Experimental investigation of chemical-looping combustion and chemical-looping gasification of biomass-based fuels using steel converter slag as oxygen carrier
Paper in proceedings, 2018
Chemical-looping combustion (CLC) is a combustion process with inherent separation of CO2, which is achieved by oxidizing the fuel with a solid oxygen carrier rather than with air. As fuel and combustion air are never mixed, no gas separation is necessary and, consequently, there is no direct energy penalty for the separation of gases. The most common form of design of chemical-looping combustion systems uses circulating fluidized beds, which is an established and widely spread technology.
Experiments were conducted in two different laboratory-scale CLC reactors with continuous fuel feeding and nominal fuel inputs of 300 Wth and 10 kWth, respectively. As oxygen carrier material, ground steel converter slag from the Linz–Donawitz process was used. This material is the second largest flow in an integrated steel mill, and it is available in huge quantities, for which there is currently limited demand. Steel converter slag consists mainly of oxides of Ca, Mg, Fe, Si and Mn. In the 300 W unit chemical-looping combustion experiments were conducted with model fuels syngas (50 vol% H2 in CO) and methane at varied reactor temperature, fuel input and oxygen-carrier circulation. Further, the ability of the oxygen-carrier material to release oxygen to the gas phase was investigated. In the 10 kW unit, the fuels used for combustion tests were steam-exploded pellets and wood char. The purpose of these experiments was to study more realistic biomass fuels and to assess the lifetime of the slag when employed as oxygen carrier. In addition, chemical-looping gasification was investigated in the 10 kW unit using both, steam-exploded pellets and regular wood pellets as fuels.
In the 300 W unit, up to 99.9 % of syngas conversion was achieved at 280 kg/MWth and 900 °C, while the highest conversion achieved with methane was 60 % at 280 kg/MWth and 950 °C. The material’s ability to release oxygen to the gas phase, i.e., CLOU property, was developed during the initial hours with fuel operation, and the activated material released 1-2 vol% of O2 into a flow of argon between 850 °C and 950 °C. The material’s initial low density decreased somewhat during CLC operation.
In the 10 kW, CO2 yields of 75-82 % were achieved with all three fuels tested in CLC conditions, while carbon leakage was very low in most cases, i.e., below 1 %. With wood char as fuel, at a fuel input of 1.8 kWth, a CO2 yield of 92 % could be achieved. The carbon fraction of C2-species was usually below 2.5 % and no C3-species were detected. During chemical-looping gasification investigation a raw gas was produced that contained mostly H2. The oxygen carrier lifetime was estimated to be about 110-170 h. However, due to its high availability and potentially low cost, this type of slag could be suitable for large-scale operation.