CARBON-CLOSURE SIZING OF A CALCIUM LOOPING TO UREA RETROFIT FOR A NIGERIAN CEMENT KILN: OXY-FUEL AND SORBENT MAKE-UP CARBON AS UREA FEEDSTOCK
Cement manufacture accounts for about half of Nigeria’s in-scope industrial carbon dioxide emissions, yet the country remains a net importer of nitrogen fertiliser, and its inland kilns lie far from any prospective geological storage. Carbon capture and utilisation is therefore the only near-term decarbonisation route available to them. This work develops, in Aspen Plus V12.1, an integrated calcium looping (CaL) capture and CO2-stripping urea synthesis complex retrofitted to the Ashaka cement plant, Gombe State, and introduces a carbon-closure framework for sizing such retrofits. The framework recognises that the carbon delivered by a CaL island to a utilisation plant is not restricted to the CO2 abstracted from the kiln flue gas: calcination of the sorbent make-up and oxy-fuel combustion in the calciner deliver additional CO2 into the same purified product stream at no incremental separation duty. A dimensionless closure factor Λ and a slipstream-sizing equation are derived and applied. For a 2.4% flue-gas slipstream (120.2 t d-1) the island delivers 36.6 t d-1 of 99.2 wt% CO2, of which 31.5% is endogenous, exactly matching the stoichiometric requirement of a 50 t d-1 urea plant (Λ = 1.00); sizing on captured flue-gas carbon alone would have oversized the slipstream by 46%. The high-pressure loop, modelled with SR-POLAR, attains 66.4% per-pass conversion at 190 °C, 150 bar and N/C = 3.0 with 0.62 wt% biuret. Integration of the carbonator and carbamate-condenser exotherms supplies the evaporation duty and reduces net thermal demand by 23.4% to 3.95 GJ t-1 urea. The levelised cost of urea is US$577 t-1 at an ammonia price of US$450 t-1, falling to US$492 t-1 on domestically sourced ammonia, against an import parity of US$580 t-1.
Authors : Abubakar, H., Kareem, S.A., Kwende, F.M. and Okoye, S.
Category : Open Access Volume (Issue) : 12(1) Date Uploaded : 24th September 2026