Straw Black Liquor Non-conventional Chemical Recovery And Alkali Metal Catalytic Reforming Of Biomass Gas For Hydrogen Rich Gas Production

It is of interests to replace direct incineration with thermochemical conversion technologies such as gasification and pyrolysis in the organic waste disposal field both at home and abroad.Agricultural and forestry biomass wastes gasification technology has been demonstrated commercially in recent years worldwide.However,the breakeven of biomass gasification project is strongly influenced by raw material costs and government subsidies.In order to reduce the particulate matter and PAHs emissions,and to increase the added value of the product gas,such as for power generation using internal combustion engine,gas turbine and fuel cell,civil fuels,and chemical synthesis,etc.,it is necessary to remove fly ash and tar from the raw gas to a very low content,and to adjust gas component to a high H₂/CO ratio.Among all the issues faced by gasification technologies,tar removal/cracking has always been of great concerns.Pulping black liquor is a special kind of biomass material,for its characteristics of high water content,high viscosity,high sodium content,and easy to swell at high temperatures.Moreover,straw black liquor has a high silicon content which disturbs it from recovery with high efficiency.Nowadays black liquor is usually recycled using Tomlinson-type recovery boilers.As is well known in the pulp and paper field,silica is a problem specific to nonwood-especially straw-pulping black liquor recovery,and the operating cost of recovery process is higher than that of wood black liquor.White mud is also not suitable for reuse due to the silica present in the straw black liquor.In this work,soda straw black liquor,which has an abundant annual output in China,will be used as the main research object,and the purpose of the work is to explore new type of straw black recovery on the basis of previous studies of researchers.The aiming process routes of the thesis are:to recover pulping chemicals using concepts of pyrolysis-leaching,electrolysis,and direct causticization to substitute or improve the conventional combustion-melting-dissolving-lime causticizing process,to recover energy in the form of hydrogen-rich gas instead of steam.The main contents of the research are as follows:?1?Improvement of analysis methods of straw black liquor basic physical and chemical properties.?2?To investigate the feasibility of pyrolysis-leaching chemical recovery process using a 10 kg/h stirring type pyrolysis experimental apparatus,and to analyze the difference of silicon distribution and total alkali recovery between char-leached and smelt-dissolved green liquor.?3?Desilication of straw black liquor using a lab scale membrane electrolysis cell adopted from the chlor-alkali industry,and the comparison of NaOH recovery efficiency difference between raw and desilicated black liquor in direct causticization procedure.To analyze the effects of silica on the recovery of NaOH in direct causticization reactions by virtue of chemical thermodynamics principle,and to clarify the mechanism of silica distribution in hydrolyzed solid and liquid phases.?4?Thermodynamic equilibrium analysis using Gibbs free energy minimization theory for hydrogen-rich gas production via straw black liquor gasification under direct causticization.?5?To evaluate the catalytic reforming effects of sodium titanates derived from direct causticization process using real biomass raw gas prepared by a fixed bed pyrolyzer other than model compounds,with the aim of tar cracking and hydrogen-rich gas production.?6?Furthermore,to evaluate the activity and stability of alkali metal catalysts?sodium and potassium?using new source of biomass raw gas produced by a screw pyrolyzer which could be operated in continuous mode and could output gas stream with relatively constant composition.The experimental and calculation results reveal that:?1?The total alkali in pyrolysis-leached green liquor was found equal to or a bit higher than that of smelt-dissolved green liquor,which means that nearly all of the soluble sodium salts-mainly carbonate and silicate-could be easily separated into green liquor by pyrolysis-leaching method.?2?Up to 40%of silicon retained in BLC-leached carbon powder as forms of insoluble silicates,thus the silicon concentration was lower in BLC-leached green liquor than that of smelt-dissolved green liquor.?3?When using NT3 as the direct causticization agent for raw black liquor,nearly all of the silica transfers to the hydrolyzed liquid phase.The NaOH yields could reach to about 80%when using desilicated black liquor in NT3 direct causticization procedure,while it is only around 60%when using raw straw black liquor as material.If silica removal efficiency could be raised higher,Na OH yields is expected to achieve to a much higher level.?4?Due to intense catalytic effects of sodium,the product gas composition of black liquor gasification system is much closer to chemical thermodynamics equilibrium state,compared to ordinary biomass such as wood or agricultural wastes.The H₂/CO of black liquor gasification product gas could reach or exceed to about 1.0,and has lower CH₄ content,as to biomass gasification,the product gas H₂/CO is only around 0.5 in general,and the CH₄content is relatively high.?5?At conditions of NT3 direct causticization,S/C mole ratio 0–0.226,temperature 1100and 1200 K,the calculation shows that a equivalence ratio higher than 0.5 is necessary when the gasifier is operated autothermally,and the corresponding gasification efficiency,product gas heating value,and gas yield are around 60%,3.5 MJ/Nm³,and 2.0 Nm³/kg,respectively.The energy consumption of direct causticization amounts to approximately 10%of the straw black liquor energy,which is roughly equivalent to lime calcination in the counterpart scale.?6?Alkali metal titanate catalysts present good ability of coking resistance and promotion of H₂ formation.Among all the catalysts tested for raw product gas reforming,including N4T5,NT3,KT2,and inert silica sand,N4T5 shows the highest activity,NT3 exhibits the best stability and satisfactory activity,and silica sand shows no activity of enhancing hydrogen yield.The H₂/CO increase to 1.8–2.0 under alkali metal titanates reforming,which is much higher than that of silica sand?about 0.35?and raw gas?about 0.25?.N4T5 raises H₂/CO mainly through promotion of water gas shift reaction,while NT3 shows catalytic effects of CH₄ reforming.KT2 has similar catalytic mechanism to N4T5 of enhancing H₂yield,however,the sintering phenomenon of which at high temperatures is intolerable serious.?7?The catalysts tested offered their catalytic effect in the order of N4T5?KT2>NT3>>silica sand.The activity of N4T5 decreases gradually in the long-term test because of the release of sodium and has a tendency to transform to NT3.?8?Most organic components in tar can be effectively reformed by alkali metal titanates?up to 99%of total removal with 200 g-tar/Nm³-raw gas?,except for a portion of xylene,naphthalene and its derivatives,biphenylene,and anthracene.The main innovations exist in:?1?Determination of the difference of silicon distribution and total alkali recovery between char-leached and smelt-dissolved green liquor.Clarity of the mechanism of silica distribution in hydrolyzed solid and liquid phases during direct causticization procedure,and the effects of silica on TiO₂ and NT3.?2?Acquisition of the output parameters thermodynamic limits of the straw black liquor direct causticization gasification process,and the acquisition of the potential of hydrogen rich gas production from black liquor.?3?Affirmation of the catalytic effects of alkali metal titanate on tar cracking and gas composition adjustment.The results of this work are expected to provide reference for improvement of straw black liquor recovery with low costs and high added-value producs,and for hydrogen-rich gas production via pyrolysis or gasification.