FuelCell Energy Inc., Danbury, CT, USA,
and James U. Watts and Jerry Hebb,
US Department of Energy National Energy Technology Laboratory, Pittsburgh, PA, USA
Coal will remain the primary fuel for power generation in the USA for some years, yet environmental concerns are generating interest in technologies such as fuel cells. An innovative project in the US will demonstrate the operation of fuel cells on coal-derived syngas, paving the way for ‘ultra-clean’ coal fired power plants.
Figure 1. US electric power generation by fuel type
Power generation in the US relies heavily on coal with 56.3 per cent of the power or 1807 billion kWh generated using coal in 1998 as shown in Figure 1. As total US coal consumption increases from 1043 to 1279 million t a year between 1998 and 2020, the average annual increase is projected to be 0.9 per cent. About 90 per cent of the coal consumed in the US is used for power generation. In the next 20 years, coal is expected to remain the primary fuel for power generation, although its share of total generation is expected to decline between 1998 and 2020 as natural gas increases its share.
As concern about the environment generates interest in ultra-clean energy plants, fuel cell power plants can respond to the challenge. Fuel cells convert hydrocarbon fuels to electricity at efficiencies exceeding conventional heat engine technologies while generating extremely low emissions. Emissions of SOx and NOx are expected to be well below current and anticipated future standards. Nitrogen oxides, a byproduct of combustion, will be extremely low in this type of power plant because power is produced electrochemically rather than by combustion. Due to its higher efficiencies, a fuel cell power plant also produces less carbon dioxide (CO2). Fuel cells, in combination with coal gasification, are therefore an efficient and environmentally acceptable means of using the abundant coal reserves both in the US and around the world.
To demonstrate this technology, FuelCell Energy, (FCE) is building a 2 MW fuel cell power plant for operation on coal derived gas as part of the Clean Coal V programme supported by the US Department of Energy. This power plant is based on the Direct Fuel Cell (DFC) technology developed by FuelCell Energy and will be installed at the Global Energy Wabash River Energy Ltd gasification facility in Indiana, USA, starting in the third quarter of 2003.
Fuel cell power
Figure 2. A schematic of the carbonate fuel cell power plant
The carbonate fuel cell derives its name from its electrolyte, which consists of potassium and lithium carbonates. Figure 2 shows a simplified flow schematic of the carbonate fuel cell power plant. Syngas from the gasification plant clean-up system is cleaned up further and moisturized. The moisturized syngas is fed to the anode side of the fuel cell where methane is internally reformed and CO is shifted to CO2 and hydrogen (H2). Spent fuel exits the anode and is consumed in the anode exhaust oxidizer to supply oxygen and CO2 to the cathode. The resulting electrochemical reactions in the fuel cell anode and cathode produce DC output, which is inverted to AC. The cathode exhaust supplies heat to the fuel clean-up, steam boiler and cogen systems as it is vented from the plant.
Figure 3. The fuel cell power plant is currently under construction
The fuel cell power plant currently under construction for this demonstration is shown in Figure 3. Two fuel cell modules, each housing four fuel cell stacks, produce the DC power. An inverter, including switchgear converts the DC power to AC. The balance of plant equipment includes fuel processing, thermal management, water treatment, instrument air system and controls. Additional syngas processing equipment, not shown here, will be provided to make the syngas suitable for fuel cell operation.
Fuel cell systems operating on coal have been studied extensively in past years. Gasification is used to convert the solid fuel to a gas, which is processed to remove sulphur compounds, tars, particulates, and trace contaminants. The cleaned fuel gas is converted to electricity in the fuel cell. Waste heat from the carbonate fuel cell is used to generate steam required for the gasification process and to generate additional power in a bottoming cycle.
At a 200 MW scale, past studies indicated that using conventional gasification and clean-up technologies, a heat rate of 7186 Btu/kWh (47.5 per cent HHV efficiency) can be achieved with Integrated Gasification Fuel Cell (IGFC) plants utilizing Texaco gasification and low temperature clean-up. This plant would require 2139 t/day coal and generate a net output of 250 MW. Later studies indicated that higher efficiencies, 51.7 to 53.5 per cent, could be achieved with higher methane producing gasifiers and by using hot gas clean-up.
Figure 4. Comparison of the combined SOx, NOx and solid waste emissions
Emissions from this plant would be below any current or anticipated future standards. Figure 4 compares the combined SOx, NOx, and solid waste emissions of existing commercial technologies, Integrated Gasification Combined Cycle (IGCC) and IGFC. IGFC technology achieves the lowest levels of pollutant emissions in addition to lower CO2 emissions and make-up water requirements. The CO2 emission is 1.54 lb/kWh and the make-up water requirement is 6.8 GPM/MWh.
Performance testing of a 20 kW sub-scale fuel cell stack was conducted at Louisiana Gasification Technology Inc. in 1993-4 by Destec. This was the world’s first test of a carbonate fuel cell on coal-derived gas. Gas from the entrained flow Destec gasifier was further cleaned-up after bulk gas cleanup by the fuel cell test facility and supplied to the fuel cell. The fuel cell operated on syngas from the gasifier and interchangeably with natural gas providing normal performance and stable operation. After completion of the test, the fuel cell was disassembled for post-test inspection. Analysis of the fuel cell components indicated no evidence of degradation and no detectable accumulation of coal gas borne contaminants in the fuel cell electrolyte or in the hardware. These results paved the way for a larger scale demonstration currently planned at Wabash River.
Clean coal demonstration
The 2 MW fuel cell power plant demonstration at the Global Energy Wabash River Energy Ltd facility near Terre Haute, Indiana will be the first and largest commercial scale carbonate fuel cell operating on coal derived gas.
The objective of this test is to demonstrate fuel cell operation on coal-derived gas at a commercial scale and to verify the efficiency and environmental benefits. Successful demonstration of this technology on coal can lead to future coal based applications at a larger scale with significant improvements in efficiency and emissions compared to existing technologies.