What is a two-stage furnace?
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What is a two-stage furnace? What are the differences and advantages/disadvantages compared to a normal gas generator? ? ?
Technical features: The structure of the vaporization furnace is shown in Figure 5. This process is a form of partial oxidation of pulverized coal. The gasifier is divided into two reaction zones; the lower section of the gasifier constitutes the first reaction zone, where pulverized coal (along with N2, O2, and steam) undergoes high-temperature chemical reactions to produce wet gas. Thereafter, wet gas at temperatures of 1400–1600°C (depending on the type of coal) enters the upper two-stage reaction zone of the gasifier. There, pulverized coal and steam (without oxygen) use the sensible heat from the gas from the first stage to carry out reactions such as the pyrolysis of coal (to remove volatiles), the vaporization of volatiles, and the vaporization of carbon, thereby producing additional gas. The unburned carbon (fly ash) from the second reaction zone is captured and returned to the first-stage coal hopper for re-combustion. If no feed is supplied to the second reaction zone of this furnace, it is the same as current single-stage gasification furnaces (Shell furnaces and PRENFLO furnaces). Figure 5 Schematic diagram of the two-stage gasifier structure. Compared with foreign dry coal powder gasification processes, the two-stage dry coal powder gasification process has the following advantages: Greater fuel adaptability. From anthracite, bituminous coal, lignite to petroleum coke, all can be gasified, and it has a wider range of ash fusion points for coal compared to other dry gasification processes. It is also suitable for coal types with high ash content, high moisture content, and high sulfur content. Due to the use of a two-stage gasification technique, in order to facilitate slag discharge, it is not necessary for the temperature in the entire reaction zone to exceed the ash fusion point; it is sufficient to ensure that the temperature in the first reaction stage exceeds this value. Due to the small space in a certain reaction zone, it is possible to increase the reaction temperature at a lower oxygen-coal ratio. Cold gas offers higher efficiency and lower oxygen consumption. Since only coal powder and steam are fed into the second stage without oxygen, pyrolysis and gasification reactions of coal occur in this stage, thereby enabling the production of more efficient gas without the need for oxygen. Thereby improving cold gas efficiency and reducing oxygen consumption. Unlike dry gasification methods such as those used by Shell, the water quenching process advocated by well-known domestic experts is employed in the chemical synthesis process; as a result, there is no need to install additional circulating cold gas pumps, which reduces power consumption. Compared with the water-coal slurry gasification process: Dry coal powder is fed in, resulting in higher gasification efficiency. The efficiency of cold gas feeding is about 10 percentage points higher than that of wet feeding. Wide coal suitability: Bituminous coal, subbituminous coal, lignite, and even petroleum coke can all be gasified; there are almost no requirements regarding the activity of the coal, and the range of ash fusion points for coal is wider compared to other gasification processes. Coal types with high ash content, high moisture, and high sulfur content can also be gasified, but the economic efficiency is slightly lower. For coals with high ash fusion point and high ash viscosity, the fluidity of the slag can be improved by adding fluxes in order to enhance the economic efficiency of gasification operations. High vaporization temperature: The vaporization temperature is around 1400–1700°C; at such high temperatures, the carbon conversion rate reaches 99%. The resulting gas is relatively clean, free of heavy hydrocarbons, with a very low methane content. The quality of the gas is excellent, as the proportion of useful gases (CO + H2) in it exceeds 90%. Low oxygen consumption: Compared with wet feed water-coal slurry gasification, the oxygen consumption is lower (15%～25%), which allows for a relatively reduced investment in the air separation unit required for this process. Pressurized operation with high production capacity per furnace: The gasification pressure of the currently operational furnaces is 3.0 MPa, allowing for a daily handling capacity of 2,000 tons of coal. The gasification furnace has no refractory brick lining, resulting in less maintenance needs. There are no moving parts inside the furnace, so its operational life is long, and no backup furnace is required. Strong production adjustment capability and long continuous operation cycle: The use of multiple burners enhances the reliability of the gasification process as well as its production adjustment capabilities. The patented gasification burner design of Shell ensures a service life of 8,000 hours. The burners used in the Demkolec power plant in the Netherlands have been in operation for nearly 10,000 hours without needing to be replaced, with a total operating time of over 7,500 hours – which provides a foundation for the long-term operation of gasification units. Affected by the pressurized feed, the maximum gasification pressure is not as high as that of wet gasification. The operating pressure for wet gasification is generally 2.8~6.5 MPa, with a maximum of up to 8.5 MPa, which helps to save energy. Due to the limitations imposed by the method of feeding pulverized coal, the gasification pressure in dry gasification is generally below 4.0 MPa. In the first reactor, the following main chemical reactions occur: CmHn+(m+n/4)O2==mCO2+n/2H2O; CmHn+m/2O2==mCO+n/2H2; CmHn+mH2O==mCO+(m+n/2)H2; CmHn+mCO2==2mCO+n/2H2; C+O2==CO2; C+1/2O2==CO; C+H2O==CO+H2; C+CO2==2CO; CO+H2O==CO2+H2. In the second reactor, the following main chemical reactions occur: CmHn+mH2O==mCO+(m+n/2)H2; CmHn+mCO2==2mCO+n/2H2; CH4+H2O==CO+3H2; CH4+CO2==2CO+2H2; C+H2O==CO+H2; C+CO2==2CO; CO+H2O==CO2+H2
The two-stage gasifier is a technology developed by Xi’an Thermal Power Institute, while there are also the staged gasification technologies from Tsinghua University and Beijing Dalike. Check it out.
There are definitely two sections, with material addition at two locations and two reaction zones. Xi’an Thermal Power Institute uses a two-section furnace, and DOW also has a two-section furnace, but it uses refractory bricks made of water-coal slurry
There is some information on the forum at http://bbs.hcbbs.com/thread-183832-1-1.html for reference
The two-stage gas generator is designed as a counterpart to the single-stage gas generator. Single-stage gas generators have lower efficiency; coal tar decomposes into asphaltic tar at high temperatures, and this tar mixes with the dust in the gas, making it easy for deposits to form within the pipes and cause blockages. As a result, these traditional single-stage generators are only suitable for kilns and industrial furnaces that require short transportation distances and have modest fuel requirements. In the gas produced by single-stage gas generators, the tar present in the gas has undergone high-temperature cracking, and it cannot be removed using electrostatic precipitators. As a result, users who rely on purified gas must use large amounts of water to wash the gas; this washing water contains phenolic substances, which can easily cause environmental pollution.
Two-stage gas generators have upper and lower gas outlets. First, coal enters the furnace from the coal bin at the top of the furnace through two sets of coal feeding valves. In the dry distillation section, the coal is thoroughly dried and distilled, gradually forming semi-coke. It then enters the gasification section, where the hot semi-coke reacts fully with the gasifying agent introduced from the bottom of the furnace. After passing through the reduction and oxidation layers within the furnace, the gas is vaporized. The ash and slag are automatically discharged from the furnace via a grate. During the dry distillation process, volatile substances are released, forming the upper-stage dry distillation gas, which accounts for about 40% of the total gas volume. This gas has a high calorific value (7400 KJ/NM) and a relatively low temperature (120°C), and it contains large amounts of tar. This tar is a product of low-temperature dry distillation; it has good fluidity and can be captured using electrostatic precipitators, where it can be used as a chemical raw material or fuel.
In the gasification section, the hot semi-coke and the gasifying agent undergo a series of chemical reactions such as oxidation and reduction, resulting in the formation of gas known as the lower-stage gas. This gas accounts for about 60% of the total gas volume. It has a relatively lower calorific value (6000 KJ/NM) but a higher temperature (450°C). Since the coal undergoes sufficient low-temperature dry distillation in the previous stage, the coal that enters the gasification section is already in the form of semi-coke, so the resulting gas contains little to no tar. The gas from the bottom of the furnace passes through devices such as cyclone separators and air coolers to remove dust and cool it down, after which it enters an intercooler. There, it merges with the upper-stage gas and is further purified using electrostatic precipitators, ensuring the quality of the purified gas and meeting the needs of users in their production processes.
Two-stage gas generators have upper and lower gas outlets. First, coal enters the furnace from the coal bin at the top of the furnace through two sets of coal feeding valves. In the dry distillation section, the coal is thoroughly dried and distilled, gradually forming semi-coke. It then enters the gasification section, where the hot semi-coke reacts fully with the gasifying agent introduced from the bottom of the furnace. After passing through the reduction and oxidation layers within the furnace, the gas is vaporized. The ash and slag are automatically discharged from the furnace via a grate. During the dry distillation process, volatile substances are released, forming the upper-stage dry distillation gas, which accounts for about 40% of the total gas volume. This gas has a high calorific value (7400 KJ/NM) and a relatively low temperature (120°C), and it contains large amounts of tar. This tar is a product of low-temperature dry distillation; it has good fluidity and can be captured using electrostatic precipitators, where it can be used as a chemical raw material or fuel.
In the gasification section, the hot semi-coke and the gasifying agent undergo a series of chemical reactions such as oxidation and reduction, resulting in the formation of gas known as the lower-stage gas. This gas accounts for about 60% of the total gas volume. It has a relatively lower calorific value (6000 KJ/NM) but a higher temperature (450°C). Since the coal undergoes sufficient low-temperature dry distillation in the previous stage, the coal that enters the gasification section is already in the form of semi-coke, so the resulting gas contains little to no tar. The gas from the bottom of the furnace passes through devices such as cyclone separators and air coolers to remove dust and cool it down, after which it enters an intercooler. There, it merges with the upper-stage gas and is further purified using electrostatic precipitators, ensuring the quality of the purified gas and meeting the needs of users in their production processes.
The two-stage gas generator has upper and lower gas outlets, enabling the production of gas with different calorific values. Its gasification efficiency and overall thermal efficiency are higher than those of single-stage generators. The coal undergoes complete carbonization in the upper section of the furnace; the gas produced in the lower section contains little tar, while the gas from the upper section contains a small amount of light tar, which reduces the risk of pipeline blockages. The gas produced by this two-stage generator has a high and stable calorific value, offers great operational flexibility, features a high degree of automation, and results in lower labor intensity. Two-stage furnace gas stations do not pollute the environment, save a significant amount of water, require little space, and have low operating costs over the long term. In this type of furnace, the grate rotation and coal feeding mechanisms can be either mechanical or hydraulic, depending on the user’s requirements. The grate is made of heat-resistant cast iron, which provides good air permeability and is suitable for gasification processes using smoke gas.
The common specifications for two-stage gas generators are: 2.0M, 2.6M, 3.0M, 3.2M, 3.4M, 3.6M.
(1) The gas at the bottom of the furnace is drawn out through 36 refractory channels, and there are 6 bottom gas control valves used to regulate the combustion balance across the entire furnace volume. (The number of refractory channels increases accordingly depending on the type of furnace.) (2) An additional central pipe is provided for extracting gas from the bottom; its functions are as follows: a) It, together with the 36 surrounding refractory channels, forms a retorting heating space, creating radial heat sources in both inner and outer circular layers. b. Together with the 36 surrounding fire-resistant channels, it forms a combustion balance system for the furnace cross-section, avoiding the problem of black holes at the combustion center in domestic two-stage furnaces, and allowing for easy adjustment of the furnace’s combustion conditions. (3) It uses a high-ash basin water seal and operates at high gasification pressure, resulting in a high degree of gasification of the generator gas and a large gas production volume. (4) The grate drive for ash removal and coal feeding utilizes a hydraulic system to achieve automatic control. (5) The water jacket is a pressure vessel with a very long service life, and the steam generated internally meets the production needs.
The common specifications for two-stage gas generators are: 2.0M, 2.6M, 3.0M, 3.2M, 3.4M, 3.6M.
(1) The gas at the bottom of the furnace is drawn out through 36 refractory channels, and there are 6 bottom gas control valves used to regulate the combustion balance across the entire furnace volume. (The number of refractory channels increases accordingly depending on the type of furnace.) (2) An additional central pipe is provided for extracting gas from the bottom; its functions are as follows: a) It, together with the 36 surrounding refractory channels, forms a retorting heating space, creating radial heat sources in both inner and outer circular layers. b. Together with the 36 surrounding fire-resistant channels, it forms a combustion balance system for the furnace cross-section, avoiding the problem of black holes at the combustion center in domestic two-stage furnaces, and allowing for easy adjustment of the furnace’s combustion conditions. (3) It uses a high-ash basin water seal and operates at high gasification pressure, resulting in a high degree of gasification of the generator gas and a large gas production volume. (4) The grate drive for ash removal and coal feeding utilizes a hydraulic system to achieve automatic control. (5) The water jacket is a pressure vessel with a very long service life, and the steam generated internally meets the production needs.
The main feature of a two-stage furnace is that it divides the gasification of coal into two reaction zones, or combustion zones. The coal that has not been completely burned in the first reaction zone undergoes further combustion in the second reaction zone, which also increases the time that the coal stays in the gasification furnace to some extent, thereby improving the conversion rate of carbon. For powder coal gasifiers, it can reduce the amount of ash carried; for water-coal slurry gasifiers, it enables better contact between coal particles and oxygen.
There are two types of two-stage furnaces: 1. Tsinghua water-coal slurry two-stage furnace (Shanxi Fengxi Group); 2. Fixed-bed two-stage furnace (the lower section uses jacketed saturated steam and air as gasifying agents, with 60% of the generated gas being sent out, while the remaining 40% is sent to the upper section for coal carbonization; the semi-coke produced in the upper section is then sent to the lower section for gasification); 3. Two-stage furnace developed by Xi’an Thermal Power Institute. Please be careful not to get them mixed up!