Catalytic Effects Of Metallic Oxides On Combustion Characteristics Of Biomass Micron Fuel (BMF)

Biomass direct combustion is the most widely used, simplest and cheapest way amongbiomass utilization approaches. But its lower burning temperature and efficiency has longlimited its application. Hence, Huazhong University of Science and Technology (HUST)developed biomass mocron fuel (BMF) high temperature combustion technologyaccording to the dust explosion and high temperature combustion principle. Researchshows that, air combustion temperature of BMF can reach1260℃, and oxygen-enrichedcombustion can go as high as1600℃. But the former still can not meet the requirementsof large-scale industrial application, the latter consumes large amounts of oxygen,resulting in higher cost. Therefore, Consider solid phase catalytic combustion of BMF,study the influences of catalytic additives on the ignition, burnout characteristics, averagereactivity, comprehensive combustion characteristics, combustion temperature, heatingrate as well as exhaust gas emission feature of it, is a new and significant method toefficiently use BMF combustion technology.Alkaline metal oxides MgO, CaO and dolomite whose main components are Ca andMg have been selected as additives to carry out the thermogravimetric analysis, static mixcumbustion and cyclone combustion experiment of BMF-additive mixtures.Thermogravimetric analysis analyzed the ignition, burnout, comprehensivecombustion characteristics, average reaction activity and kinetics characteristics of BMFsamples with1%wt,2%wt and3%wt additives. Rresults show that, except samples with3%wt MgO and3%wt CaO, all the other samples with additives have lower ignitiontemperature than BMF. None of the three additives has significant improvement forburnout characteristics of BMF, but addition of2%wt CaO and2%wt dolomite promotethe conversion rate around burnout temperature.2%wt CaO and2%wt dolomite hashigher average reaction activity while MgO has lower one than BMF.1%wt MgO,2%wtCaO and2%wt dolomite can improve comprehensive combustion properties of BMF, andwith the increase of addition percentage, the impact of CaO and dolomite was increasedwhile MgO was decreased. Activation energy calculated by Coats-Redfern method showsthat, except sample with3%wt CaO, all catalytic combustions have lower activation energy than BMF in volatile burning stage,1%wt MgO and2%wt CaO and dolomite withall percentages reduced activation energy of fixed carbon burning stage, BMF combustionand its catalytic combustion both follow lst order reaction mechanism. Take all theparameters into consideration, the optimal samples and their order is:2%wtdolomite>2%wt CaO>1%wt MgO> BMF, and it is concluded that MgO and CaO has certainsynergistic catalytic effect.Static mix comsution experiment carried out SEM, XRD, XRF analysis of ashescollected from mixed samples with2%wt additives after being calcined under600℃inmuffle furnace for2hours, the influences of the selected additives on slaggingperformance of BMF has investigated. Results indicate that addition of2%wt MgO, CaOand dolomite results in a linear increase of ash content of BMF, and cause a increase inagglomeration and fusibility degree. XRD analysis shows that, under600℃main mineralconstitutes of sample ashes are monomers including CaCO3, SiO2, KCl and MgO etc,without the formation of complex dispersion. Combined with biomass slaggingcharacteristics index including silicon ratio, aluminum to silicon ration, iron to calciumratio, it can be deduced that all the three metallic compounds caused increase in BMFslagging degree.For mixtures of BMF with2%wt additives, cyclone combustion experiment wascarried out on the basis of present cyclone furnace burning system of our lab. Bycomparing furnace temperature and exhaust emission features under different combustionconditions of BMF, the optimal excess air coefficient and feed rate was determined as1.26and360g/min respectively, when the highest temperature is1260℃. Under this condition,catalytic combustion of BMF samples with2%wt additives was carried out. The furnacetemperature and exhaust gas measurement results show that, under the optimalcombustion conditions, CaO and dolomite has no significant effect on the heating rate aswell as the highest furnace temperature below1000℃, MgO has slight negative impact.While over1000℃, dolomite and CaO can raise the highest temperature up to1280℃and1275℃respectively, and sample with MgO has lower temperature than BMF. Fluegas analysis shows that, CaO and dolomite can reduce CO emissions, while MgOincreased it slightly; MgO can reduce emissions of the NOx in flue gas, while CaO and dolomite has opposite result.