Analysis Of Tar Removal Method In Biomass Gasification Process

Energy is inseparable from the survival and development of human beings. With the development of economy and social progress, people's demand for energy is increasing day by day. At present, the main body of the world's energy structure is still fossil energy, but the proven reserves of fossil energy such as coal, oil, and natural gas have a useful life of 220, 40, and 60 years, respectively. With the reduction of its reserves, the sustainable development of the economy will be seriously affected. In addition, harmful substances such as CO2, NOx, SOx and dust emitted during the use of fossil energy directly cause harm to the environment, human health and equipment, such as global warming, ecosystem imbalance, acid rain and smog, etc., seriously threatening human survival. Therefore, the development and utilization of new energy sources is imminent.

As the fourth largest energy source after coal, oil and natural gas, biomass energy has the advantages of wide distribution, large reserves, renewable and low emission of pollutants in the process of utilization, and is also the only alternative fossil resource that can be stored and transported. Therefore, it has attracted worldwide attention, and the research and development of clean, efficient and renewable energy utilization technology has become the focus of energy development strategies of various countries. After pretreatment, biomass is similar in performance to ordinary fossil fuels, and its utilization method is also similar to that of fossil fuels. In theory, it can be used directly after a slight modification in the existing fossil energy utilization technology. However, due to the wide variety of biomass, the actual utilization is much more complicated than that of fossil fuels. Generally speaking, biomass energy conversion and utilization technologies mainly include biochemical technology and thermochemical technology, of which thermochemical utilization is further divided into biomass combustion, gasification and pyrolysis. Because biomass is rich in volatile matter, it has a high reaction rate at a lower temperature, which is more suitable for gasification reaction. Therefore, biomass gasification has become the most potential thermochemical conversion method.

Biomass gasification is the process of oxidizing carbon in solid fuels to generate combustible gas using oxygen or oxygen-containing substances in the air as gasification agents. In this process, it is also accompanied by the reaction of carbon and water vapor and carbon and hydrogen. Biomass gasification can convert low-grade solid biomass into high-grade combustible gas, but some by-products such as tar are inevitably produced during biomass gasification. Tar is in gaseous state at high temperature and is completely mixed with the gas produced by gasification, while at low temperature (generally below 200°C) it condenses into a viscous liquid state, which is easy to combine with water, coke, etc., blocking the gas pipeline and making the gasification equipment run. Difficulty occurs. In addition, tar is prone to produce carbon black and other particles during combustion, which seriously damages gas utilization equipment, such as gas turbines, and greatly reduces the utilization value of gasification gas. Therefore, minimizing and controlling the tar production in the gasification process is of decisive significance to the development and promotion of biomass gasification technology.

Definition And Classification Of Tar:

Tar is an organic substance produced in the process of biomass pyrolysis and gasification. It is a macromolecular polynuclear aromatic hydrocarbon, which is mainly generated by thermal decomposition of cellulose, hemicellulose and lignin in biomass. When biomass is heated, its organic molecular bonds will be broken, resulting in small molecules in the gaseous state, while larger molecules are called primary tar. Primary tars are generally some fragments of the original biomass raw material structure. Under the condition of gasification temperature, primary tars are not stable and will further react to become secondary tars. If the temperature is further increased, part of the tar will also be converted to tertiary tar. The diagram below shows a simple illustration of how tar is produced.

The composition of tar is quite complex, and it is estimated that it contains more than 1,000 kinds of organic substances. At present, there are more than 200 kinds of tar components that can be detected, of which there are about 7 kinds with a mass fraction greater than 5%, namely benzene, naphthalene, toluene, xylene, styrene, phenol and indene. Bergman et al. of the Energy Research Center of the Netherlands (ECN) classified tars into 5 categories according to molecular weight and boiling range:

(1) Heavy tar, a component that cannot be detected by gas chromatography.

(2) Heterocyclic aromatic hydrocarbon tars, containing heteroatoms such as N and O, with high water solubility, usually pyridine, phenol, cresol, quinoline and isoquinoline, etc.

(3) Light aromatic hydrocarbons (1 ring) tar, generally toluene, ethylbenzene, xylene and styrene.

(4) Light polycyclic aromatic hydrocarbons (2 to 3 rings) tars can also condense at very low concentrations, usually indene, naphthalene, methylnaphthalene, biphenyl, acenaphthylene, fluorene, phenanthrene and anthracene.

(5) Heavy polycyclic aromatic hydrocarbons (4-7 rings) tars can condense under low concentration and high temperature conditions.

Among the 5 types of tars, the first, second and fifth categories belong to the category of heavy tars, which are very harmful but easy to remove; the fourth category is moderate tars that cannot be removed by cooling (not at room temperature). Condensation), this kind of substance is the key and difficult point of tar purification; the third category belongs to light tar, and the utilization process is less harmful, but it cannot be removed by general methods and requires advanced treatment.

Tar Removal Method:

The gas tar removal method includes the removal method inside the gasifier and the removal method outside the gasifier. The in-furnace removal method is to suppress the production of gas tar by adjusting the structure and operating conditions of the gasifier. It is more scientific and reasonable in terms of principle and economy, but the actual results are not ideal, which will generate waste gas and reduce the gasification efficiency, resulting in gasification. The structure of the furnace is complex, which reduces the flexibility of system operation and makes it difficult to enlarge the system. It can generally be used as an auxiliary method for complete decoking. The detar method outside the furnace is the post-treatment and decoking of the gas. Regardless of the source of the gas, it can be effectively implemented independently of the gasification process, and is easier to replicate, transplant and scale up, so it has received more attention. According to whether the tar molecules are destroyed, the decoking method outside the furnace is divided into the recovery method (mechanical/physical method) and the elimination method (catalytic cracking and thermal cracking).

3.1 In-furnace tar removal method

The essence of the furnace decoking method is to change many parameters that affect the production of tar, and fundamentally solve the tar problem, also known as the parameter method. The gasification process of biomass is affected by many parameters, such as temperature, oxidant, biomass properties, biomass particle size, operating pressure, material ratio, catalyst type and amount, and gasifier type, etc. score has a greater impact. Using rice husk as raw material, the effect of temperature on the content of tar in the generated gas was studied, and it was found that the content of tar increased with the decrease of gasification temperature. The effects of temperature (750～840℃), steam biomass ratio (S/B=0.8～1.2) and pressure (0.1～0.25MPa) on tar content were studied in bubbling fluidized bed gasifier, and it was found that the temperature increased, The tar content decreases and the S/B value increases, the tar content decreases, and it is found that the higher the pressure, the higher the tar content. In non-woody biomass gasification experiments under different operating conditions, ER was found to reduce gas quality and gasification efficiency. If ER is regarded as a variable function, the optimal ER range is 0.28 to 0.32, the low calorific value of the gas produced under the optimal ER is 5.39 MJ/m3, the gas output is 2.86 m3/kg, and the gasification efficiency is 73.61%. The mass concentration of tar is 4617mg/m3. The results of the study on the effect of different gasification temperatures on biomass-derived products in fluidized bed gasifiers show that biomass gasification is significantly affected by gasification temperature, and different gasification temperatures will lead to products with different composition ratios. . The parameter method is generally used in combination with other methods in biomass gasification.

3.2 Out-of-furnace tar removal method

3.2.1 Recycling method

The recovery method is also known as a physical (mechanical) method. This method does not decompose and remove the tar, but transfers the tar from the gas phase to the condensed phase through physical operations, which has both decoking and dust removal effects. The recovered tar can be directly used as a product, or further refined to obtain high value-added aromatics. Numerous studies have found that recovery methods are quite effective in removing tar. At present, the most common recycling methods are divided into two categories: wet method (dry-wet method) and dry method.

3.2.1.1 Wet method (dry and wet method)

The wet method is also called liquid (water or oil) washing method, and its principle is to use liquid to wash and remove part of the tar in the combustible gas. Because tar is acidic, adding a small amount of alkali during the removal process can improve the removal efficiency. This method is widely used for moderate and deep removal of biomass gas tar.

(1) Washing method

Water washing is the most common method of wet detaring, including water washing towers and wet electrostatic precipitators. Washing towers mainly include washing towers, venturi towers, impact bubbling washing towers and packing towers. It is found that the water washing method can remove a large amount of dust and reduce it to 10-20mg/m3, and the water washing wastewater can be effectively purified by the microbial method. The water washing tower is simple in structure and low in price, and can completely condense and remove all heavy tars. But light tar droplets and gaseous/liquid fumes can be carried away by the airflow and are therefore the least efficient. In the experiment, it was found that washing the packed tower with water can reduce the amount of tar and dust from 600mg/m3 to 150mg/m3. According to the principle of sudden change in pressure, the venturi tower can remove the heavier substances in the gaseous state, so the decoking efficiency is high, which can reach 50% to 90%, and the volume content of solids and tar droplets in the gas at the outlet of the venturi scrubber is low. at 10mL/m3. The detar efficiency of the impact bubbling washing tower is about 70%, the equipment structure is simple, it can be connected in series, and the efficiency of the three towers in series is higher than 95%.

Wet electrostatic precipitators work by passing gaseous steam through a high-voltage, negatively-charged area to negatively charge the particles, and then pass through a positively-charged polar plate to remove the charged particles from the steam. This method is very effective for removing liquid particles, and the tar will fail when it is in a gaseous state. Therefore, before applying an electrostatic precipitator to remove tar, the gasification gas must be cooled to avoid high temperature. The wet electrostatic precipitator has a removal efficiency of more than 90% for particles with a particle size of 0.01 to 1 μm, but this method has high manufacturing and operating costs, and is usually used in large-scale gasification systems.

The water washing method has low cost and simple operation, but a large amount of tar is lost, causing waste of energy; the waste water generated during the purification process will cause secondary pollution; if the waste water is treated, the subsequent treatment process is cumbersome and the operating cost is high. Therefore, in order to make the water washing method for removing tar more widely used, a suitable wastewater treatment method must be found.

(2) Oil washing method

The mechanism of oil washing to remove tar is to use the compatibility of oil absorbent and tar, that is, the effect of van der Waals force. Using water scrubber and oil scrubber respectively to absorb biomass gasification tar, it was found that the water scrubber absorbs 31.8% of the tar, while the vegetable oil scrubber can achieve a maximum removal rate of 60.4%. The study found that 95.4% of the tar in the gasification gas could be removed by combining a vegetable oil scrubber with a rice husk charcoal adsorption bed. The absorption rate of oil-based absorbent for biomass gasification tar was studied, and it was found that vegetable oil had the highest removal efficiency for heavy tar, with an efficiency of 60.4%; for the removal of light tar, the absorption efficiency was diesel > vegetable oil > biodiesel > Oil > Water; Considering that the volatilization of light components in diesel will introduce impurities into the gas, vegetable oil is the best absorbent in a comprehensive judgment. In addition to the advantages of water washing, the oil-based detaring method can avoid the pollution of phenol-containing wastewater, but the method is difficult to separate and recover, and the operation cost is high.

3.2.1.2 Dry method

The dry method, also known as the adsorption (filtration) method, is different from the wet method in that it avoids the problem of water pollution caused by wet purification, but purifies the tar gas through filtration technology. It mainly uses the porous characteristics of the adsorbent to allow the gas to pass through the adsorbent material, or to pass the gas through a filter equipped with filter paper or ceramic core to adsorb and remove the tar in the gas. According to the characteristics of many impurities contained in biomass gas, the purification method of multi-stage adsorption can be adopted. However, the actual purification effect is not ideal, mainly due to the deposition and adhesion of tar, which reduces the removal efficiency. At the same time, the filter material adhered to the tar is also difficult to handle, so it is often used in conjunction with other purification devices. The effect of pyrolysis-adsorption two-step method to remove tar from biomass gas was investigated: at 800 ℃ pyrolysis temperature, when air or water vapor was used as auxiliary medium, the removal rate of heavy tar was 77%-92%; It shows good selective adsorption performance; activated carbon shows good adsorption performance for all tar light oils, but the disadvantage is that it can also adsorb combustible gases, thereby reducing the gasification efficiency. The synergistic adsorption of vegetable oil and rice husk charcoal to remove tar was studied, and the results showed that the adsorption and adsorption were effective for the removal of heavy tar and light tar, respectively, and could remove 95.4% of tar; since porous carbon can be directly gasified from biomass process to help improve the economics of the detar process. The study found that the adsorption capacity of coal-based activated carbon for naphthalene was 18.75mg/g, and the adsorption capacity of apricot shell activated carbon could reach 29.95mg/g. In general, the adsorption method is feasible and efficient for the tar removal process, but this aspect is still in the exploratory stage of basic research, and no pilot-scale demonstration system has been found.

3.2.2 Thermochemical method

Chemical removal method refers to the conversion of tar into small molecules of usable gas through a series of chemical reactions under certain reaction conditions such as temperature and pressure, so as to improve the conversion rate and utilization rate of biomass. Removing tar by thermochemical method can eliminate the hidden danger of tar damage to equipment and environmental pollution, and at the same time effectively recover energy. Thermochemical decoking methods mainly include thermal cracking and catalytic cracking.

3.2.2.1 Thermal cracking

Tar thermal cracking refers to breaking chemical bonds by heating above 1100 ℃, so that macromolecular compounds are cracked into small molecular gaseous compounds. A large number of literatures have proved that when the temperature is below 900 °C, the cracking rate of tar increases with the increase of temperature. At 900 °C, the cracking rate of tar reaches about 60%. Only when the temperature exceeds 1100 °C, the cracking rate can continue. The highest temperature can reach 99%, but the high temperature has high requirements for equipment, large energy consumption and high cost of cracking, and it is also easy to generate coke. Therefore, in actual production, the tar content is reduced by adding water vapor and oxidizing substances, that is, by reacting water vapor or oxidizing substances with some components in the tar to generate combustible gases such as CO, H2 and CH4, thereby reducing the coke content. generate.

3.2.2.2 Catalytic cracking

Catalytic cracking is the most economical and effective method for tar conversion at present. At the same time, the small molecule compound is reformed with H2O and CO2 to generate synthesis gas H2+CO, and the output of gasification synthesis gas is improved. Biomass tar cracking catalysts are mainly divided into two categories: natural and non-natural catalysts. The former mainly includes some natural ores such as dolomite, olivine, and ilmenite; the latter mainly includes alkali, alkaline earth metal catalysts, and iron-based metal catalysts.

(1) Alkaline/alkaline earth metals: The alkaline earth metal catalysts used for biomass tar cracking mainly include CaO and MgO, which have good catalytic cracking effect of tar, and are not only widely sourced, but also inexpensive. The study found that when the mass fraction of CaO was 2% to 6%, the dew point of tar decreased to 37 to 60 °C, the tar removal rate reached 44% to 80%, and the gas production increased by 17% to 37%. The catalytic cracking efficiencies of pure CaO and MgO tars were investigated at 800 °C, up to 96% and 97%, respectively.

K and Na in alkali metals are often used as catalysts for biomass gasification. Alkali metal ions have a certain chelating effect. Mixing the catalyst containing alkali metal elements with biomass in the process of biomass gasification can not only speed up the reaction rate of biomass gasification, but also reduce the formation of tar and reduce the energy consumption of gasification.

(2) Natural ore catalyst

Using natural ores with large reserves, easy availability, low price, and direct use to crack biomass tar as catalysts can not only reduce the time-consuming, energy-consuming, consumable and pollution problems of catalyst production, but also has the possibility of large-scale commercial application. . Ore catalysts are mainly dolomite, olivine and so on. The molar ratio of calcium and magnesium in dolomite is close to 1:1. After calcination and activation, it will form a special porous structure with a large specific surface area, and the active components (CaO, MgO) will change, and the tar decomposition ability will become stronger. By placing dolomite at the bottom of the reactor and placing coal-based activated carbon at the upper part of the reactor, it was found that the removal rate of tar reached 91.9%. Scholars have also found that the catalytic ability of dolomite is better than that of olivine, but its wear resistance is far less than that of olivine.

(3) Iron-based metal catalysts

Iron-based metals include iron, cobalt, and nickel, which have good catalytic cracking capacity for biomass tar. At present, this kind of catalyst mainly supports iron-based metals on alkaline earth metal compounds, natural ores, ion exchange resins and molecular sieves to improve the specific surface area, dispersion and sintering resistance of iron-based metal catalysts. In recent years, experiments have proved that iron-based metals can be loaded on lignite, and the catalytic effect is good.

Usually, nickel is not used alone, but the nickel alloy is evenly distributed on the carrier with a larger specific surface area, which increases the contact area with the tar and improves the catalytic activity of the nickel-based catalyst. Nickel-based catalysts have high catalytic efficiency. At lower temperature (750°C), the catalytic capacity is higher than that of alkali metals and dolomite. However, carbon deposits and sulfur-containing gases will appear on the surface of nickel-based catalysts, which will deactivate nickel-based catalysts. , making it impossible for long-term recycling, and at the same time, the high price of nickel increases the cost of biomass gasification, making it difficult for large-scale industrial production.

Fe is usually uniformly distributed on the carrier in the form of elemental or Fe2O3, and the catalyst is generally prepared by impregnation method, ion exchange method and coprecipitation method. Experiments found that the iron state has a greater ability to crack tar than the oxidized state. The catalyst tar cracking efficiency is over 60% in the metallic state, but only 18% in the oxidation state. If Ni, Cu, Co and other catalyst promoters are added, the catalytic performance of the catalyst can be greatly improved.

The research and application of biomass gasification technology has become mature, the influence of gasification tar has been paid more and more attention, and a variety of removal methods that can be practically applied have been developed. After analyzing the advantages and disadvantages of various tar removal methods, there is still a lack of a simpler, more stable, efficient and cheap tar purification technology. At present, the selection of the final method should be reasonably selected according to specific requirements. One or more methods can be used to remove tar. However, the rapid development of biomass pyrolysis and gasification technology requires the continuous maturity of tar technology. In addition, tar can be extracted into tar pitch, carbon black, etc., which can be used as asphalt paint, adsorbent, anti-corrosion coating and other waterproof materials or chemical raw materials. Therefore, the upgrading of tar will also have a good development prospect.

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