Fate of Alkali Metals and Phosphorus of Rapeseed Cake in Circulating Fluidized Bed Boiler Part 2: Cocombustion with Coal

This paper is part 2 in a series of two papers describing the fate of alkali metals and phosphorus during cocombustion of rapeseed cake pellets with different fuels in a 12 MW(th) CFB boiler. In the first part (Piotrowska, P.; Zevenhoven, M.; Davidsson, K.; Hupa, M.; Amand, L.-E.; Barisic, V.; Coda Zabetta, E. Energy Fuels 2010, 24, 333-345), wood was applied as a base fuel for the cocombustion tests. In this second paper, coal was used. Cocombustion with coal has been proven to be a strategy to improve the combustion of rapeseed cake. This paper presents the fate of alkali metals and phosphorus during successful cocombustion of up to 25% of rapeseed cake pellets on an energy basis with coal. Tests with and without addition of limestone were performed. The fuels were analyzed according to standard fuel analyses and chemical fractionation. Elemental analyses of outgoing streams were performed by means of wet chemical analysis. In addition, SEM/EDX analyses of outgoing ashes and deposit samples collected with a deposit probe were performed. The SO(2) and HCl emissions were analyzed. Mass balances were calculated for all cocombustion tests. Gaseous alkali chlorides were measured before the convective pass at a flue gas temperature of 800 degrees C using an in situ alkali chloride monitor (IACM). At the same place HCl and SO2 were measured, and deposit samples were collected with a deposit probe. Rapeseed cake cocombustion caused an increase in alkali metals and phosphorus. However, no heavy bed agglomeration or deposits were observed. This is due to interactions between alkali metals and aluminum silicates from coal. No formation of gaseous alkali metal chlorides was detected in the beginning of the convection pass by means of IACM. Phosphorus was present in the deposit samples up to about 9 wt %(P2O5) in the leeward side of the deposit probe when no lime was supplied to the combustion chamber. Addition of limestone resulted in a higher deposition rate and lowered emissions of HCl and SO(2).