The Research On The Mechanism Of High-grade Fuel From Municipal Solid Waste By Hydrothermal Carbonization And Application Studies Of Hydrochars

The harmless,volume reductive and resourceful disposal of municipal solid waste?MSW?have always been an indispensable part in the construction of ecological civilization and environmental protection.Hydrothermal carbonization technology?HTC?can convert low grade biomass into high grade solid fuel,which has drawn many attentions.According to the characters of MSW?high moisture,low energy density and large capacity?,this work focused on the research about HTC of MSW based on component groups.Through the study,the mechanisms of HTC of single component and full component of MSW are explored and revealed.Meanwhile,the comprehensive heat utilization process of MSW hydrochars and its process energy evaluation are mastered and quantified.Firstly,the HTC process and product characteristics of six typical component groups were studied.The results showed that the hydrochars yield followed the trend of waste textiles>waste wood?waste food>waste paper in HTC temperature range.However,low carbonization degree of waste rubber was observed.And almost no substantial carbonization reaction was happened during HTC of waste plastic,only transforming from granula form to molten state.The solid hydrochars and souble were the main products during HYC process,while gas accounted for only a small proportion.HTC process could retain most of C elements in hydrochars,while significant O element was removed.The energy density and fuel utilization grade of hydrochars could be improved after HTC.It could be speculated that the HTC can discard the dross and select the essence of solid waste.Although there were some similar reactions and mechanisms involved in during HTC process,but the HTC conversion and hydrochars forming mechanisms of typical component groups were different due to their physical and chemical characteristics.Therefore,based on the above analysis,the conversion mechanisms and possible conversion pathway?molecular level?of six typical component groups during HTC process were proposedSecondly,the process characteristics and regularities of HTC of MSW?210²80oC,30⁹0min?were obtained.Experimental results showed that due to the interaction between the component groups,the MSW hydrochar's yield was higher than that of waste paper,waste wood and waste food,while was lower than waste textile.The caramelization,Maillard reaction,aromatization and compound-related crosslinking during the HTC of MSW resulted in promoting the energy and density of hydrochars.The effect of HTC reaction temperature was higher than that of residence time.Besides,the effect of residence time was more obviouse under low temperture range.With the increase of temperature,the effect was weakened gradually.It's worth noting that high HTC reaction parameters not only significantly reduced the fuel yield,but also reduced the available value of hydrochars as solid fuel due to its high ash proportion.Based on the research on HTC of typical component groups,the forming and reaction mechanisms of hydrochars and its affiliated intermediates reaction during HTC of MSW were proposed.It's revealed that the process mechanisms of MSW hydrochars were formed by dissolving carbonization,decomposing carbonization,surface carbonization and physical reforming.Then,the effectd of HTC temperature and residence on combustion,pyrolysis and char CO₂-gasification characteristics of MSW hydrochars were investigated by a thermogravimetric analyzer.The results showed that the effect of HTC temperature on the thermal behavior of hydrochar was greater than the residence time.Because of the reduction of volatile matter and the increase of fixed carbon,the ignition temperature and the initial decomposition temperature were all higher than that of raw MSW.But the char CO₂-gasification activity of hydrochars were improved and the gasification conversion rates were obviously faster.The decomposition and reformation of the substances during HTC led to the disappearance of the first combusting loss peak of hydrochars which were obtained at high HTC parameters.But the second combusting loss peak rate was gradually increased.Moreover,the second pyrolysis loss peak was increased obviously,resulting in a more concentrated and intense pyrolysis behavior.As a consequent,the comprehensive pyrolysis release indexes of hydrochars were bigger than that of MSW.These confirmed that the HTC technology can conver MSW into an alternative solid fuel with comparatively good combustibility and comprehensive pyrolysis release index.The high HTC reaction temperture was the key condition to achieve the excellent CO₂-gasification performance.But prolonging the residence time has a negative effect on the char gasification.The kinetic analysis results suggested that the n-order reaction model could fit the combustion and pyrolysis of hydrochars very well.And the char CO₂-gasification behavior could be described by the mixed reaction model.Finally,the mass and energy balance analysis models coupling HTC,mechanical pressure filtration,natural drying and thermal drying process were established.The results show that most of the irreducible water and bonded water were converted into free water during HTC process.The mass balance analysis showed that more than 78%of the water could be removed in the HTC and mechanical pressure filtration stage,and only a small amount of water needed to be removed by the subsequent thermal drying.The energy balance analysis showed that due to the avoidance of the loss of vaporization heat,the HTC of MSW could achieve good energy utilization efficiency.Comparsion to direct thermal drying process,the energy saving efficiency reached 26.94%under 230oC,30min HTC condition.However,the energy saving efficiency of high-parameter HTC process has dropped remarkably.For example,the energy saving efficiencies of 250-90 and 280-90 were only 9.59%and 3.33%respectively.Therefore,high parameter is seem not suitable for disposaling MSW when choosing HTC techonology,and it may be prefer to select low HTC parameter in the practical application.On the whole,HTC can be performed as both environment-friendly MSW management and efficient energy recovery technology which can effectively upgrad MSW to value-added and energy-density solid fuel at the source.