| As a agricultural country,China has abundant biomass energy resources,with an annual output of 650 million tons of standard coal.However,the utilization rate is only account for 0.61%.Therefore,effective development and utilization of biomass plays a positive effect on replacing fossil fuel.However,high content of alkali metals and alkaline earth metals（AAEM）in biomass causes serious problems,like slagging and fouling during biomass combustion,which limits the industrial application scale.In order to effectively solve the above problems,an environmentally friendly CO2-water leaching pretreatment method was proposed in this paper to remove AAEM from biomass.The practical application feasibility of CO2-water leaching was confirmed by the research on the removal efficiency,ash fusion temperature,combustion characteristics and process simulation.The specific research content is as follows:First,the effect of temperature and leaching time on CO2-water leaching was investigated.Three kinds of typical biomass were slected for experiment.The self-made semi-continuous leaching device was usd to study the influence of temperature/leaching time parameters on the removal efficiency.The results showed that the removal efficiency of alkali metals increased with the increase of temperature and leaching time.Since the alkali metals were mainly ionic occurrence in biomass,the removal efficiency was large.95%of K was removed by CO2-water leaching.As for alkaline earth metals,the removal efficiency decreased firstly then increased with the increase of temperature.The removal efficiency of Ca was above 50%.CO2-water leaching had little effect on the organic structure of biomass.And the removal of some water-soluble oxygen-containing molecules and ash increased the HHV in CO2-water leached biomass.The leaching solution was rich in nutrients for plant growth such as K,Mg and fulvic acid,so it has the potential to be used for farmland irrigation and realize high value-added utilization.Then,the effect of CO2-water leaching on the combustion characteristics and slagging problem were studied.The wheat straw was selected and results showed that CO2-water leaching had significant removal efficiency on K,Ca,Na,Mg and Cl,especially for K and the removal effect of CO2-water leaching was better than water leaching.CO2-water leaching can also significantly reduce the release of AAEM and Cl during biomass combustion,and after CO2-water leaching for an hour,the release of K decreased from18.23 mg/g to 2.09mg/g.The softening temperature of biomass increased from 1016 oC to1414 oC by CO2-water leaching,which inhibited the tendency of slagging and fouling.Also,the TG&DTG curve shifted to high temperature zone by CO2-water leaching.Finally,the Aspen software was used to establish the CO2-water leaching process model,and the flue gas was used as CO2 gas source.Compared with conventional biomass direct-fired power plants,the main additive equipments were CO2-water leaching unit,heat exchangers and drying units.The investment cost of the biomass power plant with an installed capacity of 25MW is 287.5million yuan,and the investment recovery period is about 11 years.In summary,CO2-water leaching can effectively remove AAEM from biomass,reduce the tendency of slagging and fouling.The pretreatment is simple and has high practical feasibility and it provides a new approach for power plant flue gas utilization. |
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