Study On The Directional Control And Mechanism Of Simultaneous Sludge High Efficiency Gasification And Ash Adsorbent Preparation

Sewage sludge is a kind of solid waste from wastewater treatment process,as well as a potential biomass resource,in which organic compounds can be converted to valuable rich-H₂ fuel gas at steam atmosphere.Sludge deep dewatering is able to reduce moisture content efficiently,at the same time the inorganic substance which can act as catalysts will retain in the dewatered sludge cake.Based on the technology of“Fenton's reagent+CaO”composite conditioning combined with deep dewatered sludge gasification proposed before,a further study was continued to clarify the mechanism of residual iron and calcium salts at different gasification stage,realizing the utilization of inorganic components and suggesting a method for the directional control of target products.The key theoretical and scientific issues involved are:?I?What was the competitive effect between internal water and steam during wet sludge gasification,?II?The directional control and catalytic mechanism of Fe/Ca-conditioner on H₂ yield during sludge steam gasification,?III?Mechanism of iron and calcium salts on S/N-containing pollutants emission at different stage of gasification,?IV?The adsorption ability of gasification ash on S/N-containing polluting gas and how to directional control the performance of ash adsorbent.With regard to these issues,detailed and in-depth research was carried out.?1?Product distribution and property was analyzed to explore the competitive effect of internal water and steam during wet sludge gasification.Because of poor reactivity,the steam had obvious effect on wet traditional sludge gasification only if at high steam concentration and temperature.With the catalytic effect of residual iron and calcium salts,the reaction between internal water and unsaturated C=C bond in aliphatic occurred easily,leading to the decrease of unsaturation degree.The ring in alicyclic compounds would be broken by forming oxygen containing structure.Residual iron and calcium salts could also promote the emission of oxygen as aldehydes and ketones,in this way the oxygen vacancy appeared and the effect of internal water on gasification was enhanced.During wet deep dewatered sludge gasification,more steam would participate in gasification reaction,resulting in the hydrolysis of benzenes in char and tar to release CH₄,CO and H₂.Although providing less steam and low temperature?1073 K?,the H₂ yield during wet deep dewatered sludge gasification can reach the value that obtained at 1273 K during wet raw sludge gasification.The kinetic results show that residual iron and calcium salts can reduce the reaction activation energy by 7.2-8 KJ/mol in the process of devolatilization.?2?The prediction model of H₂ yield during gasification in fluidized bed was built and the effect of residual iron and calcium salts at different gasification stage was clarified using response surface methodology and gasification decoupling method respectively.The ANOVA of prediction model indicated that calcium salts was the most important factor for H₂ yield improvement.The interaction between Fe²⁺/H₂O₂ and CaO was negligible.However,when Fe²⁺and H₂O₂ were added simultaneously as Fenton's reagent,the synergistic effect with CaO improved the yield of H₂.It was further found that calcium remained in sludge was in the form of Ca?OH?₂,which could promote the decomposition of macromolecular organics during devolatilization and catalyze the hydrolysis of benzenes in char and tar at heterogeneous catalysis stage.The impact of the former on final H₂ yield improvement accounted for 62.9-67.7%.The iron was mainly trivalent in sludge such as Fe₃O₄ and hercynite.With the effect of H₂O₂ oxidation and the reaction with CaO to form?CaFeO?_n,the ratio of iron in sludge on Fe²⁺dosage increased from 22.5%to 77.4%at most.Iron salts trended to catalyze the decomposition of heterocyclic structure in char and tar,and the impact of which on homogeneous reaction seems equaled to that on heterogeneous reaction at 1273 K.?3?The gasification can be divided into volatiles homogenous reforming and heterogeneous catalysis stage in a self-designed decoupling gasification reactor.The control mechanism of residual iron and calcium salts on S/N-containing pollutants emission has been discussed.During deep dewatered sludge gasification,most of sulfur would be retained in solid product by calcium salt.As a result,only 10%of sulfur released as H₂S,far less than 48-55%in raw sludge.In the case of nitrogen,calcium indirectly promoted the hydrolysis of HCN during homogenous reforming stage.At the subsequent heterogeneous catalysis stage,about 20%of NH₃ would be decomposed to N₂ under the action of calcium salts catalysis.Except for the catalytic decomposition of NH₃,iron salts can also convert unstable nitriles and pyrroles to indols,hindering the generation of NH₃ during homogeneous reaction.At 1073 K,the hydrolysis of heterocyclic nitrogen to release HCN was inhibited with the effect of iron salts.?4?With the methods of response surface methodology,on-line adsorption and in situ FT-IR measurement,the function between conditioners dosage and ash adsorption performance was built and Fe/Ca-containing gasification ash was found a certain role on NH₃?SO₂ and H₂S elimination.The ANOVA of model indicated that iron salts was important for the decolorizing ability while calcium salts was response for the adsorption capacity to liquid.In terms of pollutants adsorption,the NH₃ adsorption ability depended on carbon content and surface acid sites at 873 K.The catalytic effect of iron and calcium salts played a dominant role at 1173 K.On the surface of ash prepared from raw sludge gasification,H₂S was mainly decomposed to elemental sulfur and adsorbed.The iron and calcium salts could convert H₂S to sulfate and sulfide.At the same time,SO₂ could be also fixed in ash as sulfate and sulfite.In summary,deep dewatered sludge gasification showed advantages on H₂ yield promotion,wet sludge gasification,S/N-containing pollutants control and ash adsorption performance improvement.The target products can be directional control via optimizing the conditioners dosage,which has great guideline on the development of sludge gasification industrialization.