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Biomass gasifier using high temperature air gasification technology was designed and developed to demonstrate a new energy system based on biomass fuels, in which the heat brought into the process by the high temperature through the HTAG refractories increases the calorific value of the produced synthetic gas and reduces the amount of feedstock to be combusted. By this High Temperature Air Gasification process the pollutants emission is reduced and the formation of tat and soot particles are very much reduced.
In the High Temperature Air Gasification (HTAG) technology the preheated oxidizer provides additional energy for the gasification process and enhances the thermal decomposition of the coconut shell used as feedstock which results in improved performance of the gasification process. In this research the oxidizer temperature was elevated using the HTAG refractories and the performance parameters were studied.
Gasification is a thermo chemical process that is incomplete combustion of fuel which converts carbonaceous biomass materials into gaseous products. It involves the partial oxidation of the raw material to obtain a mixture of hydrogen, carbon oxides, water, nitrogen if air is used and small amount of methane and higher hydrocarbons[3].
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The gas produced in this process is called syn gas. The syn gas is a readily available fuel used in direct and indirect thermal applications. Mixed with air, the syn gas can be used in gasoline or diesel engines and it can also be used directly in burners. Different products are produced at different stages of the gasification. The operation of the gasifier is mainly depends on the oxidiser temperature and the type of feedstock used.
The gasifiers are mainly classified based on the flow of the oxidizer in to the gasifier. The type of gasifier is selected based on the feedstock material, size, moisture level and residue content [2]. The gasifiers are majorly classified into two types namely Fixed Bed Gasifiers and Fluidized Bed Gasifiers.
The fixed bed gasifiers are mostly used which incorporates other types such as [2], Updraft gasifiers, Downdraft Gasifiers and Cross draft Gasifiers.
The gasification of the biomass involves four distinct process namely drying, pyrolysis, combustion and reduction. In real time gasification these four processes overlay with each other. In these process distinct chemical reactions will takes place.
Drying Zone:
In the drying zone the biomass feed is dried using the hot gas evolving out of gasification process. During drying of the biomass feed the moisture present in the feed will result in production of few organic acids, which eventually corrode the gasifier walls. The potential feeds for bio-gasification has very low moisture levels. In many cases in the past the feeds with high moisture level above 25% will result in high tar production. In updraft gasifier the drying zone temperature will be in the range of 400℃ - 150℃
Biomass + Heat Dry Biomass (Loss of water)
Pyrolysis Zone:
The feedstock coming out of drying zone contains very low moisture level. When the dried feed is pyrolyzed at very high temperature and as a result the remining water and few acids produced are stripped off. In high temperature air gasification, the pyrolysis zone temperature is maintained at 600 ℃ - 400 ℃. During initial stage of pyrolysis, a large amount of tar and alcohols is released, which is found to have mix up with the producer gas resulting out of gasification. Feeds containing high moisture content will result in producer gas with high tar inclusions which at tail end needs vigorous gas cleaning filters and equipment’s. A small amount of gas is produced at the tail end of pyrolysis which contains hydrogen in large proportion.
Combustion Zone:
In combustion zone the pyrolyzed feed is combusted with limited supply of oxidiser. The combustible substance of a solid fuel is composed of predominant elements like carbon, hydrogen and oxygen.[2]. During combustion CO2 is produced by the reaction of carbon molecules with the oxidiser supplied. In gasification process the CO2 acts as a diluting agent by reducing the heating value of the gas produced. The theoretical combustion temperature yields in the range of 1400℃-1450℃ and the reactions ar4e exothermic.
C + O2 = CO2 (+ 393 MJ/kg mole) (1)
2H2 + O2 = 2H2O (- 242 MJ/kg mole) (2)
Reduction Zone:
The resulting products out of combustion process are mainly CO2, water and few organic acids resulting out of pyrolysis. These products are passed through the red-hot charcoal in the reduction zone. Where, the CO2 and water is reduced to carbon monoxide and traces of methane. The carbon monoxide is a readily available gaseous fuel and it is the ultimate end product of the gasification process. The reduction reactions are shown below,
C + CO2 = 2CO (- 164.9 MJ/kg mole) (3)
C + H2O = CO + H2 (- 122.6 MJ/kg mole) (4)
CO + H2O = CO + H2 (+ 42 MJ/kg mole) (5)
C + 2H2 = CH4 (+ 75 MJ/kg mole) (6)
CO2 + H2 = CO + H2O (- 42.3 MJ/kg mole)(7)
Since these reduction reactions are endothermic in nature it tends to reduce the temperature of the reduction zone. The reduction zone temperature is in the range of 750 ℃ - 1000℃. The calorific value of the producer gas is determined by the temperature of the reduction zone.
An updraft gasifier is one of the oldest established gasifier types. In an updraft gasifier, the feedstock is fed from the top of the reactor and oxidizer is injected from the bottom end of the reactor. In this type of gasifier, the reduction zone lies between combustion zone and pyrolysis zone. In this type the pyrolysis gases do not pass through hot bed of char and hence more amount of tar is produced. The syn gas, tar and volatiles disperse at the top and ash is disposed from bottom. The exit temperature in an updraft gasifier is lower than that of a downdraft gasifier. The updraft gasifier has several distinct advantages such as small pressure drop, high thermal efficiency and it has a little tendency towards slag formation.
The concept of the HTAG technology is preheating the oxidant (air stream, oxygen, steam) to a high temperature about some degrees for efficient operation of the gasifier. The preheating of feed gas can be done by means of a regenerative air/steam preheater known as HTAG refractory regenerators. It is a beneficial approach in gasification technology. In gasification of biomass, the material is subjected to thermal decomposition by heating the feedstock to high temperatures where the feedstock experiences chemical and physical changes which results in production of volatile gaseous products. The amount of gas that have been produced and their compositions depend on the reactor temperature, type, and characteristics of fuel material.
The char gasification stage is the rate limiting in the gasification of biomass because the devolatilization stage is very fast. The composition of the final product gas is also dependent on the various gas-phase reactions. In the absence of a catalyst, gasification of char with reactive gases such as O2, H2O and CO2 occurs at higher temperatures according to the following reaction:
Char + limited oxygen Gas +Tar + Ashes
Energy will be increased in the gasifier by the use of highly preheated air or steam which influences thermal decomposition. The operating parameters, temperature and the syn gas production will vary for the feedstock that is used throughout. The production of tars and other residues that are associated with it will be decreased by the increase in feed gas temperature.
High Temperature Air Gasification system (HTAG) is a revolutionary gasification system, which achieves outstanding fuel saving by highly preheated air gasification and high efficiency waste heat recovery. HTAG of biomass wastes results in low environmental hazards compared to other techniques. At low oxidizer temperature, the feedstock is supplied in large quantities to produce the required heat. In case of HTAG, and the concentrations of diluting agents like CO2 and NOx will be low thereby increasing the heating value of the synthesis gas. This high calorific gas will in turn increase the efficiency of the gasification process. From past researches it is inferred that the NOx emissions can be lowered up to 40 % compared to that of low temperature gasification.
The design of an updraft gasifier is also based on assumptions which are based on the type of fuel that is used as a feed. Initially time taken for gasification is taken as one hour and density of fuel as 165 kg/m³.
Specific gasification rate (SGR):
It always ranges between 100-200kgh/m². Hence here we assumed as 104kgh/m².
Fuel consumption rate (FCR):
It is the amount of fuel consumed per hour.
Here we have taken as 10kg/h.
Reactor height (H):
The total distance from top to bottom end of reactor is considered to be reactor height.
Height, H = SGR*T/Density
H = 104*1/165 = 0.63m.
Reactor diameter (D):
The reactor diameter refers to the size of reactor in terms of diameter of cross-section of cylinder.
Diameter, D = (4*FCR/SGR*π) 0.5
D = (4*10/104*3.14)0.5 = 0.35m.
Reactor area = 3.14*0.35*0.35/4 = 0.09616m².
Reactor volume = Area*Height = 0.0605m³.
Air flow rate (AFR):
The rate of flow of air needed to gasify the fuel is known as air flow rate.
AFR = ξ*FCR*SA/ρ = 0.3*10*4.905/1.25 =11.748m³/h.
Superficial air velocity:
The speed of air flow in fuel bed is known as superficial air velocity.
Superficial air velocity = air flow rate/ area of the reactor
=11.748/0.09616
=122.171m/h.
Thickness of the insulation material:
For higher efficiency, the material used for insulation is alumina silicate.
Thickness of Refractory Material=110mm
The HTAG refractory setup is the main part of the high temperature air gasification technology. A five meter length copper tube is feed in a spiral shape inside the gasifier. The copper tube is reinforced with the 80 kilograms of the refractory cement. The refractory cement is placed for cm thickness around the copper tube. The outlet of the spiral copper tube is connected to the oxidizer inlet of the gasifier setup.
This HTAG refractory setup traps the heat from the combustion chamber and transfer the traped heat he oxidizer air passes through it. It act as a tool for the waste heat recovery. It can achieve low NOx emissions more than 40%.
Biomass is defined as any material that can be used as a fuel or as a raw material for the fuel, that is derived from the recently living organisms, it excludes fossil fuels and includes materials like agricultural and forestry wastes, black liquor, sewage sludge and animal refuse [9]. A gasifier is very fuel specific and it is based on the type of fuel used.
Proximate analysis is the means of determining distribution of products when the coconut shell is heated at a particular temperature. As defined by ASTM D 121, proximate analysis separates the products into four groups: (1) moisture, (2) volatile matter, consisting of gases and vapours driven off during pyrolysis, (3) fixed carbon, the non volatile fraction of coconut shell, and (4) ash, the inorganic residue remaining after combustion [2]. Proximate analysis is the most often used analysis for characterizing coals in connection with their utilization.
Biomass sample is first grounded to fine powder and this part is divided into three parts for determination of moisture content, carbon content and ash content. Fine powdered biomass is kept in an open crucible at 100°C for 2 minutes to determine the moisture content. Then the moisture content is determined by the difference between initial weight and final weight. Another sample is kept at a temperature of 200°C for duration of about 4 minutes. The differential weight of the fuel gives the carbon content. Ash content can be determined by placing at a temperature of 400°C for a time period of 6 minutes. The weight of the residue gives the total ash content of the biomass fuel that is used.
The updraft gasifier has been designed with an insulation thickness of 110 mm and with the reactor height of about 0.63 m and the reactor diameter is about 0.35 m. The fuel consumption rate is about 10kg/h. The temperature profile is followed by a set of four thermocouples situated at different heights in the gasifier. The biomass that is used in the experiment is coconut shell.
The experiment is conducted on the convectional biomass gasifier. The experimental observations are taken for every 10 minutes as the gasification process consists of three phases: solid, liquid and gas. The transition could only be denoted by means of varying temperature at various zones such as drying zone, pyrolysis zone, combustion zone and reduction zone of the reactor. The range at which the conversion of charcoal takes place is where the gas production is high and comparatively the syn gas production gets lowered after the reaction. Fig 5.1 shows the production of syn gas from the gasifier which gets ignited and burning a paper.
In this part the performance of the conventional gasifier and the performance of the gasifier using HTAG technology were separately discussed and compared.The variations according to temperature and the constituents of the syn gas that are produced are analysed and compared graphically.
Fig 3.1 Temperature Profile of Gasification Process.
Figure 3.1 indicates the normal gasification process in which the rate at which syn gas production takes place at a constant rate. This denotes the temperature profile of the gasification process at different zones of the gasifier. Syn gas produced at 420 ̊C and tends to reduce after a certain period of time.
Fig 3.2 Ultimate Analysis of Gasification Process.
Fig 3.2 indicates the constituents of the syn gas that has been produced. The values are recorded for duration of 10 minutes so that their chemical composition shows clear variations. CO2 which is a main constituent of pollutant decreases during production of syn gas at a temperature of 420̊C and other concentrations increase for the continual production of syn gas.
Fig 3.3 Temperature Profile of Gasification Process using HTAG Technology.
Fig 3.3 shows the temperature profile of the gasification process using HTAG technology in which the feed air is preheated so that there is an increase in efficiency of the gasifier compared with that of the conventional gasifier. The gasifier with which HTAG technology is used shows an increase in temperature of about 60 °C. The reaction time is much faster for a specific period of time and then when the temperature of the oxidizer is reduced the reaction also further becomes as that of conventional gasifier. A significant change that is noticed is the reduction of tar and other volatile compounds.
Fig 3.6 Comparison of H2 gas with HTAG technology and conventional combustion.
Fig 3.6 indicates the H2 production and comparison of the conventional gasifier and the gasifier with HTAG technology. Hydrogen increases with increase in temperature. The H2 is an indicator for the production of secondary tar reactions.
Fig 3.7 indicates the CO production and comparison of the conventional gasifier and the gasifier with HTAG technology. Caron monoxide increases with increase in temperature. The CO is an indicator for the production of secondary tar reactions.
Fig 3.7 Comparison of CO gas with HTAG technology and conventional combustion.
Fig 3.8 Comparison of O2 gas with HTAG technology and conventional combustion.
Fig 3.8 shows theO2 production and comparison of the conventional gasifier and the gasifier with HTAG technology. Without oxygen the production of syn gas will be reduced.
The design of the gasifier and type of feedstock are very important in the process of gasification.The synthesis gas that has been produced using HTAG technology has a outlet temperature of 212 °C and the constituents that are majorly considered for pollution are reduced in comparison to that of the conventional gasifier.
The amount of feed that is used i.e., coconut shell can be reduced as ash and the temperature plays a vital role in increasing productivity. Reduction of tar and soot is obtained.The flame colour, size and shape depend on the combustion parameters.
The reactor size can be reduced as the amount of feed is reduced.Efficiency of the gasifier is increased,i.e., for same rate of feed as that of conventional gasifier twice that of syn gas can be produced due to high temperature of the oxidizer used.During the combustion of coconut shell, the gas is produced at a temperature of 360 ̊C in a conventional gasifier and when air is preheated the same phenomenon is observed but the overall biomass content required is less and the rate of reaction is much faster than the conventional type of gasification.
The biomass gasifier usingHTAG technology with the waste heat recovery posses a very efficient operation than the conventional gasifier. In this work an attempt has been made to develop an efficient operational gasifier which can be used by the common people for various applications. Main objective of this work is to reduce the formation of tar, reduce ignition delay and increase the efficiency. When the experimental designs are carried out properly, the expected outcome can be achieved and it would be a most desirable process. It provides best performance and reduces pollution as it uses biomass as feed.
Efficiency Enhancement of Biomass Gasification through High Temperature Air Gasification (HTAG) Technology. (2024, Feb 20). Retrieved from https://studymoose.com/document/efficiency-enhancement-of-biomass-gasification-through-high-temperature-air-gasification-htag-technology
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