Farm-Based Biogas Production, Processing, and Use in Polymer

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Farm-Based Biogas Production, Processing, and Use in Polymer Electrolyte Membrane (PEM) Fuel Cells Ralf Schmersahl,* Jan Mumme, and Volkhard Scholz Leibniz Institute for Agricultural Engineering Potsdam-Bornim e.V. (ATB), Max-Eyth-Allee 100, D-14469 Potsdam, Germany

Biogas production and utilization with polymer electrolyte membrane (PEM) fuel cells provides a clean and reliable option for decentralized energy supply. The investigative experiments confirmed the fundamental suitability of co-generation from biogas with a modified fuel cell system for residential energy supply. Biogas with a methane content of ∼60% is generated by solid-state fermentation of a mixture of grass silage and cattle manure. Subsequently, the biogas is processed in a steam reformer to produce hydrogen. A hydrogen purity of >50% is sufficient for an efficient and stable operation of a PEM fuel cell stack. Power generation via biogas and fuel cells from agricultural residues such as wheat straw can provide a constant power of up to 0.5 kW per hectare of acreage. Introduction The use of biogas fuel in polymer electrolyte membrane (PEM) fuel cells with combined heat and power generation provides a clean and reliable option to replace conventional power generation with fossil fuels. It has the advantage of being a comparatively cost-effective domestic regenerative energy source, as well as being a technology that unites high electrical efficiency with low pollution emission. In 2006, the primary energy production of biogas accounted for 41 PJ in Germany and 53.6 PJ in the European Union.1 In Germany, the technical energy potential for biogas is ∼300 PJ/ yr.2 Substrates can be classified as those suitable for liquid fermentation or those suitable for solid-state digestion (see Figure 1). The largest substrate fractions are agricultural wastes. Biogas production from liquid manure and energy crops has reached a high degree of utilization, whereas solid manure and straw are rarely used. In comparison with fluid systems, solidstate digesters are expected to have a higher process stability, allowing lower investment costs and a much-lesser need for maintenance and process energy.3 Although some findings on the suitability of various substrates and substrate mixtures for solid-state anaerobic digestion had already been gained in laboratory experiments,3-6 insufficient knowledge exists on process performance and process control. Presently, most of the biogas in Germany is used in combined heat and power production (CHP) plants that are based on reciprocating engines. Biogas CHP units have problems, in regard to meeting the rated efficiency and following the strict guidelines for exhaust emissions of CO and NOx.7 Fuel cells achieve higher electrical efficiency while producing fewer emissions of noise and pollutants than motor engines.8-10 Different types of fuel cells are being developed for different fuel sources.11,12 Biogas is analyzed as a fuel source for phosphoric acid fuel cells (PAFC),13-16 molten carbonate fuel cells (MCFC),17-19 and solid oxide fuel cells (SOFCs).20,21 Research on biogas-fueled PEM fuel cell systems is done either by modeling22 or is limited to experimental work with model gases.23-26 The findings confirm the technical feasibility of the process but do not give information about system behavior with the native biogas. * To whom correspondence should be addressed. E-mail: [email protected].

Figure 1. Technical energy potential of different agricultural resources in Germany. (Adapted from Paterson.2)

The use of biogas as a fuel requires adaptation and optimization of existing systems, because biogas differs from natural gas in several aspects. The methane (CH4) content (and, thus, the energy density) is lower and irregular. In addition, biogas exhibits various harmful components such as volatile sulfur compounds.27,28 In this paper, we have experimentally investigated biogas production with solid-state fermentation in a heap reactor, the reforming of the biogas, and the use of the generated hydrogenrich gas in a PEM fuel cell stack. In addition, the biogas was manipulated with pure gases to achieve different gas qualities and to determine PEM fuel cell system behavior for varying CH4 content. Finally, an assessment of possible future system efficiency and power generation per unit area was made. Experimental Section Heap Reactor. The biogas generation was conducted under summer conditions with a substrate mixture that consisted of 2460 kg of cattle manure and 440 kg of grass silage. A farmyard manure spreader was used to mix the components. Two thousand seven hundred (2700) kg of the mixture (25% total solids (TS), 84% volatile solids (VS)) were stored under anaerobic conditions for more than three months. Figure 2 presents the design of the 7.5 m3 pilot-plant heap reactor. During the solid-state digestion, the temperature in the

10.1021/ie071292g CCC: $37.00 © 2007 American Chemical Society Published on Web 11/09/2007

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Figure 2. Diagram of the heap reactor for solid-state fermentation.

Figure 3. Flow scheme of the fuel processor.

Table 1. Analytical Instruments species method CH4 CO2 CO

IR IR IR

H2

TCD

range

instrument

remarks

0%-100% Pronova MGA10 0%-100% Siemens Ultramat 23 0-250 ppm Siemens Ultramat 23 internal cross-sensitity compensation 0%-100% Pronova MGA22 external cross-sensitity compensation

heap was monitored and maintained using a floor heating system, to achieve mesophilic conditions in the core zone. Special emphasis was placed on measuring the daily gas production, as well as changes in the gas composition. The biogas was measured continuously by a drum meter (Ritter, model TG10) and analyzed by a gas analyzer (Pronova, model SSM 6000). The biogas was stored in a 16 m3 balloon gas holder. For some experiments, the methane content was varied by adding pure gases (CH4, CO2). Fuel Cell System. The fuel cell system consists of the PEM fuel cell unit and a steam reformer. Figure 3 shows the process flow of the fuel processor. The biogas was compressed with a mechanical compressor to a pressure of 720 mbar. The combustion gas was fed into an atmospheric burner, and the process gas was fed into two copper-impregnated activated-carbon filters to remove hydrogen sulfide completely. The purified gas passed through a mass flow controller (MFC) and, en route, was mixed with steam at a steam-to-carbon (S/C) ratio of 3.5 (the carbon comes from the methane). The preheated mixture was induced into the steam reformer and passed through the ring-shaped reforming catalyst (G-90B). Here, it was heated by the burner and by the countercurrent flow of reformate, which was transported through the inner cylinder. Thereby, a temperature of ∼720-840 °C was obtained at the outlet of the catalyst bed. The reformate passed over a heat exchanger and was directed through both shift converter stages at temperatures of 250-300 °C and cooled down to ∼45 °C, resulting in the formation of a condensate. The condensate was trapped and sent to a boiler for steam production. Subsequently, carbon monoxide (CO) was eliminated by selective oxidation at 50-60 °C.

Figure 4. Process performance of the 7.5 m3 heap reactor.

Figure 5. Reformate gas composition and reforming temperature at different load levels. Nitrogen value was calculated.

The reformate gas, with a dew-point temperature of 4045 °C, was supplied with a temperature of 60 °C to the fuel cell via a heated hose. Depending on the position of the valve, it was either passed into the anode chamber of the fuel cell stack or through a bypass directly to the outlet. Two different stacks were used. Stack A had 14 cells with Gore Primea 5621 membrane electrode assemblies (MEAs) (denoted as MEA A) and stack B was assembled from six PMembrain 300 MEAs (MEA B). Each MEA had an active surface area of 200 cm2. The operating pressure was 1.3 bar, and the operating temperatures were 45-70 °C. Process gases were taken from three sample ports, to analyze and record the composition of biogas, reformate, and anode offgas (see Table 1). The biogas volume flow was measured with a drum gas meter (Ritter, model TG5). To examine the reformer, the operational behavior for biogas with a different methane content was analyzed, and the methane conversion rate (um) and the hydrogen efficiency (ηh) were calculated, with reference to the lower heating value of methane feed and the hydrogen output.

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Table 2. Performance Data of the Fuel Processor parameter

rated dataa

thermal efficiency (%) hydrogen output (kW) methane conversion hydrogen purity (%)

CO, outlet (ppm) reforming temperature (°C) a

68 2.25 0.98 64

250 ppm occurred when >2.5% air was added. The air flow control of the fuel processor does not give a fixed reformate/air ratio. At partial load, increasing the excess air leads to a higher nitrogen content and causes inferior hydrogen purity (Figure 5). For stable operation of the fuel cell, an air supply of 5% of the process gas flow was adjusted to obtain a reformate that is entirely free of CO. Fuel Cell. Fuel cell stack A attained a maximum power output of >600 W with reformate from the biogas that contained 65% methane. With the biogas with a methane content of 55%, only 500 W was obtained; the voltage of the weakest individual cell then fell below 450 mV, which led to an automatic breakdown of load operation. Stable operation was achieved with a stochiometry coefficient of 1.3 for hydrogen and 2.5 for oxygen. Measurement results showed a strong effect of biogas composition on the maximum power output of the fuel cell, whereas the influence on cell voltage (and thereby cell efficiency) was slight (see Figure 6). The cell efficiency is strongly affected by the MEA in use. Figure 7 illustrates the improvement in performance by replacing MEA A by MEA B. Both MEAs were designed for natural gas reformates.

Table 4. Potential of Power Generation from Biogas Per Unit Area Acreage Grass Silage parameter crop (t ha-1 yr-1) biogas (m3/t) biogas (m3 ha-1 yr-1) methane (%) biogas (GJ ha-1 yr-1) electricity (GJ ha-1 yr-1) const power (kW/ha)

motor engine (30% efficient)

Maize Silage

fuel cell system (40% efficient)

motor engine (30% efficient)

8.4 123 1033 52 19.3 5.8 0.18

Wheat Straw

fuel cell system (40% efficient)

motor engine (30% efficient)

43.1 173 7456 52 139.6 7.7 0.25

41.9 1.33

fuel cell system (40% efficient)

6.7 293 1963 51 36.0 55.8 1.77

10.8 0.34

14.4 0.46

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the steam reformer requires a basic system optimization, which is a goal of ongoing work. Literature Cited

Figure 7. Comparison of the performance of two different membrane electrode assemblies (MEAs) with biogas reformate.

For a particular process design, where the enthalpy of the anode off-gas is used to supply thermal energy to the reformer, the gross system efficiency of the PEMFC was calculated from performance data of the fuel processor and the fuel cell (see column status quo in Table 3). For DC/AC conversion, the inverter efficiency (ηi) was taken into account and assumed to be 0.95.

[

(

ηsys ) ηh 2 - um + 1 -

) ]( )( )

Vc 1 1 ηh η λh λh VLHV i

(1)

An estimate was made of the attainable gross system efficiency, based on the experimental results and expected future improvements29 in reforming technology and MEA performance. Table 3 gives an impression of the future efficiencies, indicating that >40% electrical efficiency can be achieved. Energy Yield and Power Calculation. To give a sense of the future power generation capacity by biogas and fuel cells, the electrical energy production per unit area is calculated, based on published data.30 As Table 4 shows, the utilization of wheat straw via biogas and fuel cells can reach a constant power output of ∼0.4 kW/ha, whereas the exclusive use of maize for biogas production might yield a constant power of up to 1.8 kW/ha. Conclusions The study of solid-state biogas production and its use in a polymer electrolyte membrane (PEM) fuel cell system has confirmed the technical feasibility of this renewable energy path from domestic sources. The utilization of agricultural residues such as wheat straw can make a contribution to the power supply of ∼0.4 kW/ha of acreage. Solid-state anaerobic digestion with a heap reactor requires low investment costs and, therefore, could be an appropriate method for the anaerobic treatment of low-cost substrates such as solid manure and plant residues. The main advantages of batch-operated solid-state anaerobic reactors are low investment cost and little need for maintenance and process energy other than heating. PEM fuel cell systems are suited for combined heat and power production (CHP) from biogas. Biogas processing in a steam reformer results in a reformate gas with a hydrogen purity of ∼60%, which is sufficient for the efficient and stable operation of a PEM fuel cell stack. The lower hydrogen content of reformed biogas, compared to natural gas, results in a slightly lower efficiency and a distinct lower maximum power output of the PEM fuel cell. Compared to motor engines, the electrical efficiency is higher. Design and operational parameters of the fuel cell system must be optimized for biogas fuel. In particular,

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ReceiVed for reView September 26, 2007 ReVised manuscript receiVed October 18, 2007 Accepted October 19, 2007 IE071292G