Optimum temperature for sulphur retention in fluidised beds working under oxy-fuel combustion conditions

Abstract

Oxy-fuel combustion is one of the leading options for power generation with CO 2 capture. The process consists of burning the fuel with a mixture of nearly pure oxygen and a CO 2 -rich recycled flue gas, resulting in a product flue gas from the boiler containing mainly CO 2 and H 2 O. Among the possible boiler types, fluidised bed combustors are very appropriate for the oxy-fuel process because they allow the in-situ desulphurisation by feeding Ca-based sorbents into the combustor.In this work, the effect of the temperature of the combustor on the retention of the SO 2 generated in the combustion of two coals with very different sulphur content (a lignite and an anthracite) has been studied. The experimental facility used was a bubbling fluidised bed (BFB) combustor of ~3 kW th . Tests were conducted under oxy-fuel combustion mode and also under enriched-air combustion mode for comparison reasons. A Spanish limestone "Granicarb" was used as Ca-based sorbent for sulphur retention. The temperatures tested were between 800 and 970 ºC using Ca/S molar ratios between 0 and 3.It was found that in BFB combustors operating under oxy-fuel combustion conditions the optimum temperature to achieve the highest sulphur retention was 900-925 ºC, whereas operating with enriched air the optimum combustion temperature was ºC. Working at the optimum temperature, the SO 2 retentions were lower in oxy-fuel 2 combustion than in enriched air combustion conditions. It was also observed that working with lignite there was 10-15% of sulphur retention by coal ashes, however, working with anthracite the sulphur retention by coal ashes was negligible. This finding was independent of the combustion mode used, oxy-fuel or enriched air.

Simultaneous calcination and sulfation of limestone in CFBB

“…The possible reasons had been proposed by many authors and these were summarized by Tarelho et al [8] into two main explanations: (i) the temperature effect on the physical properties of the limestone and (ii) the reductive decomposition of CaSO 4 at higher temperatures. The increase of SO 2 concentration always enhances the sulfation rate of the CaO, as shown in the research of Abanades et al [23] and Diego et al [11]. Smaller sorbent particle size is favorable for the reaction of CaO and SO 2 because of the larger specific surface area smaller particles has [10,24].…”

“…CFBB can be used to combust many types of fuels including different kinds of coals, coke, and biomass [1][2][3][4][5][6][7]. Apart from its excellent fuel flexibility, other advantages of CFBB are the low NOx production due to its low combustion temperature, and low SO 2 emissions which are attributed to the in situ desulfurization using calcium based sorbents [8][9][10][11]. Limestone as an economical desulfurization sorbent has been investigated intensively in the past decades.…”

Study of Oxy-fuel Coal Combustion in a 0.1 MWth Circulating Fluidized Bed at High Oxygen Concentrations

“…10 A possible reason for this phenomenon in the present study is that all of the tests were conducted with furnace temperatures around 850°C, which is not the optimum temperature for sulfur capture. A report by de Diego et al 17 describing tests conducted using a small-scale bubbling fluidized-bed (BFB) rig has shown that, under oxy-fuel combustion conditions, the optimum temperature to achieve the highest sulfur retention was 900−925°C, whereas operations using enriched air required optimum combustion temperatures of 850−870°C. The IET/CAS facility continues to conduct research in the field of high-concentration oxy-fuel CFB technology.…”

Mathematical modeling, steady state simulation, parametric analysis and optimization of SO2 capture in a countercurrent moving bed (CaO and C) reactor

“…The coal participates by 60% in SO 2 emissions, the crude oil participates by 30% and the rest 10% of SO 2 emissions is due to the combustion of pyrites and mineral sulfur. The limestone and the lime have been used for SO 2 capture in fluidized bed or pulverized coal combustors . The use, still, of fluidized bed technology in Greece and other countries is prohibitive due to the very high replacement cost of existing combustors.…”

“…In addition, various technologies for flue gas desulfurization (FGD) can be classified into three different types: wet scrubbers, semi‐dry processes and dry processes. Wet scrubbers have been widely commercialized in industry for achieving SO 2 removal in excess of 95% . However, this kind of technology generates a large amount of wet solid waste and requires treatment of waste water.…”