飛灰/氫氧化鈣吸收劑捕捉煙道氣中二氧化碳之研究

Abstract

本研究以微分固定床反應器，於模擬噴霧乾燥和乾式廢氣除硫系統的袋式過濾器中的條件下，探討廢氣氣體成分(CO2、O2、NOx、SO2、H2O)濃度對於氫氧化鈣(HL)以及飛灰/氫氧化鈣(FA/HL)吸收劑碳酸化反應之影響。飛灰/氫氧化鈣吸收劑經由漿化、乾燥程序製備，含有矽酸鈣等水合物，具有比原料氫氧化鈣更大的比表面積和孔隙體積。 氫氧化鈣於NOx/SO2不存在下，碳酸化轉化率隨相對溼度增加而增加，CO2和O2濃度對反應無影響。在NOx/O2與CO2同時存在時，氫氧化鈣碳酸化反應受到促進，相對濕度愈高愈為顯著，CO2濃度對反應有負面影響，NOx濃度則反之，溫度則無影響。SO2與CO2/O2/NOx同時存在下，碳酸鈣分率降低，但SO2濃度愈低(<500ppm)，碳酸鈣分率愈大。 飛灰/氫氧化鈣(30/70)吸收劑於NOx/SO2不存在下，碳酸化轉化率隨相對濕度增加而增加，CO2和O2濃度對反應無影響。在NOx/O2與CO2同時存在時，促進吸收劑碳酸化之作用隨相對濕度增加而減弱，而CO2與NOx濃度則無影響。當NOx和SO2都不存在時，且在相同的碳酸化條件下，飛灰/氫氧化鈣(30/70)吸收劑的碳酸鈣分率比氫氧化鈣的高，但當NOx與O2同時存在下，兩者之碳酸鈣分率於高相對濕度下十分相近。SO2與CO2/O2/NOx同時存在下，碳酸鈣分率降低，但SO2濃度愈低(<1000ppm)，飛灰/氫氧化鈣吸收劑碳酸化程度愈高。 飛灰/氫氧化鈣吸收劑於O2/NOx/SO2不存在下碳酸化，1小時碳酸鈣分率和CO2捕捉率隨其重量配比改變而變化。在重量配比為30/70時，有最大CO2捕捉率(在60oC、70% RH、12.6% CO2下，為0.253g of CO2/ g of sorbent)。然而，於CO2/O2/NOx下碳酸化，氫氧化鈣反應1小時後，有最高的CO2捕捉率(在60oC、70% RH、12.6% CO2、5% O2、600ppm NOx下，為0.379 g of CO2/ g of sorbent)。在模擬一般燃煤廢氣組成的氣體下，飛灰/氫氧化鈣吸收劑1小時SO2捕捉率皆低於氫氧化鈣的(在60oC、70% RH下，為0.534 g of SO2/ g of sorbent)，且SO2捕捉率隨飛灰重量配比增加而降低。

A differential fixed-bed reactor was employed to study the effects of the flue gas components (CO2, O2, NOx, SO2, H2O) on the carbonation reaction of Ca(OH)2 and fly ash/Ca(OH)2 (FA/HL) sorbents under the conditions similar to those in the bag filters of spray-drying flue gas desulfurization system. The FA/HL sorbents prepared by slurrying and drying processes contained foil-like calcium silicate hydrates and had a greater surface area and pore volume than Ca(OH)2. The carbonation of Ca(OH)2 was enhanced by relative humidity and was not affected by CO2 or O2 concentration when NOx and SO2 were not present. The carbonation of Ca(OH)2 was greatly enhanced with increasing relative humidity when NOx, O2, and CO2 were present simultaneously; in this case, relative humidity had a significant effect, temperature had a negligible effect, CO2 concentration had a negative effect, but NOx concentration had a positive effect. When SO2 was present with CO2, O2, and NOx, the fraction of calcium carbonate decreased, but it increased with decreasing SO2 concentration (<500ppm). The carbonation of FA/HL(30/70) sorbent was enhanced by relative humidity and was not affected by CO2 or O2 concentration when NOx and SO2 were not present. The carbonation of FA/HL sorbent were enhanced with decreasing relative humidity by the presence of NOx/O2 with CO2; in this case, the effect of relative humidity was less marked, and the effect of CO2 or NOx concentration was negligible. Under the same carbonation conditions, carbonation conversions for FA/HL(30/70) sorbent were much higher than those for Ca(OH)2 when NOx/SO2 were absent in the gas mixture, but were close to those for Ca(OH)2 at high relative humidities when NOx and O2 were also present. When SO2 was also present, the extent of carbonation of FA/HL(30/70) sorbent decreased, but it increased with decreasing SO2 concentration (<1000ppm). The reactivity of FA/HL sorbent varied with the FA/HL weight ratio. The sorbent with a ratio of 30/70 had the highest 1 h CO2 capture(0.253g of CO2/ g of sorbent at 60oC, 70% RH, and 12.6% CO2), when CO2 alone was present in humid N2. However, Ca(OH)2 had the highest 1 h CO2 capture(0.379g of CO2/ g of sorbent at 60oC, 70% RH, 12.6% CO2, 5% O2, and 600ppm NOx) when NOx and O2 were also present.Under gas the mixture with the typical flue gas composition, Ca(OH)2 had the highest 1 h SO2 capture(0.534 g of SO2/ g of sorbent at 60oC, and 70% RH), and the SO2 capture decreased with increasing FA/HL weight ratio.