| China is an agricultural country with plenty of crop straw.If used in energy,it can be used to replace fossil fuels,improve China’s energy structure,and reduce carbon dioxide and other poisonous and harmful gas emissions.Hydrothermal carbonization is a kind of straw biomass heat conversion method,which can increase the fuel quality and combustion characteristics of biomass while also converting straw into efficient energy.The current studies are primarily concerned with the standard fuel properties of single-straw biomass hydrochar,such as ultimate analysis,heating value,hydrochar yield,energetic recovery efficiency,as well as combustion behavior.However,it is difficult to ensure that the raw material is a single straw biomass in practical applications,and research on the co-hydrothermal carbonization of straw biomass and non-straw biomass has found that co-hydrothermal carbonization has a more positive effect than single biomass hydrothermal carbonization.Therefore,it is required to examine the co-hydrothermal carbonization of multi-straw biomass,to lay a theoretical foundation for the realization of engineering and large-scale practical applications of straw biomass hydrothermal carbonization.In view of this,this experiment investigated the physicochemical properties,combustion properties,and process mechanism of hydrochar derived from co-hydrothermal carbonization of multi-straw biomass,by using ultimate analysis,approximate analysis,thermogravimetric analysis,fourier transform infrared spectroscopy and other equipment.To begin,rice straw,corn straw,and wheat straw were hydrothermally carbonized at various reaction temperatures,respectively,to investigate the physicochemical properties and combustion characteristics of the hydrochar,in order to establish a solid foundation for the co-hydrothermal carbonization of multi-straw biomass and determine the optimal reaction temperature.The results showed that with the increase of reaction temperature,the yield and volatile matter content of straw biomass hydrochars all decreased,but energy densification,fixed carbon content,carbon content,and high heating value all increased.The hydrochars prepared at 180oC-200oC exhibited high energetic recovery efficiency.The single straw biomass hydrochars largely undergoe dehydration,decarboxylation,condensation,and aromatization reactions,according to the Van Krevelen figure and Fourier Transform Infrared Spectroscopy curves.Furthermore,hydrochar prepared at 180oC has the highest comprehensive combustibility index and flammability index,and the activation energy of the fixed carbon combustion stage of hydrochar prepared at low temperatures（180oC-220oC）is comparatively low.Above all,the temperature of co-hydrothermal carbonization of multi-straw biomass was selected to be 180oC.Based on the above basis,the physical and chemical properties of the co-hydrothermal carbonization products of rice straw-corn straw and wheat straw-corn straw were further studied,respectively,and it was found that there was a synergistic effect between straw biomass during the co-hydrothermal carbonization process.The co-hydrothermal carbonization of two types of straw biomass and corn stalk could improve energy densification,energetic recovery efficiency,carbon content,and high heating value of the product,while lower its oxygen content,compared to the calculated theoretical values.The energy densification showed the best improvement impact when the mixing ratio of corn stover was set at 75%,which was 0.03 and 0.06,respectively.Based on the Van Krevelen figure and Fourier Transform Infrared Spectroscopy curves,it was found that the dehydration reaction and decarboxylation reaction were significantly improved during the hydrothermal carbonization.There are also some differences between rice straw-corn straw and wheat straw-corn straw co-hydrothermal carbonization.Compared with the calculated theoretical values,the co-hydrothermal carbonization of rice straw and corn straw can improve the hydrochar yield and fixed carbon content by 1.08%and 0.53%at most,respectively,and the volatile matter content was reduced by 0.63%at most.However,the co-hydrothermal carbonization of wheat straw and corn straw showed the opposite trend,the hydrochar yield and fixed carbon content decreased,and the volatile matter content increased.Finally,the combustion characteristics of the co-hydrothermal carbonization products were analyzed,and it was found that the combustion characteristics parameters of the co-hydrothermal carbonization products,co-hydrochar,were not only different from the theoretical calculation values,but also different from the parameters of mixed straw hydrothermal carbonization samples,which further confirmed the interaction between different biomass in the hydrothermal carbonization process.Compared with the combustion characteristics of mixed hydrochar,when the corn straw mixture ratio is 50%,co-hydrothermal carbonization could significantly increase the ignition temperature and decrease the burnout temperature.In addition,the comprehensive combustion characteristic index could be improved by 1.12×10-11min-2×K-3 and 1.30×10-11min-2×K-3.However,the change of flammability index is different if the straw biomass is different.When the mixture ratio of corn straw was 75%,the flammability index of co-hydrochar of rice straw and corn straw increased most obviously,reaching 0.54×10-7min-1×K-2,while that of co-hydrochar of wheat straw and corn straw decreased obviously.For the activation energy analysis,it can be found that when the corn straw mixing ratio is 75%,the combustion activation energy in the fixed carbon combustion stage of co-hydrochar is significantly reduced,making it more prone to combustion reaction.Overall,the multi-straw biomass will have a mutual influence in the co-hydrothermal carbonization process,which may boost dehydration and decarboxylation reactions,which will have a significant impact on the product’s physicochemical and combustion attributes.The co-hydrothermal carbonization of multi-straw biomass makes its product performance better than that of single-straw raw material hydrochar,which is beneficial to the energy utilization of straw biomass.Furthermore,due to the unique characteristics of each biomass,there are some differences in the physicochemical parameters and combustion properties of the co-hydrothermal carbonization products. |
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