Final Report Summary - BIOCELLUS (Biomass fuel cell utility system)
Commercial biomass is considered as a promising energy source as it is nearly CO2-free and well-distributed all over the world. However, the concepts developed so far for its use in small-scale decentralised power plants achieve too low efficiencies to be economically feasible. Power could on the contrary be generated efficiently in fuel cells. This of course requires the gasification of the biomass. High temperature fuel cells, among them SOFCs, are the most appropriate fuel cells for withstanding operation with wood gas: they accept not only hydrogen as fuel but also carbon monoxide and hydrocarbons, which are present in the product gas from a biomass gasifier. Due to the high operating temperature of SOFCs, the fuel processing and the thermal integration are simplified. Residual heat, available at a high temperature level, can be used in a combined heat and power plant. Biomass gasifier-SOFC systems will still have to face at least two outstanding challenges: withstanding high levels of impurities in the gas and reaching high electrical efficiencies.
The approach followed in this project was to test fuel cells directly with real wood gas and to observe the degradation caused by the different pollutants. The project was accomplished through the following research steps:
- Assembly and testing of mobile test-rigs
Two mobile SOFC test-rigs have been designed and constructed. Each test-rig was equipped with measurement, control, and data acquisition systems. Humidifiers and gas preheating equipment were also present. Before being tested onsite with real wood gas, the operation of the different test-rigs has been checked with synthetic wood gas in the laboratory. For the planar cells, no deactivation took place during operation with synthetic wood gas mixtures and the anode microstructure after testing was normal. Higher tars were shown to have a negative impact on the cell performance: they inhibit the reforming of methane. Lower tar components are converted. The tubular fuel cells manufactured at the Technical University of Munich reacted to different gas flow rates, temperatures and gas compositions as expected. However, cell power was lower than expected.
- Investigation of the syngas impact on the fuel cell performance
All three mobile test-rigs have been transported to different gasifiers in Europe. The gasifiers visited were significantly different and produce gas with diverse quality, principally concerning the tar content. The expected result from this onsite tests was the maximal tar level tolerable for the SOFC and therefore the determination of the requirements for the pre-reforming unit of the gas cleaning unit. The results of the tests suggest that the planar electrolyte supported SOFC with a Ni-GDC anode formulation could operate on and cope with the tar laden wood gas (from approximately 0 to > 10 000 mg/Nm3) without degradation due to carbon formation or other product gas impurity traces. Tubular fuel cells showed some degradation probably due to unsolved leakage problems and to handmade manufacturing.
- Hot gas cleaning system and long-term testing of a single membrane
In order to achieve high efficiencies in a process integrating a biomass gasifier and a SOFC, hot gas cleaning was necessary. Hot gas cleaning was here defined as cleaning the gas at temperature above 600 degrees Celsius. The suggested cleaning system involved a ceramic filter, dolomite based tar cracker, sodium carbonate based HCl removal, activated alumina as an alkali getter, reduced ceria based H2S cleaning and a final ceramic dust filter. Because the measured current density with a defined hydrogen mixture was lower than for the other commercial cells, it was concluded that the fuel supply was insufficient due to a partial bypass of gas by leaks in the applied cell housing.
- Design of a SOFC with internal heatpipe cooling
In order to achieve high system performance, it was shown that the introduction of heat pipes in the fuel cell design is necessary. It was also very important to develop anode materials which are resistant to carbon deposition. Different anode materials have been manufactured by the sol-gel, combustion synthesis, and Pechini methods. Ni/YSZ anode materials were modified by the addition of dopants such as platinum, palladium, and copper. Some other compositions of anode materials, produced by ECN and Siemens, have also been evaluated. The carbon deposition on the materials has been tested via gas phase analysis by mass spectrometry and gas chromatography. Information on the amount of carbon deposited, on the reactivity of the deposit, and on the kinetics of the deposition was gained. Results of the temperature programmed and isothermal deposition of carbon were that: the tested anode material samples differ in the onset temperature and in the amplitude of the hydrogen release, which corresponds to the amount of deposited carbon; the oxidation temperature of the deposited carbon varies significantly and is a measure of its reactivity. Parallel to the development of anode materials and appropriate heat pipes, two stack designs have been conceived. In the planar stack design of Prototech, layers of fuel cells alternate with layers of heat pipes. The usual horizontal temperature gradients observed in planar fuel cell stacks are transformed in vertical temperature gradients by the use of heat pipes. Simulations have been carried out to determine the vertical temperature gradients in the stack depending on the heat pipe diameters. Possible interconnect materials have been identified.
- Testing of the new SOFC concept
TU Delft constructed a new medium temperature gas cleaning unit based on the design of the one from TU Graz, but able to deliver higher flow rates which are convenient for fuel cell stacks. A planar stack, able to produce 1 kW power output, was constructed by Prototech. The short tubular stack constructed by TU Munich as preliminary unit was tested in parallel, as well as single planar fuel cells with improved anode configurations. All four test-rigs were installed at the biomass heatpipe reformer from TU Munich. It was possible to deliver real wood gas to all the test-rigs.
The planar stack was operated during approximately seven hours on wood gas and had a very stable performance. The average power out was 300 W, the maximum power output, reached while increasing the wood gas flow to the stack, was 700 W. It should also be noted here that no experiment with stack and biomass gasifier of this kind with such high power output was reported in literature. The vertical temperature gradient measured through the planar stack was higher than predicted by the model, but still lower than in stacks without heat pipes.
The excess air ratio could be reduced to 2, which is a significant improvement compared to usual systems where excess air ratios of 5-10 are used in order to remove the excess heat produced by the fuel cell. The heat pipes were responsible for about 80 % of the cooling of the stack and therefore fulfil their task very good. It was not possible to operate the tubular stack properly because of leakage problems between the fuel cells.
- Economic evaluation and environmental impact
In order to reach high electrical efficiencies, it is important to know how the biomass gasifier and the fuel cell are best combined. Four possible combinations of an allothermal gasifier and SOFC stack have been simulated with the software IPSEpro under the same realistic assumptions. The simulated plants belong to the small-scale class. The interdependency of the processes is growing, starting from the simplest configuration to the so called TopCycle. In the TopCycle, the heat produced by the fuel cell is directly transferred to the gasifier by means of liquid metal heat pipes. The comparison of the four processes shows the net superiority of the TopCycle: the net electrical efficiency is increasing from 28,0 % for the simple configuration to 43,1 % for the TopCycle. The efficiency of the TopCycle could even be improved by 10 %-points if both systems were operating under a pressure of 2,5 bar and if the exhaust gas were expanded in a microturbine. A pressurised operation is probably feasible: both the BioHPR and the Siemens fuel cells were already tested under such conditions. A parameter variation for the TopCycle further demonstrates that the excess air ratio, the fuel utilisation in the fuel cell, and the excess steam ratio in the gasifier have a determinant influence on the cycle efficiency.
An estimate of the investment costs for a TopCycle system as combined heat and power plant has been carried out. As usual for a novel concept in this early phase of development, the cost calculation is made with a high uncertainty. All calculations are, however, based on existing cost estimation from other projects and reports. The economic evaluation reveals that the expected specific costs for a single TopCycle unit of 500 kWth will be about 12 000 EUR/kWel. However, due to the principle of economy of scale, the 69th produced unit would already have specific costs lower than the 2 600 EUR/ kWel limit. The investigation for the most appropriate feedstock concluded that, when focusing on Eastern and Mediterranean Europe, organic waste appears to be more abundant and convenient in use. Olive pit will be in particular very interesting.
The project achieved several valuable and promising results:
- The short term tests of single SOFCs together with a gas cleaning system at different gasification sites showed that SOFC operation on real wood gas is in principle a possible option. The gas cleaning system proved to be able to clean the biogenous gas to gas qualities required by a SOFC. The SOFC proved to be functional with cleaned biogenous gas and - most promising result - to even tolerate tar levels up to 10 g/Nm3.
- Two stack designs realising the TopCycle have been developed. Both designs - tubular and planar - have been successfully tested in small scale. The planar design has also been tested in a 1 kW version and the cooling concept with heat pipes showed its superiority.
- Both stack designs have also been tested with real wood gas. While the sealless sealing concept of the tubular concept could not cope with the gas fluctuations in the biogenous gas, the planar stack had a high power output. No degradation due to the operation on biogenous gas could be detected. A total power output of 660 W could be measured - which is the highest power output of a SOFC system on wood gas reported in open literature.
- The long term testing was delayed due to problems at the blue tower of DM2. PSI took over as a new partner and installed the Siemens testing facilities at its gasifier in Switzerland. The testing is very successful and still ongoing. So far, very low degradation due to woody gas has been detected.
- The socio-economical studies such as a thermodynamical study, a market study and a cost analysis all showed the feasibility of such system. The TopCycle system is indeed a very promising system which should be pursued in future.
- Last but not least: dissemination activities have accompanied the whole project.