烘焙生物质-煤混烧条件下灰的腐蚀行为

Corrosion behavior of ash from co-combustion of torrefied biomass and coal

生物质经烘焙后物化特性显著改善,可大规模替代煤炭进行燃烧发电,降低CO_(2)排放。然而,燃烧过程中灰沉积、腐蚀等问题亟需解决,烘焙过程对生物质燃烧中腐蚀行为的影响尚缺乏系统的研究。鉴于此,选取玉米秆、麦草2种典型生物质,在固定床反应器上制备其烘焙样,通过逐级提取结合ICP-MS、IC测试,探究烘焙对生物质中关键成灰元素的赋存形态及含量的影响。在沉降炉系统进行烘焙前后生物质、2种动力用煤单烧以及各生物质样品与煤混烧试验,并收集其总灰。选取12CrMoVG钢为腐蚀试验金属材料,使用基于水平管式炉的腐蚀试验系统,探究了550℃时,HCl存在的模拟烟气气氛下,单烧灰负载的腐蚀情况,以及HCl、SO_(2)共同存在的模拟烟气气氛下混烧灰负载的腐蚀情况,称重获得各工况下腐蚀增重,并通过SEM-EDS分析了腐蚀后样品形貌及成分特征。最后,通过不同HCl浓度下的腐蚀试验,揭示了烘焙对生物质-煤混烧的腐蚀倾向影响。结果表明,玉米秆和麦草中Cl和K含量较高。经过270℃烘焙,玉米秆和麦草中Cl分别释放了17.84%和38.76%;S分别释放了7.78%、20.98%;Na、Mg、K、Ca、Fe总量基本不变,少量水溶态K转化为离子交换态,部分水溶态的Mg、Ca向盐酸溶态转化。相同气氛条件下,由于烘焙生物质灰中AAEM(碱及碱土金属)含量略升高,其腐蚀增重相较于原生物质呈增加趋势。混烧工况的腐蚀增重显著低于单烧工况,主要是由于气氛中SO_(2)的存在导致了硫酸盐化反应,减轻了腐蚀程度。烘焙对生物质单烧和混烧的灰成分影响较小,因此对腐蚀程度的影响有限;但腐蚀气氛中Cl的增加能显著促进腐蚀行为,由于烘焙能够脱除生物质中部分Cl,因此烘焙有减轻生物质燃烧腐蚀倾向的潜质。

The physical and chemical properties of biomass after torrefaction can be significantly improved,which enable the biomass to re⁃place coal in large scale for power generation and CO_(2) emission reduction.However,ash deposition and corrosion during the combustion are still urgent issues to be solved,and systematic investigations on the influence of torrefaction on the corrosion behavior of biomass combus⁃tion is rarely available.In view of this,two typical biomass,corn stalk and wheat straw,were selected to prepare their torrefied samples in a fixed-bed reactor,and the effects of torrefaction on the occurrence modes and contents of key ash-forming elements in the biomass were investigated through chemical fractionation analysis combined with ICP-MS and IC tests.Combustion tests of biomasses,two typical pow⁃er coals and the blends of biomass and coal were conducted on a drop-tube furnace,and the bulk ash was collected.12CrMoVG steel was selected as tested alloy for corrosion tests which conducted on a horizontal tube furnace at 550℃.The corrosion behavior of loading ash from individual combustion under atmosphere containing HCl,and loading ash from co-combustion under atmosphere containing HCl and SO_(2) were investigated.The mass increment of ash loaded-alloys after corrosion tests were weighed.The morphology and composi⁃tion characteristics of the corroded samples were analyzed by SEM-EDS.Finally,the influence of torrefaction on the corrosion propensity of co-combustion of biomass and coal was revealed through corrosion tests under atmosphere with various HCl concentrations.The results indicate that the corn stalk and wheat straw contain considerable amounts of Cl and K.After torrefaction at 270℃,17.84%and 38.76%Cl is released from corn stalk and wheat straw while 7.78%and 20.98%S is released,respectively.The total amount of Na,Mg,K,Ca and Fe is hardly changed.A small amount of water-soluble K is transformed into ion-exchange forms,and part of water-soluble Mg and Ca are transformed into HCl-soluble forms.Under the same atmosphere,the mass increment after corrosion tests of torrefied biomass ash is slightly higher than that of the raw biomass due to the slightly increased AAEM(alkaline and alkaline earth metal)contents ash after tor⁃refaction.Due to the presence of SO_(2),the sulfate reaction is caused,and the degree of corrosion is reduced,thus the mass increment af⁃ter corrosion tests of co-combustion ash is obviously lower than that of individual combustion ash.After torrefaction,the elemental compo⁃sition of bulk ash from both individual combustion and co-combustion has little change,so its influence on corrosion degree is limited.However,the increase of Cl in the corrosive atmosphere can significantly promote the corrosion behavior.Benefiting that torrefaction can re⁃move part of Cl in the biomass,torrefaction is potential to reduce the corrosion tendency during combustion.