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2018年,我国城市生活垃圾焚烧处理量已达1.02×108 t。垃圾焚烧会带来污染物形态的转移。其中,NOx是垃圾焚烧过程中产生的最主要气体污染物之一,会引起酸雨、光化学烟雾等环境问题[1-2]。目前,垃圾焚烧烟气的脱硝手段包括选择性催化还原(selective catalytic reduction,SCR)和选择性非催化还原(selective non-catalytic reduction,SNCR)2种。SCR催化剂在垃圾焚烧烟气中变性失活严重且工艺运行成本过高,故其应用相对较少;SNCR技术具有设备投资低、适应性强及经济性较好等优势,故在垃圾焚烧处理中得到广泛应用[3-4]。然而,受化学反应动力学限制,SNCR脱硝技术存在温度窗口。在理想情况下,将反应温度控制在窗口(875~1 050 ℃)内可达到60%~80%的脱硝效率[5],而当反应温度偏离此脱硝温度窗口时,脱硝效率将大幅降低[6]。由于垃圾焚烧烟气温度较低,且烟气中含有的O2、H2O及飞灰等成分均会影响SNCR脱硝效率,因此,在实际工程中,常规SNCR脱硝技术仅有约30%~50%的脱硝效率,无法满足我国日趋严苛的NOx排放标准[7-8]。因此,拓宽垃圾焚烧烟气SNCR的温度窗口、强化SNCR脱硝是垃圾焚烧行业亟待解决的问题。
近年来,研究者主要通过加入各种添加剂来改善SNCR脱硝特性,以提升其脱硝效率。所采用的添加剂包括小分子气体、碱金属化合物、有机含氧化合物及其他新型添加剂。在气体添加剂方面,张彦文等[9-10]通过对添加甲烷的SNCR脱硝实验研究及CHEMKIN模拟研究,证明了添加CH4能提高低温时的脱硝效率,并可拓宽温度窗口。这是由于CH4的氧化反应使得反应氛围中的OH和HO2等基元含量增加,促进了NH3反应形成NH2,从而促进脱硝过程。HAO等[5]发现,Na/K添加剂在850~1 150 ℃都能起到明显提升SNCR脱硝效率的作用,促进效果Na2CO3 > KCl > NaCl。ROTA等[11]通过实验研究证明了添加醇、酯、酚、羧酸、醛及醚等6类典型含氧化合物后,SNCR脱硝温度窗口均不同程度地拓宽至原来的1.5~2倍,且促进作用的强弱与含氧化合物的种类没有必然联系。在150 kW中试实验台上,JAVED等[12]研究了添加H2O2、C2H5OH、C2H4(OH)2和C3H5(OH)3对SNCR脱硝的影响,发现4种添加剂都能使最佳脱硝温度大幅度向低温区移动。其中,[C3H5(OH)3]/[NO]为2.0时,最佳脱硝温度降低了330 ℃。
不同添加剂均可不同程度地影响SNCR温度窗口和脱硝特性,但碱金属化合物的引入则可能带来烟气颗粒物浓度提升及换热器结焦问题[13-14]。气体添加剂因其在工艺实际中的存储问题等限制了其应用,而有机含氧化合物可被直接喷入炉膛SNCR反应区域,在高温烟气中会被分解氧化成无害气体排出,故可行性较好。截至目前,已有大量研究者认为,添加单一组分有机含氧化合物能拓宽SNCR温度窗口[11-12, 15]。生物油作为一种生物质热利用过程中的副产品,其成本低廉且含有丰富有机含氧化合物,但将其作为SNCR添加剂的研究相对较少。张波[16]将生物油作为循环流化床燃煤锅炉SNCR脱硝添加剂,结果表明,添加1%质量比例谷壳油会使SNCR脱硝窗口向低温方向移动约50 ℃,并拓宽了窗口宽度。
本研究针对垃圾焚烧烟气的SNCR脱硝,利用高温管式炉研究了生物油添加比例(β)、氨氮比(NSR)与氧浓度对生物油强化垃圾焚烧烟气SNCR脱硝特性及CO排放的影响,并分析了其作用机理,旨在为以生物油作为添加剂的SNCR技术的工业化应用提供参考。
Characteristics and mechanism of bio-oil enhanced waste incineration SNCR denitrification
- Received Date: 16/11/2020
- Available Online: 10/11/2021
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Key words:
- waste incineration denitration /
- selective non-catalytic reduction /
- bio-oil additives /
- CO emission
Abstract: To reveal the effect and the mechanism of adding bio-oil on the denitration characteristics of waste incineration selective non-catalytic reduction (SNCR), we studied the effects of bio-oil addition ratio (β), ammonia-nitrogen ratio (NSR) and oxygen concentration on SNCR characteristics and CO emissions in a high-temperature tube furnace. Combining SNCR primitive reaction with the analysis of bio-oil thermal decomposition products, the mechanism was analyzed. The results show that the addition of bio-oil can broaden the SNCR temperature window and significantly improve the denitration efficiency in the temperature range 700~900 °C, when β=10%, the denitration efficiency increases averagely by about 21%. During the SNCR process, CO emissions improve evidently with the increase of bio-oil addition ratio, but when the temperature is above 800 °C and the oxygen concentration is above 4%, there is almost no CO emission. The decomposition products of bio-oil at high temperature are mainly H2, CO and CH4, etc, the bio-oil under high temperature will break into small molecular gases and simultaneously generates a large amount of free radicals such as H, O, HO2 and OH to strengthen the SNCR denitration reaction at low temperature. Therefore, in the waste incineration power plant, the SNCR efficiency can be improved by adding appropriate bio-oil in the area below 900 °C.