utilisation of biogas at shatin sewage treatment works final · utilisation of biogas at shatin...

Conference 2012 - Efficient Energy Management in the Water and Waste Industry

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Utilisation of Biogas at Shatin Sewage Treatment Works

FUNG Wing Cheong and YEUNG Tak Kuai

Electrical and Mechanical Projects Division

Drainage Services Department

Abstract A significant amount of energy is consumed in the wastewater

treatment. For years, designers and operators have been working hard to reduce the

energy input. Global climate change makes the issue become more pressing than

ever. Incorporation of anaerobic digestion in a wastewater treatment plant not only

reduces the sludge to be disposed of, biogas is also produced as by-product of the

process which is a source of energy. Power and heat generation from biogas has

been practiced for nearly 30 years in Shatin Sewage Treatment Works. Through the

case of Shatin Sewage Treatment Works, this paper will illustrate how biogas could be

used to reduce the energy bill. The design considerations of the replacement of the

old dual fuel generator with the new gas engine will also be presented in this paper.

1. Introduction

The Shatin Sewage Treatment Works (Shatin STW) is the largest secondary sewage

treatment works in Hong Kong. It occupies 28 hectares of land and serves population

of 600,000 in Shatin and Ma On Shan Districts, treating 250,000 m3 of sewage per

day. The Shatin STW was commissioned in 1982 with the treatment capacity of

100,000 m3 per day. After the Stage III Extension recently, the treatment capacity has

increased to 340,000 m3 per day.

The Shatin STW is one of the leaders in the application of renewable energy in Hong

Kong. There are six dual fuel generators supplying electricity and hot water for the

use of the sewage treatment plant itself. The dual fuel engines are powered by the

biogas produced in the anaerobic digestion process. After nearly 30 years in

operation, a new generation of power generation engine is to replace the old dual fuel

engine. This paper reviews the experience of using biogas for power generation and

shares the experience in the design of the new combined heat and power generator in

the Shatin STW.

2. Biogas Production and Characteristics

2.1 Production of Biogas in Shatin Sewage Treatment Works


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The Shatin STW is a typical secondary sewage treatment plant incorporated with

biological nutrient removal (see Figure 1). Sewage entering STW is firstly screened

and degritted. It is then diverted to primary sedimentation tanks, in which around

50% of suspended solids are removed. The removed suspended solids are known as

primary sludge.

The primarily treated sewage is then fed into the aeration tanks where microorganisms

absorb or adsorb majority of the organic pollutants remained after primary treatment.

Microorganisms forming the activated sludge grow in the aeration tank.

Liquid-solids separation is done in the final sedimentation tanks. Removal of the

surplus activated sludge maintains the required population of microorganisms in the

treatment process. It is thickened and digested in the anaerobic digester together

with primary sludge.

Figure 1 Flow Diagram of Shatin Sewage Treatment Works

Removal of the organic pollutants, i.e. biochemical oxygen demand or chemical

oxygen demands, is achieved mainly through the removal of primary sludge and

surplus activated sludge from sewage. Mesophilic anaerobic digestion is adopted in

the Shatin STW and the organic matters are decomposed into simple chemicals.

Anaerobic digestion involves a series of processes, such as hydrolysis, acidogenesis,


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methanogenesis. Microorganisms break down the biodegradable materials in the

absence of oxygen and, eventually, methane is produced.

The major environmental factors in the anaerobic digestion process are (1) retention

time, (2) temperature, (3) alkalinity, (4) pH and (5) the presence of inhibitory

substances. Among them, the presence of sulphate has greater impact on the

production of biogas in the Shatin STW than in many other places. Sea water toilet

flushing brings in a large amount of sulphate in sewage and sludge. During anaerobic

digestion process, sulfate is reduced to sulfide. The sulfide formed is an inhibitor to

anaerobic digestion. High hydrogen sulfide content (H2S gas) in biogas can adversely

affect the operation of downstream combustion equipment. It is important to control

the H2S concentration in the biogas because combustion will lead to the oxidation of

sulphide into sulphur dioxide which will become sulphuric acid. Sulphuric acid will

damage the combustion equipment. In order to tackle this problem, the Shatin STW

has been using ferric chloride (FeCl3) to control sulfide. FeCl3 dosing can suppress H2S

in biogas to a level suitable for the generators downstream. FeCl3 solution is added to

the digestion tanks and sulphide is precipitated resulting less hydrogen sulphide in the

sludge and hence in the biogas.

Between January 2009 and February 2012, the monthly production rate of biogas

ranges from 304,518m3 to 561,917m

3. Besides the fluctuation in biogas production,

there are also some degree of variation in the gas composition, which can be seen in

the following table:

Compositions Monthly Average

Methane, CH4 49-65 %

Carbon dioxide, CO2 36-48 %

Hydrogen sulphide, H2S 100 – 4705 ppm

3. Existing Dual Fuel Generators

Methane is a flammable gas and its combustion gives out heat. Instead of

flaring off, there are six dual fuel generators installed in the Shatin STW

producing electricity from biogas. Each generator has a four-stroke

compression ignition engine, which operates mainly on biogas fuel with a small

percentage of diesel as pilot fuel to initiate combustion or may operate with

diesel only. The normal power rating is 1.12MW when biogas is used. An

11kV internal grid has been built in Shatin STW to receive the power from the


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dual fuel engine and the power supply company. Normally, two dual fuel

generators run in parallel operation for the Shatin STW to consume all the

biogas.

In addition to electricity generation, the system has facilities to recover heat

from the charged air, lube oil, jacket water and flue exhaust of the dual fuel

generator. Water is recirculated to supply the recovered heat to the sludge

digestion heating system in Shatin STW. External heat is required to keep

the digester at mesophlic range in winter.

After nearly 30 years in operation, equipment aging becomes apparent. The

number of breakdown increases and more diesel fuel is consumed. On the

other hand, the energy efficiency of internal combustion engine has increased

in the past few decades. For improving the energy and environmental

performance, the Drainage Services Department has decided to start replacing

these dual fuel generators.

4. Project of Biogas fueled Combined Heat and Power (CHP) Generator

4.1 Selection of CHP generator

As a start, one dual fuel engine was selected for replacement. The design

work was started in early 2010. Because of the previous experience, the

combined heat and power (CHP) generator set was selected. The CHP

generator consists of a lean burn gas engine, an alternator and an integrated

heat recovery system.

The advantages of the CHP generator include:

1. The four-stroke internal combustion engine using lean burn gas

combustion with turbo-charge and after-cool system making the

� diesel is no longer necessary as combustion is spark ignited,

� lower combustion temperatures and hence lower NOx formation from

the air-fuel mixtures,

� efficiency is improved from higher compression ratio for

combustion,

2. The tailor- made heat exchanging system is an integral part of the CHP

generator.


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The new CHP generator is expected to satisfy the following requirements:

1. The electricity output of the CHP genset shall connect with the existing

electricity distribution system in the Shatin STW. The heat recovery

system should connect with the existing hot water system which supplies

heat to the anaerobic digesters.

2. The CHP genset shall be designed for continuous operation. Together with

not more than one dual fuel genset, the CHP genset should be able to cope

with the change in biogas production rate as well as the anticipated

fluctuation in gas composition. Flare-off should be avoided as far as

possible.

3. The CHP genset is normally run at above 50% of full load to meet the

exhaust requirements. Sufficient load is therefore required to be

connected with the CHP genset to enable continuous and stable operation.

4. The genset should be installed within the space left by the old dual fuel

genset.

Two ratings of CHP genset, i.e. 1MW and 1.4 MW, were considered and the

particulars of the gensets are as follows.

Based on the gas consumption of the CHP generators and the dual fuel

generator and the historical gas production data and characteristics, it can be

shown that the 1.4 MW is the best fit to the biogas gas production in the Shatin

STW. With this combination, a new 1.4 MW CHP generator running with one

existing dual fuel generator could fully utilize the biogas in most of the time

resulting very little chance of operating more than two generators or flaring off

the biogas. The energy efficiency of the 1.4MW generator is also higher.

In 2011, the biogas production monthly average has a 6% increase from 2010, which

is still well within the operation range of the selected 1.4 MW scheme.

1 MW 1.4 MW

Fuel gas LHV in kWh/Nm³ 5.3 5.3

Loading in % 100% 75% 50% 100% 75% 50%

Gas volume Nm³/h 502 386 271 637 491 346

Electrical output in kWe 1064 798 530 1416 1.062 706

Electrical efficiency in % 40.0 39.0 36.9 42.0 40.8 38.5


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Graph 1 - Biogas production against generators consumption

4.2 Biogas Treatments and Conditioning

Compared with the existing dual fuel generator, the new CHP generator has

more stringent requirements on the biogas quality.

Currently, in the Shatin STW, ferric chloride is dosed to the anaerobic digester to

control hydrogen sulphide. Since the new CHP generator cannot tolerate too much

hydrogen sulphide (maximum 200 ppm), a separate biogas desulphurization system is

required to ensure the limit of hydrogen sulphide (H2S) in biogas not to be exceeded.

Iron sponge is being used for other CHP system in the Drainage Services Department.

However, the high hydrogen sulphide concentration and high gas flow would require a

lot of space and frequent replacement of the media. A system using chemical and

biological process for sulphide removal is selected. The system consists of scrubber,

biological reactor and sulphur separator as shown in Figure 2.

Hydrogen sulphide is absorbed in the scrubber where the pH of the absorbent is

maintained between 7.9 - 8.9. The absorbed hydrogen sulphide is oxidized in the

subsequent biological reactor where majority of the sulphide is reduced to elemental

sulphur. Sulphur will settle and leave the system. The system should be able to

1.4 MW CHP at full

load + 1DF

1.0 MW CHP at full

load + 1DF

1.4MW CHP at 50%

load

1.0 MW CHP at 50%

load

CHP = CHP

generator

DF = dual fuel

generator

0500010000150002000025000

Apr-2009 Apr-2009 Apr-2009 May-2009 May-2009 Jun-2009 Jun-2009 Jul-2009 Jul-2009 Aug-2009 Aug-2009 Sep-2009 Sep-2009 Sep-2009 Oct-2009 Oct-2009 Nov-2009 Nov-2009 Dec-2009 Dec-2009 Jan-2010 Jan-2010 Feb-2010 Feb-2010 Mar-2010 Mar-2010 Mar-2010Gas consumed by df genset (m3) Total gas production (m3)


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maintain H2S in the biogas not more than 150 ppm.

Figure 2 Block diagram of biogas desulphurization system for the CHP generator

After treatment in the desulphurization system, the biogas is further conditioned by

downstream units. Chiller, separator with demister and air blast cooler are required

to keep the relative humidity in the biogas not more than 50% at the temperature

below 40oC. Activated carbon is required to remove siloxane which could damage

the engine. A gas booster is to maintain the feed gas pressure at 300 mbar to the

engine. The conditioning system is indicated in Figure 3.

Figure 3 The Conditioning System for Biogas Treatment


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4.3 Integration of CHP generator into the Electric Power System

The CHP generator will connect with the existing electricity distribution system

(Figure 4). The CHP generator is designed for three operation modes:

(a) running alone,

(b) parallel with existing generators and

(c) future connection with the grid of power supply company

Parallel operation of the generator enables all the biogas to be consumed for heat

and power generation for most of the time. There is more than enough electrical

load to use up the generated power. Large machinery includes air blowers

supplying air for the activated sludge process and the pumps in the sewage

treatment process. The CHP generator will have automatic voltage regulator and

governor to adjust the generator output to match with the load demand.

Connection with the grid of power supply company will further enhance the

operation flexibility of the CHP generator.

Figure 4 The existing dual fuel generator no. 2 will be replaced by the CHP generator

4.4 Connecting the CHP generator into the Heat Recovery System

The CHP generator will have an integrated heat recovery system (Figure 5).


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Through the hot water recirculation system, it recovers heat from:

1. air/fuel mixture

2. lube oil

3. engine jacket water.

4. exhaust gas.

The recoverable thermal output (RTO) of the CHP generator is 1408kW. Through

the main heat exchanger, the recovered heat is transferred to the existing hot water

re-circulation system for sludge digestion heating system through the downstream

main heat exchange.

Since the heat demand of the digester may be less than the RTO or the heat recovery

system is shut down for maintenance, a radiator is provided to reject the heat to the

atmosphere so that power generation is not affected.

@ the tested figure shows slightly higher amount of recoverable heat

Figure 5 The heat exchanger system of CHP generator connecting the existing system

4.5 Energy and Environmental Benefits

4.5.1 Electricity and Thermal Energy

The new CHP generator will significant improve the energy performance of the power


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generation system in the Shatin STW. The energy efficiency of the new CHP

generator and the existing dual fuel generator are compared in the table below. It

can be seen that significant improvement is expected from the new generator. The

maximum combined energy efficiency is around 83%.

Electrical Energy Efficiency Improvement

New CHP generator #41.7% Dual fuel generator by biogas 33.77 % 23.47%

Thermal Energy Efficiency Improvement

New CHP generator #41.8% Dual fuel generator by biogas 28.30 % 47.69%

# based on biogas Lower Heat Value at 6.5 kWh/Nm³ under full electric load

4.5.2 Estimation on Power Generation and Heat Recovery

It is estimated that the CHP generator will produce 8.5M kWh electricity each year

assuming its availability is 80%. The produced electricity is equal to 23.6% of the

annual electricity consumption (36M kWh) in Shatin STW.

4.5.3 Green House Gas CO2-e Emission

The production of electricity and heat recovery can lead to reduction of greenhouse

gas emission though not directly. With the utilisation of biogas for power generation,

less power is imported from the power supply company and therefore there would be

less greenhouse gas emission offsite.

Year Electricity Generated

by Biogas (in kWh)

Thermal Energy Recovery

from Biogas (in kWh)

Avoidance of Green

House Gas CO2-e

Emission (in Tonne)

2011 8,871,086 7,433,871 8,805

5. Conclusions

Biogas being a by-product in the anaerobic sludge digestion press, nowadays, is a

valuable source of renewable energy at the sewage treatment works. With proper

treatment, the biogas could be used for power generation in the sewage treatment

works. In Hong Kong seawater flushing increases hydrogen sulphide concentration

in the biogas. The new CHP generator requires a higher quality of biogas for use.

The H2S concentration in the biogas should be below 200ppm and the relative

humidity at 50%. Siloxanes should be removed to safeguard the engine against

abrasion.


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Utilisation of biogas at the Shatin STW leads to 7,400,000 kWh heat recovery and

8,900,000 kWh power generation each year. It significantly contributes to the

energy efficiency of the sewage treatment works. The new CHP generator is

expected to further improve its energy performance.

Because of heat and power generation at the Shatin STW, the avoidance of

greenhouse gas CO2-e emission is around 8,800 tonnes each year. It indirectly

reduces greenhouse gas emission from Hong Kong.


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References

i. Metcalf & Eddy, Inc., George Tchobanoglous, Franklin L. Burton and H. David

Stensel, Wastewater Engineering Treatment and Reuse - Fourth Edition

ii. Approved EIA report EIA-022/1999 for “Sha Tin Sewage Treatment Works,

Stage III Extension - Environmental Impact Assessment Study”

iii. http://en.wikipedia.org/wiki/Anaerobic_digestion

iv. Operations record data from Shatin Sewage Treatment Works

v. Technical Description - Cogeneration Unit JMS 420 GS-B.L- B125

vi. http://www.dieselnet.com/standards/eu/hd.php

vii. http://www.dieselnet.com/standards/de/taluft.php

viii. http://en.wikipedia.org/wiki/Lean_burn#Toyota_lean_burn_engines

ix. EMSD and EPD, Guideline to Account for and Report on Greenhouse Gas

Emissions and Removals for Buildings

x. https://www.clponline.com.hk/ourenvironment/measureourimpact/carbonfootpri

nt/pages/default.aspx?lang=en

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