Fundamentals And Industrial Application Of Dual Fluidized Bed Decoupling Combustion Of Biomass Waste With High Water And Nitrogen Content

The industrial biomass waste with high water and nitrogen content is a kind of solid residue from light industry in China.With massive production,being rich in cellulose and concentrated distribution,there is a huge potential in resource utilization of those biomass waste.While the lack of large-scale and resourceful treatment technology leads to those biomass waste becoming the primary pollution source of light industry.And it seems very efficient and convenient to combust the high water-containing and nitrogen-containing biomass waste to produce steam with meeting the industrial requirement for rapid,clean and large-scale treatment as well.But the high water and nitrogen content in industrial biomass waste becomes the major bottleneck for combustion technology and limites its application.Considering the problems existing in traditional combustion of biomass waste with high water and nitrogen content,such as low combustion temperature,incompleted combustion and NO_x emission exceeding standard seriously,the so-called dual fluidized bed decoupling combustion(DFBDC)was researched and developed in this study.This technology divides the traditional combustion into fuel pyrolysis and combustion of pyrolysis products and then improves the combustion process of pyrolysis products based on the reburning theory,thus eliminating the influence of water content in fuel and reaching low NO_x emission.For developing the DFBDC process,distilled spirit lees(DSL)was used as the matrial in this study,and bubbling fluidized pyrolysis of DSL and char combustion were conducted to support the reactor design and operating conditions definition.An industrial demonstration plant with a DSL treatment capacity of 50,000 tons/year was established to verify the technical advantages and flexibility to the biomass waste with high water and nitrogen content.A DFBDC ASPEN model was established to optimize reactor structure and operating conditions.The main results from this study are summarized as follows:1.DSL pyrolysis in bubbling fluidized bed(DFB).The pyrolysis temperature had a significant effect on the yields of pyrolysis products,and an increasing temperature leaded to an obvious decrease in the yield of char and tar but increased the yields of pyrolysis gas.The excess air ratio(ER)had a little influence on char yield,while raising ER would increase the pyrolysis gas yield and CO2 content in pyrolysis gas remarkably,followed by reducing compositions content decreasing as a result of CO2 dilution.ER increasing resulted in the decrease of tar yield under low-temperature pyrolysis(500-600?)but had an insignificant effect on tar yield and compositons under high-temperature pyrolysis(700-800?).The pyrolysis temperature and water content in DSL had a significant effect on pyrolysis reaction rate.In order to obtain a high pyrolysis reaction rate and conversion,the operating temperature of industrial BFB pyrolyzer should be controlled within 600-800? with particle residence time no less than 9.9 s.2.Char combustion characteristics.The increase of pyrolysis temperature and ER improved the dehydrogenation of small aromatic rings and growth of aromatic rings,thus leading to the increase of aromatics with ring number more than 6 and cross-linked structure as well as the decrease of small aromatic rings with high reactivity.Those carbon structure changes reduced the stable combustion stage of char and extended the burnout stage,and it also attributed to the decrease of burnout reaction rate and ignition temperature.Especially for the char with pyrolysis temperature of 800? and ER of 0.2,its ash still contented a lot of unreacted carbon even combusting at 900?.Therefore,the optimal operating temperature and ER of industrial BFB pyrolyzer were 600-800? and 0.1 respectively.The combustor temperature should be no less than 900? with particle residence time more than 10 s to promote the combustion of low reactive carbon structure and decrease carbon content in fly ash.3.Industrial demonstration plant operation.According to the fundamental researches about DSL pyrolysis and its char combustion,the design and operating parameters of reactors were determined,and an industrial demonstration plant with treatment capacity of 50,000 tons/year was also established.Comparison of direct combustion and decoupling combustion of DSL showed that decoupling combustion technology could solve the problems existing in CFB direct combustion of high-water containing DSL,such as combustion zone decreasing and moving upward,instable combustion,incompleted combustion of CO and volatiles in gaseous phase and serious slagging in loop seal.Besides,the NO_x emission in flue gas for decoupling combustion was no more than 100 mg/m~3,which reeached the emission standard without any denitrification treatment.This industrial plant of DFB decoupling combustion could also keep stable operation under different boiler loads,feeding rates and operating parameters.And the NO_x emission and fuel-N conversion could be controlled below 220 mg/m~3 and 2.87%respectively,as feeding rate of DSL and boiler load were no less than 1.45 t/h and 70%respectively.4.ASPEN simulation of DFBDC process.Based on the operation results of industrial demonstration plant,an ASPEN model of DFBDC process was established.The simulation results of BFB pyrolyzer showed that the temperature of circulating bed material and water content in DSL had a significant effect on particle circulating rate for keeping pyrolyzer at a certain temperature.Therefore.a granular flow partition valve was applied in industrial plant instead of traditional loop seal,thus realizing the flexible adjustment of the bed material entering into pyrolyzer for controlling reaction temperature and avoiding the fluctuation of the temperature and pressure in combustor simultaneously.The oversize volume of pyrolyzer could not increase the reducing compositions content in pyrolysis gas and carbon conversion of char partial-gasification significantly.Thus,a novel riser pyrolyzer with small volume and high-throughput was applied in industrial plant instead of the previous BFB pyrolyzer,which reduced the bed pressure,power consumption and tempering risk greatly as well as shortened the preheating and start-up time of pyrolyzer.The successful application of riser pyrolyzer aslo proved that the combination of riser pyrolyzer and riser combustor was more suitable for DFBDC technology with combustion as the primary reaction.The simulation results of riser combustior and heat exchange system showed that the boiler effiency and steam output reduced with the increasing of flue gas temperature.And an economizer was installed in flue to slove the problem of high exhaust gas temperature,making heat loss ratio caused by exhaust gas reduce from 8.2%to 4.6%.Although the boiler effiency and steam output reduced with the increasing of water content in DSL,the influence of water content was weakened significantly as water content was lower than 35 wt.%and 30-35wt.%was the optimal water content in fuel for dual fluidized bed decoupling combustion.