Tar Removal Research Articles

Sol-gel synthesis and application of NiO, NiTiO3 and Ni5TiO4(BO3)2 on open-cell TiO2 foams for CO2 methanation

Bottleneck of fixed-bed reactors for methanation in a Power-to-Methane (PtM) context is a reliable thermal-management due to the exothermic nature of the reaction under dynamic operation. Novel titania (TiO2) open-cell foam supports with promising heat, mass, and momentum transfer properties and Ni-based catalytic coatings are presented on a technical scale. The coating with promising catalyst candidates NiO, NiTiO3 and Ni5TiO4(BO3)2 is conducted by a scalable sol–gel synthesis. Ni5TiO4(BO3)2 is known as a promising tar removal catalyst, but shows no activity for the methanation reaction. Despite a low BET-surface area, high-loaded (57 wt% Ni) mixed-oxide NiO/NiTiO3 (∼57% NiO/∼43% NiTiO3) shows promising results. Adjusted activation with respect to high-temperature strong metal support interactions (SMSI) leads to a methane-yield close to equilibrium at 400 °C (CO2 conversion: 75%, CH4 selectivity: 99%). The catalyst is stable under realistic methanation conditions for at least 120 h of operation and outperforms an industrial catalyst (20 wt% Ni-Al2O3) above 400 °C. This study reveals the underestimated potential that lies in transition metal-supported foam catalysts for direct methanation and is fundamental for further research regarding temperature control for dynamic PtM processes.

An integrated two-step process of reforming and adsorption using biochar for enhanced tar removal in syngas cleaning

Tar reforming processes conducted at typically high-temperatures for syngas cleaning using biochar in biomass gasification plants, fail to remove the thermally stable smaller aromatic compounds (such as naphthalene, indene, anthracene) thereby retaining the risk of clogging downstream equipment. In this context, integration of an additional step of adsorption at relatively lower temperature, targeting the lighter tar compounds is proposed. An integrated approach for removal of tar generated from mallee wood bio-oil, involving reforming at 750 °C and adsorption at 220 °C was systematicaly evaluated against conventional reforming process. The comparative assessment included tar analysis estimating tar yield and examining tar constituents through UV-fluorescence spectroscopy, biochar structural analysis based on Raman spectroscopy supported by reactivity studies through TGA, and biochar morphological analysis by FESEM imaging and BET analysis. Tar analysis reveals a drastic drop in the two-step process by ∼ 80%: with ∼ 60% tar reduction in reforming and ∼ 40 % of residual tar in adsorption. The removal of lower aromatics was confirmed through UV-fluorescence of residual tars. The Raman spectroscopy shows a growing predominance of higher and lower aromatics in catalyst and adsorbent respectively. These trends were explained based on cross-linking of aromatic compounds through a polymeric condensation in reforming and the penetration of light tars into the micropores in adsorption. The effects of aromaticity are reaffirmed in reactivity studies by TGA. The BET results show a manifold decrease in porosity, implying coverage of the pore surface by the adsorbing species, which is also evident in FESEM images.

Effect of steam on coke deposition during the tar reforming from corn straw pyrolysis over biochar

Coke deposition is a critical issue for catalysts in tar reforming. Steam is conducive to tar removal and hydrogen production in biomass pyrolysis, while its effect on coke formation is not well known. Thus, a two-stage fluidized bed/fixed bed reactor was used to study the effect of steam addition on coke deposition. The primary conclusions are as follows: combining steam and biochar, the tar removal efficiency is more than 90% within 20 min of time on stream (TOS). After the TOS reaches 30 min, the biochar surface is saturated with coke deposition and the specific surface area of the biochar stops decreasing. The addition of steam leads to a reduction in the combustion reactivity of the biochar. Besides, the aromatization of the biochar increase and the number of O-containing structures decreases. For the tar, the addition of steam reduces the aliphatics content and increases the O-containing aromatics content. For the gas composition, the addition of steam results in the decrease of CH4 yield while the H2 yield increases to 0.12 L/g. The H/C atomic ratio of the gas composition decreases while the O/C atomic ratio increases. The active coke is consumed and decomposed after the introduction of steam. The remaining coke is dominated by inert coke. This contributes to the relatively developed pore structure but a weakened combustion reactivity of biochar.

Gasification of woody biomass in a novel indirectly heated bubbling fluidized bed steam reformer

Within this work, a novel 50 kWth indirectly heated bubbling fluidized bed steam reformer (IHBFBSR) is presented, along with its commissioning experiments. In the IHBFBSR, heat is provided through two radiant tube natural gas burners in the bed and the freeboard area. The aim of this innovative design is sufficient heat provision for biomass steam reforming and cracking reactions and heat loss reduction, thus allowing the possibility of scaling-up to an industrial level. Experiments were performed with two woody biomass feedstocks and two bed material particle sizes under different operating conditions (steam to biomass ratio, lambda, temperature), in order to identify the setup's main characteristics. Product gas composition and quality, as well as the cold gas efficiency of the IHBFBSR were in reasonable agreement to similar systems, however carbon conversion prediction needs further improvement. H2 production and tar removal are favoured by small bed material particle sizes as well as by char accumulation in the bed area. Furthermore, air injection above the bed led to improved H2/CO ratios and lower tar yields compared to when air is used as a fluidization agent. Overall, it was shown that the IHBFBSR technology constitutes a promising development in the field of biomass allothermal gasification.

Evaluating the impact of several scrubbing systems on the tar removal efficiency from producer gas

Evaluating the impact of several scrubbing systems on the tar removal efficiency from producer gas

Catalytic reforming of volatiles from pyrolysis of biomass components over a novel Ni/magnesium slag catalyst

A novel magnesium slag (MS) supported nickel-based catalysts (Ni/MS) was prepared and tested in catalytic reforming of volatiles derived from the pyrolysis of biomass components (cellulose, hemicellulose and lignin) and two biomass feedstocks (pine and straw). The physical and chemical properties of catalysts were characterized by XRD, TPR, BET, TEM, TPO-TGA and Raman spectroscopy. Ni/MS catalyst showed a higher tar removal efficiency than conventional Ni/γ-Al2O3 under the same Ni loading. The tar conversion of cellulose hemicellulose and lignin reached 94.8 %, 92.0 % and 89.4 %, respectively over Ni/MS catalyst. In addition, the lignin content of the biomass feedstock appreciably affected the yield and dew point of tar. The highest increment in gas and H2 yield (increased by 39.0 % and 106.4 %, respectively) was observed in cellulose. The highest decrement in tar dew point (from 166.4 ℃ to 53.0 ℃) was observed in hemicellulose. The excellent tar removal of Ni/MS was attributed to the metallic Ni and Ni-Fe alloy on catalyst surface. Besides, the MgO and Fe2O3 phases in the support can retard the carbon deposition during the reaction. The results of this study are expected to provide insightful guidance for the application of Ni/MS catalysts for pyrolysis/gasification of biomass.

Nickel catalyst in coupled plasma-catalytic system for tar removal

Abstract Tar formation is a significant issue during biomass gasification. Catalytic removal of tars with the use of nickel catalyst allows to obtain high conversion rate but coke formation on catalysts surface lead to its deactivation. Toluene decomposition as a tar imitator was studied in gliding discharge plasma-catalytic system with the use of 5%, 10% and 15% by weight Ni and NiO catalyst on Al2O3 (α-Al2O3) and Peshiney (γ-Al2O3) carrier in gas composition similar to the gas after biomass pyrolysis. The optimal concentration of nickel was identified to be 10% by weight on Al2O3. It was stable in all studied initial toluene concentrations, discharge power while C7H8 conversion rate remained high – up to 82%. During the process, nickel catalysts were deactivated by sooth formation on the surface. On catalysts surface, toluene decomposition products were identified including benzyl alcohol and 3-hexen-2-one.

Hydrogen-rich syngas from wet municipal solid waste gasification using Ni/Waste marble powder catalyst promoted by transition metals.

Gasification of wet municipal solid waste (MSW) coupled with in-situ CO2 capture is an attractive option for MSW disposal, allowing chemical and energy recovery. In this study, the Ni-CaO based catalysts were prepared with waste marble powder (WMP) as an alternative to CaO and promoted by different transition metals (i.e., Fe, Cu, Co and Zn). The bimetallic catalysts were prepared by the impregnation method and characterized by different analytical techniques. The catalyst performance for wet MSW gasification was evaluated in a fixed-bed reactor at optimized conditions (850°C and 50% moisture content of MSW). The results revealed that the addition of Ni-WMP catalyst greatly enhanced the dry gas yield (DGY), H2 yield, carbon conversion efficiency (CCE) and reduced the tar content from 0.73 to 1.16N.m3/kg, 212 to 509mL/g, 61.70% to 76.40% and 9.11 to 3.9wt%, respectively, compared to without catalyst. In contrast to the Ni-WMP catalyst, the transition metal promoted catalysts showed higher catalytic activity towards H2 yield (549-629mL/g), DGY (1.19-1.30N.m3/kg), and lower tar content (3.45-2.93wt%). The results revealed that Co promoted bimetallic catalyst performed better than Fe, Cu and Zn promoted catalysts. The tar content produced was also analyzed via GC-MS (gas chromatography-mass spectrometry) to understand the effect of different catalysts on tar composition. According to experimental results, the bimetallic promoted catalysts can be ranked as Ni-Co-WMP>Ni-Cu-WMP>Ni-Fe-WMP>Ni-Zn-WMP based on H2 yield and tar removal.

Construction of Fe embedded graphene nanoshell/carbon nanofibers catalyst for catalytic cracking of biomass tar: Effect of CO2 etching

Carbon-based metal catalyst has attracted wide attention in the utilization for tar catalytic cracking because of its high hierarchical porous structure and high dispersion of metal particles. In this study, sawdust was used as raw material to prepare Fe embedded graphene nanoshell/carbon nanofibers catalyst for catalytic cracking of biomass tar by impregnation followed by carbothermal reduction in CO2 atmosphere. The properties of the catalysts were evaluated by a variety of characterization methods and based on which the evolutionary mechanism was proposed. CO2 etching improved the porosity (Vtotal = 0.4187 cm3/g and BET surface area = 225.93 m2/g) and increased the content of Fe0. During CO2 etching and catalysis of FeCl3, the prepared catalyst formed a structure in which a large amount of carbon nanofibers/graphene shells wrapped metallic iron deposited on the porous carbon support. The stacking of carbon nanofibers enhanced the adsorption capacity of the catalyst to tar macromolecules. The coating of the graphene shell to the metal nanoparticles prevented the agglomeration and sintering of metal active sites, which increased the stability of the catalyst. The results indicated that the Fe@C800C1 catalyst showed good catalytic performance at 700 °C, and the tar conversion efficiency reached up to 90.4%. After 4 cycles, the tar removal efficiency still maintained at 90.0%.

Catalytic conversion characteristics of tar at different temperatures during circulating fluidized bed gasification

Char catalytic conversion of tar at different temperatures was carried out on a laboratory-scale circulating fluidized bed to providing theoretical support for the removal of tar. Various modern detection techniques were used to analyze the gas, tar and char, such as the gas chromatograph, the specific surface area analyzer and the gas chromatograph mass spectroscopy (GC-MS). The efficiency of tar conversion (ETC) could be effectively improved by the increasing temperature, while transformation characteristics of the organic compounds of the used tar also changed. When the temperature increased from 850 °C to 900 °C, the catalytic capability of char was improved, resulting a reducing of the heavy polycyclic aromatic hydrocarbon compounds (heavy PAH compounds) of the produced tar. When the temperature increased from 900 °C to 950 °C, the catalytic capability of char weakened, so the heavy PAH compounds increased, caused by the intensifying of tar thermal cracking. At the tested temperature (850–950 °C), the conversion of naphthalene, the most abundant organic compound in the used tar, has not been seem a significant change, showing a selectivity of the char catalyst.

Performance study on simultaneous tar removal and bio-syngas methanation by combining catalysis with nonthermal plasma

In this work, simultaneous toluene removal and syngas methanation using the combination of packed-bed dielectric barrier discharge and Ni/γ-Al2O3 catalyst were conducted with the research object of simulated gasification gas (SGG) containing toluene. The effects of reaction temperature, H2/CO ratio and H2O addition on the reaction performances of both toluene removal and SGG methanation were investigated. The results show that high-efficiency simultaneous toluene removal and SGG methanation can be achieved at 400°C under plasma catalysis treatment. When the H2/CO is 3.2, the toluene removal rate and the tar removal rate are close to 100% and 97%, the CO conversion rate and CH4 selectivity approach about 88% and 97%, and the energy efficiencies in toluene removal and SGG methanation processes can reach 9.7 g/(kW·h) and 17.3 mol/(kW·h). Both the high H2/CO ratio and the H2O addition can promote toluene removal and SGG methanation, and reduce the amount of carbon deposition but increase its graphitized degree. Moreover, high H2/CO ratio can rise the heating value of SGG and achieve higher energy efficiencies in both toluene removal and SGG methanation processes, while the H2O addition is difficult to obtain high CH4 selectivity to increase the heating value and is not conducive to the promotion of energy efficiency in SGG methanation process. In addition, for these two simultaneous processes, the SGG methanation process exerts a significant inhibiting effect on toluene removal, but process of toluene removal has less influence on the other process due to the lower concentration of toluene.

Replacement of Palm Methyl Ester to Rapeseed Methyl Ester for Tar Removal in the Nong Bua Dual Fluidized Bed Gasification Power Plant

The blockage problem of tar in a biomass power plant is a main problem and it must be removed. Rapeseed methyl ester (RME) was imported and used as scrubbing solvent to scrub tar at a 1 MWel Nong Bua prototype Dual Fluidised Bed (DFB) gasifier in Nong Bua district, Nakhonsawan province, Thailand. Using local oil in Thailand is an attractive choice from economic viewpoint. Pervious lab test study on naphthalene solubility in different local oils in Thailand was investigated. Local palm methyl ester (PME) shows the competitive performance to the RME. In this research, PME was tested to scrub tar in an oil scrubber at the Nong Bua DFB gasifier plant. Gravimetric tar content after passed a PME scrubber was measured and compared to that from a RME oil scrubber. The results show that both solvents have similar tar removal performance due to their contents of ester, methanol, and glycerin are similar. In addition, viscosity of both solvents has no significant effect on tar removal. From the current research, therefore, PME has been used as solvent in an oil scrubber at 1 MWel Nong Bua prototype DFB gasifier with technical and economic reasons.

Solubility of Tar Model Compounds in Various Solvents for Tar Removal in a Dual Fluidized Bed Biomass Gasification Process

Production of high quality product gas via biomass steam gasification is a promising technology. However, impurities in the product gas, namely tars, cause problems in the downstream gas processing operations and thus they need to be removed efficiently. Oil scrubbing is an effective solution for tar removal due to its non-polar characteristic which is similar to tar nature. In this research, solubility values of five simulated tar compounds were experimentally investigated for selecting the new scrubbing solvent. The simulated tar compounds investigated represent those found in the dual fluidized bed steam gasification of wood biomass, which are: naphthalene, biphenyl, anthracene, fluoranthene, and pyrene. The scrubbing solvents tested in this research are classified into biodiesels, vegetable oils, and diesel. Biodiesel used are rapeseed methyl ester (RME) and 2 different palm methyl esters (denoted as PME1 and PME2). Vegetable oils are sunflower oil, refined palm oil, Thai rice bran oil, and crude palm oil. All of the solubility tests were performed in the laboratory-scale test-rig at 30, 50, 70, and 80°C. Biodiesels are found to be the effective solvent in dissolving the tar compounds. PME1 shows the similar tar removal performance to RME but is more readily available; therefore, PME1 is chosen to be used as a scrubbing solvent at the Thailand 1 MWel prototype DFB gasifier at Nong Bua district in Nakhon Sawan province, Thailand.

Mechanism of coke formation and corresponding gas fraction characteristics in biochar-catalyzed tar reforming during Corn Straw Pyrolysis

Formation of coke deposition is the main problem that restricts the development of tar reforming and even biomass thermal conversion. Biochar, prepared from gasification at 800 °C, was used for the catalytic reforming of biomass pyrolysis tar at 650 °C, monitoring the syngas formation during coke deposition. The principal consequences and conclusions are as follows: the coke yield increases from 10.9% to 18.9%, and the tar removal efficiency decreases from 89.0% to 56.7% when the feeding time of biomass is extended (10–50 min). The number of O-containing groups on the biochar surface decreases, and the proportion of minor aromatic rings increases. Biochar is more efficient in the catalytic removal of aliphatic components from tar than aromatic components. Catalytic reforming leads to the development of tar in the direction of increased aromatization. The addition of biochar has essentially no effect on CO yield and CO2 yield, while the CH4 yield decreases and the H2 yield increases. In addition to tar reforming, CH4 cracking is one of the pathways for coke generation on the biochar surface. The number of physical adsorption sites (nN2) and adsorption equilibrium constant (b) of biochar are obtained based on the N2 adsorption test.

Synthesis and characterization of different activated biochar catalysts for removal of biomass pyrolysis tar

The use of low-cost and high-efficiency catalysts is a promising approach to remove tar and increase the gas yield in the catalytic reforming of biomass pyrolysis volatiles. In this study, using rice husk biochar (RHC) as a precursor, four typical activated biochar catalysts were prepared using both chemical (H3PO4 and KOH) and physical (CO2 and H2O) activators. The experiments demonstrated that high surface areas were obtained upon activation. The chemical activators introduced inorganic elements in the biochar, while physical activators were beneficial to the formation of high micropore volume. The effects of varying the catalyst type, catalyst to feedstock ratio, and catalytic cracking temperature on the tar decomposition efficiency and gas yield were studied. The results showed that maximum tar decomposition efficiency (91.75%) and gas yield (704.85 mL/g) were achieved using the RHC-KOH catalyst at a catalytic cracking temperature of 800 °C and a catalyst to feedstock ratio of 2:1. The introduction of the catalysts changed the gas composition, and the yield of H2/CO increased significantly. The increase in catalytic cracking temperature promoted the secondary cracking reaction of tar and the biochar self-gasification reaction in addition to facilitating the cyclopolymerization of some tar components.

A novel low-cost tar removal technology for small-scale biomass gasification to power

Small-scale decentralized biomass gasification could be an effective way to reduce air pollution and improve living quality through electrification in rural areas. Tar removal is a challenge at the small-scale and its high cost limits the application of small-scale gasification-to-power systems. A simple, compact, and low-cost tar removal technology for small-scale application is investigated. Hot raw producer gas is partially oxidized under fuel rich condition in an internal combustion engine. Tars carried by the producer gas are destroyed under high temperature and pressure from compression and partial oxidation. High tar cleanup efficiency with low energy consumption is achieved. Economic analysis of the technology is evaluated on a 20 kWe gasification-to-power system. The capital expenditures (CAPEX) and the operating expenditures (OPEX) reduction are 40% and 17%, respectively. The OPEX can be further reduced. The large cost reduction is mainly attributed to the high tar toleration of the proposed technology.

Preparation and characterization of char supported Ni[sbnd]Cu nanoalloy catalyst for biomass tar cracking together with syngas-rich gas production

In order to realize the efficient biomass tar cracking into syngas-rich gases, several Aspen wood char (ASC) supported metal catalysts, including the Ni/ASC, Cu/ASC and bimetallic Ni-Cu/ASC catalysts, were synthesized with the method of one-step pyrolysis. The structure of the NiCu/ASC catalyst with NiCu nanoalloy encapsulated in carbon nanofibers (CNFs) was observed on the basis of the XRD, SEM and TEM analysis. The catalytic performances of the prepared catalysts on tar cracking were assessed in a two-stage fixed-bed pyrolysis-cracking/reforming device at the catalytic cracking temperatures of 600-800 °C. In a comparison to the single metal catalysts, the bimetallic Ni-Cu/ASC catalyst exerted a better performance on tar removal and possessed a higher selectivity to H2 and CO. The tar conversion efficiency and syngas (H2 + CO) yield, over the NiCu/ASC catalyst at 800 °C, reached up to 93.2% and 581.07 ml/g with the total volumetric content of 72.2% together with H2/CO of 1.1, respectively. The Ni-Cu/ASC catalyst resulted in the lowest contents of two-ring and three-ring aromatics accompanied with highest contents of one-ring aromatics in tar. The Ni-Cu/ASC catalyst was reasonably stable because of its good resistance to sintering and carbon deposition.

Comparatively investigation of real MSW and biomass tar treatment by a rotating gliding arc

Effective tar removal is the bottleneck in gasification application with municipal solid waste (MSW) and biomass as feedstocks. Herein, a rotating gliding arc plasma reactor equipped with a specific designed swirling generator and pre-heating systems is firstly proposed to treat real tar rather than tar surrogates. The composition of MSW and biomass tar is comparatively analyzed. The influence of vapor concentration, CO2 concentration and the discharge current on the destruction efficiency and distribution of gas products are systematically emphasized. The destruction efficiency of major components from both MSW and biomass tar can reach up to over 90%. The order of destruction efficiency of the components tends to be η (Monocyclic species) > η (Bicyclic species) > η (Tricyclic species). Amongst, species with oxygen-containing functional group such as –OH is prone to be destructed compared with those aromatic hydrocarbon species. Similar changing trend of MSW and biomass tar destruction efficiency dependent on operating parameters is detected, with slight differences attributed to the differences of element and group content.

Insights into the mechanism of tar reforming using biochar as a catalyst

Biochar is an efficient catalyst for tar removal from syngas during biomass gasification. The aim of this research is to investigate the mechanism of tar reforming using biochar as a catalyst. A series of in situ steam tar reforming experiments were carried out using a two-stage fluidized-bed/fixed-bed reactor at 800 °C. Mallee wood biochar (106–250 μm) was activated in 15 vol% H2O balanced with Ar for different times (0–50 min) and then used as a catalyst for tar reforming. The on-line gas composition, light tar composition and the pore structure of biochar were analysed using mass spectrometer (MS), GC–MS and synchrotron small angle X-ray scattering (SAXS) respectively. An increased ratio of H2/CO was observed after reforming with biochar compared to reforming without biochar. The destruction of light tar compounds, especially the non-oxygen-containing compounds, was significantly enhanced when activated biochars were used. Steam activation increased the specific surface area (SSA), micro- and mesopore volumes in biochar while the values stayed almost unchanged during tar reforming. Results indicate that the micro- and mesopores in biochar promote the diffusion of both small and large tar molecules into the internal surface of biochar. However, the catalytic activity of biochar for tar reforming mainly depends on the content of O-containing functional groups in biochar. The O-containing functional groups facilitate the dissociation of tar molecules to form tar radicals, giving rise to the enhanced tar removal efficiency. Moreover, the formation of tar radicals over O-containing functional groups appears as the rate-limiting step in the process of catalytic reforming of tar over biochar catalysts.

Catalytic Hot Gas Cleanup of Biomass Gasification Producer Gas via Steam Reforming Using Nickel-Supported Clay Minerals

Amongst the issues associated with the commercialization of biomass gasification, the presence of tars has been one of the most difficult aspects to address. Tars are an impurity generated from the gasifier and upon their condensation cause problems in downstream equipment including plugging, blockages, corrosion, and major catalyst deactivation. These problems lead to losses of efficiency as well as potential maintenance issues resulting from damaged processing units. Therefore, the removal of tars is necessary in order for the effective operation of a biomass gasification facility for the production of high-value fuel gas. The catalytic activity of montmorillonite and montmorillonite-supported nickel as tar removal catalysts will be investigated in this study. Ni-montmorillonite catalyst was prepared, characterized, and tested in a laboratory-scale reactor for its efficiency in reforming tars using naphthalene as a tar model compound. Efficacy of montmorillonite-supported nickel catalyst was tested as a function of nickel content, reaction temperature, steam-to-carbon ratio, and naphthalene loading. The results demonstrate that montmorillonite is catalytically active in removing naphthalene. Ni-montmorillonite had high activity towards naphthalene removal via steam reforming, with removal efficiencies greater than 99%. The activation energy was calculated for Ni-montmorillonite assuming first-order kinetics and was found to be 84.5 kJ/mole in accordance with the literature. Long-term activity tests were also conducted and showed that the catalyst was active with naphthalene removal efficiencies greater than 95% maintained over a 97-h test period. A little loss of activity was observed with a removal decrease from 97% to 95%. To investigate the decrease in catalytic activity, characterization of fresh and used catalyst samples was performed using thermogravimetric analysis, transmission electron microscopy, X-ray diffraction, and surface area analysis. The loss in activity was attributed to a decrease in catalyst surface area caused by nickel sintering and coke formation.