We determined the interaction between reaction conditions and the composition of reaction products to optimize the operating conditions for producing liquid fertilizer containing humic acid using hydrothermal liquefaction of cotton stalks.A response surface analysis method with three factors and three levels was used to investigate the effects of reaction temperature(X1,260-340℃),reaction time(X2,30-90 min),and material mass fraction(X3,5%-10%)on the yield of humic acid in water-soluble fertilizers.Regression model analysis of variance showed that reaction temperature,reaction time,and material mass fraction all significantly impacted the yield of humic acid.Among them,the material mass fraction was the most important parameter.The optimal reaction conditions for the yield of humic acid were reaction temperature 300℃,reaction time 90 minutes,and material mass fraction 10%.Under these conditions,the yield of humic acid was 4.10%,which is higher than the humic acid content specified in the national standard for humic acid water-soluble fertilizers(not less than 3%;NY 1106-2010)and is in good agreement with the predicted value.GC-MS analysis showed that the water-soluble products of cotton stem hydrothermal liquefaction mainly contained phenols and their derivatives,i.e.,ketones,aldehydes,alcohols,and organic acid compounds.
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