Tar Removal Research Articles

Effects of loading methods and oxidation degree of support on the tar reforming activity of char-supported Ni catalyst using toluene as a model compound

It is necessary to remove the tars present in gasification systems of biomass or low-rank coal to prevent the blockage and corrosion of downstream equipment. In this study, catalysts were prepared using different Ni loading methods: impregnation, pressure impregnation, and ion exchange methods using coal char as a catalyst support for tar removal. The catalyst prepared by ion exchange method showed considerably better performance for the steam reforming of toluene (model tar) compared with the other two catalysts. The oxygen-containing groups of low-rank coal acted as cation-exchange sites, and the oxidation significantly increased the number of oxygen-containing groups. Therefore, the intrinsic correlation between oxidation degree of support and catalytic property was further explored. ICP analysis, XRD spectroscopy, and TEM images confirmed that Ni loading and Ni crystallite size (NCS) significantly depended on the oxidation degree of support. When coal was oxidized with 30% H2O2 and 30% HNO3, the Ni loadings of catalysts were 9.9 wt% and 12.1 wt%, and the NCS were 3.5 nm and 4.5 nm, respectively. Compared to HNO3, the catalyst prepared using H2O2 pretreatment exhibited a higher toluene removal efficiency.

Experimental Study on Product Gas and Tar Removal in Air–Steam Gasification of Corn Straw in a Bench-Scale Internally Circulating Fluidized Bed

Excessive consumption of fossil fuels and its negative global effects have resulted in increasing interest in biomass energy. This paper presents an experimental study on biomass gasification react...

Evaluation of sorbents for high temperature removal of tars, hydrogen sulphide, hydrogen chloride and ammonia from biomass-derived syngas by using Aspen Plus

Biomass gasification is a promising technology to produce secondary fuels or heat and power, offering considerable advantages over fossil fuels. An important aspect in the usage of producer gas is the removal of harmful contaminants from the raw syngas. Thus, the object of this study is the development of a simulation model for a gasifier including gas clean-up, for which a fluidized-bed gasifier for biomass-derived syngas production was considered, based on a quasi-equilibrium approach through Gibbs free energy minimisation, and including an innovative hot gas cleaning, constituted by a combination of catalyst sorbents inside the gasification reactor, catalysts in the freeboard and subsequent sorbent reactors, by using Aspen Plus software. The gas cleaning chain simulates the raw syngas clean-up for several organic and inorganic contaminants, i.e. toluene, benzene, naphthalene, hydrogen sulphide, hydrogen chloride and ammonia. The tar and inorganic contaminants final values achieved are under 1 g/Nm3 and 1 ppm respectively.

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.

Applications of microwave energy in gas production and tar removal during biomass gasification

Microwave heating has the potential to promote gas production and tar removal during biomass pyrolysis or gasification based on its advantageous characteristics such as rapid heating and selective heating.

Application of Dolomite for tar mitigation in downdraft gasifiers: An experimental analysis

The biomass gasifiers have been adopted to generate producer gas widely across the globe. The producer gas finds application in automotive sectors in spark and combustion ignition engines. Furthermore the producer gas is used in power production and thermal applications. Inspite of the wide applications of biomass gasifiers it is not broadly employed due to its inherent disadvantages. The formation of tar along with the producer gas is a major bottleneck in applying producer gas as a viable alternative fuel. The tar mitigation methods viz., physical, thermal and catalytic tar removal methods has been employed to refine the producer gas. From published literature it is evident that many bulk and nano catalysts have been investigated for tar cracking. The dolomite mineral is a potential material for tar cracking in producer gas but has not been investigated widely. In this study the tar cracking capability of dolomite has been investigated in a downdraft gasifier. Two important tar cracking influencing parameters viz., dolomite tar cracking temperature and dolomite quantity required for tar cracking has been investigated. From the experimental study it is eviden that the optimum dolomite operating temperature is 700 °C and the optimum dolomite quantity is 200 g.

Removal of toluene as a biomass tar surrogate by combining catalysis with nonthermal plasma: understanding the processing stability of plasma catalysis

The processing stability of plasma catalysis was understood in terms of tar removal.

Microwave reforming with char-supported Nickel-Cerium catalysts: A potential approach for thorough conversion of biomass tar model compound

Tar abatement and removal is most challenging facing industrialization of biomass gasification technology and its cost-effective removal is indispensably dreamed. Thorough, low-cost and stable removal of toluene, a major compound of biomass tar, was achieved by the microwave catalytic reforming method. A self-designed microwave tube furnace was employed, and toluene was converted with char-supported nickel-iron and nickel-cerium catalysts in an ex situ hot gas conditions. Under the optimal conditions, a 100% toluene conversion efficiency could be achieved, and the hydrogen concentration in the product gas could reach 28.2 vol%. The energy efficiency was 98.97 g/kW h. According to duration tests, the conversion efficiency of nickel-cerium catalysts could still be higher than 90%, even after 8-h successive runs. Comprehensive characterizations of fresh and spent catalysts were conducted, showing agglomeration was the main challenge instead of carbon deposition, which was previously commonly believed. Furthermore, in the pursuit of advancing industrial tar abatement, the depletion and thermal stability of char-supported catalysts were evaluated through a custom-designed gasification-atmosphere-thermogravimetry-mass spectrum analysis. The study provided a breakthrough for the integration of thermal, catalysis and microwave effects for toluene conversion, and the mechanism of char-supported catalysis under specific microwave conditions was also highlighted.

Characteristics of Tar Thermal Cracking and Catalytic Conversion during Circulating Fluidized Bed Char Gasification

Thermal cracking and char catalytic conversion of tar were carried out on a laboratory-scale circulating fluidized bed to provide theoretical support for the removal of tar. The used char and gas were characterized using a portable infrared analyzer, a specific surface area analyzer, proximate analysis, etc. Catalytic conversion of tar was more effective than thermal cracking. It could inhibit carbon deposition and improve the quality of syngas. The effect of temperature and tar amount on the catalytic conversion of tar during circulating fluidized-bed char gasification was investigated. The effect of temperature on the catalytic conversion of tar was remarkable. The efficiency of tar conversion (ETC) increased significantly with increasing temperature. A tendency for catalytic conversion of tar at different temperatures was revealed. As the amount of tar feeding increased, the ETC would decrease because of the carbon deposition. In this process, a maximum increase of low heating value (LHV) was discovered and so there was an optimum amount of tar treatment for a certain amount of char.

VOC degradation by microwave-induced metal discharge and thermal destruction: a comparative study

The effective treatment of volatile organic compounds (VOCs) is essential because of their direct effects on air pollution and human health. This paper introduces microwave-induced metal discharge as a highly effective and byproduct value-added approach to degrade high-concentration toluene as a model VOC. The effect of the factors that influence the discharge intensity, including the metal type (Fe, Cu, Ni, Zn) and amount, was investigated. Degradation efficiency of toluene can reach 79.76% under optimal discharge condition. In addition, the discharge method was compared with traditional thermal destruction at 700 °C, 900 °C and 1100 °C. The gaseous and liquid cracking products of toluene produced by the microwave-induced metal discharge method were almost similar to those obtained under thermal destruction at 900 °C; however, the solid-phase discharge products were nanoparticles that demonstrated good graphitization, while the thermal destruction products were amorphous microparticles. This work offers an effective and flexible way to degrade high-concentration VOCs and also provides an application reference for biomass tar cracking and removal of other organic pollutants.

Experimental study on catalytic pyrolysis of biomass over a Ni/Ca-promoted Fe catalyst

Herein, we evaluated the potential of a Ni/Ca-promoted Fe catalyst for sustainable hydrogen production by tar abatement at 600–750 °C and 0.1 MPa N2. The catalyst was prepared through impregnation into the biomass (wheat straw) structure, and nickel incorporation led to the formation of a nickel iron alloy. We found that the yields of light aromatic hydrocarbons and hydrogen gas could be significantly improved at a given Fe/Ca/Ni ratio, and maximum hydrogen yields of 72.57 mL/g and 91.19 mL/g can be achieved by using a catalyst composed of 5% Fe + 1.5% Ca + 0.8% Ni at 600 °C and 750 °C, respectively. Analysis of the biomass char revealed details regarding the catalytic pyrolysis process. In particular, the dispersion of iron was promoted by calcium and nickel that caused the action between iron and nickel to be strengthened, which were confirmed through the formation of Fe-Ni alloy. Our results suggested that the interactions of the Fe/Ca/Ni catalyst depend on not only the presence of Ca2Fe2O5 but also the stability of the reaction between nickel and iron under the specified reaction conditions. Ultimately, the Fe/Ca/Ni catalyst was found to contain an active phase (Fe-Ni alloy) that played an important role in tar removal during the wheat straw catalytic pyrolysis process.

Lab-scale and pilot-scale two-stage gasification of biomass using active carbon for production of hydrogen-rich and low-tar producer gas

Laboratory- and pilot-scale two-stage air gasification of various types of biomass was performed to produce hydrogen-rich and low-tar gas. Typical tar removal apparatuses, such as scrubbers and electrostatic precipitators, were not employed and the only tar removal agent used was coal-based active carbon. The hydrogen content of all producer gases was remarkably high, whereas the tar content was low (maximum H2: 29 vol%; minimum tar: 0 mg/Nm3). The producer gases from the pilot-scale experiment also exhibited an exceedingly high hydrogen content and low tar content (maximum H2: 27 vol%; minimum tar: 3.3 mg/Nm3). The pilot-scale operation continued without any external heat supply via autothermal operation. A gas engine was connected to the pilot-scale gasification plant to generate electricity. The generated power ranged from 20 to 35 kWe depending on the type of biomass used. This indicated that the two-stage gasification process could be applied to decentralized power generation. To summarize, the present study provided positive experimental results of the two-stage gasification process in terms of tar reduction, hydrogen production, and power generation. Furthermore, studies on long-term operation are needed to obtain more reliable results.

Toluene microwave cracking and reforming over bio-char with in-situ activation and ex-situ impregnation of metal

Removal of biomass tar is of importance for biomass gasification technology, since the existence of tar can cause tremendous problems, such as lowering heat transfer efficiencies and blocking the filters and pipes, etc. A typical one-ring aromatic hydrocarbon (i.e., toluene) is chosen as model compounds to investigate tar cracking and reforming performance with microwave heating, catalyzed by biomass-derived char (Bio-char). The adopted chars consist of original char obtained by microwave or electrical heating, an in-situ activated char as well as metal-impregnated char. For the cracking process, original char achieved with microwave heating exhibits better catalytic activity, compared to that achieved with electrical heating. It is further compared activated char is more active to toluene cracking and an activated char impregnated by Ni shows the most desirable performance to this process. A process of mixed reforming comprised by steam and dry reforming presents an evident superiority in terms of toluene conversion and catalyst stability, which can achieve an average conversion of 95.19% and a minor mass loss of 2.2% occurred in the used char during 120 min. It is calculated energy efficiency of a microwave-assisted combined reforming system at lab-scale is up to 57.8%.

An improved tar–water separation process of low–rank coal conversion wastewater for increasing the tar yield and reducing the oil content in wastewater

Complete removal of tar from wastewater is one of the most challenging problems in the pyrolysis of low–rank coal. In this study, the properties and composition of tar are comprehensively analyzed, and the reasons for the difficulty of oil–water separation are expounded. A separation method using tar fraction as extractant is also developed. Results show that the oil content in the wastewater can be reduced to less than 300 mg·L−1 when the boiling point of the fraction is 120–140 °C and the volume ratio of the extractant to the wastewater system is 1:4 at room temperature without adjusting the pH value of the wastewater. This condition can meet the requirements of follow-up treatment devices and reduce the biological toxicity of wastewater. The revamping scheme and important operation parameters of the distillation unit are optimized through simulation. The proposed method is simple, has low investment cost, and does not require buying chemicals. The method exhibits good industrial feasibility and is expected to solve the problem of oil–water separation in coal pyrolysis.

In-situ removal of toluene as a biomass tar model compound using NiFe2O4 for application in chemical looping gasification oxygen carrier

Efficient removal of tar is a major challenge for biomass gasification. A scheme based on chemical looping gasification (CLG) provides a promising alternative for converting biomass into syngas with low tar content. The current study investigates the reactivity of NiFe2O4 oxygen carrier for toluene (biomass tar model compound) removal. The NiFe2O4 oxygen carrier shows a dual-function of oxidation-catalysis for toluene cracking and significantly promotes toluene cracked into carbon and H2. A suitable temperature for toluene cracking is determined at 850 °C. As the weight hourly space velocity (WHSV) increases by approximately a factor of nine, the toluene removal decreases slightly by 2.78%. The toluene removal does not significantly decrease with the crystal phase transformation of the oxygen carrier. Addition of steam significantly eliminates the carbon deposition, which decreases to 4.97% at S/C (steam/toluene) ratio of 1.20. The catalytic activity of NiFe2O4 initially remained stable for a long time, and then started showing a slight decrease after transitory activation during the long-term experiment (82 h). These results fully demonstrate that the NiFe2O4 is a good oxygen carrier for tar removal in biomass CLG.

Thermal and Catalytic Cracking of Toluene Using Char from Commercial Gasification Systems

Tar formation hinders the development of biomass gasification technologies. The use of pyrolytic char as a catalyst for removing tar has been widely investigated; its large specific surface area and pores distribution make it a good candidate for the cracking of heavy hydrocarbons. The present work assesses the catalytic activity of char from a commercial gasifier. Thermal degradation tests in N2 and in CO2 proved that the char is suitable for high-temperature applications (catalytic cracking) and showed release of CO and H2, which might affect the catalytic performance of the char when used for tar removal applications. For inspecting the potential of the char for tar removal, toluene was chosen as model tar. Through GC-FID, toluene removal efficiency and the amount of benzene produced from its decomposition were evaluated. Tests up to 1273 K resulted in tar removal efficiencies as high as 99.0%, and empty reactor tests allowed for discerning the effects of thermal and catalytic cracking. The catalytic activity of the char was more pronounced at 1173 K, as char increased the toluene removal efficiency from 39.9% (empty reactor) to 60.3%. The results confirmed that gasification char, like pyrolytic char, has a high potential for catalytic tar removal applications.

Catalytic reforming of biomass pyrolysis tar using the low-cost steel slag as catalyst

In this work, the possibility of steel slag as an effective and low-cost catalyst for the decomposition of biomass pyrolysis tar has been explored based on the high content of iron oxides for sustainable syngas production from biomass. By simple calcination treatment at 800 °C, the loose structure of the steel slag was formed with the main chemical composition of Fe2O3 and MgFe2O4. The steel slag exhibited good catalytic activity on the cracking of biomass pyrolysis tar, and even higher tar conversion efficiency can be obtained by reusing the steel slag, leading to the increase in syngas yield. The presence of additional steam can further promote the tar reforming reactions, leading to the significant increase in H2 and CO. At 800 °C, the tar conversion efficiency reached 94.1% with a high gas yield of 493.5 mL/g. The interaction between steel slag and reductive gases resulted in the reduction of iron oxides into Fe3O4, and more pores were formed for the spent steel slag, which can enhance the contact between active sites and reactants. These characteristics indicate that steel slag has the potential to be used as an efficient catalyst with excellent stability in the long-term biomass tar removal applications.

Evaluation of the catalytic performance of different activated biochar catalysts for removal of tar from biomass pyrolysis

Biomass derived chars via simple synthesis methods play an important role in different applications due to their porous structure, low cost and potential for the catalysis research field. This work prepared three typical activated char catalysts using KOH, H3PO4 and ZnCl2 as activators and rice husk as precursor for the catalytic decomposition of tar from biomass pyrolysis. Results showed that high surface areas were achieved and inorganic elements were introduced by applying the activators. The char obtained by KOH activation exhibited excellent catalytic performance on tar decomposition due to the high surface area and the presence of potassium compounds. Activation using H3PO4 led to a more heterogeneous pore size distribution of biochar, also exhibiting high catalytic performance, while the activation of ZnCl2 might promote the agglomeration of zinc and rice husk containing inorganic matters, resulting in relatively lower tar conversion efficiency. The tar catalytic decomposition led to the significant increase in the yield of product gas, particularly the combustible gas components such as H2, CO and CH4. GC-MS test results showed that macromolecular tar components were generally cracked and phenol was the main components of the residue tar, representing the good selectivity of the catalysts. The catalysts also showed excellent stability for tar cracking process and high catalytic performance was still achieved after five cycling tests. The biochar catalysts remained good porous structure with high surface area and the metals phases were well retained after five recycles, indicating that the biochar catalysts have the potential for long-term practical applications.

Non-thermal plasma degradation of tar in gasification syngas

The dielectric barrier discharge (DBD) technology was examined using a non-thermal plasma reactor for the degradation of tar formed in a fluidized bed gasification reactor which is the absence of oxygen in the syngas. Under the same gasification conditions, the effect of specific energy density (SED) on the tar removal efficiency was investigated. The experimental results indicated that the removal efficiency of tar in the syngas increased with SED, demonstrating the possibility of tar removal by the DBD technology. The removal efficiency of tar reached above 85% at an SED of 706 J/L. The gaseous and liquid products were further analyzed in a gas chromatography and mass spectrometer (GC–MS). During discharge, the content of aromatic and long chain aliphatic hydrocarbons in the treated syngas was found to decrease, while the amount of the gaseous hydrocarbons increased. Most of the aromatic hydrocarbons were found to be converted into less toxic substances of smaller molecules containing less numbers of carbon atoms. Based on the analytical results above, the degradation mechanism of tar in the DBD process was also discussed with toluene as a model.

Catalytic supercritical water gasification of aqueous phase directly derived from microalgae hydrothermal liquefaction

Microalgae (N. chlorella) hydrothermal liquefaction (HTL) was conducted at 320 °C for 30 min to directly obtain original aqueous phase with a solvent-free separation method, and then the supercritical water gasification (SCWG) experiments of the aqueous phase were performed at 450 and 500 °C for 10 min with different catalysts (i.e., Pt-Pd/C, Ru/C, Pd/C, Na2CO3 and NaOH). The results show that increasing temperature from 450 to 500 °C could improve H2 yield and TGE (total gasification efficiency), CGE (carbon gasification efficiency), HGE (hydrogen gasification efficiency), TOC (total organic carbon) removal efficiency and tar removal efficiency. The catalytic activity order in improving the H2 yield was NaOH > Na2CO3 > None > Pd/C > Pt-Pd/C > Ru/C. Ru/C produced the highest CH4 mole fraction, TGE, CGE, TOC removal efficiency and tar removal efficiency, while NaOH led to the highest H2 mole fraction, H2 yield and HGE at 500 °C. Increasing temperature and adding proper catalyst could remarkably improve the SCWG process above, but some N-containing compounds were difficult to be gasified. This information is valuable for guiding the treatment of the aqueous phase derived from microalgae HTL.