Adapting calciners operating under CO<inf>2</inf> enrichment for CCS

Abstract

The combustion-generated CO 2 emissions from cement (and lime) plants are significantly augmented by the CO 2 release of the calcination reactions. The exhaust gas CO 2 concentrations of cement plants are almost double those of other fossil-fuel-fired industries. Cement plants are, therefore, eminently suitable candidates for carbon capture and storage (CCS) projects. Due to both technical and financial constraints, CCS has thus far not been demonstrated at the full scale. Apart from CO 2 storage costs, the main technical challenges relate to the effective use of the oxygen, which substitutes the nitrogen in the combustion/cooling air, and the minimization of air-inleakage so that a CO 2 enrichment in excess of 95% is assured. Modification of almost all the components of a cement plant, including the clinker cooler, kiln and the kiln burner, calciner and cyclones, is necessitated. The most critical issue is the inhibition of the calcination rate consequent of the enriched calciner CO 2 . Small-scale experiments reveal that an increase in the calcination temperature of about 80K can be expected for an increase of the gas-phase CO 2 partial pressure of 80%. However, the effect on the gas residence time remains relatively unknown, as the particles/stones were static during the experiments. At the full scale, the increase in the temperature window for the onset of the calcination reactions and the inhibition of calcination under enriched CO 2 concentrations need to be quantified. To this end, a CCS research project was initiated involving research institutions, technology providers, leading consulting firms and cement producers. Some of the findings of the project as well as an independent study which was conducted in parallel are summarized in this paper. Presented here are the results of computational modelling for both bench-scale, pilot-scale and full plant size data. General purpose computational fluid dynamics (CFD) codes do not account for the all-important combustion/mineral interactions. To remedy this failing, an in-house MI-CFD (mineral interactive computational fluid dynamics) program has been developed which adequately simulates these interactions of calcination and combustion/gasification. Several competing and counterbalancing aspects of calciner operation under CCS are modelled in detail by the MI-CFD code: 1) the inhibiting of the calcination reactions by the higher CO 2 partial pressures which tend to raise the temperature of the gas stream in which the meal (CaCOs) particles are immersed, 2) the consequent increase in meal particles' temperature and thus calcination reactions occurring at a higher meal particle temperature which somewhat counterbalances aspect 1 and finally 3) the increase in gas-phase density due to the higher CO 2 concentrations, which results in lower upward gas velocities and increased the particle residence times and, to a smaller extent, also compensates for the effects of aspect 1. MI-CFD simulations are presented herein for three calciner sizes of 5600 tpd, 1000 and 500, operating under CCS conditions. The operating conditions of plants are for partial oxycombustion and CO 2 recycling within the calciner as well as full plant CCS conditions where clinker cooler design, kiln burner and pyroprocessing are integrated to target CO 2 enrichment level of 95%. The results show that, provided suitable arrangements are made for the oxy-fuel firing such that hot spots are eliminated, good calcination is achieved for all three cases. The predictions also accord with the trials on a calciner operating under oxy-fuel conditions [12]. After application and validation of the MI-CFD program it is possible to design and/or adapt existing plants of various geometries under CCS, for partial enrichment (calciner) or comprising of an oxy-fuel cooler, kiln burner and calciner constructed for full-scale CCS operation.