Char Bed Research Articles

Progressive inhibition of [formula omitted] reaction in wood char packed bed by in-situ evolving [formula omitted]: Experimental insights and theoretical analysis

Previous research on Char reactions with gas phase compounds under micro-thermogravimetry systems shows that hydrogen inhibits heterogeneous char reactions. However, its impact on larger gasification systems with evolving hydrogen profiles remains largely unexplored. This study examines a macro-scale wood char bed to understand the influence of in situ evolving hydrogen on char reactions. When subjected to a specific steam flux, carbon conversion and pore morphology changes are mainly confined to the bed’s upstream, with the downstream char retaining its original characteristics. Numerical investigations reveal over 75 % of species production and consumption occurs within the initial 20 % of bed height. Fourier-transform infrared spectroscopy confirms hydrogen-induced inhibition in downstream segments, showing a shift from C–OH to C–H bonds. Particle-scale analysis indicates significantly higher rates of hydrogen diffusion and adsorption compared to H2O, impeding downstream C+H2O reactions. Increased temperature, higher reactant concentrations, or reduced residence time can overcome this inhibition, enhancing conversion rates. These findings are critical for optimizing steam-to-biomass ratios in oxy-steam gasification systems for generating hydrogen-rich syngas.

Conversion in a char bed reactor of tars and syngas from a wood gasifier

Cleaning syngas, particularly reforming tars, is still a major bottleneck to the development of gasification. In this work, syngas was simultaneously sampled and analysed downstream and upstream of a tar conversion reactor using a real syngas produced with a commercial gasifier.At 800 °C and 2 s residence time, tar conversion reached 51a% compared with only 11 % in an inert bed. The tar content in the syngas decreased from 8.1 to 4.2 g/Nm3. Almost all the tertiary tars were converted, and only benzene, toluene, and naphthalene persisted. The energy content of the syngas increased by 5 % thanks to simultaneously increase of the LHV and syngas flowrate. LHV's increase in mainly due to H2 production.

Evaluating the effect of recovery boiler operation of green liquor dregs concentration using multivariate analysis

Poor settling and filterability of green liquor dregs has been a persistent problem in many kraft pulp mills. While the concentration and settling/filtering behaviors of dregs are expected to be related to how black liquor is burned in recovery boilers, the effect of boiler operation is not well understood. A systematic study was conducted to examine how recovery boiler operation may affect the dregs concentration in the raw green liquor (RGL) at three kraft pulp mills using SIMCA, a multivariate data analysis (MVDA) program. Daily average boiler operating data from three kraft mills were analyzed over a 3-year period. Results of both principal component analysis (PCA) and partial least squares regression (PLS) suggest that the main boiler operations contributing to high dregs concentrations in RGL are low liquor firing load, low bed temperature, poor char burning, and unstable char bed.

Steam reforming of tar using biomass gasification char in a Pilot-scale gasifier

Gasification char is a residual material produced during the biomass gasification process. Considered as industrial waste, it is typically disposed of through incineration or landfilling, thereby incurring significant economic costs. Nonetheless, gasification char is characterized by its high carbon content and surface area making it an economical alternative to common catalysts and catalyst support materials. In this study, char from a pilot-scale downdraft gasifier was used to reform the tar generated from the same gasifier. Reforming was performed both in the presence and absence of steam. The aim is to convert condensable hydrocarbon derivatives (tars) into non-condensable lower molecular weight products such as H2 and CO. Reforming test conducted with 0.18 kg/h steam for 2 h at 750 °C and char bed weighing 600 g resulted in a reduction of tar concentration from 2407 mg/Nm3 to 20 mg/Nm3. The same test conditions were also responsible for an increase in H2 production from 15 to 26 vol%. The combined effect of steam and char bed suggests that both upgrading the producer gas as well as cleaning it can be made possible in a single process.

Evaluation of the suitability of integrated bone char- and biochar-treated groundwater for drinking using single-factor, Nemerow, and heavy metal pollution indexes.

The treatment of contaminated groundwater using integrated bone char and biochar beds has been studied. The bone char and biochar were made in a locally built double-barrel retort utilising cow bones, coconut husks, bamboo, neem trees, and palm kernel shells at 450°C and were graded into 0.05- and 0.315-mm sizes. Eight groundwater treatment experiments (BF2-BF9) were performed in columns with bed heights of 8.5-16.5cm to remove nutrients, heavy metals, microorganisms, and interfering ions from groundwater using bone char, biochar, and a combination of bone and biochar. The water samples were analysed for twenty-one water quality parameters including pH, total dissolved solids, conductivity, turbidity, fluoride, chloride, sodium, and potassium. The rest were total coliforms, faecal coliforms, total heterotrophic bacteria, Escherichia coli, manganese, and total iron. The effectiveness of the treatment processes was assessed using the Ghana standard authority and the World Health Organisation's recommended values for drinking water quality. The results were shared using a simplified single-factor index, Nemerow's pollution index, and a heavy metal pollution index with decision-makers as a technology for groundwater treatment in rural communities in Africa. Bone char was more effective in removing total heterotrophic bacteria than any of the other treatment agents tested. This is because of its compact nature and small particle size. The quality of water treated by BF3, BF5, BF6, BF7, BF8, and BF9 was fit for drinking based on the single-factor and heavy-metal pollution evaluation because they have the lowest level of pollution. However, Nemerow pollution analysis found only BF5 to be the most suitable for public use.

Experimental and artificial intelligence study on catalytic reforming of tar over bio-char surface coupled with hydrogen production

Tar problem is an obstacle in biomass gasification. Naphthalene was used as tar model compound to investigate the tar catalytic removal over char bed coupled with hydrogen production. The influence of char properties, residence time and atmosphere on tar reduction was taken into consideration. Results show pinewood char acted as quite good performance for catalytic removal of naphthalene. With the increasing of duration time, deactivation would occur, which brought down the tar conversion efficiency. The addition of H2O could inhibit the carbon deposition and promote hydrogen yield via in-situ gasification. At 800 °C, the naphthalene conversion rate slightly declined to 95.77% even at 182 min under 10% steam atmosphere. The H2 yield was around 8.98 mol/(mole of naphthalene) at initial 12 min. The pinewood char pore analysis results confirmed the inhibition of carbon deposition by steam. Artificial neural network (ANN) models coupled with genetic algorithm (GA) and particle swarm optimization (PSO) were built for prediction of naphthalene conversion and hydrogen yield. The BET surface area, potassium content, penetration time, duration time, temperature and atmosphere were used as input variables. Modeling results show that the PSO-ANN model and relative impact analysis could be effectively used for modeling and analysis of tar catalytic conversion over char bed.

A study on pyrolysis of wood of different sizes at various temperatures and pressures

In this work, three different particle sizes (0.5 ∼ 1 mm, 2 mm and 5 mm) of pine wood were pyrolyzed at the temperature range between 350 and 800 °C and pressures between 1 and 8 bar in a pressurised fixed bed reactor. A monolayer of different particle sizes of biomass samples was also pyrolysed at various pressures to assess by comparison the effect of intra-particle vs inter-particle secondary reactions on char formation. The relatively small particle and reactor size as well as the slow heating, enable assessing the effects of particle size and pressure in the absence of heat transfer limitations. The liquid and solid products were characterized by various techniques, including gel permeation chromatography (GPC), gas chromatography with mass spectrometry detection (GC–MS), elemental analyser and Raman spectroscopy. The combustion reactivity of chars obtained at various conditions was tested by thermogravimetric analysis (TGA). Higher temperatures promote tar and gas yields at the expense of solid residues. The chars produced at higher temperatures show greater aromaticity and lower combustion reactivity. The larger particle size can decrease tar formation at 350 °C, while smaller particle sizes decrease tar yield through the inter-particle secondary reaction within the hot char bed at 500 °C and above. For larger particle sizes, a stronger intra-particle secondary process can make the chars more ordered, thus leading to a lower combustion reactivity. Higher pressure can hinder the vaporization of fragments and promote the formation of secondary chars, thus less tar and more char were obtained. Chars produced at higher pressures show a lower combustion reactivity due to the higher aromaticity of secondary chars. Therefore, higher temperature, larger particle size and higher pressure can all produce more ordered chars with lower combustion reactivity.

DFT study on the chemisorption and reforming of naphthalene over bio-char: The detailed mechanism of carbon deposition and hydrogen production

Tar removal over char bed is a potential method for syngas cleaning. The heterogeneous interaction mechanism between char surface and tar molecule is crucial and still not fully revealed. In this work, the detailed interaction between bio-char surface and naphthalene was studied by experiment and density functional theory (DFT) at molecular level. Three bio-char models and corresponding reaction paths were proposed. The geometric optimization and frequency calculation of each species were conducted. Mulliken charges and electrostatic potential results of char models indicate that oxygenated functional groups could weaken the electron cloud density of adjacent carbon atoms and make the carbon atoms positively charged, which appears as a “Lewis acid center” and benefit the adsorption of tar molecule. The orbital electron results show that the band gaps of three char models are 1.2965 eV, 2.7903 eV and 3.4366 eV, respectively, indicating the unpaired electrons on char surface will enhance the reactivity of char compared with oxygenated functional groups. The potential energy evolution shows that the initial adsorption of naphthalene on char surface is an exothermic reaction. The activation energies for naphthalene decomposition over char surface are 207.83 kJ/mole, 164.01 kJ/mole, and 159.17 kJ/mole, which are much lower than that of thermal decomposition (446 kJ/mole). The chemisorption of naphthalene on char surface changes the electron density distribution, which greatly enhance tar decomposition. Experimental results proved that the oxidative pyrolysis char is rich in micropores and oxygenated functional groups, which largely enhanced the naphthalene reduction efficiency over bio-char surface.

Fluidized Bed Pyrolysis of Biomass: A Model-Based Assessment of the Relevance of Heterogeneous Secondary Reactions and Char Loading

A model of non-catalytic fast fluidized bed pyrolysis of biomass has been developed with the aim of investigating the relevance of secondary heterogeneous reactions between primary products of biomass decomposition and the char accumulated in the bed. The fate of biomass and the extent of biomass and char holdup in the pyrolyzer have been modeled by considering entrainment and elutriation of biomass and char particles, char attrition, as well as bed drain/regeneration. The kinetics of primary and secondary pyrolitic reactions is modeled according to a semi-lumped reaction network using kinetic parameters selected among published correlations. The rate of heterogeneous volatile–char secondary reactions has been modeled borrowing a kinetic expression from the neighboring area of tar adsorption/decomposition over char during biomass gasification. Model computations are helpful to assess the role of heterogeneous vapor–char secondary reactions. The sensitivity of the pyrolyzer performance on char loading in the bed as a result of combined char entrainment, elutriation, attrition, and bed drain/regeneration, the extent of gas-phase backmixing, the process temperature, the gas superficial velocity and residence time, and the particle size is demonstrated and discussed. Model results provide useful guidelines and pinpoint future research priorities for optimal design and control of fluidized bed pyrolyzers.

The catalytic effect of inner minerals on the combustion of oil shale char: Characteristics and mechanism

To determine the effects of minerals inside char particles and bed materials, the process and mechanism of char combustion in a fluidized bed were studied using a Micro Fluidized Bed Reactor Analyzer. A demineralization procedure using HCl and HF was employed to remove the minerals from pyrolyzed char samples. Raman spectroscopy was used to study the carbon structure of the remaining solid char after demineralization at 500 °C, 600 °C, 700 °C and 800 °C. From the regular changes in band area ratio and full width at half maximum, it can be concluded that as pyrolysis temperature increases, the carbon structure becomes highly ordered, the plane lattice size gradually increases, and the reaction activity decreases from 0.245 s−1 to 0.138 s−1.Furthermore, minerals both within the char particle and in the external bed had marked catalytic activity on char combustion, and their combined catalysis was most notable. CaO and Fe2O3 were the dominant active minerals components contributing to catalytic combustion of char in this study. The catalytic action of CaO was stronger than that of Fe2O3. The activation energy of char combustion ranged from 57.70 kJ/mol to 91.53 kJ/mol, and it significantly decreased in the presence of the bed materials.

Pyrolysis of RDF and Catalytic Decomposition of the Produced Tar in a Char Bed Secondary Reactor as an Efficient Source of Syngas

One of the technical limitations of refuse-derived fuel (RDF) pyrolysis is the high content of tar in its gas products. In order to resolve this problem, a two-stage RDF pyrolysis with a catalyst based on char from RDF pyrolysis is proposed. This paper presents the results of municipal waste pyrolysis beginning in an oven heated to 480 °C and ending with catalytic tar cracking carried out in the temperature range from 800 to 1000 °C. Thermal and catalytic pyrolysis with a char catalyst containing a minimum of 6% Fe resulted in increases in the CO and H2 contents in gas products and decreases in CO2 and CH4. At 1000 °C, the mass ratio of gaseous products to liquids was greater than 6. The residence time of the gases in the catalytic zone was about 3–5 s. The reactor was a good source of hydrogen and carbon monoxide.

Kraft recovery boiler operation with splash plate and/or beer can nozzles — a case study

In this work, we study a boiler experiencing upper furnace plugging and availability issues. To improve the situation and increase boiler availability, the liquor spray system was tuned/modified by testing different combinations of splash plate and beer can nozzles. While beer cans are typically used in smaller furnaces, in this work, we considered a furnace with a large floor area for the study. The tested cases included: 1) all splash plate nozzles (original operation), 2) all beer can nozzles, and 3) splash plate nozzles on front and back wall and beer cans nozzles on side walls. We found that operating according to Case 3 resulted in improved overall boiler operation as compared to the original condition of using splash plates only. Additionally, we carried out computational fluid dynamics (CFD) modeling of the three liquor spray cases to better understand the furnace behavior in detail for the tested cases. Model predictions show details of furnace combustion characteristics such as temperature, turbulence, gas flow pattern, carryover, and char bed behavior. Simulation using only the beer can nozzles resulted in a clear reduction of carryover. However, at the same time, the predicted lower furnace temperatures close to the char bed were in some locations very low, indicating unstable bed burning. Compared to the first two cases, the model predictions using a mixed setup of splash plate and beer can nozzles showed lower carryover, but without the excessive lowering of gas temperatures close to the char bed.

Effect of temperature and reaction atmosphere on nitric oxide emission during a char grate‐fired process in local flue gas recirculation

AbstractLocal flue gas recirculation (LFGR) is an effective technology for reducing nitrogen oxide (NOx) emissions from coal‐fired industrial boilers. The temperature and reaction atmosphere changes when flue gas is recycled, thereby affecting both the grate‐fired process and NOx emission. In this paper, the boundary of LFGR was simulated by changing the experimental parameters. On a small‐scale one‐dimensional fixed‐bed system, the effects of temperature, O2 flux, and CO, CO2, and recycled NO concentrations on oxidation‐reduction layering and char nitrogen conversion during the char grate‐fired process were studied. The effect of temperature and recycled flue gas components on nitric oxide (NO) emissions during the char grate‐fired process was then analyzed based on the mass proportion of oxygen‐absent and oxygen‐present parts. The results show that, with the introduction of recycled flue gas, increasing the temperature will also increase the reduction layer mass and proportion, and, subsequently, inhibit NO emissions; increasing the O2 flux will reduce the reduction layer proportion and subsequently promote NO emissions; increasing the CO and CO2 concentrations will reduce the NO emitted from the oxidation layer, yet has limited effects on the entire char bed; and recycled NO will significantly reduce the NO emissions. The effect of LFGR‐induced changes in temperature and reaction atmosphere on NO emissions can be ascribed to the negative effect of the increase in O2 flux and the positive effect of the increase in temperature and CO, CO2, and recycled NO concentrations.

Effect of Steam on the Tar Reforming during Circulating Fluidized Bed Char Gasification.

In this work, the real tar was introduced into the circulating fluidized bed gasifier by pre-mixing tar and char. The effect of steam on the tar reforming characteristics at both 850 and 900 °C was investigated by combining the analysis of the rate of tar conversion, the change of tar content, and char physical structure. The test results indicated that steam could effectively promote the tar conversion. Therefore, the content of tar in the final gas could be reached as low as 32 mg/Nm3. It was found that the effect of steam on the different components of tar was in difference. Among the various components, polycyclic aromatic substances were more inclined to decompose. The results of BET confirmed that the distribution and structure of pore were obviously developed at the presence of steam, and the abundant pore structure further improved the catalytic performance of the char on the tar conversion in turn.

Simulation of Prosopis juliflora Air Gasification in Multistage Fluidized Process

A multistage atmospheric fluidized bed gasifier was developed using the Aspen Plus simulation process. The innovative gasification reactor aims to yield a high-quality product gas as it conducts pyrolysis, combustion, and reduction in different zones. In addition, it uses gas as a heat carrier and has a fluidized char bed in the reduction zone to enhance the in-situ tar reduction. In order to study the feasibility of the gasifier, an evaluation of the product gas and the process efficiency is required. The proposed model was based on the reaction rates and hydrodynamic parameters of the bubbling bed. Four different stages were initially considered in the simulation process: decomposition of the feed, partial volatile combustion, char reduction, and gas solid separation. The gasification reactor was operated over a temperature range of 800–1000 °C and an isothermal combustion reactor was operated at 1000 °C. In addition, the air to biomass mass ratio was varied from 0.2 to 0.5. It has been validated and displayed very good agreement with published data. Effects of gasification reactor temperature, air to biomass ratio, and gasifier dimensions on the composition of product gas were investigated Results showed that the principal component is CO and its concentration in the product gas increases with increase in gasifier temperature but decreases with increasing air to biomass ratio. The results also gave a relatively high value of the lowered gas caloric value and acceptable cold gas efficiency which help the sizing of gasifiers and the choice of optimal operating conditions.

High-quality tar production from coal in an integrated reactor: Rapid pyrolysis in a drop tube and downstream volatiles upgrading over char in a moving bed

A purposely built integrated reactor, which comprised a drop tube and a moving bed assembly located at the bottom of the drop tube, combined rapid pyrolysis and in situ volatiles upgrading over char for the production of high-quality tar from coal. Rapid pyrolysis took place when coal particles flowed through the hot drop tube, and then the produced volatiles and char were introduced into the moving bed for further pyrolysis and volatiles upgrading. The operation parameters of rapid pyrolysis alone in the drop tube were first investigated to ensure a high yield of the initial tar for further upgrading in the moving bed. The effects of the temperature and char bed thickness on further pyrolysis and volatiles upgrading were then determined to obtain the optimized operating conditions in the moving bed. Under the optimized conditions, the tar yield could be as high as 11.4 wt% and reached 89.8% of the Gray-King assay yield. The light fraction in the tar and light tar yield reached 69.0% and 7.9 wt%, respectively, which was much better than that obtained from the rapid pyrolysis alone in the drop tube. The experimental results could confirm the feasibility of the integrated pyrolysis-upgrading process for the production of high-quality tar from coal.

Residual gasification char applied to tar reforming in a pilot-scale gasifier: Performance and evolution of char properties for perspective cascade uses

The solid residue of gasification, or gasification char, is in many cases considered as a waste material inevitably generated during the operation of gasifiers. However, it is a good candidate to substitute synthetic catalysts for tar reforming and it is a carbon-rich material with a good potential for carbon negative applications as biochar. In this work, a wood-derived gasification char was tested for the upgrading of producer gas generated from pilot-scale gasification of wheat straw. The effect of a char bed in combination with partial oxidation was assessed under different conditions. The presence of the char bed markedly reduced the total tar concentration, but the bed material suffered deactivation. Partial oxidation prolonged the activity of the char bed by promoting reforming reactions at the surface of char particles and further reduced the tar content below 1 g/Nm3. Post-experimental characterization of the spent chars revealed a decreased surface area, a more ordered carbon phase and an increased resistance to thermal degradation. The results show that under optimized conditions, gasification char is a promising substrate for tar reforming and the upgrading of producer gas to syngas. Moreover, the changes in the char suggested possible cascade uses of the spent material for carbon sequestration.

A tandem pyrolysis-upgrading strategy in an integrated reactor to improve the quality of coal tar

Coal pyrolysis and volatiles upgrading were intelligently coupled into an integrated reactor with two sections that enabled tar quality to be improved. The upper drop tube served as the rapid pyrolysis section to maximize the primary yield of tar. Subsequently, the pyrolysis char together with the released volatiles was introduced into the lower moving bed section, wherein the formed char bed was endowed with a catalytic upgrading function by enhancing volatiles-char interactions. This design succeeded in the consecutive pyrolysis-upgrading process by recombining its pyrolysis products. In the first-step rapid pyrolysis, a much higher yield of tar than the Gray-King assay yield was obtained at 600 °C, but the light fraction (boil pointing < 360 °C) in tar was as low as 43.0%, which was identified as poor quality. After the second-step catalytic upgrading at 600 °C, the resultant tar continued to maintain a relatively higher yield, which was 91.2% of that determined by Gay-King assay, despite being cracked over char. Accordingly, the light fraction in tar reached 57.0%, i.e., a 32.6% increase compared with the drop tube pyrolysis, which enabled a higher yield of light tar. This finding could be attributed to the shift of heavy tar into light tar and gas at the expense of the forming coke over char bed. The experimental results demonstrated that the proposed tandem rapid pyrolysis-volatile upgrading strategy had its unique superiorities in terms of guaranteeing both the quantity and quality of coal pyrolysis tar compared with other pyrolysis processes.

Thermodynamic analysis of methanol synthesis combining straw gasification and electrolysis via the low temperature circulating fluid bed gasifier and a char bed gas cleaning unit

The phase-out of fossil fuels in the heavy transportation sector will require energy-dense biofuels like methanol, and will likely require that a wide range of biomasses are utilized. In this framework, gasification of straw and subsequent upgrading to methanol represents a potentially advantageous conversion route. In this study, the established low-temperature circulating fluid bed (LTCFB) gasifier is coupled to a partial oxidation (POX) and char bed reactor, which enables a relatively robust and effective conversion of tars - making the product gas suitable for methanol synthesis. Five scenarios producing methanol via traditional air-separation units and electrolysis were thermodynamically modeled and analyzed in Aspen Plus. The analysis showed state-of-the-art biomass-to-methanol energy efficiencies up to 54–56% and overall carbon conversions above 57%. A parametric analysis on the POX temperature revealed the potential to increase efficiency and the carbon conversion up to 58% and 68%, respectively. The proposed systems outperform alternative systems framed on straw gasification, and exceed in terms of efficiency and overall carbon conversion other solutions based on wood-gasification.

Catalytic conversion of gaseous tars using land, coastal and marine biomass-derived char catalysts in a bench-scale downstream combined fixed bed system

The catalytic activity of biochar for tar removal was evaluated in a bench-scale combined fixed bed reactor by comparison of gaseous tar catalytic cracking behaviors over land (Corn stalks, Cs), coastal (Reed, Re) and marine (Sargassum horneri, Sh) char catalyst. The experiments demonstrated that the tar yield after addition of the biochar was reduced significantly; the tar conversion efficiency reached to 94.6% for catalytic at 850 °C with 50 mm char bed length using Re char. And the yield and composition of gas also changed markedly. The percentage of H2 and CO in the product gas were obviously increased. Sh has a higher H2 content (49.3% of the total gas content), whereas, CO dominated in the gas products for Cs (45.4%) and Re (48.1%). The results from GC–MS analysis illustrated that the increase in temperature promoted the tar cracking and also promotes the polymerization of some tar components.