| As a new energy, biomass energy has drawn attention widely in recent years. The biomass straw as the great agricultural waste production in our country need reasonable utilization. Despite the low cost, high yield of straw, but also contains large amounts of K, and the presence of alkali metal will make biomass slagging, poly group, corrosion in thermal conversion process, which hindered the biomass energy utilization. Corn straw, a rich source of this K, has made a deep insight to the releasing and conversion characteristics of alkali, and made an analysis on physical structure and chemical analysis.The experiments of pyrolysis of biomass corn straw was conducted in N2 atmosphere in 300~1000℃. All experiments was carried out on a horizontal tubular reactor. Combustion experiments was done in 30% O2 atmosphere(400℃、600℃、800℃). Analysis includes alkali metal analysis(ICP-AES), occurrence form of alkali metal, microscopic structure of sample char(SEM), distribution of AAEMs(EDX), and alkali metal compound type of ash(XRD). Pyrolysis process was monitored by CO2 gas analyzer.Pyrolysis experiment found that duration of CO2 exhalation was long and the pyrolytic conversion rate of char sample was higher in 300℃~500℃. CO2 released at60 s in 600℃~1000℃, this was in consistant with the white smog(main componets in tar precipitation) in experiment. And alkali metal released in this process. Biomass is rich of potassium and less of sodium. Dry stage is mainly occured below 200℃, in which a small amount of K element released. K released quicly in 300℃~400℃, the amount of K released in this stage was below 10% of the total K. Compared with former stage, K released slowly in 400℃~600℃. More K element was released when pyrolysis temperature was beyond 600℃. Nearly 20% of total K released in 600℃~800℃. 53%of total K was released in 1000℃. This showed that alkali metal K is easy to release in the pyrolysis process. Release characteristics of alkali metal Na and K was similar, but the amount of Na is less than K. Ca and Mg was relatively stable in the pyrolysis process.Elemental analysis of biomass burning ash in 400℃~800℃ showed that the burning of straw was very quick, and combustion products was easier to slagging as temperature increased. In contrast to the weight loss ratio, the weight loss ratio in400℃~600℃ increased more quick than in 600℃~800℃. However, the change of retention rate of K was just the opposite. Amount of released K reached maximum at120 s in 400℃. However, the time point is 60 s in 800℃, followed by a stable process.Finally, the microscopic structure and alkali metal occurrence states of pyrolysis and combustion products was analyzed.Pyrolysis tar samples in 300℃~700℃ exist some surface holes but still holds the same fiber structure. The surface is rich of K element, molten Si and O elements will appear as temperature increase. Si element is as high as 15.06% under 800℃, the main occurrence state is Si O2, thus the sample has a clear boundary.Chemical fractionation experimental results showed that most of K element is in the form of soluble salt and ion exchange. 90.91% of K element is in the form of water soluble.Precipitation of organic K is the major occurrence states in300℃~500℃. Precipitation of soluble salt form is the major occurrence states in600℃~800℃. The precipitaiton form were potassium sulphate, potassium silicate, and mainly in the form of potassium chloride. |
PDF Full Text Request