Experimental Research On The Effect Of Ashing Temperature On Dissolution Characteristics Of Straw Ash

With the rapid development of utilization technologies of renewable energy,utilization of biomass as energy is increasingly concerned by researchers.Straw is the main biomass in China with annual production of about 800 million tons.Thermo-chemical conversion of biomass to energy,such as combustion,gasification and etc.,are most popular at present.As a result of a large amount of straw ash are produced.There are plenty of nutrients,such as potassium,phosphorus,calcium,magnesium and so on,in straw ash.The return of ash to farmland can realize the circulation of these nutrients in agricultural ecosystem.However,high temperature in the process of straw combustion(high ashing temperature)leads to ash softening,slagging and sintering.These nutrients in the sintered ash cannot be dissolved in soil,and thus unaccessible to the crops.Understanding the effects of ashing temperature on the dissolution characteristics of straw ash is of great significance for the sustainable utilization of agro-stalks as energy.In this paper,three typical agro-stalks in China—corn stover,wheat straw and rice straw—were chosen as experimental materials.Based on preliminary experiment and the analysis of common temperature in thermo-chemical conversion technology of biomass,the temperature range of 400-800 °C was chosen for ash preparation and investigation.X-ray Fluorescence(XRF),supplemented by Inductively Coupled Plasma-Mass Spectrometry(ICP-MS),Scanning Electron Microscope(SEM),Flame Photometer(FP)and Potassium-ion Selective Electrode(PISE),were applied to measure elemental and soluble components in straw ash or straw.According to mass conservation law,the water solubilities of ashes and elements were calculated.Effects of ashing temperature on water solubility of ash and recyclable content of nutrients in straw were studied,and the equilibrium relationships between these parameters and the temperature were analyzed.And the relationships of water solubility of potassium in corn stoversh with the temperature and residence time were discussed.The main contents and conclusion are following:(1)Effects of ashing temperature on water solubility of both the three kinds of straw ashes and potassium in the ashes were studied.Results showed that the water solubility of the three kinds of straw ashes,in decreasing order of abundance,were wheat straw > corn stover > rice straw at different temperatures.With the increase of ashing temperature from 400 °C to 800 °C,the water solubilities of ashes from corn stover,wheat straw and rice straw decreased from about 38%,50% and 38% to about 10%,9% and 6%,respectively.In the three kinds of straw ashes,potassium,sodium,sulfur and chlorine were easily water-soluble elements,while phosphorus,calcium,magnesium,silicon,iron and aluminum were water-insoluble elements.The water solubility of potassium in the ashes of corn stover,wheat straw and rice straw decreased from about 82%,85% and 85% to about 32%,48% and 15% as the temperature increased from 400 °C to 800 °C.The water solubility of sulfur was more than 75%,and that of sodium and chloride decreased differently with the increase of temperature.(2)Effects of ashing temperature on the recyclable content of nutrients in the three kinds of straws and that of potassium in corn stover were studied.Results showed that there was no significant difference for the recyclable content of nutrients in the three kinds of straws at different temperatures.With the increase of temperature from 400 °C to 800 °C,the recyclable content of nutrients in corn stover,wheat straw and rice straw decreased respectively from 4.2%,4.9% and 5.2% to 0.8%,0.6% and 0.7%,and that of potassium in corn stover decreased from about 89% to about 20%.It can be seen that the recycling proportion of nutrients in straw in nature could be effectively improved at low-temperature combustion condition.(3)The equilibrium relationships of ash solubilities and potassium solubility in ashes with the temperature for corn stover,wheat straw and rice straw were investigated.Results showed that the solubilities of the former two decreased slowly from 400 °C to 600 °C.Then they decreased fast from 600 °C to 700 °C,and lastly those decreased from 700 °C to 800 °C were insignificant.For rice straw ash,these parameters decreased linearly with increasing temperature.(4)Effects of residence time on the water solubilities of both corn stoversh and potassium in the ash were preliminarily discussed.Results showed that the water solubilities of corn stoversh and potassium in the ash vary considerably at different temperatures and residence time.When the temperature was below 600 °C,the effects of residence time on these solubilities were ignorable.At the temperature of 700 °C,the water solubility of the ash and potassium decreased respectively from about 18% and 49% to about 8% and 29% with increasing the residence time from 10 minutes to 240 minutes.At the temperature of 800 °C,when the residence time increased from 10 minutes to 120 minutes,the water solubilities of corn stoversh and potassium in the ash decreased from about 11% and 42% to about 7% and 31%,respectively.As the residence time continued to be extended,water solubilities of both of them remained almost unchangable.