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Selecting the right substitution

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World Cement,

Tahir Abbas and Michalis Akritopoulos, Cinar Ltd., discuss the key factors cement plants should consider when increasing the substitution rate of alternative fuels in their production process.

A higher substitution rate of alternative fuels necessitates low-CAPEX solutions for kilns and calciners, without having a negative effect on clinker production and emissions. Major plant modifications, however, need to be evaluated in conjunction with future net zero CO2 emission targets that may require several step-wise carbon mitigation measures, beyond conventional and already tried-and-tested technologies.

Over the last 30 years, the cement industry has reported a more than 30% reduction in CO2 emissions (compared to a 1990 baseline) through improved energy efficiency and increased usage of alternative fuels and raw materials. Several multinational cement and lime corporations are devising strategies aimed at achieving future CO2 emission targets as proposed by the Global Cement and Concrete Association (GCCA); focusing on carbon capture, utilisation, and storage (CCUS) and H2 as a fuel in its transition to 2050 emission reduction targets.­ There are a number of R&D initiatives where CO2 enrichment and its capture technologies are being developed. At present, oxy-combustion and CO2 enrichment are studied as attractive CO2 capture options, whereas H2 is perceived as one of the major thermal energy sources in the near future, being produced onsite through the application of electrolysers.

Co-firing H2 with biomass will produce water vapour (H2O) together with CO2. Depending on the carbon content of the co-firing fuel, water vapour may be removed later, hence, leaving a highly concentrated CO2 gas stream for CCUS, due to the fact that CO2 capture costs decrease as concentrations in the flue gases increase. Under the co-firing conditions for H2 and/or oxygen enrichment, the alternative fuel (AF) chips may burn faster or could be partially replaced with less expensive, coarser AF fractions. This, in turn, may reverse the current trend of increasing calciner residence times, enabling the calciner to achieve a higher thermal substitution rate (TSR) of AF. There could even be a tendency in the future for more compact kiln, clinker cooler, and calciner designs, as the industry has observed the previous transformation from the wet/dry process to the short dry kilns of today.

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