Energy ratio (NER)

The energy efficiency of bio fuels can be analyzed by

finding Net Energy ratio (NER). Net energy ratio is

defined as the ratio of total energy outputs to total energy

inputs.

Ein =Egrowth+Eharvest+Edewatering +Edrying

+Eoilextraction+Etransesterification (MJ ha-1

)

Eout = CV *T*1000*Where, CV= Calorific value of Micro algal biodiesel (MJ

Kg-1

) = 41 (MJ Kg-1

) (Q.Wu.et al., 2006)

T= Quantity of biodiesel (tonne)

=brake-thermal efficiency of a CI Engine (20%)

Water

Water

Water CO2 AlgaeLight

Cultivation

Harvesting/

Drying

Transesterification

Extraction

Liquid Algae

Algae Cake

Green Crude Biomass

Glycerol Bio-Diesel


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2.4 Assumptions:

The various parameters and values used for this study are

listed in Table 1.

Table 1. Parameters used for the study (Liaw.et al,2010)

Stage Inputs Unit value

Growth Diesel fuel

consumption

L/ha 10

Electricity

Consumption

KWh/ha 41404

Algae

dewatering

Electricity

use

KWh/ha 30788

Lipid

extraction

Natural

gas

Consumption

MJ/ha 141994

Electricity

consumption

KWh/ha 12706

3. ResultsNet Energy balance, GHG Balance and NER for algal

biodiesel production based on current technology, has been

determined.

3.1 Net Energy Balance Assessment of large scale raceway

ponds.

(i)Algal Cultivation:

Diesel Use in growing microalgae = 10 L/hectare/year.

Calorific Value of Conventional Diesel = 44,800 KJ/kg

Density of Diesel = 0.832 kg/L.

Therefore, Mass of diesel in 10 L = 8.32 kg

Energy consumption of Diesel = 372.74 MJ/hectare/year.

Electricity Use in growing microalgae = 41,404 KWh =

149054.4 MJ/hectare/year.Total energy consumption in cultivation = 149427.14

MJ/hectare/year

(ii)Biomass Processing:

Biomass processing (dewatering, drying) electricity use

= 30,788 KWh/hectare/year.

= 110836.8 MJ/hectare/year.

Energy consumption in biomass processing is 110836.8

MJ/hectare/year .

(iii)Extraction and Esterification :

Electricity Consumption in expeller = 12,706 KWh =

45741.6 MJ/hectare/year

Natural gas required = 1, 41,994 MJ/hectare/year

Total power consumption in extraction and esterificationis 187735.6 MJ/hectare/year.

(iv)Energy Available in Biodiesel Produced:

Total quantity of biodiesel produced = 40,000

kg/hectare/year.

Calorific Value of Biodiesel = 37,800 KJ/kg

Total energy available = 1512000 MJ.

(v)Net Energy Assessment:

Net Energy generated = 1512000 - 149427.14 -

110836.8 - 187735.6 = 1064000.46 MJ/hectare/year.

Net energy generated is 1064000.46 MJ/hectare/year

from Microalgae Biodiesel.

A cumulative energy analysis of algae biodiesel process

is shown in Fig.4

Fig 4: Energy assessment of Microalgae biodiesel.

3. 2 Carbon balance of large scale raceway ponds

(i)Algae Cultivation:

Diesel Use in growing microalgae = 10 L/hectare/year.

Electricity Use in growing microalgae = 41,404

KWh/hectare/year.

Carbon dioxide emission for diesel = 22.2 gallons/pound.

I.e. Carbon dioxide emissions = 22.2 x 8.34 L/kg =

185.15 L/kg

Density of Diesel = 0.832 kg/L.

Therefore, Mass of diesel in 10 L = 8.32 kg

So, carbon dioxide emission = 8.32 x 185.15 L =

1540.45 L of CO2

= 0.154 m3

of CO2

Density of CO2 = 1.842 kg/m3.

Therefore, weight of CO2 = 1.842 x 0.154 = 0.28

kg/hectare/year.

From electricity usage = 41,404 x 0.8 = 33,123.2

kg/hectare/year of CO2

Therefore, CO2 emission in growth stage = 33123.5

kg/hectare/year.

Total Carbon Dioxide emissions = 33123.78

kg/hectare/year.

(ii)Carbon Dioxide Sequestration by Microalgae:

1 gram of algal biomass consumes 1.8 grams of CO2

So, 91,000 kg/hectare of algae consumes (91,000 x 1.8)

= 1, 63,800 kg/hectare/year.

Total Carbon Dioxide sequestered during growth of

microalgae is 1, 63,800 kg/hectare/year.

(iii)Biomass Processing:

Dewatering electricity use = 30,788 KWh/hectare/year.Carbon Dioxide emissions = 30,788 x 0.8 = 24,630

kg/hectare/year.

Carbon Dioxide emissions in Biomass Processing are

24,630 kg/hectare/year.

(iv)Extraction and Esterification:

Electricity Consumption in expeller = 12,706

KWh/hectare/year

Carbon Dioxide emission = 12,706 x 0.8 = 10,164.8

kg/hectare/year.

Natural gas required = 1, 41,994 MJ/hectare/year

Total power consumption = 1, 41,994 x (33/1000)

= 4685.80 kg of CO2

Total Carbon dioxide emission during extraction is10164.8 + 4685.80 = 14, 850.60 kg.


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energy balance and exergy analysis of large scale algal biomass production

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