| OCR Text |
Show reducing conditions. A negative sulphur retention of about 100% at a bed temperature of 890 C and at an in-bed ai r rat i 0 of 0.75 was reported by Bramer (7) which indicates that decomposition of CaS04 could also occur at a temperature of 880 C. Reactions (1) to (3), given below, show the decomposition mechanism of CaS04 • CaS04 + CO CaO + SO,- + C~ CaSO + 4CO CaS + 4CU2 (£) 3CaS04+ CaS 4CaO+ 4S02 (3) (1 ) Apart from CO, other reduci ng agents 1 ike Hz and Carbon can also reduce CaS04 • Laboratory scale investigations of Spltsbergen et al (8) show that on 1 y small amount of CaO and S02.. were produced at 850 C and cons i derab 1 e amount of CaS was not converted. His findings based on CaS + 20 CaS04 (4) CaS + 3/202 CaO + S02 (5) reactions (4) and (5) give support to thought that during staged combustion S02 is produced by react ion (5). The maximum retent i on at 830 Cis in agreement with the finding of Tatebayashi (4) and Va1k et al (5). The S02 retention at a high level of air staging (PAF of 0.84) is always poor and is not affected by change in temperature. This ineffectiveness of bed temperature, shown in Figure 6 could be due to a balance between the S02 pick up by limestone in the regions where oxidizing conditions prevail and the release of S02 (from CaS04, in reducing regions) a 1 ready captured el sewhere in the combustor. It is apparent from Fi g 7 that lower emissions are obtained at lower secondary air ratio. At 40% secondary air, the poor reduction in SOz emission is due to a large proportion of bed being under reducing condltions. Two additional runs at a total air velocity of 1.0 m/s and 2.0 m/s at 70:30 air staging were carried out to investigate the effect of fluidizing velocity. Lower sulphur retention was achieved at both the fluidizing velocities of 1.0 m/s and 2.0 m/s and a 0.95 PAF. Fig 7 also shows the effect of bed temperature on SO) retention at different fluidizing velocities. This figure clearly indlcates that at a bed temperature of 880 C limestone is ineffective in capturing S02 at all three velocities. CONCLUSION: (i) Combustion efficiency decreases as the level of air staging increases. (ii) The zero tests for unstaged combustion without limestone show little effect of bed temperature on S02 emissions at 40% excess air . (iii) For a given excess air level and bed temperature, increasing the level of air staging or lowering the PAF causes an increase in S02 emission without limestone addition. At PAF of 0.84, 20% higher S02 emissions than those at a PAF of 0.95 were observed. (iv) A 73% reduction in S02 emission was obtained by using limestone at a 3:1 Ca/S ratio under normal oxidizing combustion conditions at 830 C. Increase in the bed temperature to 880 C causes sulphur retention to fall to 43%. (6) |
