Thermochemical conversion of biomass: Gasification by flash pyrolysis study

Thermal decomposition of the basic components of dried biomass (cellulose, lignin, wood) is studied in inert atmosphere. Glucose is studied for comparison. The experiments are performed in an alumina porous bed reactor heated at temperatures between 600 and 1000°C. Flash pyrolysis (heating rate 250°C/s) allows the production of a medium heating value synthetic gas with gas phase conversion thermal efficiency of up to 95 percent. The weight percent of carbon gasified during the pyrolysis reaches 90 percent for cellulose and 70 percent for wood. Light hydrocarbons (CH₄, C₂H₄, C₂H₂, C₂H₆) provide about 50 percent of the energy recovered in the gas. Ethylene represents 5 percent (vol) of the pyrolysis gas. The overall ethylene and acetylene yield is markedly increased at high temperatures for short gas residence times. The pyrolysis reactions are endothermic. The total amount of energy required for pyrolysing wood at 850°C roughly corresponds to 20 percent of its heating value.     

Characterisation of oils from the fluidised bed pyrolysis of biomass with zeolite catalyst upgrading

Biomass in the form of pine wood was pyrolysed in an externally heated 7.5 cm diameter, 100 cm high fluidised bed pyrolysis reactor with nitrogen as the fluidising gas. A section of the freeboard of the reactor was packed with zeolite ZSM-5 catalyst. The pyrolysis oils before and after catalysis were collected in a series of condensers and cold traps. In addition, gases were analysed off-line by packed column gas chromatography. The compositions of the oils and gases were determined in relation to the primary fluidised bed and after catalysis at increasing catalyst bed temperatures from 400° to 550°C. The oils were analysed by a number of techniques to determine composition, including liquid chromatography, gas chromatography/mass spectrometry. Fourier transform infrared spectroscopy and size exclusion chromatography. The results showed that the oils before catalysis were highly oxygenated; after catalysis the oils were markedly reduced in oxygenated species with an increase in aromatic and polycyclic aromatic species.

The gases evolved from the fluidised bed pyrolysis of biomass were CO₂, CO, H₂, CH₄, C₂H₄, C₃H₆ and minor concentrations of other hydrocarbon gases. After catalysis the concentrations of CO₂ and CO were increased. The conversion of oxygenated compounds was mainly to H₂O at lower catalyst temperatures and CO₂ and CO at high catalyst temperatures. Detailed analysis of the oils showed that there were high concentrations of biologically active polycyclic aromatic species in the catalysed oil which increased with increasing catalyst temperature. The oxygenated compounds in the uncatalysed oil were mainly phenols and carboxylic acids. After catalysis these decreased in concentration with increasing catalyst temperature     

Singlet methylene removal by saturated and unsaturated hydrocarbons

The technique of laser flash phyotolysis/laser absorption has been used to obtain absolute removal rate constants for singlet methylene, ¹CH₂ (ã¹A₁), with various saturated and unsaturated hydrocarbons. The removal rate constants for CH₄, C₂H₆, C₃H₈, C₂H₄, C₃H₆, C₂H₂, CH₂CCH₂, and C₆H₆ were found to be in excellent agreement with previously reported results. Removal rate constants were also measured for n-C₄H₁₀, i-C₄H₁₀, n-C₅H₁₂, c-C₃H₆, c-C₆H₁₂, 1-C₄H₈, cis-2-C₄H₈, trans-2-C₄H₈, and 1-C₄H₆, and determined to be (3.17 ± 0.15), (2.53 ± 0.11), (3.35 ± 0.24), (1.63 ± 0.08), (3.77 ± 0.21), (3.80 ± 0.20), (3.67 ± 0.16), (3.43 ± 0.16) and (4.05 ± 0.18) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹, respectively. This series of hydrocarbons forms the basis of a larger series of compounds containing a wide variety of organic functional groups. The removal rate constants are reported here, both as a series within its own right, and as a reference point for future work.     

Energy-efficient conversion of propane to propylene and ethylene over a V2O5/CeO2/SA5205 catalyst in the presence of limited oxygen

Oxidative conversion of propane to propylene and ethylene over a V₂O₅/CeO₂/SA5205 (V:Ce=1:1) catalyst, with or without steam and limited O₂, has been studied at different temperatures (700–850 °C), C₃H₈/O₂ ratio (4.0), H₂O/C₃H₈ ratio (0.5) and space velocity (3000 cm³ g⁻¹ h⁻¹). The propane conversion, selectivity for propylene and net heat of reaction (ΔHr) are strongly influenced by the reaction temperature and presence of steam in the reactant feed. In the presence of steam and limited O₂, the process involves a coupling of endothermic thermal cracking and exothermic oxidative conversion reactions of propane which occur simultaneously. Because of the coupling of exothermic and endothermic reactions, the process operates in an energy-efficient and safe manner. The net heat of reaction can be controlled by the reaction temperature and concentration of O₂. The process exothermicity is found to be reduced drastically with increasing temperature. Due to the addition of steam in the feed, no coke formation was observed in the process.     

Low-temperature catalytic partial oxidation of hydrocarbons (C1–C10) for hydrogen production

The catalytic partial oxidation of hydrocarbons to provide hydrogen for fuel cells, mobile or stationary, requires high temperatures (900°C), multireactors and incurs the highest incremental costs for the gasoline fuel processor. New experimental data between 500°C and 600°C, supported by equilibrium calculations, show that hydrogen with low carbon monoxide concentrations can be produced from liquid and gaseous hydrocarbons, thus simplifying the reactor chain. Low sulphur refinery feeds (C₄–C₆, C₄–C₁₀), simulated natural gas (C₁–C₃) and single compounds are used and safety procedures discussed. Results from laboratory reactors with 1 wt% rhodium on mixed oxide catalysts show that hydrogen rates of 43,000 lH₂/h/l reactor (power density 129 kWth/l reactor) are produced with RON=95 feeds. However, the cost and availability of rhodium limit the catalyst rhodium content to 0.1 wt% when 31,100 lH₂/h/l reactor were measured. Optimisation and reactor scale-up for heat management is in progress.     

Upgrading of a wood-derived oil over various catalysts

The upgrading of a bio-oil using a fixed bed micro-reactor operating at 1 atm, 3.6 WHSV and 330–410°C over various catalysts is reported. The catalysts used were HZSM-5, silicalite, H-mordenite, H-Y and silica-alumina. The yield of hydrocarbons as well as the extent of deoxygenation, coke formation and conversion of the non-volatile portion of the bio-oil were used as measures of catalyst performance. The maximum hydrocarbon yield when HZSM-5 was used occurred at 370°C and was 39.3 wt% of the bio-oil. For the other catalysts, the hydrocarbon yields increased with temperature and were up to 22.1 wt% for silicalite; 27.5 wt% for H-mordenite; 21.0 wt% for H-Y; and 26.2 wt% for silica-alumina at 410°C. The hydrocarbon selectivity with HZSM-5 and silicalite catalysts was mostly for gasoline range hydrocarbons (C₆ to C₁₂) and for H-mordenite and H-Y for kerosene range hydrocarbons (C₉ to C₁₅). The hydrocarbon fraction obtained with silica-alumina did not produce any defined distribution. The pore size, catalyst acidity and catalyst shape selectively affected the product distribution. The overall performance followed the order: HZSM-5 > H-mordenite > H/Y > silica-alumina, silicalite.     

The oxidation of benzene under conditions encountered in waste incinerators

The oxidation of benzene was studied as a function of residence time (τ_res=0–2.5 s), temperature (850–960 K), and oxygen concentration (O₂=0.2–2.3%) in a heated laminar flow reactor at atmospheric pressure. Nitrogen, doped with 350 ppm benzene, was injected downstream of the burned gas from a near stoichiometric flame of methane + air. Gas samples were taken at different heights up the reactor and analyzed using GC-FID/TCD and HPLC techniques. Phenol and partially oxidized hydrocarbons such as acetaldehyde, formaldehyde, and acrolein were found with concentrations up to 50 ppm. At relatively low temperatures, the conversion of benzene was observed to proceed considerably more slowly at higher oxygen concentrations. Measured concentration profiles were modeled using detailed reaction schemes. A modified mechanism for the oxidation of benzene called BenWas was constructed from the mechanism of Zhang and McKinnon (Combust. Sci. Technol. 107 (1995) 261) by incorporating a submechanism for benzoquinone (OC₆H₄O) and by updating and enlarging the reaction scheme of cyclopentadiene (C₅H₆). The agreement between observed and predicted concentration profiles, e.g., of phenol (C₆H₅OH), acetylene (C₂H₂), and carbon monoxide (CO), was considerably improved by the use of the BenWas mechanism for rich and lean conditions, mainly due to the introduction of an additional pathway for phenyl oxidation (C₆H₅ + O₂ = OC₆H₄O + H) and due to the changed kinetics of the oxidation of cyclopentadienyl (C₅H₅) in C₅H₅ + O₂ = C₅H₄O + OH. The measured retardation of benzene oxidation with higher amounts of oxygen can be explained by the formation and reactions of peroxy radicals.     

Quality of gas produced from wheat straw in a dual-distributor type fluidized bed gasifier

A 400 kW, dual distributor type fluidized bed gasifier was used to investigate the production rate and composition of the gas produced from wheat straw at various equivalence ratios (0.17, 0.20, 0.25, 0.35) and fluidization velocities (0.28, 0.33 and 0.37 m s⁻). The results showed that the equivalence ratio was the major parameter affecting the gas composition. The equivalence ratio of 0.25 appeared to be the optimum with respect to the quality of the gas. The mole fractions of the combustible components reached their maximum values at this equivalence ratio. A typical gas composition at the equivalence ratio of 0.25 was 7% H₂, 7% hydrocarbons (CH₄, C₂H₂, C₂/H₄ and C₂H₆), 14% C0₂, 22% CO and 50% N₂. The higher heating value of the produced gas (6.3–7.3 MJ Nm⁻³) obtained at this equivalence ratio appeared to be higher than most values reported in the literature for several types of biomass fuels.     

中药渣固废的碳酸钾催化热解特性与产物分布规律

以木本中药渣为原料,采用浸渍法负载不同含量K₂CO₃催化剂;通过热重实验,分析中药渣催化热解特性和热解特征参数,并采用Starink法进行动力学分析,计算催化热解反应的表观活化能;使用固定床热解炉,优化催化热解反应条件,考察不同K₂CO₃负载量对热解产物分布的影响规律。热重结果表明,K₂CO₃能显著降低中药渣的初始热解温度和最大热解温度,从而降低热解快速失重段的反应活化能;且K₂CO₃负载量越大,催化热解效果越好。热解实验证实：K₂CO₃含量为中药渣催化热解反应的最主要影响因素,它可加速生物基大分子的低温解聚和热解中间产物的催化裂解,既可降低热解油产率,又能大幅提升H₂、CO和C₂H₆等小分子低碳烃气体的产率,且有利于提高热解气的H₂/CO比例。     

The origin of soot in flames: is the nucleus an ion?

There are two schools of thought on how soot originates in a fuel-rich flame. On the one hand, the ionic theory postulates that small ions, such as C₃H₃⁺, act as nuclei, so that species such as C₂H₂ and C₄H₂ add on to them, and occasionally liberate H₂ in a repetitive growth process. Once these ions become large (≈2000 a.m.u.) they supposedly dissociate and produce an uncharged, but large, hydrocarbon “molecule,” which can grow, coalesce or coagulate to give soot particles. Simultaneously this dissociation produces a very small ion, which repeats the process of adding on C₂, C₃, and C₄ species, etc. The other school of thought believes that fairly similar processes occur, but the species involved are not ions, but uncharged radicals and molecules. This present study has spectroscopically monitored the level of sooting in the earliest stages of its production in a premixed, oxyacetylene flame at 1 atm. If soot originates from ions such as C₃H₃⁺, the addition of a relatively large quantity of easily ionized cesium removes C₃H₃⁺ ions from the flame. In that case there should also be less soot produced. When either distilled water or a strong aqueous solution of CsCl was nebulized into the sooting flame, the intensity of the emission fell by the same amount. This was by only 1% in the earliest part of the burned gas, but rose to a larger drop farther downstream of the reaction zone. Thus cesium itself has no effect on the sooting level early in this premixed flame, indicating that there is no evidence here for ions acting as nuclei for soot. However, the addition of water alone does inhibit the production of soot.     

Mechanism of soot initiation in methane systems

Thermochemical and kinetic evaluations of the very rapid elementary radical reactions consuming the C₂H₂ produced in a chlorine catalyzed polymerization of CH₄ are presented. An earlier examination of the data and mechanism leading to C₂H₂ supports a methyl and chloromethyl recombination path to C₂ hydrocarbons. The relative yield of CH₃ and CH₂Cl depends on the excess of methane.

In the CH₄, system consumption of C₂ species to ultimately form benzene is shown to proceed by a stepwise addition of CH₃ radicals to C_nH_m species. When n is even the dominant species is an unsaturated polyolefin molecule. When n is odd the dominant species is a conjugated, unsaturated radical such as allyl, pentadienyl, benzyl, etc. Mono-olefins or saturated molecules are rapidly stripped to these species by radical catalyzed dehydrogenations. In the current system chloromethyl radicals are equivalent kinetically to methyl and play a dominant role. Their addition to unsaturated species produces chlorinated radicals that dechlorinate rapidly or recombine with chloromethyl to produce dichlorohydrocarbons that dehydrochlorinate very rapidly.

A very important reaction in the sequence is the isomerization of propenyl and chloropropenyl radicals to allyl and chlorallyl by a 1–3 H (or Cl) atom shift. Its high pressure Arrhenius parameters at 1300 K are estimated to be log [k(sec⁻¹)] = 13.7 − 37/θ = 13.7 - 37/0 where 0 = 2.303 RT in kcal/mol. It appears likely that benzene conversion to soot also proceeds via a CH₃/CH₂Cl radical, sequential addition mechanism.

Stoichiometry considerations applied to the product yield distribution support the role of methyl and chloromethyl predicted by the proposed mechanism. Ionic pathways are shown to be insignificant in the formation of aromatics.     

超临界工质布雷顿循环热力学分析

  [目的]  N₂O、C₂H₆、SF₆用于制冷剂或朗肯循环的工质,这些工质的临界点和物性特征使其具有作为超临界布雷顿循环工质的潜力。  [方法]  采用自行开发的MATLAB程序并调用美国国家标准与技术研究所(NIST)发布的REFPROP物性数据库,对超临界N₂O(S-N₂O)、超临界C₂H₆(S-C₂H₆)、超临界SF₆(S-SF₆)布雷顿循环进行热力学分析,并与超临界CO₂(S-CO₂)布雷顿循环进行对比。选择再压缩循环方式,分别计算得到了透平入口温度为300～550 ℃、压力为15～25 MPa,预冷器出口温度为32 ℃和47 ℃的各种工况。  [结果]  热效率计算表明:S-N₂O、S-C₂H₆、S-SF₆再压缩循环均表现较高的热效率,且比相对应的S-CO₂再压缩循环的热效率高,再压缩循环热效率总是随着透平入口温度的提高而提高,但提高压力不一定总是提高循环热效率,提高预冷器出口温度导致循环热效率显著下降。流量计算表明,S-N₂O、S-C₂H₆、S-SF₆、S-CO₂循环的总质量流量和透平入口体积流量均远高于同等参数条件的蒸汽朗肯循环,但这四种超临界工质循环的透平出口体积流量相近。  [结论]  S-N₂O、S-C₂H₆、S-SF₆、S-CO₂循环均有潜在应用价值。     

Coal oxidation at low temperatures: oxygen consumption, oxidation products, reaction mechanism and kinetic modelling

Coal oxidation at low temperatures (i.e. <100 °C) is the major heat source responsible for the self-heating and spontaneous combustion of coal and is an important source of greenhouse gas emissions. This review focuses on the chemical reactions occurring during low-temperature oxidation of coal. Current understanding indicates that this process involves consumption of O₂, formation of solid oxygenated complexes, thermal decomposition of solid oxygenated complexes and generation of gaseous oxidation products. Parameters, such as mass change, heat release, oxygen consumption, and formation of oxidation products in the gas or solid phase, have been used to qualitatively and quantitatively describe the oxidation process. Reaction mechanisms have been proposed to explain the characteristics of consumption of O₂, and formation of oxidation products in the gas and solid phases. Various kinetic models have also been developed to describe the rate of oxygen consumption and the rates of formation of gaseous oxidation products in terms of the rate parameters of the relevant reactions, oxidation time, temperature, and initial concentration of oxygen in the oxidising medium. Further research emphasis should be placed on the formation of the complete reaction pathways proceeding in the oxidation process and on the development of kinetic models applicable for predicting the self-heating and gas emission in a coal seam or stockpile.     

Method of gasification in IGCC system

A coal gasifier is designed to operate at the temperature range of 1200–1300 °C. The 1200 °C sets the lower limit to the carbon reforming efficiency of the high temperature reformer, and the 1300 °C is the lower limit of the fluid temperature of coal slags, below which they may be collected as non-fluid slag. The gasifier is connected to two syngas burners where a portion of product syngas is combusted with O₂ gas and produce ultra hot H₂O and CO₂ gases, these two gases enter into the gasifier and maintain the gasifier temperature at above 1200 °C and reform carbon into syngas. The temperature of the gasifier is controlled by the flow of O₂ gas into the syngas burner, where O₂ gas is completely consumed and none left to enter into the gasifier. This removes any possibility of forming oxidated products, and compressed CO₂ gas spray coal powder into the gasifier column and non-fluid slag is collected at the bottom. A higher level integration of oxidation–reduction cycle is shown for a IGCC system, wherein the exhaust gas of syngas turbine drives the reduction reaction of coal gasification.

A smooth and uniform temperature control within the gasifier assures high efficiency of carbon reforming and quality of product syngas. Conventional Lurgi gasifier relies on its large heat capacity and accumulating coal slag along the inner walls of the gasifier has made the gasifier bigger, lately as large as a three story building. The gasifier of the present design is constructed much smaller in its size, but with greater reforming efficiency.     

The chemistry of sodium with sulfur in flames

An experimental and analytical program of sodium/sulfur chemistry has been conducted in a series of fuel rich and lean H₂/O₂/N₂ flames, with and without added sulfur, and covering a wide range of temperatures and stoichiometries. Fluorescence measurements of OH and Na profiles together with sodium line reversal temperature profiles provided a broad data base for kinetic modeling. Analysis indicated NaSO₂ to be the only significant sodium/sulfur product formed in the lean flames. NaOS is dominant in the rich flames, coupled with small contributions from NaSO₂, NaSH, NaS and NaS₂. A bond dissociation energy of D₀(Na---SO₂) = 197 ± 20 kJ mol⁻¹ is derived. Calculations indicate that the linear or triangular structures for NaOS both co-exist in approximately equal proportions in flames. Analyses based on results developed in the study show that Na₂SO₄ formation is kinetically limited and cannot be a significant gas phase flame product at sodium levels much below 100 ppm. Na₂SO₄ induced corrosion in combustion systems must result from heterogeneously formed Na₂SO₄.     

Application of heat pump in rectification decreases energy consumption in the chemical industry

In the rectification, in essence, most heat energy brought in at the boiler of the column at the higher temperature is transformed into the condensation heat of the vapor, which is taken away at the lower temperature. The application of the heat pump makes it possible to transform the condensation het of the vapor to the higher temperature, by which the utilization is realizable. But, it is necessary to bring into the system the noble electric energy. The realization is judged not only by the energy efficiency, but also by the exergy efficiency. An example is given of the separation of ethylbenzene from the mixture C₈H₁₀/C₈H₈. The problem of the calculation is that the ethylbenzene vapors are not an ideal gas.     

Hydrogen production by biomass gasification in supercritical or subcritical water with Raney-Ni and other catalysts

Gasification of peanut shell, sawdust and straw in supercritical or subcritical water has been studied in a batch reactor with the presence of a series of Raney-Ni and its mixture with ZnCl₂ or Ca(OH)₂. The main gas products were hydrogen, methane, carbon dioxide, and a small amount of carbon monoxide. Different types of Raney-Ni, containing different metal components such as Fe, Mo or Cr, have different influences on the gasification yield and hydrogen selectivity. The catalysis effect can be improved obviously by adding ZnCl₂ or Ca(OH)₂. Increasing the reaction temperature or adding ZnCl₂ and Ca(OH)₂ could improve the mass of H₂ in gas products and reduce the mass of CH₄ and CO₂ at the same time. The possible mechanism is that ZnCl₂ can decompose the biomass particle by accelerating cellulose hydrolyzation in high-temperature water, increasing more specific surface to admit catalysts, while Ca(OH)₂ can absorb CO₂ to produce CaCO₃ deposit, which can drop out from the reactant system, and which will drive the reaction to get more hydrogen. With respect to the biomass conversion to gas product and selectivity of H₂ at low temperature, the series of Raney-Ni has shown many advantages over other catalysts; thus, this kind of catalyst has great potential to be utilized in the hydrogen industry for the gasification of biomass.     

The ignition of oxetane in shock waves and oxidation in a jet-stirred reactor: An experimental and kinetic modeling study

The ignition and oxidation of oxetane have been studied in a single-pulse shock tube under reflected shock wave conditions and also in a jet-stirred reactor (JSR). These experiments cover a wide range of conditions: 1–10 atm, 0.5 ≤ φ ≤ 2.0, 800–1780 K. The ignition delays of oxetane measured in a shock tube have been used to propose an overall dependence of ignition delay time on the concentrations of each component in the gas as: τ = 10^−13.5 exp(13389/T₅)[C₃H₆O]^−0.36[O₂]^−0.59[Ar]^0.088 (units: seconds, moles per cubic decimeters, and Kelvin). Concentration profiles of the reactants, intermediates, and products of the oxidation of oxetane were measured in a JSR. A numerical model, consisting of a detailed kinetic reaction mechanism with 423 reactions (most of them reversible) of 63 species describes the ignition of oxetane in reflected shock waves and its oxidation in a jet-stirred reactor. Fairly good agreement between the observations and the model was obtained. The major reaction paths have been identified through detailed kinetic modeling.     

Preparation of activated carbon from rockrose char. influence of activation temperature

The influence of the activation temperature on textural characteristics of activated carbon prepared by partial gasification of a carbonized product (C-600) from rockrose wood (Cistus ladaniferus L.) was studied. Activations were effected in air, CO₂, and steam. The temperature ranged between 450° and 750°C in air and between 750° and 950°C in CO₂ or steam. Burnoff was 40%. Techniques used in the characterization study of the samples were gas adsorption (N₂, 77K; CO₂, 298K), mercury porosimetry, and density measurements. As temperature was increased, the microporosity increased for activations of C-600 in air, whereas the macroporosity decreased in CO₂ and in steam. The development of mesoporosity was greater when activating in steam at the lowest temperature. The product of steam activation at 750°C had the best textural characteristics.