Combustion characteristics of wheat straw in a fixed bed

Combustion of wheat straw was investigated in a fixed-bed reactor for two air flow rates, 1,151 and 2,125 kg/m²h. Measured temperatures, mass loss, and gas composition were used to evaluate the combustion characteristics in terms of ignition front speed, burning rate, and quality of combustion in general. At lower value of air flow rate difference between the ignition rate and burning rate was significant, which can be easily noticed by the measurement results. At higher value of air flow rate, there was still a small difference between the ignition and burning rate, while the content of oxygen in the flue gas was 10%, which leads to the conclusion that a change in the method of delivering the combustion air is necessary.     

Hydrogen-rich syngas produced from co-gasification of municipal solid waste and wheat straw in an oxygen-enriched air fluidized bed

The gasification characteristics of solid waste and wheat straw were investigated in an oxygen-rich atmosphere by using a laboratory-scale continuous fluidized bed reactor in the range of oxidation equivalent (ER) of 0.2~0.5 and reaction temperature of 600 °C~900 °C. Gasification of biomass and waste is an economical method for hydrogen production. When air is used as a carrier gas to gasify municipal solid waste, increasing the oxygen concentration can effectively increase the hydrogen concentration of the syngas. The product distribution of gasification reaction under different mixing ratios and reaction parameters was obtained. As is shown in the results, first, when the ER is between 0.2 and 0.5, if ER decreases by 0.1, the hydrogen concentration of gas production will increase by about 30%; second, if the oxygen concentration increases by 5%, the hydrogen concentration of gas production will increase by about 14%, and the calorific value of gas production will increase by about 14–18%; third, after adding wheat straw in a ratio of 1:1, due to the reduction of plastics, the overall yield of syngas decreased, but the yield of hydrogen increased, and the concentration of hydrogen in syngas increased by 6.4%.     

细颗粒木屑在鼓泡流化床中流化速度的研究

流化床气化反应器是生物质能源热化学转化反应的关键设备,流化条件的控制和确定,直接影响到转化反应的质量和效率.试验研究了在分布板开孔率为2%～4%,以粒径d＜0.28 mm的细颗粒木屑为原料,床层高度为0.4D,0.8D,1.2D,1.6D,2.0D(D=183 mm,D为有机玻璃管内径)的冷态条件下木屑的流化速度变化情况.结果表明:以木屑为原料,分布板开孔率和床层高度对细颗粒木屑的临界流化速度的影响不大.     

Effect of the type of gasifying agent on gas composition in a bubbling fluidized bed reactor

It is commonly accepted that gasification of coal has a high potential for a more sustainable and clean way of coal utilization. In recent years, research and development in coal gasification areas are mainly focused on the synthetic raw gas production, raw gas cleaning and, utilization of synthesis gas for different areas such as electricity, liquid fuels and chemicals productions within the concept of poly-generation applications. The most important parameter in the design phase of the gasification process is the quality of the synthetic raw gas that depends on various parameters such as gasifier reactor itself, type of gasification agent and operational conditions. In this work, coal gasification has been investigated in a laboratory scale atmospheric pressure bubbling fluidized bed reactor, with a focus on the influence of the gasification agents on the gas composition in the synthesis raw gas. Several tests were performed at continuous coal feeding of several kg/h. Gas quality (contents in H₂, CO, CO₂, CH₄, O₂) was analyzed by using online gas analyzer through experiments. Coal was crushed to a size below 1 mm. It was found that the gas produced through experiments had a maximum energy content of 5.28 MJ/Nm³ at a bed temperature of approximately 800 °C, with the equivalence ratio at 0.23 based on air as a gasification agent for the coal feedstock. Furthermore, with the addition of steam, the yield of hydrogen increases in the synthesis gas with respect to the water–gas shift reaction. It was also found that the gas produced through experiments had a maximum energy content of 9.21 MJ/Nm³ at a bed temperature range of approximately 800–950 °C, with the equivalence ratio at 0.21 based on steam and oxygen mixtures as gasification agents for the coal feedstock. The influence of gasification agents, operational conditions of gasifier, etc. on the quality of synthetic raw gas, gas production efficiency of gasifier and coal conversion ratio are discussed in details.     

Heat transfer characteristics in a pulsating fluidized bed in relation to bubble characteristics

A pulsating fluidized bed is operated with two sequential durations designated as an on‐period with injecting fluidization gas and an off‐period without it. The heat transfer coefficient between a vertically immersed heater and bed in a pulsating fluidized bed is measured under various pulse cycles and fluidized particles. The obtained results are compared with those in a normal fluidized bed with continuous fluidization air injection. The relationship between heat transfer coefficients and bubble characteristics, evaluated using a digital video camera, has also been investigated. For certain fluidized particles and operating pulse cycles, the fluidization of particles and the increment of heat transfer coefficients can be obtained under a mean air velocity based on a pulse cycle duration smaller than the minimum fluidization air velocity in a normal fluidized bed. Under the pulse cycles where a static bed through the whole bed is formed in the off‐period duration, the improved heat transfer rate over that in a normal fluidized bed can be measured. This may be attributed to large bubble formation. As heat transfer in the pulsating fluidized bed is obstructed with increasing time to keep a static bed due to the excessive off‐period duration, it is indicated that there is an optimum off‐period duration based on the heat transfer rate. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 31(4): 307–319, 2002; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10038     

Studies on the operation of loop-seal in circulating fluidized bed boilers

Loop-seal, considered heart of a circulating fluidized bed (CFB), returns solids captured by cyclone to the base of the riser while preventing direct flow of gas from high pressure riser to the low-pressure cyclone. This non-mechanical valve is used in thousands of CFB systems yet only a limited information is available on its working. Present research studies the flow of solids through a loop-seal and the effect of several design and operating parameters on it. This experimental study was conducted in a loop-seal 110 mm × 448 mm × 400 mm high connected to a riser 152 mm diameter and 5180 mm high. Majority of the experiments was done with 171 μm sand though several other size and type of solids were studied for their flowability. It was found that for the solids to flow through the loop-seal a minimum level of aeration, in excess of that required for minimum fluidization was required. The length of the horizontal passage connecting the supply and recycle chambers of the loop-seal had an important effect on the solids flow. For example, the minimum aeration for the onset of solids flow increases with increase in this length. The pressure drop per unit length across the passage also increased with the passage length. The air fed into the supply chamber is split such that the superficial air velocity in the supply chamber (or the standpipe) remained below the minimum fluidization velocity of the particles while the remaining air conveys solids through the horizontal passage. Present study showed that the solids flowing through the horizontal passage are neither fully fluidized nor moving packed or suspended solids. It moves as a segregated flow of solids driven by hydrostatic pressure and fluid drag.     

Development of CO2 emission factors from a large circulating fluidized bed boiler

Greenhouse gas emissions of the nation should be precisely assessed in order to effectively tackle the problems stemming from climate changes. To do so, country-specific data that reflect a nation’s distinct characteristics must be applied to more precisely assess greenhouse gas emissions. In this research, carbon emission factors were assessed for emission sources making up over 53% of the domestic anthracite consumption with calorific values of fuels and elementary analysis. Furthermore, oxidation rates were assessed based on measurement results of unburned carbon produced from tested power plants to calculate CO₂ emission factors. Ultimately, the CO₂ emission factor was calculated at 106,747 kg/TJ, about 9% higher than the anthracite emission factor presented by the Intergovernmental Panel on Climate Control. Such differences are assumed to mainly come from disparities of anthracite properties along with combustion technology and differences of oxidation rates depending on the combustion conditions. Therefore, continued research on a wide variety of fuels and energy consumption facilities should be conducted in order to establish country-specific data, which will help more accurately assess greenhouse gas emissions and subsequently will lead to reliable greenhouse gas reduction goals.     

Performance evaluation of an innovative 100 kWth dual bubbling fluidized bed gasifier through two years of experimental tests: Results of the BLAZE project

In this work, the results of two years of experimental tests on an innovative dual bubbling fluidized bed gasifier are reported. These are related to the activities of the BLAZE project (Horizon 2020) for the integration of steam biomass gasification and solid oxide fuel cell. Several tests were carried out on the pilot-scale reactor at various operating conditions, and in this work the results are reported in terms of dry gas composition and yield, organic and inorganic contaminants (tar, particulate matter, H₂S). The compact design of the gasifier (a single reactor with two concentric chambers and in-situ hot gas cleaning and conditioning) reduces the heat losses and produces close to nitrogen-free syngas. Preliminary tests using a filter candle filled with conventional catalyst, installed in the freeboard of the gasifier, show that the tar content dropped to about 2 g/Nm³, and the H₂ concentration increased up to 41%_vol,dry.     

Physicochemical properties and gasification reactivity of the ultrafine semi-char derived from a bench-scale fluidized bed gasifier

Zhundong coalfield is the largest intact coalfield worldwide and fluidized bed gasification has been considered as a promising way to achieve its clean and efficient utilization.The purpose of this study is to investigate the physieochemical properties and gasification reactivity of the ultrafine semi-char,derived from a bench-scale fluidized bed gasifier,using Zhundong coal as fuel.The results obtained are as follows.In comparison to the raw coal,the carbon and ash content of the semi-char increase after partial gasification,but the ash fusion temperatures of them show no significant difference.Particularly,76.53％ of the sodium in the feed coal has released to the gas phase after fluidized bed gasification.The chemical compositions of the semi-char are closely related to its particle size,attributable to the distinctly different natures of diverse elements.The semi-char exhibits a higher graphitization degree,higher BET surface area,and richer meso-and macropores,which results in superior gasification reactivity than the coal char.The chemical reactivity of the semi-char is significantly improved by an increased gasification temperature,which suggests the necessity of regasification of the semi-char at a higher temperature.Consequently,it will be considered feasible that these carbons in the semi-char from fluidized bed gasifiers are reclaimed and reused for the gasification process.     

Gasification of biomass to hydrogen-rich gas in fluidized beds using porous medium as bed material

Experiments were carried out to study the characteristics of biomass gasification in a fluidized bed using industrial sand and porous medium as bed materials. Analysis was conducted to investigate the effects of different operation parameters, including bed material, gasification temperature (600 °C–900 °C), oxygen enrichment in the gasifying agent (21 vol.% to 50 vol.%), and steam flow rate (1.08 kg/h to 2.10 kg/h), on product yields and gas composition. The results of gas chromatography show that the main generated gas species were H₂, CO, CO₂, CH₄, and C₂H₄. Compared with industrial sand as bed material, porous medium as bed material was more suitable for gasifying biomass to hydrogen-rich gas. The physical characteristics of porous structure are more favorable to heat transfer, producing the secondary crack of heavy hydrocarbons and generating more hydrogen and other permanent gases. The product yields of hydrogen-rich gas increased with increasing gasification temperature. The hydrogen concentration improved from 22.52 vol.% to 36.06 vol.%, but the CO concentration decreased from 37.53 vol.% to 28.37 vol.% with increasing temperature from 600 °C to 900 °C under the operation parameters of porous bed material at a steam flow rate of 1.56 kg/h. With increasing oxygen concentration, H₂ concentration increased from 12.36% to 20.21%. Over the ranges of the examined experimental conditions, the actual steam flux value (e.g., 1.56 kg/h) was found to be the optimum value for gasification.     

Influence of chemical and thermodynamic parameters on the flue gas desulphurization efficiency in a circulating fluidized bed

IneductionEven there are W successful Mods Of flue gasdesulPhuriZation (FGD), people have been searching fOrnew ones, which are more econondcal, with higherefficiency and more reliable. Lap, Thyssen and WullffMasclunen developed the teChnulogy of circulatinfluldized bed flue gas desulPhurization (CFBFGD) inlate 1980s, which is similar with circulating fluidized bedboilers in enhancing chendcal reachvity As flue gas andabsothent are Inixed in theulent bed, SO2 is absothedand changed in…     

Effect of operating parameters on sulphur capture in a pressurized circulating fluidized bed combustor

Experiments are conducted to investigate the effect of system pressure, Ca/S ratio and primary air velocity on sulphur capture in a pressurized circulating fluidized bed (PCFB) combustor. The pressure inside the PCFB combustor is varied from 200 to 700 kPa. The Ca/S ratio is varied from 1.6 to 3.0. The primary air velocity ranges from 3 to 7 ms^?1. The bed temperature is maintained at 750°C. The sulphur capture increases with system pressure in the present range of experimental investigations. The sulphur capture also increases with Ca/S ratio up to a certain ratio and then shows a decreasing trend for the given operating conditions. A semi‐empirical model is developed for explaining the sulphur capture mechanism in the pressurized circulating fluidized bed combustor under batch combustion conditions. The experimental data are validated with the model predictions and a reasonable agreement has been observed. Copyright © 2002 John Wiley & Sons, Ltd.     

Selective non-catalytic reduction of flue gas in a circulating fluidized bed

Selective non-catalytic reduction can meet the requirements of the new National Emission Regulation due to the low NOx emission characteristic of circulating fluidized bed boilers. In this work, ammonia was injected into the simulated circulating fluidized bed flue gas as a reducing agent. Optimum reaction conditions were obtained as: temperature of 920°C, residence time of 0.6 s, and a normalized stoichiometric ratio (NSR₁) of 1.5. H₂, as an additive, made a shift of 170°C towards a lower temperature, while CH₄ made a shift of 80°C.     

Study on gas holdup in a fluidized flotation column from bed pressure drop

This paper investigated the overall gas holdup characteristics in a cocurrent three-phase fluidized flotation column with liquid as the continuous phase. The air, water, and glass beads with a diameter of 3 mm were, respectively, used as the gas, liquid, and solid phases in the flotation column. The gas holdup studies were carried out in a plexiglass column with 0.05 m in internal diameter and 2.2 m in height. Bed pressure drop measurements were used to calculate the fractional gas holdup. During the measurements, the superficial gas and liquid velocities, respectively, varied from 0.42 to 2.55 cm/s and from 6.47 to 10.82 cm/s. Detailed experimental investigations were carried out to study the effects of static liquid height, initial static bed height, gas velocity, liquid velocity, and frother concentration on gas holdup in a cocurrent three-phase fluidized flotation column. It was found that the gas holdup increased with the flow rate of air and decreased with an increase in the water flow rate. Certain effect of the static bed height on gas holdup was observed when the gas velocity varied. But the increase in the static liquid height resulted in the decrease in gas holdup when the gas velocity varied.     

Investigation of syngas exergy value and hydrogen concentration in syngas from biomass gasification in a bubbling fluidized bed gasifier by using machine learning

In this study, an artificial neural network (ANN) model as a machine learning method has been employed to investigate the exergy value of syngas, where the hydrogen content in syngas reached maximum in bubbling fluidized bed gasifier which is developed in Aspen Plus® and validated from experimental data in literature. Levenberg-Marquardt algorithm has been used to train ANN model, where oxygen, hydrogen and carbon contents of sixteen different biomass, gasification temperature, steam and fuel flow rates were selected as input parameters of the model. Moreover, four different biomass samples, which hadn't been used in training and testing, have been used to create second validation. The hydrogen mole fraction of syngas was also evaluated at the different steam to fuel ratio and gasification temperature and the exergy value of syngas at the point where the hydrogen content in syngas reached maximum were estimated with low relative error value.     

Combined-gasification of biomass and municipal solid waste in a fluidized bed gasifier

Due to the environmental problems associated with burning of fossil fuels and population growth, more attention has been paid to develop renewable energies in recent years. Among all options for renewable energy utilization, biomass gasification is more popular because of environmental benefits and economic issues. In the present study, a series of experiments were carried out to study the influence of blending ratio, reaction temperature, equivalence ratio (ER) on co-gasification characteristics of pine sawdust (SD) and municipal solid waste (MSW). By increasing the blending ratio from 100% SD to 100% MSW, CO and CH₄ respectively increased from 16.7 to 18.8 vol% and from 4.1 to 5.1 vol%, while an opposite trend was found for H₂ and CO₂. Over the ranges of the experimental conditions used, the tar content and gas yield varied from 5.4 to 10.1 g/Nm³ and 1.34 to 1.15 Nm³/kg, respectively.     

Effect of CaO on tar reforming to hydrogen-enriched gas with in-process CO2 capture in a bubbling fluidized bed biomass steam gasifier

Previous studies showed that calcium oxide (CaO), when added to a biomass steam gasifier, could play the role of both CO₂ sorbent and tar reforming catalyst, and thereby produce more hydrogen. However, most of these works focused on the former role with little attention to tar reforming aspect of CaO. Therefore, this work aims primarily at studying the tar reforming effect of in-bed CaO. To this end, an in-depth analysis of the effect of CaO on tar yield and composition is presented. The present work also studies the role of CaO as a CO₂ sorbent to enhance hydrogen production from steam gasification of biomass in a bubbling fluidized bed. The influence of different operating parameters, temperature (T) and steam to biomass (S/B) ratio, as well as the effect of using in-bed CaO on gas and tar production is investigated. Results show that the maximum H₂ and minimum CO₂ concentration of 63.07% and 18.68%, respectively are obtained at T = 650 °C and S/B = 3.41. The maximum H₂ yield of 256.81 ml g⁻¹-biomass was obtained at T = 700 °C and S/B = 3.41, at which the minimum tar content of 6.45 g N m⁻³ was also received. Compared to a bed of sand alone, a 20% higher H₂ concentration, an almost double H₂ yield and a 67% reduction in tar content were obtained when a bed of CaO was used. Moreover, shifting the tar species from higher to fewer ring structures as a result of in-bed CaO can reduce tar dew point by 11 °C and tar carcinogenic potential by almost 60%.     

Performance of a water gas shift pilot plant processing product gas from an industrial scale biomass steam gasification plant

In this paper, the performance of a commercial Fe/Cr based catalyst for the water gas shift reaction was investigated. The catalyst was used in a water gas shift pilot plant which processed real product gas from a commercial biomass steam gasification plant with two different qualities: extracted before and extracted after scrubbing with a rapeseed methyl ester gas scrubber. The performance of the WGS pilot plant regarding these two different gas qualities was investigated. For this reason, extensive chemical analyses were carried out. CO, CO₂, CH₄, N₂, O₂, C₂H₆, C₂H₄, and C₂H₂ and H₂S, COS, and C₄H₄ S were measured. In addition, GCMS tar and NH₃ analyses were performed. Furthermore, the catalyst's activity was observed by measuring the temperature profiles along the reactors of the water gas shift pilot plant. During the 200 h of operation with both product gas qualities, no catalyst deactivation could be observed. A CO conversion up to 93% as well as a GCMS tar reduction (about 28%) along the water gas shift pilot plant was obtained. Furthermore, a specific H₂ production of 63 g H₂ per kg biomass (dry and ash free) was reached with both product gas qualities. No significant performance difference could be observed.     

A study of solid circulation rate in a circulating fluidized bed

An experimental investigation under cold condition was made to study the effects of some operating/design parameters and non-mechanical L valve configuration on the solid circulation rate in a 4.5 m tall, 0.15 m diameter circulating fluidized bed with riser flow rate varying from 1400 litres/min to 2000 litres/min and bed inventory from 15 kg to 25 kg of sand of average sizes 200 μm, 400 μm and 500 μm. Solid circulation rate was estimated by measuring velocity of sand particle travelling through a vertical Perspex tube section at the bottom of the return leg. It was found to be in the range of 2.8 to 12.3kg/m²s, 0.07 to 9.1kg/m²s and 0.12 to 2.23kg/m²s for sand sizes of 200 μm, 400 μm and 500 μm, respectively for a horizontal L valve. Two mathematical correlations have been developed from the experimental results to predict solid circulation rate as a function of riser flow rate, aeration flow rate, total bed inventory and particle size used.