Inhibition of steam gasification of biomass char by hydrogen and tar

The influence of hydrogen and tar on the reaction rate of woody biomass char in steam gasification was investigated by varying the concentrations in a rapid-heating thermobalance reactor. It was observed that the steam gasification of biomass char can be separated into two periods. Compared with the first period, in the second period (in which the relative mass of remaining char is smaller than 0.4) the gasification rate is increased. These effects are probably due to inherent potassium catalyst. Higher hydrogen partial pressure greatly inhibits the gasification of biomass char in the first and second periods. By calculating the first-order rate constants of char gasification in the first and second periods, we found that the hydrogen inhibition on biomass char gasification is caused by the reverse oxygen exchange reaction in the first period. In the second period, dissociative hydrogen adsorption on the char is the major inhibition reaction. The influence of levoglucosan, a major tar component derived from cellulose, was also examined. We found that not only hydrogen but also vapor-phase levoglucosan and its pyrolysates inhibited the steam gasification of woody biomass char. By mixing levoglucosan with woody biomass sample, the pyrolysis of char proceeds slightly more rapidly than with woody biomass alone, and gas evolution rates of H₂ and CO₂ are larger in steam gasification.     

Char and char-supported nickel catalysts for secondary syngas cleanup and conditioning

Tars in biomass gasification systems need to be removed to avoid damaging and clogging downstream pipes or equipment. In this study, Ni-based catalysts were made by mechanically mixing NiO and char particles at various ratios. Catalytic performance of the Ni/char catalysts was studied and compared with performance of wood char and coal char without Ni for syngas cleanup in a laboratory-scale updraft biomass gasifier. Reforming parameters investigated were reaction temperature (650–850 °C), NiO loading (5–20% of the weight of char support), and gas residence time (0.1–1.2 s). The Ni/coalchar and Ni/woodchar catalysts removed more than 97% of tars in syngas at 800 °C reforming temperature, 15% NiO loading, and 0.3 s gas residence time. Analysis of syngas composition indicated that concentrations of H₂ and CO in syngas significantly. Furthermore, performance of the Ni/coalchar catalyst was continuously tested for 8 h. There was slight deactivation of the catalyst in the early stage of tar/syngas reforming; however, the catalyst was able to stabilize soon after. It was concluded that chars especially coal char can be an effective and inexpensive support of NiO for biomass gasification tar removal and syngas conditioning.     

Co–gasification of coal/biomass blends in 50 kWe circulating fluidized bed gasifier

This paper reports gasification of coal/biomass blends in a pilot scale (50 kWe) air-blown circulating fluidized bed gasifier. Yardsticks for gasification performance are net yield, LHV and composition and tar content of producer gas, cold gas efficiency (CGE) and carbon conversion efficiency (CCE). Net LHV decreased with increasing equivalence ratio (ER) whereas CCE and CGE increased. Max gas yield (1.91 Nm³/kg) and least tar yield (5.61 g/kg of dry fuel) was obtained for coal biomass composition of 60:40 wt% at 800 °C. Catalytic effect of alkali and alkaline earth metals in biomass enhanced char and tar conversion for coal/biomass blend of 60:40 wt% at ER = 0.29, with CGE and CCE of 44% and 84%, respectively. Gasification of 60:40 wt% coal/biomass blend with dolomite (10 wt%, in-bed) gave higher gas yield (2.11 Nm³/kg) and H₂ content (12.63 vol%) of producer gas with reduced tar content (4.3 g/kg dry fuel).     

Hydrogen production by biomass gasification in a fluidized-bed reactor promoted by an Fe/CaO catalyst

Biomass gasification for hydrogen production was performed in a continuous-feeding fluidized-bed with the use of Fe/CaO catalysts. The relationship between catalyst properties and biomass gasification efficiencies was studied. The findings indicated that only CaO was involved in the enhancement of char gasification, resulting in an increased hydrogen production. However, CaO was also easily deactivated by biomass tar. The characterization results indicated that when CaO was impregnated with Fe, Ca₂Fe₂O₅ formed on the surface of the support. Ca₂Fe₂O₅ decomposed polyaromatic tar but was not effective in char gasification. The synergistic effects between Fe and CaO that effectively enhanced biomass gasification mainly involved combustion and pyrolysis, and the biomass gasification products, i.e., char and tar, were further gasified, indicating that tailor-made Fe/CaO catalysts prevented CaO deactivation by tar, thus promoting biomass gasification and hydrogen production.     

Catalytic effects observed during the co-gasification of coal and switchgrass

We are investigating catalytic gasification of coal char using biomass-derived potassium salts. Alkali metal salts, especially those containing potassium, are excellent promoters of gasification reactions but are generally considered too expensive for commercial use. Fast-growing biomass, which contains large quantities of potassium, may prove to be an excellent source of inexpensive gasification catalyst. A series of CO₂-char gasification tests were performed in a thermogravimetric analyzer (TGA) to evaluate the catalytic activity of alkali-rich biomass-derived materials. Both switchgrass char and switchgrass ash displayed catalytic activity in mixtures with coal char produced from Illinois No. 6 coal. The results obtained with switchgrass ash were especially impressive, with an almost eight-fold increase in coal char gasification rate at 895°C in a 10:90 mixture of coal char and switchgrass ash. These results give encouragement that biomass could be the source of inexpensive, coal gasification catalysts.     

Exergy analysis of biomass staged-gasification for hydrogen-rich syngas

Using Aspen Plus simulations, exergy analyses of hydrogen-rich syngas production via biomass staged-gasification are carried out for three configurations, namely, staged-gasification with pyrolysis gas combustion and char gasification (C-1), staged-gasification with pyrolysis gas reforming and char gasification (C-2), and staged-gasification with pyrolysis gas reforming and char combustion (C-3). The results show that, for the gasification and reforming processes, the exergy loss of pyrolysis gas with tar reforming is less than that of char gasification. As for the system, it is conducive to generating hydrogen by making full use of the hydrogen element (H) in biomass instead of the H in water. The benefits of C-1 are that it removes tar and produces higher yield and concentration of hydrogen. However, C-2 is capable of obtaining higher exergy efficiency and lower exergy loss per mole of H₂ production. C-3 theoretically has greater process performances, but it has disadvantages in tar conversion in practical applications. The appropriate gasification temperature (T_G) are in the range of 700–750 °C and the appropriate mass ratio of steam to biomass (S/B) are in the range of 0.6–0.8 for C-1 and C-3; the corresponding parameters for C-2 are in the ranges of 650–700 °C and 0.7–0.8, respectively.     

The effect of temperature on various parameters in coal,biomass and CO-gasification: A review

Numerous reviews have been found related to individual studies on coal gasification (CG) and biomass gasification (BG). However, this review deals mainly with the CO-gasification (COG) of numerous types of coal and biomass and then compares their results with those obtained using coal and biomass gasification in detail. There are several process parameters which have a direct effect on the gasification process and among them temperature is the most significant one. In this paper, the production of H₂, CO₂, CO, CH₄, and other hydrocarbons in CG, BG and COG with the variation of temperature is reviewed in detail. As it mainly influences the gaseous products and their characteristic behaviour, this review takes into account the effect of temperature on various other parameters such as carbon conversion, gas yield, calorific value, cold gas efficiency as well as tar and char contents in various kinds of coals, biomasses and their mixtures under catalytic or non-catalytic conditions.     

1MW木粉气化发电系统的运行特性分析

通过对海南三亚木粉发电系统的运行状况分析，结果发现：气化过程中所夹带的飞灰以及所生成的焦油是影响发电系统正常运行的主要因素，显热损失和飞灰中没有完全气化的碳损失是导致气化发电系统效率下降的主要原因。同时考查了气化过程中污染物排放情况及添加焦油裂解催化剂对气体成份和焦油裂解的影响，对改进今后生物质气化发电系统的大型化设计具有一定的指导意义。     

Promoting effect of biomass ash additives on high-temperature gasification of petroleum coke: Reactivity and kinetic analysis

The effect of biomass ash (rice straw ash (RSA) and cotton straw ash (CSA)) on the gasification reactivity and the evolution of physicochemical structure of petcoke char was investigated. The catalytic effect of CSA was significantly higher than that of RSA, and the catalytic effect of biomass ash was enhanced at lower gasification temperature and for higher blending ratio of biomass ash. The promoting effect of biomass ash was related to the increase of active AAEM content, the decrease of order degree of carbon structure and the development of surface structure in char gasification after biomass ash addition, which was more significant for CSA, at lower temperature and for higher blending ratio. Moreover, the shrinking core model was suitable for char gasification, and the additions of RSA and CSA reduced the activation energy of petcoke char gasification from 199.84 kJ mol⁻¹ to 159.85 kJ mol⁻¹ and 62.75 kJ mol⁻¹, respectively.     

Development of renewable biomass energy by catalytic gasification: Syngas production for environmental management

Gasification is a process in which biomass is decomposed into small quantities of solid char and ash and large quantities of gaseous products in the presence of one or more fluidizing agents. In this paper, a lab scale fluidized bed gasifier was used to explore the effects of different kinds of calcined dolomites (CD-1, CD-2, and CD-3) on tar conversion and composition during air-steam gasification of biomass. Results showed that all dolomites have a good performance in terms of tar reduction but CD-2 is more reactive than two other dolomites with respect to tar destruction. When the temperature was lower than 850^?C, conversion of tar was relatively low; however, with temperature increasing further (>850 ^?C), tar conversion was greatly enhanced.     

Improving the performance of fluidized bed biomass/waste gasifiers for distributed electricity: A new three-stage gasification system

Methods to increase the conversion of char and tar in fluidized bed gasifiers (FBG) are discussed, with the focus on small to medium-size biomass/waste gasifiers for power production (from 0.5 to 10 MW_e). Optimization of such systems aims at (i) maximizing energy utilization of the fuel (maximizing char conversion), (ii) minimizing secondary treatment of the gas (by avoiding complex tar cleaning), and (iii) application in small biomass-to-electricity gasification plants. The efficiency of various measures to increase tar and char conversion within a gasification reactor (primary methods) is discussed. The optimization of FBG by using in-bed catalysts, by addition of steam and enriched air as gasification agent, and by secondary-air injection, although improving the process, is shown to be insufficient to attain the gas purity required for burning the gas in an engine to produce electricity. Staged gasification is identified as the only method capable of reaching the targets mentioned above with reasonable simplicity and cost, so it is ideal for power production. A promising new stage gasification process is presented. It is based on three stages: FB devolatilization, non-catalytic air/steam reforming of the gas coming from the devolatilizer, and chemical filtering of the gas and gasification of the char in a moving bed supplied with the char generated in the devolatilizer. Design considerations and comparison with one-stage FBG are discussed.     

Two-step steam pyrolysis of biomass for hydrogen production

In this study, different char based catalysts were evaluated in order to increase hydrogen production from the steam pyrolysis of olive pomace in two stage fixed bed reactor system. Biomass char, nickel loaded biomass char, coal char and nickel or iron loaded coal chars were used as catalyst. Acid washed biomass char was also tested to investigate the effect of inorganics in char on catalytic activity for hydrogen production. Catalysts were characterized by using Brunauer–Emmet–Teller (BET) method, X-ray diffraction (XRD) analyzer, X-ray fluorescence (XRF) and thermogravimetric analyzer (TGA). The results showed that the steam in absence of catalyst had no influence on hydrogen production. Increase in catalytic bed temperature (from 500 °C to 700 °C) enhanced hydrogen production in presence of Ni-impregnated and non-impregnated biomass char. Inherent inorganic content of char had great effect on hydrogen production. Ni based biomass char exhibited the highest catalytic activity in terms of hydrogen production. Besides, Ni and Fe based coal char had catalytic activity on H₂ production. On the other hand, the results showed that biomass char was not thermally stable under steam pyrolysis conditions. Weight loss of catalyst during steam pyrolysis could be attributed to steam gasification of biomass char itself. In contrast, properties of coal char based catalysts after steam pyrolysis process remained nearly unchanged, leading to better thermal stability than biomass char.     

制焦条件对稻秆与大同烟煤共气化特性影响的实验研究

在自行设计搭建的气化实验装置上进行不同制焦条件下稻焦-大同烟煤焦的混合焦样气化特性实验。对比不同工况下气化特性曲线发现:在本实验工况下,稻-煤焦样经机械掺混后的气化特性优于相同工况下浸渍混合后的气化特性;混合后制焦所得焦样气化特性优于先制焦后混合处理所得焦样的气化特性;煤和稻焦热解温度对混焦的气化特性影响不同,热解温度对生物质焦以及随后气化特性的影响大于对煤焦的影响;无论是稻焦还是煤焦,热解时间对混焦的气化特性影响均不明显。通过上述热解条件对稻-煤气化特性的影响,为煤与生物质共气化的工业应用提供指导。     

生物质气化发电燃气焦油脱除方法的探讨

生物质气化发电技术的最大难点之一就是如何除去燃气中含有的焦油等污染物,这些成分会对燃气轮机或内燃机等设备造成一定的影响.因此生物质气化发电过程中燃气焦油的脱除是目前国内外重点研究和解决的课题之一.文章在研究国内外大量有关文献资料的基础上,深入阐述了气化过程中焦油产生的机理、影响焦油生成的因素以及焦油的脱除方法,重点探讨了目前较为有效的焦油热化学脱除方法,即焦油的热裂解和催化裂解方法,以期为生物质气化发电燃气焦油的脱除提供一些思路和参考.     

掺混生物质焦和煤焦共气化的热重分析

采用热重分析法对煤焦和几种生物质焦及其混合物的气化过程及动力学规律进行了分析,分析结果表明煤焦的失重与生物质焦相比差别明显,混合物中的生物质焦所占比重越大、发生失重的温度越低。进一步采用Coats—Redfern方法对样品气化过程进行了动力学分析,得到了生物质焦、煤焦及其混合物表观活化能。     

浑源煤焦CO_2气化反应的影响因素及动力学特性分析

研究了热解温度、热解时间以及气化温度对浑源煤焦CO2气化反应的影响,并获得了气化反应的动力学模型.结果表明：浑源煤焦的气化活性随热解温度的提高而降低;每个热解温度都对应着一个最佳热解时间,且存在最佳热解时间随温度升高而缩短的趋势;提高气化温度能够显著提高煤焦的气化反应性能,气化温度对气化反应的影响大于热解温度的影响;低温度煤焦的气化活性随气化温度的提高而增加更为剧烈;900℃及以上的高温使活性点数增加,从而使煤焦间的活性差距分布均匀;浑源煤焦的气化反应适宜用体积模型来描述,所求取的动力学参数之间存在补偿效应,其等动力学温度约为1 199.6℃.     

Kinetics of CO2 gasification of biomass char in granulated blast furnace slag

The CO₂ gasification reactions of biomass char in granulated BFS (blast furnace slag) were isothermally investigated using a thermogravimetric analyzer with the temperature ranging from 1173 K to 1323 K. The effects of temperature, biomass type and granulated BFS on the kinetic characterizations of CO₂ gasification of biomass char were illuminated. The kinetic mechanism models and parameters were obtained through a novel two-step calculation method. The results indicated that the CO₂ gasification reactivity of biomass char as conversion and gasification index increased with the increase of temperature and it could be promoted through granulated BFS. The CO₂ gasification reactivity of CS (cornstalk) char with lower alkali index was lower than that of PS (peanut shell) char. The A₄ model (Avrami-Erofeev (m = 4) model) and A₃ model (Avrami-Erofeev (m = 3) model) were demonstrated as the best appropriate models for CO₂ gasification of CS char and PS char, respectively. The gasification activation energy of CS char ranging from 155.08 to 160.48 kJ/mol was higher than that of PS char whether with or without granulated BFS. Granulated BFS could decrease the activation energy of CO₂ gasification of char at any biomass type.     

生物质半焦气化的反应动力学

利用热重分析仪研究了CO2气氛下的生物质半焦的反应性。研究发现，所研究的4种生物质半焦都表现出了相同的反应性趋势。其反应性随着转化率的增加而增加。这可能是由于生物质焦样中的碱金属含量，尤其是钾的含量较高的原因。对比生物质气化反应动力学参数研究表明，4种焦样的气化行为可以用收缩核模型来描述，并求出了4种生物质焦样的反应动力学参数。在不同的CO2分压下进行了花生壳焦样的反应性实验研究，发现焦样的反应性正比于反应气体浓度，求出了花生壳焦样的反应动力学方程式。     

Impacts of char structure evolution and inherent alkali and alkaline earth metallic species catalysis on reactivity during the coal char gasification with CO2/H2O

In this work, we studied the effects of char structural evolution and alkali and alkaline earth metallic species (AAEMs) catalysis on the reactivity during the char gasification with CO₂, H₂O, and their mixture. The gasified chars with different carbon conversion levels were prepared, and their physicochemical structures were characterized via nitrogen adsorption and FT‐Raman techniques. The concentrations of AAEMs in different modes were obtained by the sequential chemical extraction method. The reactivities of the raw and gasified chars were analyzed by the thermogravimetric analysis. The gasification atmospheres had varied effects on the physicochemical structure of coal char. The gasified char obtained in the CO₂ atmosphere had a lower aromatic condensation degree compared with that obtained in the H₂O atmosphere, irrespective of the temperature. The impact of the atmospheres on the specific surface area of the char varied with the temperature because H₂O and CO₂ have different routes of development of pore structure with coal char. A large specific surface area facilitates the exposure and dispersion of more AAEMs on the surface of the channel, which is conducive to their contact with the gasification agent to play the catalytic role. Thus, the reactivity of the gasified char is well correlated with its specific surface area at different gasification temperatures. In the absence of AAEMs, the chemical structure of coal char becomes the dominant factor affecting the reactivity.     

焦油主要组份在循环灰下催化裂解的实验研究

在以循环流化床锅炉循环灰为热载体，部分气化产生的半焦为锅炉燃料，煤气为燃气轮机燃料的煤的部分气化联合循环中，降低焦油产率，提高煤气产率是一个技术关键，以焦油的两种主要组份苯和甲苯为研究对象，利用固定床实验台实验研究了一种混煤形成的循环灰条件下的裂解反应特性，测定了裂解反应动力学参数，探讨了循环灰对焦油裂解的催化机理。实验结果表明，与石英砂条件相比，循环灰极大地促进了焦油的裂解程度，气态裂解产物总量