生物质半焦气化及应用研究现状

介绍了近些年来生物质半焦气化及应用的研究现状,主要分析了不同气化剂和催化剂对生物质半焦气化的影响、生物质半焦作为催化剂对生物质气化及焦油产物催化裂解及其吸附重金属等吸附性能的研究。     

生物质半焦气化及应用研究现状

介绍了近些年来生物质半焦气化及应用的研究现状,主要分析了不同气化剂和催化剂对生物质半焦气化的影响、生物质半焦作为催化剂对生物质气化及焦油产物催化裂解及其吸附重金属等吸附性能的研究。     

Catalytic gasification of char from co-pyrolysis of coal and biomass

The catalytic gasification of char from co-pyrolysis of coal and wheat straw was studied. Alkali metal salts, especially potassium salts, are considered as effective catalysts for carbon gasification by steam and CO₂, while too expensive for industry application. The herbaceous type of biomass, which has a high content of potassium, may be used as an inexpensive source of catalyst by co-processing with coal. The reactivity of chars from co-pyrolysis of coal and straw was experimentally examined. The chars were prepared in a spout-entrained reactor with different ratios of coal to straw. The gasification characteristics of chars were measured by thermogravimetric analysis (TGA). The co-pyrolysis chars revealed higher gasification reactivity than that of char from coal, especially at high level of carbon conversion. The influence of the alkali in the char and the pyrolysis temperature on the reactivity of co-pyrolysis char was investigated. The experimental results show that the co-pyrolysis char prepared at 750 °C have the highest alkali concentration and reactivity.     

Secondary char formation in the catalytic pyrolysis of biomass

Samples of four types of wood and pure cellulose, both untreated and impregnated with salt (Na₂CO₃, K₂CO₃, NaCl, KC1), were pyrolysed. Two experimental systems with different geometries and secondary reaction patterns were used, viz. a McBain thermogravimetric balance and a Gray-King retort. The substrates were pyrolysed under a stream of nitrogen in the thermobalance and in some of the Gray-King runs, using a modified retort. Salt impregnation was found to modify weight loss rates and to increase the charcoal yield in the presence of an inert carrier gas in both experimental systems. Longer residence times of volatiles in the hot zone gave rise to larger charcoal yields from untreated substrates. However, long residence times of volatile matter over Na₂CO₃-impregnated cellulose were found to be detrimental to char formation. These results indicate that primary volatiles may undergo secondary reactions through competitive pathways, either polymerizing to form char or cracking to form lighter volatiles. Long residence times of light volatiles appear to enhance the latter pathway in the presence of Na₂CO₃.     

Simulation of biomass char gasification in a downdraft reactor for syngas production

A steady state, one‐dimensional computational fluid dynamics model of wood char gasification in a downdraft reactor is presented. The model is not only based on reaction kinetics and fluid flow in the porous char bed but also on equations of heat and mass conservation. An original OpenFOAM solver is used to simulate the model and the results are found to be in good agreement with published experimental data. Next, a sensitivity analysis is performed to study the influence of reactor inlet temperature and gas composition on char conversion, bed temperature profile and syngas composition. In addition, the evolution of the complex reaction mechanisms involved in mixed atmosphere gasification is investigated, and the most suitable operating parameters for controlling syngas composition are evaluated. Our simulation results provide essential knowledge for optimizing the design and operation of downdraft gasifiers to produce syngas that meets the requirements of various biofuel applications. © 2015 American Institute of Chemical Engineers AIChE J, 62: 1079–1091, 2016     

Modeling of catalytic gasification kinetics of coal char and carbon

Calcium- and potassium-catalyzed gasification reactions of coal char and carbon by CO₂ are conducted, and the common theoretical kinetic models for gas-carbon (or char) reaction are reviewed. The obtained experimental reactivities as a function of conversion are compared with those calculated based on the random pore model (RPM), and great deviations are found at low or high conversion levels as predicted by theory. Namely, calcium-catalyzed gasification shows enhanced reactivity at low conversion levels of <0.4, whereas potassium-catalyzed gasification indicated a peculiarity that the reactivity increases with conversion. CO₂ chemisorption analysis received satisfactory successes in both interpreting catalytic effects and correlating the gasification reactivity with irreversible CO₂ chemical uptakes (CCU_ir) of char and carbon at 300 °C. In details, calcium and potassium additions led to significant increases in CCU_ir and correspondent high reactivities of the char and carbon. Furthermore, CCU_ir of char and carbon decreased with conversion for calcium-catalyzed reaction but increased for potassium-catalyzed one, corresponded to the tendency of their reactivity. The RPM is extended and applied to these catalytic gasification systems. It is found that the extended RPM predicts the experimental reactivity satisfactorily. The most important finding of this paper is that the empirical constants in the extended RPM correlate well with catalyst loadings on coal.     

Gasification of woody biomass char with CO2: The catalytic effects of K and Ca species on char gasification reactivity

The effects of alkali and alkaline earth metals such as potassium (K) and calcium (Ca) on CO₂ gasification reactivity of Japanese cypress (hinoki) char under various temperatures (1123-1223 K) and CO₂ concentration (20-80 vol.%) were studied using thermal gravimetric analysis. The presence of K and Ca compounds in char improved the reactivity of hinoki char for CO₂ gasification catalytically. It was also confirmed that K and Ca compounds can be supported on char to exhibit an enhanced catalytic effect during CO₂ gasification of K-char and Ca-char. The char gasification rate increased with the increase of CO₂ concentration at higher temperatures (1173-1223 K), however at lower temperature (1123 K) the gasification rate decreased at 80% CO₂. The retardation of char gasification rate at higher CO₂ concentration is caused by the inhibition effect of CO: CO is disproportionated on alkali metal catalysts to CO₂ and carbon, and affected the CO₂ gasification rate. The dependence of char gasification rate on reaction temperature yielded a straight line in an Arrhenius-type plot which indicated that there was no significant change in the gasification mechanism in the temperature range of 1123-1223 K.     

On the intrinsic reaction rate of biomass char gasification with carbon dioxide and steam

The gasification of beech wood char and oil palm shell char with carbon dioxide and steam was studied. To avoid heat and mass transport limitations during gasification, the amount of char, particle size and flow rate were varied in isothermal experiments. A rate expression of the Langmuir–Hinshelwood-type was applied to match the experimental data at different partial pressures and reaction temperatures in the intrinsic regime. Furthermore, the reactive surface area (RSA) of the biomass chars was determined as a function of the degree of conversion by the temperature-programmed desorption technique (TPD). The results show that the reaction rate is in general proportional to the RSA. The surface related reaction rates for the studied biomass chars are comparable to surface related reaction rates for coal chars at similar reaction temperatures.     

Reactivity and structural change of coal char during steam gasification

This study is intended to clarify the relationship among the reactivity of coal char with steam, structural change in residual carbon, and ash behavior. Steam gasification of various coal chars and demineralized chars was carried out in a fixed-bed reactor. After gasification, the reacted char was analyzed using laser raman spectroscope (LRS), and scanning electron microscope, energy dispersive X-ray spectroscope (SEM/EDX) mapping. Results of SEM images and EDX-mappings revealed that novel parallel analysis of cross correlation between EDX-mapping and LRS-mapping was found to be very effective for the comprehensive evaluation of ash behavior and carbonaceous structure. As the gasification reaction proceeds, the reactivity of the char was varied; existence of Si and Al seemed to suffocate the char reactivity.     

胜利褐煤负载镍生物质气化催化剂实验研究

为获得较低温度下生物质焦油高效裂解气化的廉价型催化剂,基于褐煤富含含氧官能团特点,通过离子交换法将镍负载到胜利褐煤上制备褐煤负载镍催化剂(Ni/SLC)。研究了溶液p H值和炭化温度对催化剂物理化学性质的影响,得到Ni/SLC催化剂最佳制备条件,最后在两段式移动床石英反应器中将催化剂用于玉米芯挥发分的催化气化,研究了催化剂对生成气产量和碳平衡的影响。结果表明:p H值为11,炭化温度为650℃时制备的Ni/SLC比表面积最大达到266.3 m2/g,镍微晶尺寸较小为5.0 nm。催化气化实验表明:650℃下Ni/SLC催化剂具有高的焦油裂解活性,气体产量高达43.9 mmol/g,相当于无催化实验气体总产量的3.3倍;水蒸气气化可将热解焦油完全气化,气体产量为85.1 mmol/g,H2产量高达61.9 mmol/g,占气体总量的72.7%,说明Ni/SLC催化剂可作为生物质催化气化制氢的潜在催化剂。     

An investigation of alumina-supported catalysts for the selective catalytic oxidation of ammonia in biomass gasification

Alumina-supported catalysts containing different transition metals (Ni, Cu, Cr, Mn, Fe and Co) were prepared and tested for their activity in the selective oxidation of ammonia reaction at high temperatures (between 700 and 900 °C) using a synthetic gasification gas mixture. The catalysts were also characterised for their acidic properties by infrared studies of pyridine and ammonia adsorption and reaction/desorption. The Ni/Al₂O₃ and Cr/Al₂O₃ catalyst displayed the highest selective catalytic oxidation (SCO) activity in that temperature range with excellent N₂ selectivities. FT-IR studies of adsorbed pyridine and NH₃ indicate that Lewis acid sites dominate and that NH₃ adsorption on these sites is likely to be the first step in the SCO reaction. FT-IR studies on less active catalysts, particularly on Cu/Al₂O₃ allowed the detection of oxidation intermediates, amide (NH₂), and possibly hydrazine and imido and nitroxyl species. The amide and hydrazine intermediate gives credence to a proposed SCO mechanism involving a hydrazine intermediate, while the proposed imide, =N–H, and/or nitroxyl, HNO species could be intermediates in incomplete oxidation of NH₃ to N₂O.     

A kinetics study on catalytic and non-catalytic steam gasification of biomass

An experimental study on catalytic and non-catalytic steam-gasification of sawdust has been conducted in a microsystem by exposing a small quantity (about 1 g) of sawdust to flash pyrolysis in an atmosphere of steam in the temperature range of 350 to 650°C. The results have been evaluated on the basis of loss of weight of sawdust and a detailed gas analysis through Orsat analyser and gas chromatograph. From the rate of gas evolution, i.e. volume fraction of gas collected with time, kinetic analysis was attempted through appropriate solid—gas-reaction models. Results of this analysis has been reported in the present paper.     

Characterization and catalytic gasification of the aqueous by-product from vacuum pyrolysis of biomass

Spruce wood residues were treated in a vacuum pyrolysis Process Development Unit with a throughput capacity of 28 kg/h. Two aqueous phase condensate samples with COD concentration varying between 190 and 255 g/L were produced and sequentially extracted with dichloromethane and ethylacetate solvents. The soluble organic matter was composed of acidic, phenolic, alcoholic and ketonic compounds. The insoluble fraction was sequentially distilled at 100 and 110°C under atmospheric pressure. Mainly water was recovered in the first distillate, while the second distillate contained 30.4% formic and acetic acids, 69.4% water and 0.2% residual organic compounds. The distillation residue was rich in oxygen and was essentially insoluble in any organic solvent. The two aqueous phase pyroligneous samples were treated in Bat-telle's Thermochemical Environmental Energy System (TEESr?), a registered service mark of Onsite*Ofsite, Inc. of Duarte, California, U.S.A. The results of the tests showed that similar results were obtained with either feedstock. In batch tests a COD reduction of 99% was achieved. The product gas composition was typically about 49% methane, 5% hydrogen, 1 % ethane and 45% carbon dioxide. Tests in a continuous stirred-tank reactor produced reproducible data which can be used for process scale-up. Catalyst lifetime was identified as needing further improvement. The preliminary results demonstrated the technical feasibility of the catalytic gasification process as a useful step in the recovery of energy from the secondary condensate stream and the cleanup of the by-product water from vacuum pyrolysis of wood.     

Nature,activity and stability of active sites during alkali metal carbonate-catalysed gasification reactions of coal char

The activity pattern of potassium carbonate-containing coal char during gasification with low pressures of steam, is shown to consist of at least three stages of activity. The relative importance of these stages, that occur at different burn-off levels, is influenced by the pretreatment temperature, the catalyst loading and the sequence of catalyst addition and coal pyrolysis. An explanation for this is given, based on the assumption of three processes occurring simultaneously during pretreatment: oxygen retention by potassium, intercalation of potassium and potassium carbonate crystallization. Comparison with activated carbon results supports the interpretation of the results. It follows that the maximum activity of alkali metals correlates with the oxygen content of carbon and that intercalation depends on the degree of graphitization of carbon.     

Reaction kinetics and mass transfer studies of biomass char gasification with CO2

Gasification kinetics of wheat straw char with CO₂ was investigated using Thermogravimetric apparatus (TGA). The main objective was to identify the diffusional and surface reaction phenomena that may occur during biomass char gasification experiments with CO₂. The effects of temperature (750–900 °C) and particle size (<60–925 μm) on gasification rate of char-CO₂ reaction were determined. The 50% conversion (r₅₀) rate showed that the reactivity increases with temperature, and it decreases as the particle size increases. The important diffusional parameters such as effective diffusivity, effectiveness factor and Thiele modulus were calculated based on the experimental data and the results showed that the impact of physical factors is prominent at high temperatures and large particle sizes. It was found that char gasification within the temperature range studied followed the chemically controlled reaction regime and the influence of pore diffusion was negligible for fine powder particles.     

气化过程中煤焦破碎成灰的模型模拟

基于渗流理论,建立了气化过程中单颗粒煤焦的破碎成灰模型,研究了煤焦结构对气化反应性能的影响,煤焦结构的演变过程以及最终的残灰粒径分布。模拟结果表明:随着初始孔隙率的增加,煤焦在气化前期的气化反应和破碎程度更为剧烈。对于初始孔隙率较小的煤焦,在气化末期,当转化率超过某一数值时,煤焦粒径急剧下降,这与文献报道的实验结果吻合。残灰粒径分布曲线在超微米区(1—10μm)存在一个峰值粒径,这是内在矿物质的聚合与煤焦破碎相互竞争的结果。随着初始孔隙率的增加,残灰的峰值粒径呈下降的变化趋势,残灰主要集中在粒径较小的范围内。     

Reactive intermediate in the alkali-carbonate-catalysed gasification of coal char

Based on results from a variety of experimental measurements, a detailed mechanism is postulated for the action of the inorganic catalyst in char gasification. In this mechanism, a catalyst such as potassium carbonate in contact with char undergoes a chemical and physical transformation to form a molten potassium oxide film that covers the char surface. This film serves as an oxygen transfer medium between the gaseous reactant (H₂O or CO₂) and the char. At the catalyst/char interface, an oxidation-reduction reaction occurs and the anions in the catalyst react with the oxidized char to form a phenolate-type functional group that subsequently splits out CO. The anions are replenished by reaction between the oxidizing gas (H₂O or CO₂) and the oxide at the gas/catalyst interface. Net transport of oxygen from gas to char occurs by diffusion of the species in the molten catalyst film.     

Optical pyrometric measurements of surface temperatures during black liquor char burning and gasification

Surface temperatures of single black liquor droplets in high temperature furnace environments were measured using a two-colour optical pyrometer with a quartz optical fibre probe. The experimental conditions were 750–900 °C and in oxidizing and gasifying environments. Surface temperatures were calculated based on measurements at 650 and 1050 nm using an algorithm which accounted for reflection of radiation from the particle surface. The method gave reliable temperatures under combustion conditions during char burning. The droplet temperature during this period reached a maximum which depended upon droplet size and oxygen content. The flame which appeared during the early part of the devolatilization stage apparently contained enough soot particles to obscure the droplets; this resulted in unreliable indicated temperatures. The indicated surface temperature during the latter half of devolatilization was probably closer to the actual surface temperature, but may have been influenced by the combustion of volatile gases near the particle surface. The surface temperature during drying was too low relative to the surrounding furnace temperature to be measured.     

The effect of steam on pyrolysis and char reactions behavior during rice straw gasification

Steam gasification of biomass can generate hydrogen-rich, medium heating value gas. We investigated pyrolysis and char reaction behavior during biomass gasification in detail to clarify the effect of steam presence. Rice straw was gasified in a laboratory scale, batch-type gasification reactor. Time-series data for the yields and compositions of gas, tar and char were examined under inert and steam atmosphere at the temperature range of 873-1173 K. Obtained experimental results were categorized into those of pyrolysis stage and char reaction stage. At the pyrolysis stage, low H₂, CO and aromatic tar yields were observed under steam atmosphere while total tar yield increased by steam. This result can be interpreted as the dominant, but incomplete steam reforming reactions of primary tar under steam atmosphere. During the char reaction stage, only H₂ and CO₂ were detected, which were originated from carbonization of char and char gasification with steam (C + H₂O→CO + H₂). It implies the catalytic effect of char on the water-gas shift reaction. Acceleration of char carbonization by steam was implied by faster hydrogen loss from solid residue.