Sludge gas powers fuel cells
by the CADDET Japanese National Team

What are fuel cells?
Fuel cells are electrochemical devices which convert the energy of a chemical reaction directly into electricity and heat. They are similar in principle to primary batteries except that the fuel and oxidant are stored externally, enabling them to continue operating as long as reactants are supplied.

Each cell consists of an electrolyte sandwiched between two electrodes. Fuel is oxidised at the anode, liberating electrons which flow in an external circuit to the cathode. The circuit is completed by a flow of ions across the electrolyte that separates the fuel and oxidant streams.

Cells, each with a power output of a few tens or hundreds of watts are usually 'stacked' into modules to provide a larger output. Other components of a fuel cell system are a fuel processor and power conditioner (including an inverter if alternating current is required).

Phosphoric acid fuel cells, like the one featured in this article, have a phosphoric acid electrolyte and a platinum or platinum-ruthenium catalyst on carbon electodes. There is presently about 44 MW of installed capacity world-wide. Overall system electrical efficiency is about 43%.

Background
Fuel cells are more efficient than conventional thermal power generation systems which produce mechanical energy which is then converted into electricity by a generator. This is because fuel cells convert the energy of a chemical reaction directly into electricity and heat. Fuel cells also have environmental advantages: they create virtually no noise or vibration, and their waste products are mainly water and carbon dioxide. Less of the latter is produced compared to fuel burnt to produce electricity in conventional plant. In addition, the heat generated in the reaction process makes fuel cells very suitable for co-generation systems.

The Sewage Works Bureau of Yokohama city collects all the sludge from its 11 sewage treatment plants using pipelines to take the sludge to two sludge treatment centres (the North Centre and the South Centre) where it is treated for final disposal. Together, the treatment centres produce 84,000 m3/day of sludge digestion gas by anaerobic fermentation (see Table 1). Some of the gas is used to incinerate dewatered, digested sludge, but 60% of it is used in two co-generation systems, one in each of the treatment centres. With seven gas engine generator units and a total electrical output of 7,180 kW, the two co-generation units annually generate energy worth JPY 500 million (where JPY is the Japanese yen).

Table 1: Sewage sludge treatment in Yokohama (1996)

 

 

 Received sludge

 Digested sludge

 Dewatered sludge

 Ash of incinerated sludge

 Digestion gas production

 Gas used for power generation

 Electricity generated by gas

 Total power consumption

 

 

 m3/day

 m3/day

 m3/day

 tonne/day

 m3/day

 m3/day

 kWh/day

 kWh/day

 Sludge treatment centre

 North

 6,000

 2,300

 290

 28

 50,000

 32,000

 62,000

 74,000

 South

 7,300

 2,000

220

 18

 34,000

 18,000

 42,000

 105,000

 

 Total

 13,300

 4,300

510

 46

 84,000

 50,000

 104,000

 179,000

The energy generated covers over half the power needs of the sludge treatment centres, and heat is recovered from the exhaust gas and engine cooling water for use in the digestion process.

Aqua-power
In 1994, aware of the advantages of using fuel cells in co-generation systems, the Sewage Works Bureau started a joint research project, the 'Aqua-power Project', in co-operation with two fuel cell manufacturers. Experimental fuel cell power generation systems were installed in each sludge treatment centre and, in 1996, the project succeeded in operating a 200 kW fuel cell system using sludge digestion gas at the North Centre.

The sludge treatment centres receive sludge with a water content of 98­99%. It is dewatered by centrifugal force to a solid content of about 5% and then decomposed in sludge digesters at 36°C for about 30 days.


Flow diagram of experimental fuel cell system.

The fuel cell
The experimental fuel cell system in the North Centre consists of a standard 200 kW fuel cell package designed to use natural gas as fuel but with pre-treatment equipment added to allow sludge digestion gas to be used. A flow diagram of the system is shown above.

First, a desulphuriser in the pre-treatment equipment removes any sulphur compounds remaining in the sludge. An absorber then removes minor constituents that are damaging to fuel cells, such as chlorides and ammonia.

Sludge digestion gas consists of about 60% methane and 40% carbon dioxide. Initially, the pre-treatment process included a methane concentrator to increase the amount of methane in the purified gas by pressure swing absorption. However, the project team later found that the fuel cell can operate at its designed output without methane enrichment, making this step unnecessary.

In the fuel cell package, the preheated digestion gas first enters a reformer, which adds steam to the gas and reforms methane into hydrogen gas using a platinum catalyst. The reformer also produces carbon monoxide which is hazardous to the platinum catalyst in this type of fuel cell, so a carbon monoxide converter reacts steam with the reformed gas to produce more hydrogen and carbon dioxide. In the fuel cell unit, the hydrogen fuel is oxidised on the anode (positive electrode) to produce electrons, which flow through the external circuit, and protons, which migrate through the electrolyte to the cathode (negative electrode) where they combine with oxygen from the air to produce water. High-temperature waste heat generated by this chemical reaction is removed by cooling plates incorporated into the cell unit; a heat exchanger makes the heat available for use in the Centre. Finally, an inverter converts the direct current produced by the fuel cell to alternating current.

Operational testing
After it was confirmed that the pre-treatment equipment could provide usable gas in the fuel cell unit, testing of the power generation system was carried out in three phases:

Phase 1: The stability and repeatability of the system were verified in comparison with natural gas operation by running the system using digestion gas enriched to 90% methane.

Phase 2: Different methane concentrations (90%, 75% and 60%) and their effect on power generation characteristics were evaluated and the necessity for methane enrichment was investigated. Table 2 shows the results of this phase.

Tab le 2: Effects of different concentrations of methane on power generation

 Methane concentration

 Hours of operation

 Electricity output

 Power generating efficiency

 90%

 319

 100-200 kW

 38% or more

 75%

 47

 100-150 kW

 38% or more

 60%

 4,938
(3,097)*

 100-200 kW
(200 kW)

 38% or more

*Operation at the rated output of 200 kW.

Phase 3: Long-term testing of continuous power generation at the rated output of 200 kW without methane enrichment was conducted to establish the reliability of the system.

The phase 2 testing confirmed that the fuel cell could use digestion gas without methane enrichment. However, the fuel feeding system could not supply enough gas to generate power at the rated output of 200 kW. To remedy this, changes were made to the pipe diameters in the fuel supply line. After these modifications were made, the fuel cell system maintained power generation at the rated output over a long test period (3,000 hours) without serious trouble. Test operations were completed in October 1997.

Conclusion
Before this project, methane enrichment was considered crucial for fuel cell systems using digestion gas. The Aqua-power Project has revealed that this is not always necessary. The omission of a methane concentrator will reduce the initial costs and running costs of digestion gas, fuel cell power generation in the future. This discovery is one of the most significant results of the joint research.

The Sewage Works Bureau of Yokohama will now evaluate the durability, reliability and maintainability of the system, based on the results of the operational testing. It also plans to continue the joint research with the aim of obtaining design specifications for fuel cell systems fuelled by digestion gas at its other sludge treatment facilities.

For more information contact the CADDET Japanese National Team in Tokyo.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to
Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.