Towards negative emissions in the cement industry

Abstract

Negative emission technologies aim to remove historic CO2 from the atmosphere. Bioenergy with carbon capture and storage (BECCS) is regarded as a possible negative emission technology. This thesis aims to study the technological and economic feasibility of implementing BECCS in the cement industry. Four different types of biomass - rice husk pellets, wood pellets, sewage sludge, and municipal solid waste were chosen to substitute 30% of the coal in a model of a European cement plant. With an emphasis on retrofitting, three CO2 capture technologies, absorption using monoethanol amine (MEA), calcium looping (CaL) based capture, and oxyfuel combustion capture were chosen. This thesis compares the techno-economic performance of the BECCS technologies in a cement plant using the key performance indicators (KPIs) of specific primary energy consumption per CO2 avoided (SPECCA), cement production costs and costs of CO2 avoided. Mass balance results and estimates of capital and operating expenses were used to calculate the KPIs. The parameters for each biomass-CO2 capture combination are compared to each other and compared to the reference value of a cement plant which uses 100% coal without CCS. The results were subjected to sensitivity analysis. CO2 accounting was performed for a defined boundary to estimate the carbon footprint of the BECCS technologies. The BECCS technologies studied have a lower rate of cement production as a result of co-firing biomass in existing boilers. This can be attributed to the low calorific value of biomass. Of the three CO2 capture technologies, oxyfuel combustion capture is the least energy consuming option (1.8 MJ/tCO2, with wood pellets) and the highest range of SPECCA is visible in the case of MEA (8.6 MJ/tCO2, with municipal solid waste). The cement production costs increase by 42 to 89% compared to the costs without CO2 capture. The cost of CO2 avoided is between 45 €/tCO2 (wood pellets with oxyfuel) to a higher range of 96 €/tCO2 (sewage sludge with MEA). The variation in costs is significantly affected by the type of biomass used. CaL technology has a moderate performance in energy consumption and costs. SPECCA obtained for CaL process is in the range of 4.1 to 4.4 MJ/tCO2 and the cost of CO2 avoided is in the range of 57 to 74 €/tCO2. CaL also entails the highest CO2 capture rates, in comparison with MEA and oxyfuel technologies. In terms of total CO2 emissions avoided, CaL based CO2 capture gives the highest CO2 avoided for the system boundary defined. Therefore, when the CO2 removed from the atmosphere through the growth of biomass is included, the net CO2 emissions are the least for CaL capture technology. The reason is the additional thermal energy requirements for the retrofitted CaL process units, which are met with 100% biomass. Although theoretically, a net negative value of CO2 emissions was visible in the case of CaL, it must be noted that upstream process emissions are not included. But in practice, it is still unclear as to whether negative emissions are attainable with the information at hand.