Investigation of Partial CO2 Capture Using Ash from CFB Boilers with Limestone Injection

Abstract

Circulating Fluidized Bed (CFB) power plants have been gaining popularity globally due to its ability to utilize less expensive solid fuels, in-situ capture of SOx, low NOx emission and flexible operating characteristics. Higher CO2 emission from sulfur capturing CFB plants, on the other hand, is a major shortcoming of this technology. Additionally, fly ash generated from CFB power plants contains high amount of sulphate and unreacted lime due to sulphur capture in the furnace, which restrict its use in many commercial applications. To turn these shortcomings into positive use, an investigation was conducted to explore if the fly ash produced in a CFB power plant can capture a part of the CO2 released from it. Present work found that unreacted CaO in fly ash from some CFB boilers can indeed be utilized to partially capture CO2 in its flue gas. Experiments were conducted in a reactor with a batch of samples and continuous supply of simulated flue gas at 500-750 °C and 30-80 °C for dry and hydrated fly ash respectively from the 197 MWe Point Aconi power plant. It was shown that CO2 capture through conversion into CaCO3 was initially 100%, but decreased with time. The capture rate increased with temperature up to 700 °C for dry fly ash and 50 °C for hydrated ash; thereafter it started to reduce. Effect of partial pressure of CO2 (17 kPa to 31 kPa) on carbonation reaction was minor, but, the duration of the high initial CO2 capture was inversely proportional to the partial pressure. For baseline tests, the capture characteristics of high calcium lime were compared with that of CFB ash at the same conditions. In most cases, carbonation reaction in such lime followed a similar pattern as in fly ash. The capture characteristics were also similar for bottom ash (particle size, 116 μm to 275 μm) of CFB boiler, but the amount of CO2 captured by bottom ash was significantly lower than that by dry fly ash at the same condition. Thus the present work showed its potential for use of dry or hydrated ash in reducing CO2 emission from CFB power plants at low cost.