Evaluation of 100% alternative fuel combustion under oxyfuel conditions in a pilot-scale burner for application in retrofit oxyfuel cement kiln

Abstract

The cement industry is one of the largest industrial CO₂ emitters, where CO₂ is emitted from combustion and clinker production processes. This study focuses on substituting fossil fuels with alternative fuels under oxyfuel conditions to advance the technical feasibility of carbon capture technology. Combustion tests are performed in a 300  kW pilot-scale facility, where the burner has been optimized to replicate a retrofit oxyfuel cement kiln burner. Coal combustion in air is the reference case for the tests. The alternative fuels used in the tests are solid recovered fuel (SRF), wood and co-combustion of 90 %_th wood − 10 %_th sludge. Combustion scenarios are studied under both air and oxyfuel conditions with two different flue gas recirculation ratios (RR). Axial measurements of the flame temperature, heat flux and concentrations of gases are measured and evaluated. The burner and combustion chamber are modeled with CFD simulations. The boundary conditions of the SRF combustion in both air and oxyfuel conditions are modeled and the results are validated with the corresponding experimental data. The O₂ and CO₂ concentration during combustion of alternative fuels under oxyfuel conditions, measured 300  cm from the burner, are on average 3 ± 2 vol-% and 82 ± 5 vol-%, respectively. The average heat flux for the alternative fuels, 33  cm from the burner, is 122 ± 15  kW/m² and increases to form a plateau between 100 and 200  cm from the burner at 177 ± 16  kW/m². Compared to coal, the used alternative fuels are milled to a larger particle size, have on average 3.3 times higher volatile matter content, have faster devolatilization rates and have a longer flame shape. The oxyfuel case with higher RR resembles the air case in terms of the temperature profile, heat flux profile and inlet gas momentum. The combustion of alternative fuels is stable in both air and oxyfuel conditions and the flames compared to coal are wider, longer and less intense. CFD simulations of the prototype burner are conducted and validated against experimental data for 100 % SRF combustion. The model offers useful insight into the combustion of SRF fuel, it is particularly accurate for conventional air operating conditions.