| Biomass energy is one kind of carrier of solar energy, abundant in resource,transportable, storable compared with solar energy and wind energy. As carbon-containning material, biomass can be converted into fuel in solid, liquid and gaseous form, which makes biomass an ideal substitute for fossil fuels.Corn is the most widely cultivated crop all over the world, and also the second largest food crop in China with wide distribution. The energy utilization of corn stalk is thus an important issue to be solved for the consideration of waste treatment and open-burning inhibition. The industrial utilization of straw material for biofuel production can implement the energy derivation from waste with controllable CO2 emission.Thermochemical conversion technology, including direct combustion, pyrolysis and gasification, etc., could convert straw biomass into gas, liquid and solid fuel. So,it becomes the most rapidly developing route for straw utilization, with great potential for industrilization. In the process of thermal treatment of straw materials, tar conversion, gas composition adjustment, corrosion and deposition issues with alkali and chlorine in the ash, and also the agglomeration of bed material, provide some key issues associated with competitive technology development.Calcium oxide, CaO, as an additive in thermal conversion process, will be active in catalyzing the complicated reaction of macromolecular intermediates, inhibiting the emission of sulfide, alkali metal and chlorine compounds, reducing the fouling and corrosion problems. At the same time, CaO can be applied as a CO2 absorption material through the carbonation reaction under thermal conditions, resulting in CO2 in-situ separation and fixation, which is conducive to the control of CO2 emissions.The mechanism of CaO addition on the thermal conversion process was systematically studied for the development of directional thermal conversion technology of straw biomass. Fuel characteristics of corn stalk was analyzed for compositional and physicochemical properties. The decompostion behavior and ash melting point of straw in thermal process was analyzed. With relatively high oxygen content, calorific value of biomass is low, resulting in low energy density. Corn stalk ash samples were regarded with serious slag tendency. Under thermal conditions, the conversion process was di,vided into three stages. Volatile release perdominants at temperatures between 150 to 600?. And above 580?, the slow decomposition of charcoal residue proceeds for a long time. Under combustion conditions, volatiles released from pyrolysis would be involved in the combustion with oxygen, which would accelerate the progress of whole reaction. The volatile combustion stage is carried out below 400?, with fixed carbon combustion stage succeeded.The effect of CaO addition on pyrolysis and combustion of corn stalk was studied by thermogravimetric analysis. CaO addition exhibited some blockage to the release of moisture and volatile, hindering the heat conduction and gas release. And CaO act as a promotion for the transformation of macromolecules at low temperature and the rapid proceed of pyrolysis process. At temperatures below 700 ?, CaO exhibited absorption effect on CO2 during pyrolysis and combustion. Also the CO2 absorption process, with its exothermic property, showed some influence on the process thermal effect. Certain negative effect of CaO on the combustion process was observed from the point of increased ignition point, decreased volatilization rate and increased burnout temperature of fixed carbon.CaO addition has positive effect on reducing the slagging rate, especially when the addition ratio reaches 30%. The slagging tendency of corn stalk ash decreased greatly when the addition ratio was above 30 percent.The changes in ash composition were studied using XRD analysis. K and Si were in large content in ash,with SiO2, KCl and KAlSi3O8 as the prominent compound.KC1 and KAlSi3O8, with low melting point material,are the main factors causing the slagging and fouling problem. The addition of CaO increases the Ca content which weakens the influence of the low melting K compound. More Ca2SiO4 and other high melting substances helps in improving the overall anti-slagging performance of the corn stalk ash.From SEM analysis, corn stalk ash showed a smooth molten state. Obvious scaly changes were observed with increased CaO addition ratio. Regular CaCO3 crystal form, mixed with the flaky material, was observed when the addition ratio reaches above 30 percent, indicating the additive CaO had a significant effect on the melting of the corn stalk ash. Ash melting point analysis also proved the effect of CaO addition.The results of CaO addition on the thermal conversion behavior and ash property of corn stalk would be useful and informative for the development of competitive conversion technology, providing fundamental data and mechanism analysis. |
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