Experimental Study On Co-hydrothermal Preparation Of Low-Chlorine And Low-Alkali Fuel Based On Sludge And Corn Stover By Response Surface Methodology

The rapid development of global industrialization and human society has led to the increasing production of sludge and corn stover,most of which are not used in an environmentally friendly form.Compared with the traditional treatment method,the co-hydrothermal preparation of fuel can properly treat sewage sludge and corn stover reduces the pretreatment cost of biomass fuel and is an environmentally friendly treatment method.Based on response surface methodology（RSM）,a hydrothermal high-pressure reactor was designed to prepare water at medium temperature（160-260°C）,holding time（30-90min）,and mixing ratio of sludge and corn stover（0%-100%）.The thermal carbon test was carried out and the parameters such as higher heating value（HHV）,mass yield,dealkalization and dechlorination efficiency of sludge and corn stover co-hydrothermal were analyzed and optimized.After hydrothermal treatment,the HHV of sludge hydrothermal char ranged from 5210.7-6548.0 k J/kg and decreased with the increase of reaction temperature.However,they are all lower than the HHV of the original sludge sample of 8185.8 k J/kg.The HHV of corn stover-hydrothermal char ranged from 18567.5-21102.4k J/kg with the increase of temperature,and both were higher than the original HHV of corn stover by 15500k J/kg,and its yield was much lower than that of sludge hydrothermal charcoal.Temperature and mixing ratio have a great effect on yield,and time has little effect.The yields of corn stover and sludge showed the same trend with the reaction conditions,which was essentially the hydrolysis of macromolecules,and decreased significantly with the increase of temperature.The yield of corn stover hydrothermal char is very low,ranging from26.74-61.26%,while the yield of mixed hydrothermal char is 41.46-72.81%.The addition of corn stover in the co-hydrothermal experiment significantly increased the HHV of the hybrid hydrothermal char,but at the expense of yield.Compared with the original sludge and corn stover,the content of alkali metal elements,alkaline earth metal elements and Cl under each hydrothermal condition was measured,and the removal efficiency of the five elements was calculated based on the yield.Combined with the analysis of the response surface,the stability of the five elements in the removal process can be expressed as:Mg>Ca>K>Na>Cl.Temperature,time and blending ratio have a very significant negative effect on the removal efficiency of Na（the response surface shows a concave shape）,while the effect on the removal efficiency of K is negligible.Compared with temperature or time,the effect of blending ratio on the removal efficiency of Na is much greater.When the blending ratio was fixed,the removal efficiency of K always fluctuated in a small range regardless of temperature and time,which was related to tightly bound extracellular aggregates.At the same temperature or time,the removal efficiency of K from the sludge-hydrothermal char is much greater than that of straw,which may be due to the fact that lignin can fix K through physical adsorption and van der Waals force.The results also show that there is a significant interaction between temperature,time and mixing ratio on the removal efficiency of Ca,while the removal efficiency of time,temperature and mixing ratio and Mg is very small,which is related to the strong stability of Mg.Sludge and corn stover have a synergistic effect on the removal of Ca,probably because the special network structure of lignin contains a large number of active groups such as hydroxyl,carboxyl and carbonyl,which can form complexes with Ca²⁺.Sludge and corn stover have a synergistic effect in dechlorination.The-OH released by the corn stover contributes to the release of Cl in the hydrothermal char to the liquid phase,while the hydrogen ions and HCl in the sludge hydrothermal system further increase the hydrolysis of the corn stover.This synergistic effect may promote the increase of dechlorination efficiency,and the maximum removal rate can reach 95%.In the range of 185-260°C,the structure of the hydrothermal carbon changes,and the whole hydrothermal carbon changes from dense to loose,which may make some Cl dissolved in the liquid phase re-adsorbed on the hydrothermal carbon.Through the comparison of aluminum-silicon ratio,iron-calcium ratio and contamination index,it can be found that:pure sludge hydrothermal carbon has a stronger tendency to slagging and contamination than pure corn stover hydrothermal carbon;some pure corn stover hydrothermal carbon has a higher Cl content;the index of hybrid hydrothermal carbon is between the two.The ideal hydrothermal conditions should not only meet the requirements of dechlorination and dealkalization,but also meet the requirements of slagging,fouling and clean discharge.After multi-objective optimization,the ideal hydrothermal process should obtain the maximum hydrothermal carbon yield,HHV,and dealkalization and dechlorination efficiency through lower reaction conditions.Combined with the analysis of actual factors,errors and the importance of indicators,the optimal hydrothermal carbonization condition is that the hydrothermal temperature is 246.14℃,the residence time is90min,and the mixing ratio（Sludge:Corn stover）is 57.18%.Sludge and corn stover co-hydrothermally prepare hydrothermal carbon,which avoids the use of chemical additives,reduces the pretreatment cost of solid waste and low-grade biomass fuel,and improves the quality of fuel,realizing two low-quality sludge and straw.Clean thermal conversion of energy.Under the optimal hydrothermal carbonization conditions,clean fuels with low alkali,low chlorine,low slagging and fouling tendency were obtained.