THE PURGE CREDIT: SORBENT MAKE-UP OPTIMISATION AND CONSISTENT CARBON ACCOUNTING FOR A CALCIUM LOOPING-UREA COMPLEX HOSTED BY A CEMENT KILN
Statement of the problem: In every published assessment of calcium looping (CaL), the deactivated sorbent purge is treated as a waste that must be disposed of, and the make-up rate is consequently minimised. Where the host is a cement kiln this premise is inverted: the purge is a calcined raw meal that the kiln can consume. Aim of the research: This paper aims to quantify the purge credit for a CaL-urea complex retrofitted to the Ashaka cement plant, Nigeria, derive the economically optimal make-up ratio, and establish a consistent carbon-accounting framework that separates transferred, temporarily stored and permanently avoided emissions. Methodology: A marginal-cost model was developed incorporating raw-meal displacement, avoided kiln calcination fuel per BS EN 197-1:2011 and NIS 444-1:2003, recovered purge sensible heat, and carbon-closure sizing from the companion study. Sorbent deactivation was described by the Grasa-Abanades expression, with mean carrying capacity weighted over the particle age distribution. Results obtained: The purge credit reduces the marginal cost of make-up from US$47.2 per tonne of limestone at a power-plant host to US$3.5 per tonne at a cement host, a 93% reduction. The optimal make-up ratio rises from the conventional F0/FCO2 = 0.115 to 0.30, increasing mean CaO carrying capacity from 0.172 to 0.315 and cutting solids circulation by 35% (247 to 162 t d-1). The kiln can absorb twenty times the purge generated (0.50% of CaO demand at ratio 0.30), with sulphur load at 1.3% of clinker allowance. Raising make-up increases endogenous carbon fraction from 31.5% to 52.3%, contracting required flue-gas slipstream from 1.17% to 0.42% of the kiln. Recommendation: It is recommended that CaL retrofits hosted by cement kilns adopt F0/FCO2 = 0.30, return the purge to the raw-meal circuit, and fire the calciner with biogenic fuel and decarbonised power to achieve carbon neutrality, as the complex is otherwise a net permanent emitter of 12.1 t d-1 despite temporarily storing 36.5 t d-1 in urea.
Authors : Abubakar, H., Kareem, S.A., Kwende, F.M. and Okoye, S.
Category : Open Access Volume (Issue) : 11(2) Date Uploaded : 24th September 2026