CO_2-capture from coal combustion using chemical-looping combustion — Reactivity investigation of Fe, Ni and Mn based oxygen carriers using syngas

摘要

Chemical-looping combustion (CLC) is a combustion technology where an oxygen carrier is used to transfer oxygen from the combustion air to the fuel, thus avoiding direct contact between air and fuel. The system is composed of two reactors, a fuel and an air reactor, where the flue gas from the fuel reactor contains CO_2 and H_2O and the flue gas from the air reactor contains only N_2 and some unreacted O_2. Thus, CO_2 and H_2O are inherently separated from the rest of the flue gases, and no major energy is expended for this separation. This paper investigates the use of three promising oxygen carriers based on the metals Mn, Fe and Ni together with syngas from coal gasification. Reactivity investigations in a laboratory fluidized bed reactor under alternating oxidizing and reducing conditions, using 50% H_2 and 50% CO for the reducing period, and 5% O_2 for the oxidation period, found that the reactivity of all three was high at 950°C. For the Mn and Ni based oxygen carriers, there was full conversion of the fuel gas using bed masses of 46 and 173 kg oxygen carrier per MW of syngas. The Fe based carrier had a somewhat lower reactivity, but the gas yield to CO_2 was still high. The effect of temperature was investigated, and manganese oxide showed high reactivity and conversion interval in the temperature range 650-950°C. For Ni and Fe based carriers the degree of conversion decreased as a function of temperature, although the initial reactivity was high. At lower temperatures there was formation of both carbon and methane for some of the oxygen carriers. This was associated with low degrees of gas yields, and is not expected in a real system. For comparison methane was also used as fuel. The nickel based carrier showed a much higher reactivity in comparison to manganese- and iron oxide at 950°C. Thus one implication of the results in this paper are that when using syngas as fuel, the cheaper and more environmentally sound Mn or Fe-based particles may be better candidates compared to Ni. On the other hand, when using natural gas, which has a high content of methane, Ni based particles would be the preferred oxygen carrier.