Co2 Abatement by the Combustion of H2-rich Fuels in Gas Turbines

As a response to the threat of global climate change, a number of technologies have been proposed in which the carbon dioxide produced by combustion of fossil fuels at power stations is captured and stored. A leading option is based on the pre-combustion decarbonisation of fossil fuels to produce hydrogen-rich fuel gases. Such decarbonisation processes integrate the production of a synthesis gas with combined cycle power generation: Synthesis gas (essentially a mixture of hydrogen and carbon monoxide) is produced from a fossil fuel by partial oxidation and/or steam reforming. Carbon monoxide is converted by a water-shift reaction to CO2. The CO2 is then removed from the fuel gas stream using a regenerable solvent and sequestered. The fuel gas, consisting predominantly of hydrogen, is burnt in the gas turbine of a combined cycle. If synthesis gas is produced from natural gas using air as the oxidant, the decarbonised turbine fuel is approximately a 50:50 mixture of hydrogen and nitrogen (volume basis). The paper examines barriers to the availability of suitable turbines for the combustion of the hydrogen-rich fuels produced in decarbonisation processes. Studies carried out on a model of a Modern Reference Engine (MRE) are reported. The MRE model was developed to represent stateof-the-art technology for the gas turbines available from leading manufacturers. Potentially advantageous effects of a hydrogen-rich fuel on turbine performance are reported. The technical modifications needed to burn hydrogen-rich fuels are mainly confined to the combustor, fuel system and control system. Suppression of NOx emissions is a key issue. Hydrogen-rich gas cannot, at present, be burnt in existing pre-mix combustion systems. The implications of suppressing NOx emissions by adding steam to the hydrogen-rich gas are examined. The overall conclusion is that the future availability of gas turbines suitable for use in decarbonisation processes for CO2 abatement is not in jeopardy due to any major technology barriers. Hydrogen is potentially an attractive fuel. Development work is required, but the timescale and costs involved are not prohibitive. These conclusions are supported by views expressed by manufacturers, users, and other informed parties. PRODUCTION OF A HYDROGEN-RICH FUEL GAS The cheapest source of hydrogen at present is from fossil fuels but their carbon content is released to atmosphere as CO2. Such hydrogen production processes can be readily adapted to recover the CO2 but a cost and efficiency penalty is incurred. Audus et.al.(1996) show that the cost of CO2 capture (and compression to 100bar) is approximately 20US$/tCO2 and it adds about 30% to the cost of hydrogen production. The overall effect is that the energy content of decarbonised hydrogen is considerably more costly than that of the source fossil fuel. From this previous work, it is known that the production of electricity from decarbonised hydrogen is unlikely to be competitive with alternatives, such as post-combustion capture of CO2, unless a significant synergy effect can be gained by linking the synthesis gas production step with power generation. The process to produce a hydrogen-rich fuel gas for power generation is conceptually the same for any fossil fuel (coal, natural gas, refinery residues, etc.); it is an integration of hydrogen production with combined cycle power generation. The fuel is partially oxidised to produce a ‘synthesis gas’ containing hydrogen and carbon monoxide. The carbon monoxide is reacted with steam to produce CO2 and additional hydrogen. CO2 is recovered, usually by an absorption process, and sent to a long-term store. Finally, the fuel gas (the combustible content of which is almost entirely hydrogen) is fired in the gas turbine of a combined cycle power plant. The process is depicted in Figure 1 using coal feed in an adaptation of an integrated gasification combined cycle (IGCC). (IGCC is generally regarded as demonstrated technology that has to be made less expensive before it is widely adopted.) In this adaptation of an oxygen-blown IGCC process the fuel could be almost pure hydrogen; nitrogen is available from the air-separation plant (ASU) used to produce oxygen and can be added to the hydrogen fuel gas as a diluent. Figure 1: The production of decarbonised electricity via a hydrogen-rich fuel gas. Most hydrogen is made commercially in a steam-methane reformer; process air/oxygen is not required. The other main process option is partial oxidation in which oxygen is one of the reactants. There are many variants and hybrids of the two routes including; autothermal reforming, and gasheated reforming. Woodfin (1997) describes various options. IEA GHG is interested in the promotion of the process illustrated in Figure 2. In this process a generation efficiency in the region of 45-50% can be obtained using near-term technology. The component technologies are used in the chemical and other industries but it has not been demonstrated as an integrated process. The hydrogen-rich fuel gas is approximately 53%vol H2 and 43%vol N2. Figure 2: Precombustion decarbonisation combined cycle Audus et al. (1998) conclude that the further development of this process should be focussed on: •= reducing the capital costs of synthesis gas production and CO2 capture. O2 Gasifier Shift conversion