The use of a biomass gasifier that drives a gas turbine integrated with a fuel cell, is a potentially very attractive way to generate electricity and heat with a high efficiency and very low emissions. At Delft University of Technology in The Netherlands,a biomass gasifier has been set up and a conceptual design developed for modelling the integrated system.


Figure 1. Flow sheet of the solid oxide fuel cell modelled in Aspen Plus
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During the last five years market liberalization combined with technology developments such as micro-turbine generator packages and high temperature fuel cells have improved the opportunities for small-scale, distributed power generation. Meanwhile, the drive to globally reduce CO2 emissions has led to an increase in the use of cogeneration plants, and biomass in place of fossil fuels.

At Delft University in the Netherlands, attempts are being made to integrate a fuel cell with a micro gas turbine to create system configuration in which biomass is used as the fuel source.

Biomass gasification

Solid fuels (coal, biomass and waste) can be converted into usable energy in an efficient, reliable, environmentally benign- and sustainable way by gasification and subsequent conversion of the product gas into electricity and heat. And with the increasing drive to use renewable sources there has been a lot of research effort (especially in the Scandinavian countries) particularly in the field of fluidized bed gasification of biomass.


Figure 2. Flow sheet of integrated SOFC / micro gas turbine configuration
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Yet in spite of all the work that has been carried out, there are still a number of uncertainties with respect to the different processes (fuel pretreatment, feeding, gasification process conditions, cleanup and gas turbine operation), necessary to enable an optimized design of a complete biomass gasification combined cycle (BIGCC) systems.

The Thermal Power Engineering department of Delft University of Technology is doing theoretical and experimental research in the area of energy production from biomass and organic waste to help solve some of the uncertainties.

Delft has built a 1.5 MWth process development unit (PDU) to obtain the experimental data needed to develop and validate the models.

Because the main ‘active’ components of a BIGCC are included in the PDU, it is possible to obtain experimental information about the different gaseous, solid and liquid components during their passage through the system as well as study integration issues.

The PDU basically consists of a dedicated compressed air-supply system, three independently controlled solids feed systems, a pressurized bubbling fluidized bed gasifier, a high temperature ceramic filter and a gas turbine combustion section which includes a pressurized heated air supply system, simulating compressor air.

Process demonstration units with a thermal capacity of between 0.3 and 1.5 MWth, like the Delft PDU are especially suitable to gain experience in this technology, because:

  • Gasification experiments can be performed under well determined and controlled process conditions
  • The composition of product gas (and solids) can be determined in several relevant positions in the unit
  • The results can be applied to predict the behaviour of larger scale installations.

The test rig consists of an air/steam blown pressurized bubbling fluidized bed gasifier (PFBG) with a ceramic channel-type filter and a gas turbine combustor for the low calorific value (LCV) gas produced.

Biomass, coal and additive are metered from big bags onto a conveyor belt using three independently controlled screw feeders and the mixture is then transported into a double valve lock hopper system followed by screw feeding into a vessel. The material is then fed pneumatically into the bed through a feed point in the bottom plate and directed toward the central nozzle.

The feeding system was originally designed for a coal-fired pressurized fluidized bed combustor and had to be modified to increase the capacity sufficiently for the gasification experiments. These changes were:

  • Installation of a biomass (pellet) crusher
  • Enlargement of the set up for the biomass big bag
  • Improvement of the control structure to detect bridging in the lockhopper system
  • Installation of nitrogen pulse system on upper lockhopper bunker to ensure continuous flow in case of indicated bridging
  • Improvement of the hopper above the pneumatic transport line to guarantee continuous flow by preventing blockage and bridging
  • Installation of nitrogen pulse system in the pneumatic transport line to compensate the influence of pressure pulses upstream (of the filter cleaning system).

The cylindrical gasifier consists of two sections: a bed section which is cooled by an annular heat exchanger preheating the fluidization air and a thermally insulated freeboard section. The gasifier is placed inside a pressure vessel. The air and steam enter the bed at the bottom of the bed section through a central nozzle with a large number of radially, outward directed small holes.

The premixed coal, biomass and sorbent material are injected pneumatically into the bottom of the bed through a pipe which extends vertically upward into the bottom of the bed through the air distributor plate. The bed content is kept constant during the experiments by an automatic bed material removal system, installed at the bottom of the bed. Secondary air and nitrogen can be injected into the lower part of the freeboard. The freeboard region is equipped with a number of traversible probes at different heights above the bottom plate, which are used to determine axial and radial temperature and concentration profiles (gas composition, tar and solids loading).

SOFC

A Solid Oxide Fuel Cell (SOFC) using the biomass as the fuel source was selected for a number of reasons. These include:

  • The high operating temperatures in the SOFC ensures that most light hydrocarbons will be oxidized
  • External reforming will not be necessary when using a SOFC
  • The system in which a SOFC operates does not require CO2 recycling such as is the case with Molten Carbonate Fuel Cell (MCFC) systems, enabling a simple system configuration
  • The tolerance to impurities of a SOFC is expected to be higher than of a MCFC
  • SOFCs do not have a theoretical service life limit
  • SOFCs have a solid electrolyte and do not have the stability problems that may occur with fluid or molten electrolytes
  • SOFCs have a better tolerance than MCFCs to overload, underload and short-circuiting.

In the SOFC model, hot air and fuel (both 800°C) are supplied to the fuel cell. The pressure drop in the fuel cell is assumed to be 0.05 bar and is modelled to occur in the air- and fuel- heater. In a SOFC, the membrane lets only oxygen ions flow from the oxidant side to the cathode side, where the oxygen reacts with the fuel. For modelling reasons, the incoming air is first separated in two streams, one containing pure oxygen, which flows through the membrane, and one stream containing the remainder of the air.

In the Delft model, the allowable temperature range of the fuel cell determines the mass flow of the oxygen and thus the mass flow of the fuel gas. The SOFC membrane operates best at an average temperature of approximately 900°C. The total temperature range of the fuel cell then becomes 800°C to 1000°C. Thus, at the outlet of the anode and cathode, the depleted fuel gas and depleted air flows must have a temperature of approximately 1000°C. This temperature can be controlled by varying the oxygen mass flow, which enables variation of the amount of oxidation of the fuel gas and thus variation of the heat production.

The modelling programme then determines the mass flow of the fuel gas that should enter the anode in order to obtain the utilization factor, which has been set to 85 per cent. By changing the mass flow of the oxygen through the membrane, the mass flow of the fuel gas is changed, and thus the obtained temperature of the gas streams leaving the fuel cell is changed. This is done iteratively until both outlet streams have reached a temperature of approximately 1000°C. The air and fuel gas mass flows determine the operating point of the fuel cell.

The fuel and pure oxygen react in a reactor at a temperature of 900°C. The heat produced in this reactor is transferred to a heat splitter.

The amount of electrical power that is generated in the SOFC is calculated according to the model. This amount of electrical power is separated in the heat splitter as a process product. The remaining heat is used to heat the air and fuel streams leaving the fuel cell. The heat is divided so that both streams obtain the same temperature when leaving the fuel cell.

SOFC/micro gas turbine

The micro gas turbine considered in the study is a Capstone 28 kWe micro gas turbine that is closed. Preferably, the gas turbine is used at normal operating conditions. However, to enable integration of the SOFC, some adjustments have been made. A flow sheet of a possible configuration is given in Figure 2.

The compressor mass flow is 0.27 kg/s. The leakage losses in the compressor are added to the outlet gas stream of the heat exchanger. A relatively small amount of the fuel gas produced by the gasifier is then burned with the compressed air, giving a fuel cell inlet temperature of 800°C at the cathode inlet gas stream.

The pressure drop in the combustion chamber is set to 0.1 bar. After combustion, the hot, vitiated air is led to the anode side of the fuel cell.

Gases leave the SOFC at a temperature of about 1000°C, and are further heated in the second combustion chamber, using the remaining calorific value of the fuel gas. After combustion, the temperature of the flue gases reach a about 1250°C. This gas is led to the turbine, although normally the turbine inlet temperature in the micro gas turbine is about 885°C.

Cooling of the turbine blades or the application of other, more heat resistant materials in the blades (e.g. ceramics) is not desirable since it would result in extra costs. Therefore, the hot flue gases at the outlet of the second combustion chamber are cooled down to 885°C. This is done by adding a ‘cold’ stream from the compressor (160°C) to the gas stream entering the turbine.

When the gas entering the turbine is too hot, the outlet temperature of the turbine is expected to be too high (900°C) to be led into the heat exchanger since the normal operating temperature of the heat exchanger is some 660°C.

In order to ensure the turbine operates at the optimum operating point, the mass flow entering the turbine is set at its specific operating point. This is done by separating the mass flow into two flows: one leading to the turbine in the micro gas turbine, and the other to an additional turbine, which is assumed to operate with the same isentropic efficiency as the turbine in the micro gas turbine. The gas leaving the micro turbine is led to the heat exchanger, while the hot gas leaving the additional turbine can be used for heating purposes or as a process stream.

It has been assumed that the fuel gas produced by the gasifier can be directly applied in the fuel cell. This means that the fuel gas is clean enough and has the right temperature and pressure. The air flow through the compressor determines the optimum amount of fuel supplied to the fuel cell.

After utilization in the fuel cell, both the used fuel and air are led to the second combustion chamber. The pressure drop in the combustion chamber is set equal to the pressure drop in the first combustion chamber.

To enable simulation of the micro gas turbine using the gas produced by the gasifier as the fuel source, an additional turbine has been applied. The fuel gas is a low calorific value (LCV) gas, and in order to obtain the same temperature increase of the flue gases, more fuel is needed when compared to the amount needed when fossil fuels are used as the fuel source. This extra amount of gas may cause the turbine to operate outside of its optimum operating region. The additional turbine is used in the same manner as described above for the SOFC/micro gas turbine configuration.

Simulation results

The LCV gas is a mixture of N2, H2O, CO2, H2, CO, and light hydrocarbons and has a lower heating value (LHV) of 4.1 MJ/kg. Almost two-thirds of the generated electrical power is generated by the SOFC, while the remainder is generated by the turbines. The thermodynamic efficiency (based on the Lower Heating Value and the electrical output) is 54 per cent.

The SOFC/micro gas turbine has a high thermodynamic efficiency based on the electrical output and the lower heating value of the fuel gas, and the configuration is a very promising concept for small scale decentralized generation of power and heat using biomass as a fuel source.

Several assumptions have been made in order to obtain the simulation results. Some of these results need further investigation to check the validity of these assumptions.

Firstly, the second combustion chamber is expected to be operating with a very low calorific value gas entering with a temperature of 1000°C. The lcv fuel gas leaving the fuel cell after utilization has a calorific value approximately 15 per cent of the entering gas, which complicates the combustion of the gas.

More research is needed to see whether burning of the very low calorific gas at these high temperatures is possible without co-combustion of natural gas and what emissions are evolved.

Secondly, more data is needed for the validation of the SOFC model because the available data is too scarce. Several assumptions have been made in the model and should be verified, such as the total current output of the fuel cell and the internal resistance.

Experiments should be done to obtain more quantitative information on the effect of different input parameters on the cell performance, such as utilization factor of both oxygen and fuel, the current output per unit cell surface and the effect of the impurities occurring in gas obtained by gasification of biomass on the fuel cell and the micro gas turbine.

Discussion

Integration of the system in the Delft pressurized fluidized bed gasification test rig needs careful consideration. When the SOFC/micro gas turbine system configuration is integrated with the gasifier system, several adjustments have to be made.

Firstly, the gas obtained from the gasifier has to be cleaned at high temperatures and pressures. The fuel gas is obtained at a pressure of 4 bar and a temperature of some 800°C, and this is the required temperature for use of the fuel gas in the SOFC. To what extent this high temperature gas cleaning is required depends on the exact tolerance limits of the SOFC to impurities.

Secondly, the mass flow of LCV fuel gas produced by the gasifier is 2200 kg/h. For the proposed system of small-scale power generation, a mass flow of some 185 kg/h is required, so only a slipstream of the LCV gas will be used. The pressure of the fuel gas (4 bar) does not need any adjustments before it is led to the system.

Further reading

“Biomass-based, small-scale, distributed generation of electicity and heat using integrated gas turbine-fuel system” by B. J. P. Buhre, J. Andries, Delft Universtiy of Technology. This paper was presented at the ASME Turbo Expo 2000, May 8-11, 2000, Munich, Germany.

Final Report: “Pressurised fluidised bed gasification of biomass, hot gas filtration and pressurised combustion of low calorific value product gas,” by J. Andries, W. de Jong, P.D.J. Hoppesteyn.