Char Bed Research Articles

Gasification of char from wood pellets and from wood chips: Textural properties and thermochemical conversion along a continuous fixed bed

In fixed bed gasifiers, the char bed gasification zone where char is converted into syngas plays a major role in terms of efficiency and control of the process. This zone is particularly complex as many phenomena compete, i.e. heterogeneous and homogeneous chemical reactions, gas flow in porous medium and flow of solid particles.This paper investigates the mechanical and thermochemical behavior of the char bed gasification zone and focuses particularly on bed compaction. To achieve this, a low-density biomass char from wood chips and a high-density one from wood pellets were gasified in a pilot scale continuous fixed bed reactor.Measurements of profiles were taken along the char bed for temperature, gas species concentration, char composition, char bed density and char particle velocity using fine instrumentation and specific char and gas sampling techniques.In our operating conditions, the char bed reactive zone is 3 times longer for chips (45cm) than for pellets (16cm). We show that pelletization has no effect on: char bed compaction, final char conversion (about 95%) and syngas quality (16% H2 and 13% CO).Finally, we discuss char bed compaction and the main phenomena that control it in order to propose a line of inquiry for modeling.

Raman spectroscopy of coal macerals and fluidized bed char morphotypes

Raman spectroscopy analyses were carried out on macerals from a set of coals with increasing rank and their corresponding char morphotypes in order to improve the understanding of the structure of coal char produced in a fluidized bed reactor.The Raman parameters obtained on collotelinite and fusinite are different from those obtained on the corresponding fused char and fusinoid morphotypes. The highest differences occur between collotelinite and corresponding fused chars, suggesting that major structural changes occur in collotelinite during pyrolysis, while those occurring in fusinite are less pronounced.Raman parameters of the different fused char and fusinoid portions are similar, suggesting that the heat treatment enhanced the formation of similar structures. However, the fused char portions not only display different optical patterns from fusinoid chars but also have slightly higher reflectance.

Low-NOx coal combustion via combining decoupling combustion and gas reburning

Through separating coal pyrolysis and char combustion and letting the pyrolysis gas burn in its passing through the combusting char bed, the so-called decoupling combustion has been proven to be effective for lowering NOx emission of coal and biomass combustion. The original design of the decoupling combustion was based on coal pyrolysis incurring with the heat transferring from the char combustion zone, thus causing the difficulty of scale-up in matching the reactions of slow coal pyrolysis and quick char combustion. An idea of using partial gasification to replace such a pyrolysis was proposed to allow the combination of reburning part of the gasification gas over the combusting char layer and the decoupling combustion of the left gasification gas by letting it pass through the char bed. This article compared first the effectiveness of reducing NOx emission by a few different combustion methods including reburning of gasification gas and the decoupling combustion based on coal pyrolysis in a laboratory scale dual-stage reactor for five different kinds of coal. It was shown that both the decoupling combustion and reburning of gasification gas represented the most effective low-NOx combustion methods, which can reduce the NOx emission by 30–40% in comparison with the traditional combustion without air-staging and reburning. The fuel ratio of fixed carbon to volatile matters of the coal also affected the effectiveness of NOx reduction, the lower the fuel ratio, the higher the NOx reduction effect was. The actual running test of a 1.4MW industrial boiler based on combining the decoupling combustion and reburning part of the partial gasification gas reduced the NOx emission by 33% in comparison with the traditional industrial boiler. The result showed a successful scale-up of the technology and verified the idea of reducing NOx emission by combining the decoupling combustion and the reburning of coal gasification gas.

Gas Upgrading in a Downdraft Fixed-Bed Reactor Downstream of a Fluidized-Bed Coal Pyrolyzer

A new two-stage gasification process, consisting of a fluidized-bed (FB) pyrolyzer and a downdraft fixed-bed (DFB) gasifier, has been proposed to gasify powder coal for fuel gas production with low tar generation. Aiming at developing the new technology, this paper investigated the means for coal pyrolysis gas upgrading, with the focus on tar removal by both thermal cracking with or without the presence of oxygen and catalytic cracking or reforming over a char bed in a fixed-bed reactor downstream of a FB coal pyrolyzer. The presence of oxygen and the adoption of a char bed evidently facilitated the tar removal performance and also improved the produced fuel gas quality. Analyzing the tar sample collected at the outlet of the tar removal reactor with gas chromatography-mass spectrometry (GC-MS) clarified that the oxidation by O(2) and the char-catalyzed reforming both exhibited certain selectivity to the tar-containing chemical species. In addition, the property of char played an important role on its catalytic activity. The higher the specific surface area, the better the activity of char for removing tar. The spent char showed a much reduced specific surface area of micropores but evidently elevated the specific surface area of mesopores, even by up to 3 times. The tests with the metal oxides impregnated onto the demineralized char demonstrated that both Ca and Fe oxides enabled better catalytic activity for tar removal than Na and Mg oxides. This study clarified as well that the viable operating conditions for tar removal in a char bed were at 1100 degrees C with an excessive air ratio (ER) of 0.04 and gas residence time above 1.3 s.

Nascent Biomass Tar Evolution Properties under Homogeneous/Heterogeneous Decomposition Conditions in a Two-Stage Reactor

In order to study the mechanism of biomass tar formation and elimination in a two-stage downdraft gasifier, the nascent rice straw pyrolysis tar evolution properties under homogeneous/heterogeneous decomposition conditions have been investigated in a constructed lab-scale two-stage reactor by varying factors as temperature, concentration and reforming agents of CO2/H2O/O2, and char bed heights. The nascent tar was produced in the first stage reactor and then decomposed in the second stage with different reforming agents or char beds. In the first stage, the results showed that nascent pyrolysis tar yields increased with increasing pyrolysis temperature, tar was mainly produced during 200–400 °C, and 400–500 °C would be a proper pyrolysis temperature range in commercial operation due to little effect on tar yields in higher temperature. In the second stage, it can be observed that nascent biomass tar was converted into polycyclic aromatic hydrocarbons (PAHs) (even soot), thermally stable one ring aromatics, and noncondensable gases in homogeneous conditions with increasing temperature. Different effects were obtained in varying tar species under different homogeneous reforming agents. However, benzene, toluene, styrene, phenol, and naphthalene are the most typical compounds, accounting for 50–75% in total tar concentration at 900 °C in all decomposition conditions. Char bed can selectively reduce PAH species remarkably and increase the toluene yields. As for the three reforming agents, steam showed the highest efficiency in tar elimination, while CO2 and O2 present will induce OH, H, and O radicals formation, which increases hydrocarbon conversion. The mechanism of tar destruction in a two-stage downdraft gasifier can be concluded as follows: nascent tar yields from the pyrolysis stage will be first reformed into PAHs, thermally stable one ring aromatics and noncondensable gases in the throat region, and then PAHs species are almost completely decomposed by the char bed, which are the main troublesome tar components in syngas, and finally the syngas with low tar was obtained.

Biomass waste gasification – Can be the two stage process suitable for tar reduction and power generation?

A pilot scale gasification unit with novel co-current, updraft arrangement in the first stage and counter-current downdraft in the second stage was developed and exploited for studying effects of two stage gasification in comparison with one stage gasification of biomass (wood pellets) on fuel gas composition and attainable gas purity. Significant producer gas parameters (gas composition, heating value, content of tar compounds, content of inorganic gas impurities) were compared for the two stage and the one stage method of the gasification arrangement with only the upward moving bed (co-current updraft). The main novel features of the gasifier conception include grate-less reactor, upward moving bed of biomass particles (e.g. pellets) by means of a screw elevator with changeable rotational speed and gradual expanding diameter of the cylindrical reactor in the part above the upper end of the screw. The gasifier concept and arrangement are considered convenient for thermal power range 100–350kWth. The second stage of the gasifier served mainly for tar compounds destruction/reforming by increased temperature (around 950°C) and for gasification reaction of the fuel gas with char. The second stage used additional combustion of the fuel gas by preheated secondary air for attaining higher temperature and faster gasification of the remaining char from the first stage. The measurements of gas composition and tar compound contents confirmed superiority of the two stage gasification system, drastic decrease of aromatic compounds with two and higher number of benzene rings by 1–2 orders. On the other hand the two stage gasification (with overall ER=0.71) led to substantial reduction of gas heating value (LHV=3.15MJ/Nm3), elevation of gas volume and increase of nitrogen content in fuel gas. The increased temperature (>950°C) at the entrance to the char bed caused also substantial decrease of ammonia content in fuel gas. The char with higher content of ash leaving the second stage presented only few mass% of the inlet biomass stream.

Mathematical Model Development of Raceway Parameters and Their Effects on Corex Process

Corex is an alternative ironmaking process and raceway is one of the important areas to maintain the stability of the furnace. The raceway parameters are well established for blast furnace operation. But for Corex process, it has not yet been established and optimized. Thus, a mathematical model was developed to determine various raceway parameters such as RAFT (raceway adiabatic flame temperature), tuyere gas velocity and kinetic energy. The model provides an idea about the raceway geometry, zone temperature and kinetic energy accumulated in tuyere gas. Besides, all the raceway parameters have been analyzed to find out their effects on the Corex process. It is found that RAFT influences the gasification reaction kinetics and higher RAFT generates more CO in reduction gas, which improves the metallisation degree of the DRI in shaft. It is also found that increased gas velocity and kinetic energy generate more fines and demand more coke to maintain char bed permeability. High coke rate increases the production cost and lowers the production of hot metal.

Experimental investigation on the changes in bed properties of a downdraft biomass gasifier

Biomass gasifiers of fixed bed, downdraft type are generally used for driving internal combustion engines. As part of research work, a versatile, throat type, biomass gasifier was developed. The gasifier had facilities for bed temperature measurements, pressure measurements, physical observation, sampling of bed particles, bed agitation, etc. The generated producer gas was cleaned and cooled in downstream components namely, cyclone separator, dust filter and gas cooler such that water had no direct contact with producer gas. The dry cleaned producer gas was flared directly at the exit pipe. Bed temperatures and bed char properties viz., weight, volume, density and volatile matter content along the gasifier axis, wood feeding rate, gasification air and producer gas flow rates, etc. were observed and analyzed during the gasification of wood pieces. Temperature stratification was found to depend on biomass bed height, phase of operation, location of reaction zones and reactor pressure. Specific gasification rate for wood pieces was found to be 75 kg h-1 m-2 which was lesser than that for loose bioresidues. Mass balance was also done for the gasification system. The producer gas to wood ratio was determined to be 2.39 at an optimum equivalence ratio of 0.3. The volume of wood piece or char decreased by as high as 86% even before it enters the reduction zone of the gasifier. International Journal of Engineering, Science and Technology, Vol. 2, No. 6, 2010, pp. 98-106

Decomposition and gasification of pyrolysis volatiles from pine wood through a bed of hot char

Pine wood was pyrolyzed in a fixed bed reactor at a heating rate of 10°C and a final temperature of 700°C, and the resultant volatiles were allowed to be secondarily cracked through a tubular reactor in a temperature range of 500–700°C with and without packing a bed of char. The thermal effect and the catalytic effect of char on the cracking of tar were investigated. An attempt was made to deconvolute the intermingled contributions of the char-catalyzed tar cracking and the char gasification to the yields of gaseous and liquid products. It was found that the wood char (charcoal) was catalytically active for the tar cracking at 500–600°C, while at 650–700°C, the thermal effect became a dominant mode of the tar cracking. Above 600°C, the autogenerated steam gasified the charcoal, resulting in a marked increase in the yield of gaseous product and a significant change in the gas composition. An anthracite char (A-char), a bituminous coal char (B-char), a lignite char (L-char) and graphite also behaved with catalytic activities towards the tar cracking at lower temperature, but only L-char showed reactivity for gasification at higher temperature.

Design of a Lab-Scale Two-Stage Rice Husk Gasifier

Abstract Rice is a major product in Thailand and a large quantity of rice husk are generated annually in the rice milling industries, which can be used for energy exploitation. A commercial 400 MWe, two-stage gasifier has been constructed in Lopburi province using rice husk as the fuel. It is necessary to perform a further development and study of the optimization conditions to achieve a desired operating of the plant. A design procedure was developed for a 50 kWth lab-scale gasifier, and the calculations were made based on the essential parameters such as volumetric flow rate, minimum fluidized velocity, residence time to find out the dimension parameters of the reactor. On the basis of the air supplied rate, the dimensions of the first stage was calculated to provide a sustaining condition of the fluidization. The calculated diameters and heights of the lower part and upper part of the first stage were found to be 0.09, 0.13, 0.50, and 0.50 m, respectively. The height of gasification and char reduction chamber in the second stage was estimated to be 1.40 m and the calculated throat diameter was 0.10 m, whereas, the height of the char bed was variable. This design can facilitate varying parameters such as equivalence ratio, superficial velocity, and temperature. The optimization design is obtained from the experiments and from the theoretical and experimental information available in the literature.

Influence of Auxiliary Fuel Injection to Tuyeres on Flame Temperature and Flooding Limits in the FINEX® Process: A Theoretical Investigation

The FINEX® melter gasifier, in contrast to the blast furnace, uses oxygen instead of air for the blast, and the packed bed consists of coal char but not of coke. In this paper, the theoretical raceway flame temperature for FINEX® conditions has been calculated for different injection rates of auxiliary fuels and steam. A regression equation has been derived from these results. Moreover, a flooding limit has been taken into account when auxiliary fuels were injected in order to understand the practical operational troubles such as peaks of reducing gas flow and temperature. The theoretical investigations show, that the available heat level in the raceway has been sufficient enough to inject the auxiliary fuels, such as pulverized coal (PC) and natural gas (NG) at the same rate and even higher than regarding the blast furnace. The permeability of the FINEX® char bed is lower in comparison to the coke bed in the blast furnace. High injection rates of auxiliary fuels require enhanced char stability and bed voidage.

High-density circulating fluidized bed gasifier for advanced IGCC/IGFC—Advantages and challenges

Coal-fired Integrated Gasification Combined Cycle (IGCC) and Integrated coal Gasification Fuel-cell Combined cycle (IGFC) are being developed as high-efficiency electric power generation technology. However, the highest theoretical gross thermal efficiency of the conventional IGCC/IGFC is still below 52%. In order to obtain higher power generation efficiency, an advanced IGCC (A-IGCC) or advanced IGFC (A-IGFC) system making use of the exergy recuperation concept by recycling waste heat from gas turbine or fuel cells for steam gasification of coal and biomass was proposed in our laboratory. Corresponding to this system, a novel high-density triple-bed combined circulating fluidized bed (TBCFB) gasifier, composed of a downer pyrolyzer, a bubbling fluidized bed char gasifier, and a riser combustor, was proposed to replace traditional gasifiers such as the entrained flow bed gasifier. The new system is expected to more effectively utilize the waste heat from gas turbines or fuel cells and the heat produced by the combustion of the unreacted char in the riser combustor for pyrolysis and gasification of coal and biomass. In this short review, the advantages and future challenges in the development of high-density TBCFB gasifier are presented and discussed.

Downdraft gasification of pellets made of wood, palm-oil residues respective bagasse: Experimental study

The downdraft gasification technology has an increased interest among researchers worldwide due to the possibility to produce mechanical and electrical power from biomass in small-scale to an affordable price. The research is generally focused on improvement of the performance and optimizing of a certain gasifier, on testing different fuels, on increasing the user-friendliness of the gasifier and on finding other uses for the product gas than in an IC-engine, for example liquid fuel production. The main objective with the gasification tests presented here is to further contribute in the field by studying the impact of the char bed properties such as char bed porosity and pressure drop on the gasification performance as well as the impact of fuel particle size and composition on the gasification process in one and the same gasifier. In addition, there is very little gasification data available in literature of “before disregarded” fuels such as sugar cane bagasse from sugar/alcohol production and empty fruit bunch (EFB) from the palm-oil production. By pelletizing these residues, it is possible to introduce them into downdraft gasification technology which has been done in this study. The results show that one and the same reactor can be used for a variety of fuels in pellet form, but at varying air–fuel ratios, temperature levels, gas compositions and lower heating values. Gasification of wood pellets results in a richer producer gas while EFB pellets give a poorer one with higher contents of non-combustible compounds. In this gasification study, there is almost linear relation between the air–fuel ratio and the cold-gas efficiency for the studied fuels: Higher air–fuel ratios result in better efficiency. The pressure drop in the char bed is higher for more reactive fuels, which in turn is caused by low porosity char beds.

Tar Formation and Destruction in a Fixed-Bed Reactor Simulating Downdraft Gasification: Equipment Development and Characterization of Tar-Cracking Products

The aim of the present study is to examine operating parameters that would reduce the residual tar content in the fuel gas in downdraft gasifiers and eventually eliminate it altogether. A two-stage fixed-bed reactor has been employed to simulate elements of tar cracking in a downdraft gasifier. In this reactor, tar is generated by pyrolysis in the first stage and cracking and gasification take place in the second stage. Modifications to a previous configuration of this reactor are described, which have enabled the use of smaller char particle sizes in the second stage and the generation of a more complete inventory of the reaction products. In this work, the effect of the temperature and the presence of char on product distributions are reported. Increasing the temperature from 700 to 1000 °C resulted in a decrease in the quantity of tar recovered and an increase in the total amount of CO released. The amount of CH4 released increased between 700 and 800 °C before remaining steady up to 1000 °C. The CO2 content of the gas was relatively constant between 700 and 800 °C and increased as the temperature increased from 800 to 1000 °C. The amount of water and light hydrocarbons (C2−C5 alkanes and alkenes) sharply decreased at 1000 °C. The presence of char in the second stage had significant effects on tar cracking and product distributions. These effects were more obvious on the concentrations of CO, CO2, and H2O, which may be a result of reduction reactions taking place with the carbon in the packed char bed. These reactions appear to be more significant at temperatures between 900 and 1000 °C, where the rates of gasification are expected to increase.

Energy and Exergy Analysis of the Teduction Zone in a Downdraft (Biomass) Gasifier

This article describes the energy and exergy analysis of the reduction zone in a downdraft biomass gasifier. A simplistic formulation for describing the pyrolysis and oxidation of these products has been presented for initialization. Equilibrium and kinetic models are used to predict the reduction products leaving the reduction zone and thus the 1st law efficiency. In the reduction zone, exergy destruction due to chemical, physical, compositional, internal heat transfer and heat loss to the surrounding has been quantified to describe 2nd law efficiency. The comparison of equilibrium and kinetic models is carried out with experimental data for general validity. Parametric analysis of char bed length and inflow temperature on gas composition, un-converted char, exergy destruction, 1st law and the 2nd law efficiency has also been carried out. Simulation results identified a critical char bed length (where all char gets consumed) for a given feedstock, which depends on residence time and reaction temperature in the reduction zone. Near critical char bed length, predictions show high calorific value of gas with relatively less exergy destruction and thus optimum reactor performance. The accuracy of the prediction depends on the validity of initial input conditions.

Influence of Coal Size on the Performance of Corex Process

The size distribution of coal used in the Corex process is of prime importance. It controls the permeability and stability of the char bed and thus the performance of the furnace. The particle size of the coal is monitored by its mean particle size (MPS) and the fraction of -6.3 mm in coal charge. A lower fraction of -6.3 mm in coal and higher coal MPS are desirable for improved performance. The variation in MPS and fraction -6.3mm of coal influences the fuel rates making the furnace unstable and thereby decreases the production. Theoretical calculations have been attempted using the actual plant data to know the impact of coal size on the permeability of the bed in the melter gasifier (MG). Statistical analysis was employed to investigate the influence of coal size on the performance of the Corex process. Attempts were made to establish the relationship between the coal size charged and performance indicators of the Corex process. It was found that a stable and improved performance of the Corex process requires MPS of coal in the range of 19-22 mm and fraction -6.3 mm in coal less than 15 %. A regression equation for the fuel rate including coal size has been derived. The present paper discusses the influence of coal fines and MPS on the performance of the Corex process.

A new experimental Continuous Fixed Bed Reactor to characterise wood char gasification

Two-stage fixed bed gasification is one of the most promising technologies for low and medium energy production from biomass. In industrial processes, control and optimisation is often based on constructor know-how rather than on an understanding of the mechanisms involved. We present a new original tool, the Continuous Fixed Bed Reactor (CFiBR), which was specifically designed and built to enable a fine understanding of the limiting stage of a gasifier: the char bed gasification zone. The reactor, the instrumentation, the operating procedure and set-up tests are described in detail. The potential of the reactor is demonstrated through the characterisation of the gasification of a continuous wood char bed. Temperature profiles and gas concentrations along the 65 cm bed were established and showed that the most reactive zone was the first 10 cm of the char bed. Accurate energy and mass balances provided relevant information regarding the contributions of the main reactions involved in the fixed char bed gasification process.

CFD-Based Modeling of Kraft Char Beds • Part 2: A Study on the Effects of Droplet Size and Bed Shape on Bed Processes

This is the second of two papers. The first paper presented the char bed burning model used in this work. In this paper, the effects of liquor droplet size and bed shape on kraft char bed processes and lower furnace combustion are studied using a computational fluid dynamics (CFD)-based furnace model. The simulations show that droplet size affects not only the amount of in-flight conversion and the amount of combustible material reaching the bed, but also to what degree char carbon conversion in the char bed is due to direct oxidation or gasification. Droplet size also affects char bed burning indirectly through bed shape. Char bed shape affects the degree to which combustion air jets impinge on the bed surface, increasing carbon conversion by direct oxidation. In addition, changes in bed shape over time affect char bed processes. The results suggest that when a char bed starts to grow, the local carbon input to the bed could increase with bed height. In addition, carbon conversion decreases in locations of air jet impingement if the bed shape is not maintained.

CFD-Based Modeling of Kraft Char Beds • Part 1: Char Bed Burning Model

Understanding the physical and chemical phenomena governing char bed burning is important for stable and efficient operation of the recovery boiler. Stand-alone char bed models have been developed and evaluated to increase knowledge of the complex char bed conversion processes. Available computational fluid dynamics (CFD)-based recovery furnace models previously have been applied to evaluate lower and upper furnace processes rather than to investigate the behavior of the char bed. In this first of two papers, a stand-alone char bed model for predicting the chemical processes of char bed burning was modified and implemented into a CFD-based furnace model. The char bed model, which is fully coupled with the gas phase and a simplified black liquor droplet model, solves equations describing the mass and energy balance on the surface of the char bed and includes carbon conversion via direct oxidation, gasification reactions, and sulfate reduction. The model was tested by simulating a 3150 tons dry solids/day recovery boiler. Simulations for two different primary air distributions and using two sulfate reduction degrees in droplets arriving to the bed were performed to evaluate the char bed model. The effects on bed conversion processes were clearly revealed by the new char bed model. The model gives a reasonable description of the chemical processes occurring on the char bed in steady state. In the second part of this work, the model is applied to study the effects of droplet size and bed shape.

Wood Combustion in an Overfeed Packed Bed, Including Detailed Measurements within the Bed

Measurements of local gas and tar concentrations, temperatures, and properties of samples of bed solids are reported for wood combustion in an overfeed packed bed. A novel probe is introduced for the measurement of tar concentrations in situ, which collects tar samples on small filter elements. It is shown to give credible values of local tar concentrations at different locations in the bed. Particle sizes and properties are measured by analysis of samples from the bed materials taken after shut-down. A previously developed detailed numerical model of packed bed char combustion is extended by the addition of a one-step pyrolysis reaction and compared with experimental measurements. The model is shown to give good agreement with measured quantities using physically reasonable values of the pyrolysis reaction rate parameters.