Study On The Incineration Process Of Finished Leather Waste

China is a large tanning country,the annual output of finished leather is about 600million square meters,accounting for about 20%of the world’s leather production.At the same time,a large number of finished leather wastes are also produced.Because of the complex composition,it is easy to pollute environment by discarding them.So the finished leather solid waste treatment has become one of the key work in the leather industry,and it is also an important issue for the sustainable and green development of the leather industry.As a mature thermal treatment technology,incineration is widely used in waste reduction.Based on this,the study of combustion characteristics of finished leather waste can provide theoretical basis and application guidance for its inclusion in municipal waste incineration system,also appropriate measures can be taken to address the pollution generated at a later stage.In this thesis,based on chrome tanned leather waste,then,the finished leather waste is divided into two types according to the presence or absence of coating on the surface of the finished leather:uncoated finished leather waste and coated finished leather waste.The combustion behavior and combustion kinetics of two finished leather wastes were investigated by TG-DSC;the gas emission characteristics of two finished leather wastes during combustion were evaluated by TG-FTIR;the incineration conditions of two types of finished leather wastes and the degree of chromium oxidation and transformation of chromium forms at different temperatures were analyzed by using a tube furnace to simulate the actual incineration conditions.The main conclusions of this thesis are as follows:（1）The finished leather waste has a high calorific value（～16 MJ/kg）,elemental carbon is the main component accounting for about 44 wt.%,the combustible elements C,H,N and S account for about 62%,and the weight loss after incineration treatment is more than 94%,which means that the finished leather waste can be used as a biomass fuel,but the presence of nitrogen and sulfur elements means that polluting gases such as NO_X and SO_X will be generated,therefore,it is necessary to prevent and control the polluting gases generated.（2）The combustion characteristics of the two kinds of finished leather waste is different.The main combustion stage of uncoated leather is one-stage,which is mainly the combustion process of collagen fiber;the main combustion stage of coated leather is two-stage,which represents the combustion process of collagen fiber and polymer film-forming material respectively.The increase of flammability and burnout rate of finished leather waste with the increase of heating rate means the combustion performance became better.The results of Flynn-Wall-Ozawa（FWO）method calculations showed that the average activation energies of uncoated finished leather stage 1,stage 2 and the whole stage were 119.82,287.83 and 261.98 k J/mol,respectively;the average activation energies of coated finished leather stage 1,stage 2,stage 3 and the whole stage were 89.48,289.28,225.28 and 218.83 k J/mol,respectively.It can be seen that the overall phase of the reaction activation energy of the coated leather is lower than that of the uncoated leather,which means that the coated leather is more combustible.Coats-Redfern（C-R）integral method analysis shows that uncoated finished leather waste is a two-step kinetic process consisting of pre-heat decomposition and collagen fiber combustion,while coated finished leather waste is a three-step kinetic process consisting of pre-heat decomposition,collagen fiber combustion and polymer film-forming substances combustion.Among them,the pre-heat decomposition stage can be represented by the assumed random nucleation and its subsequent growth model（A）.Random nucleation means that one new product randomly forms at reactive sites（nucleation points）in the reactants lattice,and the growth rate of nucleation may be expressed via the increasingly formed nuclei radius,which provides the site for the next step of the combustion of collagen fibers.The oxidative combustion stage of collagen fibers can be represented by the power law model（P）,and the rapid oxidative combustion of collagen fibers is achieved in this stage.The chemical reaction mechanism means that there is a linear relationship between the reaction rate and the remaining reactants amount,and the combustion stage of polymeric film-forming substances can be represented by the second-order chemical reaction model（F2）.The TG-FTIR results showed that the main gases produced during the combustion of the finished leather waste were CO₂ accompanied by the formation of CO,NO₂,NH₃,SO₂,hydrocarbons and carbonyl compounds.（3）The results of orthogonal experiments gave the best incineration conditions for uncoated finished leather waste as 900℃,holding 30min under 5 L/min air flow rate;the best incineration conditions for coated finished leather waste as 900℃,holding 60min under 5 L/min air flow rate.Combined with the actual industrial incineration conditions and the insignificant effect of holding time and air flow rate on the Cr（VI）content,the actual incineration conditions for the finished leather waste were suggested to be 850°C and 30 minutes of combustion at 5 L/min.Sintering experiments showed that increasing the temperature significantly inhibited chromium oxidation,implying that it is feasible to increase the combustion temperature to inhibit chromium oxidation.The XRD results showed that Cr₂O₃ was the main crystalline phase in the sintered ash,but the Cr（VI）content in the ash exceeded the emission limit,implying that the incinerated ash needed to be re-treated or utilized to eliminate the potential risk.