Thermal Cracking Of Tar Research Articles

Generation features and kinetics of gas products in bio-tar catalytic cracking by char for biomass gasification

This research focused on the reaction features and kinetics of tar catalytic conversion by char based on the generation and evolution of gas products. To probe the roles of heat and catalysis in tar conversion, a micro fluidized bed reaction analyzer (MFBRA) was used to assess tar thermal cracking (TC) and catalytic cracking (CC) by char at 1073, 1123, 1173, and 1223 K. The tar cracking behavior of the individual gas components (H2, CO, CO2, and CH4), generation rates, and reaction kinetics were thoroughly investigated. The results show that TC and CC included a fast (Stage-1) and slow (Stage-2) conversion stage, in which Stage-1 contributed more than 80% of the total conversion. Compared to TC, tar CC required a longer reaction time but obtained higher conversions to individual gas components and the total gas products. The final conversion ratios (XCC/TC) of H2, CO, CO2, and CH4 were kept above 1 at the tested temperature. The char catalytic action was very remarkable, especially at low temperatures. In tar CC, the generation rate of individual gas components slowly declined after quickly reaching the peak, forming a relatively stable zone. The volume reaction model (VRM) well described the tar conversion behavior with and without char. Compared to TC, char lowered the activation energy (Ea) of gas generation. For individual gas components (H2, CO, CO2, CH4) and the total gas products, the corresponding reduction degree of Ea was 25.56%, 24.18%, 22.14%, 4.75%, and 21.71%, respectively, further displaying the promoting impact of char. The evaluated generation features and kinetics of individual components in the gas products will benefit the in-situ tar removal in the biomass gasification process.

Comparison of reaction characteristics and kinetics between tar thermal cracking and steam reforming in a micro fluidized bed reaction analyzer

In this study, a micro fluidized bed reaction analyzer (MFBRA) was used to investigate the reaction characteristics of tar thermal cracking and steam reforming at different temperatures (T, 750, 850, and 950 °C) and steam partial pressures (SP, 10%, 20%, and 30%). The reaction kinetics, including gas components generation (H2, CO, CO2, and CH4) and carbon (C) conversion in gas products, were calculated and further compared. For the two processes, the behaviors of gas generation and C conversion can be classified as Stage-1 and Stage-2 based on the different increases. By raising temperature and SP, the yields of H2/CO/CH4 and C conversion increased obviously, while CO2 showed a growing and subsequently declining trend. A similar phenomenon also existed in the relationship between reaction rate Vs. gas yield/C conversion. Compared to thermal cracking, the reaction time in tar steam reforming became much longer; the yields of H2/CO/CH4 and the C conversion increased significantly, while CO2 decreased. In the meantime, there was a clear reduction in all reaction rates of gas generation and C conversion. Furthermore, in tar steam reforming, the activated energy (Ea) of gas components generation (H2, CO, CO2, and CH4) decreased from 114.92 to 90.48, 86.54–74.49, 32.39–28.87, and 55.34–42.06 kJ/mol, respectively; and Ea of total C conversion lowered from 87.74 to 65.75 kJ/mol. Additionally, SP had no impact on Ea during tar steam reforming. To ensure the validity of the test results by MFBRA, the kinetics of tar thermal cracking was compared with that in the literature.

Insight into tar thermal cracking and catalytic cracking by char: Characteristics and kinetics

In this research, the non-isothermal and isothermal reaction behavior of tar catalytic conversion by char was tested by a thermogravimetric analyzer (TGA) and a micro fluidized bed reaction analyzer (MFBRA), respectively. The TGA analysis shows that the obvious catalytic action can be found at a temperature above 873 K. In the MFBRA measurement, the roles of heat and catalysis during tar catalytic conversion from char were examined by comparing tar thermal and catalytic cracking. The results show that, regardless of tar thermal and catalytic cracking, the isothermal reaction behavior can be divided into the primary reaction stage (S1) and the secondary reaction stage (S2). In S1, the reaction behaviors in terms of reaction time, tar conversion, and gas yields, were very similar, indicating the dominant role of heat. In S2, the process of tar catalytic conversion needed a longer reaction time with higher tar conversion and more gas product emission, manifesting the significant role of catalytic action. In the examined temperature range of 1023–1223 K, the temperature effect on gas components during tar catalytic cracking by char included promotion (CO, CH4, H2, and CO2), suppression (C2H6, C3H6, and C3H8), and competition between them (C2H4). For the reaction kinetics, the activation energy of each gas component generation and the total gas product in S2 was much higher than that in S1. This was much related to the catalytic conversion of hard-to-crack components in the tar sample. The findings were beneficial to understanding tar catalytic reaction by char deeply and providing data support for the design and optimization of the tar removal unit in a gasification process.

Decomposition of biomass gasification tar model compounds over waste tire pyrolysis char

Gasification of biomass produces a syngas containing trace amounts of viscous hydrocarbon tar, which causes serious problems in downstream pipelines, valves and processing equipment. This study focuses on the use of tire-derived pyrolysis char for tar conversion using biomass tar model compounds representative of tar. The catalytic decomposition of tar model compounds, including methylnaphthalene, furfural, phenol, and toluene, over tire char was investigated using a fixed bed reactor at a bed temperature of 700 °C and 60 min time on stream. The influence of temperature, reaction time, porous texture, and acidity of the tire char was investigated with the use of methylnaphthalene as the tar model compound. Oxygenated tar model compounds were found to have higher conversion than those containing a single or multi-aromatic ring. The reactivity of tar compounds followed the order of furfural > phenol > toluene > methylnaphthalene. The conversion of the model compounds in the presence of the tire char was much higher than tar thermal cracking. Gas production increased dramatically with the introduction of tire char. The H2 potential for the studied tar model compounds was found to be in the range of 40%–50%. The activity of tire char for naphthalene removal was compared with two commercial activated carbons possessing a very well-developed porous texture. The results suggest that the influence of Brunauer-Emmett-Teller surface area of the carbon on tar cracking is negligible compared with the mineral content in the carbon samples.Graphical abstract

Thermocracking of a heavy fraction of the oil residue in a mixture with shale

The process of tar thermal cracking in a mixture with crushed oil shale to obtain components of motor fuels and raw materials for the process of thermal cracking is investigated in this paper. The optimization results of technological parameters (shale concentration, temperature, and duration) are presented and the material balance (mass.%) of the process is made. It was found that during single-stage processing under relatively mild conditions (5 MPa, 425C, feed space velocity of 1.0 h-1), a deep destruction of tar is achieved (the yield of the gasoline fraction from boiling point to 200C is ~12 wt.%; medium distillates with boil. point 200370C-43-44 mass.%; raw materials for thermal cracking with boil. point above 370C ~15-16 wt.% on per the original tar). The generating coke-like products and the V and Ni contained in the raw materials are deposited on the mineral part of the shale and removed from the reaction zone with the liquid products of the process.

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.

Comparison of tar thermal cracking and catalytic reforming by char in a micro fluidized bed reaction analyzer

In this study, a micro fluidized bed reactor analyzer (MFBRA) was adopted to test the isothermal reaction characteristics of tar catalytic reforming by char and thermal cracking by quartz sand from 1023 to 1223 K. The behaviors of tar conversion, the evolution of gaseous products generation, and the corresponding reaction kinetics were comprehensively analyzed. Compared to thermal cracking, the yield of catalytic cracking is higher. It can not only promote the tar conversion but also upgrade the fuel gas quality even at a low temperature of 1023 K, especially for CO, CH4, CO2, and C3H6. The tested activation energies (Ea) of tar conversion and gas generation (CH4, C2H6, C3H6, H2, CO2, and CO) in catalytic reforming were about 60.27, 45.48, 56.22, 71.34, 62.35, 51.51, and 60.75 kJ/mol, respectively, corresponding that of 78.16, 63.19, 71.16, 83.19, 84.33, 90.81 and 130.72 kJ/mol in thermal cracking. The lower Ea indicated the good catalytic activity of char on tar conversion and the change of gas generation paths by choosing char as a catalyst. Finally, the kinetics was compared with the results in the literature to verify the feasibility of MFBRA and the accuracy of testing results. This research deepened the understanding of tar catalytic reforming by char and was beneficial for developing a biomass gasification process with low tar generation.

Experimental study on the cracking of tar derived from municipal solid waste gasification to produce syngas

ABSTRACT The cracking of real tar obtained from a pilot-scale gasifier of municipal solid waste was studied. The tar was both thermally and catalytically cracked under N2 atmosphere. Thermal cracking experiment was conducted at 600°C, 700°C, and 800°C, while catalytic cracking experiment using waste wood-derived biochar as catalyst was conducted at 700°C. Biochar was produced by conducting slow pyrolysis of wood in the lab-scale. Thermal cracking of tar at 800°C significantly improved the H2 yield in the produced gas compared to thermal cracking at 600°C and 700°C. The results indicated a significant improvement upon the application of biochar compared to the thermal method in terms of H2 yield, tar conversion, and H2/CO ratio of the produced gas. The H2 yield from catalytic cracking at 700°C was only slightly lower than the yield from thermal cracking at 800°C. The weight of biochar was reduced by around 10% after the tar cracking experiment, which indicated that the biochar was stable under the experimental condition. Further study by conducting the tar cracking experiment using producer gas as carrier gas could elucidate the potential of the developed waste wood-derived biochar for industrial application.

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.

Bond cleavage and reactive radical intermediates in heavy tar thermal cracking

Thermal cracking is an important step of heavy tar processing, in which covalent bonds are initially cleaved to form free radical fragments. The free radical fragments, namely the reactive intermediates, react with each other to form lighter products and coke. Quantity and activity of these reactive radicals determine the products distribution and process operation. Since few studies can be found about the reactive radicals from thermal cracking of heavy tar, we quantify their generation and analyze their activity using two tars provided by ExxonMobil Chemical Company. The tars are heavy fractions of steam cracking of naphtha and heavier hydrocarbon streams. It is found that these heavy tars crack significantly at 350 °C. The reactive radicals generated in 10 min for Tar-1 at 350 and 400 °C are 2.62 and 4.73 mmol/g-tar, respectively, and 1.27 and 3.88 mmol/g-tar, respectively, for Tar-2. The bonds cleaved in 10 min at 400 °C are about 1.50% and 1.30% of total covalent bonds in Tar-1 and Tar-2, respectively, and the bond cleavage activation energies are 31.00 and 38.80 kJ/mol, respectively. The differences in bond cleavage were analyzed from the viewpoint of the composition and structural differences between the two heavy tars, in terms of asphaltene content, aromaticity, and substitutive degree of aromatic ring, for example. The reactive radicals were further categorized into low activity ones and high activity ones and were correlated with stable radicals which have survived the cracking.

Characteristics of tar abatement by thermal cracking and char catalytic reforming in a fluidized bed two-stage reactor

With respect to the newly proposed fluidized bed two-stage (FBTS) gasification process, the researches on tar thermal cracking and catalytic removal by char bed layer were conducted on a FB two-stage reaction apparatus. It shows that the effect of char on tar removal was much related to the reaction temperature and residence time of tar-containing fuel gas in the tar reformer. Under the tested experimental conditions, increasing reaction temperature and residence time can effectively promote tar abatement and generate more H2, CO and CH4. For catalytic reforming by char, the higher reaction temperature and the longer residence time, the lower tar content in fuel gas. Compared to thermal cracking, catalytic reforming of tar by char was more effective in tar removal efficiency and inhibition of carbon deposition, upgrading the quality of fuel gas. The researches indicated that the suitable operational conditions for tar removal by char in the FB two-stage gasification process were at reforming temperature of 1000 °C, and residence time above 0.9 s.

Molecular dynamics simulations of Zhundong coal pyrolysis using reactive force field

In this paper, the pyrolysis process of a typical Zhundong coal was studied using the reactive molecular dynamics simulations (ReaxFF-MD) to investigate the secondary reactions of tar which has a great influence on coal conversion behavior. The coal molecular model was built based on a combination of experiments and classical coal model. First, simulations were carried out at 1400–3000K for 250ps to investigate the effect of temperature on Zhundong coal pyrolysis process. The results show that the secondary reactions of tar occur at temperature higher than 2600K, contributing to an increase of char yield and production of large amount of H2 and CO. The secondary reactions of tar need more energy compared to the initial thermal cracking of coal. Then, the secondary reaction mechanisms of tar were revealed by analyzing the evolution of pyrolysis species at 3000K. Ethenone (C2H2O) is found as one of the main intermediates from thermal cracking of tar. There are two pathways of the secondary reaction of tar leading to char formation: (1) one tar fragment reacts with another leading to the generation of char; (2) the radicals from tar decomposition attack the char leading to the growth of char fragment accompanied by the release of H2 and CO. Finally, the effect of heating rate on Zhundong coal pyrolysis was studied. The results show that lower heating rate leads to higher gas yield and lower tar yield due to the secondary reactions of tar which mainly undergoes two stages: thermal cracking resulting in gas formation and polymerization leading to char formation.

Gasification characteristics of biomass for tar removal by secondary oxidant injection

Gasification experiments for sawdust were conducted using a fixed bed reactor at 900 °C by varying the secondary oxidant injection ratio to determine the optimal conditions for tar removal along with the enhancement of gasification efficiency. Secondary oxidant was injected as an oxidant at the top zone of the gasifier in varying ratios of 10–30% of the total amount of oxidant. This method was based on the primary method of tar removal and gasification efficiency improvement by thermal cracking of tar. Various gasification performance parameters were evaluated and tar content was estimated by measuring the fluctuation of weight of the activated carbon filter. The results showed that the concentration of tar in the producer gas decreased by injecting the secondary oxidant, even though syngas yield decreased. The recycling potential of the char produced in the gasification experiments was also assessed with the purpose of utilizing char as an adsorbent by determining its surface area and pore volume. The results demonstrated that the char produced from the gasification experiment had similar quality to that of the activated carbon used in this experiment.

Microwave-induced cracking of pyrolytic tars coupled to microwave pyrolysis for syngas production

Herein a new process is proposed to produce a syngas-rich gas fraction (>80vol% H2+CO) from biowaste based on microwave heating within two differentiated steps in order to avoid tars production. The first step consists of the microwave pyrolysis of biowaste induced by a char-based susceptor at 400–800°C; tars, char and syngas-rich gas fractions being produced. The tars are then fed into the second step where a portion of the char from the first step is used as a bed material in a 0.3:1wt% ratio. This bed is heated up by microwaves up to 800°C, allowing thermal cracking of tars and additional syngas (>90vol% H2+CO) being then produced. This new concept arises as an alternative technology to the gasification of biowastes for producing syngas with no need for catalysts or gasifying reagents to minimise tars production.

Modeling of tar thermal cracking in a plasma reactor

This paper deals with the theoretical modeling of a high temperature reactor devoted to the thermal cracking of tars contained in a gas coming from a biomass gasifier. The aim of this paper is to give an insight into the chemical reactions that take part in this conversion but also into the reactions that may occur in such environment. The investigation is carried on using the simplest continuous reactor configuration that exists (completely stirred tank reactor), which allows to address the effects of chemical reactions without taking into account the mixing. This paper presents the complete set of reactions taken into account. It can also be noticed that this model also describes possible gasification of char particles entering the reactor. An in-depth analysis of the weight of the involved reactions is proposed for the nominal conditions of the reactor operation. The results of the model are also compared to the results of a previous one, highlighting the benefits of the actual model. These results are also compared to values obtained during operation of a pilot plant. Finally, the influence of five of the operating parameters of the reactor is theoretically investigated. It shows that these parameters can have an important impact on reactor operation, but also that conditions can be found that always lead to a conversion of at least 95% of the incoming tars.

Biomass gasification under high solar heat flux: Experiments on thermally thick samples

In this study, thermally thick samples of beech wood are exposed to radiative heat flux above 1MW/m2 (1000 suns). It was motivated by the fact that concentrated solar energy allows to achieve temperatures higher than 1200°C where char gasification, tar thermal cracking and tar steam reforming can take place. It is achieved using a new experimental device made of an artificial sun and a new reaction chamber, that monitors the sample mass throughout a run and can trap the produced tars using a liquid nitrogen cooled tar condensing device. Thanks to this experimental device, it is possible to compute the average wood consumption rate as well as drying water, char, gas and tar production rates. The produced light gases are also analyzed using microGC. Furthermore, a radiometer is used to monitor surface temperature, which is around 1500°C. First, a new behavior has been highlighted. Under high radiative heat flux, a char crater which mirrored incident heat flux distribution, is formed inside of the sample. Then, using this device, the impact of two major parameters was tested: wood fiber orientation relative to the solar flux and initial moisture content. Wood fiber orientation (end grain and with the grain) was shown to only have a minor impact on the production rates, gas composition and crater formation. Three initial moisture contents (0, 9 and 55%wb) were tested. It was shown that increasing the sample moisture leads to direct drying steam gasification of the char produced by the pyrolysis. Moreover, steam also promotes tar steam reforming and therefore decreases the tar yield. Finally, form an energetic point of view, the dry samples can achieve an energetic conversion efficiency of 90%, capturing up to 72% of the incident solar power in chemical form.

Semi-empirical model for estimating molar fraction of syngas produced by thermal cracking of wood tar

To optimize the efficiency of the wood gasification, tar which limits the use of syngas in thermal engines must be reduced. The quantity, quality and composition of syngas produced after thermal cracking of tar were analysed, and mathematical equations formulated based on Newton’s bivariate polynomial interpolation were used to estimate the molar fraction of the gases. The conceptual analysis of the problem was based on experimental data of temperature and residence time, and the semi-empirical model was simulated using Matlab to predict the composition of the syngas and the tar content in the gas mixture. The thermal cracking of tar increased the amount of syngas produced; in particular, the molar fraction of carbon monoxide and hydrogen. The gasification energy efficiency also improved with temperature and residence time, as the quantity of tar was reduced during cracking. The model was validated by comparing the experimental and numerical data. Keywords: Syngas, Gasification, Pyrolysis, Residence time, Thermal cracking of tar, Semi-empirical model

Tar reduction in downdraft biomass gasifier using a primary method

This work present a novel primary method, for tar reduction in downdraft gasification. The principle of this new technology is to change the fluid dynamic behaviour of the mixture, formed by pyrolysis product and gasification agent in combustion zone; allowing a homogeneous temperature distribution in radial direction in this reaction zone. To achieve the change in the fluid dynamic behaviour of the mixture; the entry of gasification agent to combustion zone is oriented by means of wall nozzles in order to form a swirl flow. This modification in combination with the extension of the reduction zone, will allow, to increases the efficiency of the tar thermal cracking inside the gasifier and the extension of the Boudouard reactions. Consequently, the quantity of tar passing through the combustion zone without cracking and the concentration of tar in the final gas, decrease significantly in relation with the common value obtained for this type of reactor, without affecting significantly the heating value of the producer gas. In this work is presented a new design for 15 kW downdraft gasification reactor, with this technology implemented, the tar content obtained in the experiments never overcome 10mg/Nm3, with a lower heating value of 3.97MJ/Nm3.

Effects of the Preheated Temperature and the Change in Gas Composition on Soot Formation in the Gas Reforming Process by Partial Combustion Method

Partial combustion can be an effective way to reform fuel gases. The possible application we assume is reforming of tar in a producer gas from woody biomass gasification. The reforming process of tar by partial combustion of the producer gas is conducted in a gas reformer by the combination of oxidative cracking and thermal cracking of tar. The partial combustion type gas reformer is an apparatus stage subsequent to the biomass gasifier. The inverse diffusion flame is formed when the oxidizer for partial combustion is supplied to the producer gas. Cracking and polymerization of the tar occur simultaneously at proximity of the inverse diffusion flame. This polymerization of tar into soot is, however, a significant problem in the gas reformer, e.g. the reduction of the carbon conversion ratio and the change in temperature field by radiation from soot. The experimental and numerical study have been performed to clarify effects of hydrogen addition to the oxidizer on soot formation and growth of the polycyclic aromatic hydrocarbons (PAHs) that is precursor of soot. The main results from experiments are as follows. The increase in the preheated temperature of model producer gas suppresses soot formation. This is because the increase in preheated temperature leads the fraction of toluene to be low. TSI (Threshold Sooting Index) of toluene (model tar) is relatively high within components in the model producer gas.

An Analysis of Wood Pyrolysis Tar from High Temperature Thermal Cracking Process

The objective of this study is to investigate and analyze the tar yields and structural changes of tar formed during a high temperature thermal cracking process. Experiments were conducted in order to study the effect of temperature on the yield and characteristic chemicals of tar. All experiments were carried out with parawood under steady-state conditions. The temperature of the pyrolyzer was controlled at 600°C and pyrolysis gases were carried to the reformer by nitrogen. The thermal cracking tests were performed at six different temperatures: 700, 800, 900, 1,000, 1,100, and 1,200°C. The samples of tar were analyzed by gravimetric analysis, gas chromatography-flame ionization detector, and gel permeation chromatography techniques in order to get information and an understanding about the tar, such as tar yield, compositions, and average molecular weight. From the experimental results and quantity analysis, the amount of tar decreased continually to 90% reduction as temperature increased from 600 to 1,200°C resulting from thermal cracking of tar into the lighter products. In chemical analysis, at a low temperature range, the aromatic compounds were dominant in tar compositions. Meanwhile, these phenomena were adverse at higher temperatures and polycyclic aromatic hydrocarbons dominated. These resulted in higher than the average molecular weight as the temperature increased.