Cinder Agglomeration Fluidized Bed Pulverized Coal Gasification Technology (CAGGTM) and Its Industrial Applications

Thread Content

0 Introduction The Gray Fusion Fluidized Bed Coal Gasification Technology (CAGGTM) is a coal gasification technology with independent intellectual property rights in China, developed by Shaanxi Qinen Tianji Technology Co., Ltd. and Shaanxi ** Coal Gasification Technology Co., Ltd. (Patent numbers: ZL03262731.9, ZL200420085932.2, ZL200520079458.7, ZL200520079695.3). This powder coal gasification technology boasts advantages such as mature technology, good performance, environmental friendliness, and a wide range of suitable raw materials. In 2004, the **National Development and Reform Commission, the Nitrogen Fertilizer Industry Association, and other organizations officially recommended this technology to small and medium-sized fertilizer manufacturers across the country as the preferred method for localizing the use of raw coal in production. 1 Introduction to Ash-Melting Fluidized Bed Pulverized Coal Gasification Technology: In conventional fluidized bed coal gasification processes, it is necessary to maintain a certain carbon content in the furnace (usually above 40% in order to preserve a reducing atmosphere within the furnace). Moreover, under fluidized conditions, it is difficult to separate the slag from the material layer, and both the ash and fly ash have relatively high carbon contents. The gasification temperature is often kept below 950°C; at this temperature, only coal types with high reactivity, such as lignite and bituminous coal, can be used to produce gas of good quality. The ash agglomeration fluidized bed coal gasification technology utilizes the principle of jets to incorporate an ash agglomeration and separation device at the bottom of the fluidized bed, thereby creating localized high-temperature areas within the bed. This allows the ash to agglomerate into balls, and the difference in weight is used to separate these ash balls from the semi-coke. As a result, low-carbon content ash can be continuously and selectively discharged without the formation of slag. Consequently, the carbon content within the bed remains high, and the bed temperature stays at 950–1100°C, which broadens the range of coal types that can be used. 1.1 Characteristics of the gray fusion fluidized bed coal gasification technology: (1) It is suitable for a wide range of coal types, and can be used for gasifying various feedstocks such as lignite, bituminous coal, anthracite, and coke dust ； (2) Can use coal dust as a raw material, and is suitable for the gasification of coal with high ash content, high sulfur content, high ash melting point, and low activity ； (3) The gasification furnace has a simple structure; it is a single-stage fluidized bed with no rotating components inside, making it easy to manufacture and maintain ； (4) The high-temperature zone at the center of the gasification furnace causes the ash to agglomerate into ash balls, enabling effective separation between coal powder and these ash balls; this improves the conversion rate of carbon and reduces the carbon content in the ash ； (5) The fly ash entrained in the gas is separated and recovered by the first-stage cyclone dust collector and returned to the furnace for further combustion and gasification, thereby increasing the carbon conversion rate ； (6) The gas contains no tar, the washing wastewater has a low phenol content, and purification is simple ； (7) High bed temperature, high gasification intensity ； (8) Easy to control, reliable in operation ； (9) This technology possesses independent intellectual property rights in China, and the equipment can be entirely manufactured domestically. At the same scale, compared with foreign gasification technologies, the investment required is only about 50% of that. 1.2 Ash fusion polymerized fluidized bed pulverized coal gasification process The process flow of the ash fusion polymerized fluidized bed pulverized coal gasification process is shown in Figure 1, and it mainly includes systems for coal feeding, gasification, dust removal, and waste heat recovery. (1) Coal feeding system: The raw coal powder with a particle size of 0–Φ6 mm (with a moisture content kept below 5%) supplied by the coal preparation system enters the coal hopper, coal lock, and feed hopper in sequence. The amount of coal fed is controlled by screw conveyors, and the coal powder is blown into the lower part of the gasification furnace using nitrogen or carbon dioxide. (2) Gasification system: In the gasifier, pulverized coal goes through five main processes: drying and dehydrating the coal, removing volatiles, gasifying the coal, fusing the ash and slag, and separating the ash and slag from the bed. Steam and oxygen enter through the distribution plate, primarily to fluidize the bed material. Steam and oxygen enter through the central tube, creating a high-temperature combustion zone near the nozzles that causes the ash to agglomerate into balls. Steam and oxygen enter through the annular tube; at a certain gas flow rate, this facilitates the separation of the ash balls from the semi-coke. Under conditions without slag formation, low-carbon ash can be continuously and selectively discharged. In the gasification furnace, pulverized coal reacts with the gasifying agents oxygen and steam to produce gases such as CO, H2, CH4, CO2, and H2S; the resulting gas is drawn out from the top. The upper part of the gasification furnace has a larger diameter, which reduces the flow velocity of the airflow containing fine ash; most of the fine ash and the partially reacted semi-coke fall to the lower part of the furnace to continue reacting, while only a portion of the fine ash and semi-coke is carried out of the furnace with the airflow. (3) Dust removal system: The high-temperature gas discharged from the upper part of the gasification furnace enters two-stage cyclone separators in sequence. The hot fly ash separated from the first-stage separator is controlled by high-temperature-resistant valves and blown into the gasification furnace using steam through a material leg to undergo further combustion and gasification, thereby increasing the carbon conversion rate; a small amount of fly ash separated from the second-stage separator is discharged outside the furnace. (4) Waste heat recovery system and gas purification system: The hot gas, after being dust-separated by a cyclone, enters the No. 1 waste heat boiler, the steam superheater, the No. 2 waste heat boiler, and the boiler feedwater preheater in sequence; finally, it goes to a scrubber for washing and cooling, and the resulting gas is sent to the next processing stage. 2 Industrial application of ash fusion fluidized bed pulverized coal gasification technology: Through industrialization and demonstration projects, the atmospheric-pressure ash fusion fluidized bed pulverized coal gasification technology has become increasingly mature and reliable. The Tianjiang project began operations in June 2005, while the Taihua project started operating in January 2007. The detailed design and procurement of key equipment for the Tianji Luhua project have been completed, with operations scheduled to begin in 2008. Furthermore, the innovation project heavily supported by the National Development and Reform Commission – the industrial demonstration unit for gasification using ash fusion powder under a pressure of 1.0 MPa – began operation in January 2007 and has performed well. Below, the industrial application of the ash fusion fluidized bed pulverized coal gasification technology will be discussed with a focus on energy consumption. 2.1 The gray fusion fluidized bed coal gasification technology features good environmental performance: (1) The dust-containing exhaust gases generated at various dust discharge points in the coal conveying system are cleaned using box-type and bag filters before being released; the coal powder collected by these filters is then returned to the system. (2) The ash fusion fluidized bed powder gasifier produces gas continuously; under normal operation, no waste gases are emitted from the facility, except during furnace drying, at the beginning of operation, and in case of accidents. (3) The slag from the gasification furnace can be handled in a sealed manner, preventing dust from being generated; the resulting slag can be used as raw material for manufacturing building materials. (4) Secondary cyclone dust removal allows for sealed collection and treatment, or emission after humidification. It can be processed into shaped coal as a feedstock for fixed-bed gasifiers, or it can be transported in a sealed manner to circulating fluidized bed boilers for combustion, or used as a raw material for the production of chemical products such as carbon black. (5) Through primary and secondary cyclone dust removal as well as advanced dust removal, it is possible to ensure that the pollutant levels in the gas washing water meet environmental protection requirements. Since the wastewater contains only compounds such as NH3-N, COD, and volatile monophenols, without tar, it can be simply stripped and reused within the system, achieving zero emissions. 2.2 Main energy-saving measures adopted in the ash fusion fluidized bed pulverized coal gasification technology and their effects (1) The ash fusion fluidized bed pulverized coal gasification technology uses continuous pulverized coal gasification; compared with traditional batch gasification, there is no need to vent the purge gas, which represents the greatest energy-saving and emission-reduction advantage. (2) The thermal energy of the high-temperature gas exiting the gasification furnace can be recovered in large quantities; there is an excess of steam generated as a by-product. For each ton of ammonia produced, 0.75 tons of steam at 1.3 MPa can be supplied externally, worth 45 yuan. The gas cooler at the outlet of the gasification furnace uses soft water as a cooling agent, allowing for the recovery of 2512.08 MJ/h of heat; this translates to 33.2 kg of standard coal being consumed per ton of ammonia, equivalent to 10.29 yuan. The combined value of the thermal energy recovery from ammonia in these two items is 55.29 yuan per ton. (3) The fine ash captured by the secondary cyclone dust collector can be used as a raw material for producing chemical products such as carbon black; 96 kg of fine ash can be recovered per ton of ammonia, worth 27.96 yuan. (4) For the selection of pumps, **recommended energy-saving products** should be used. (5) Insulation measures should be applied to high-temperature gas pipelines to reduce heat loss and lower energy consumption. 2.3 Energy consumption in atmospheric pressure ash fusion fluidized bed gasification technology Below is a comparison of the energy consumption per thousand cubic meters of gas produced by the atmospheric pressure ash fusion gasification furnace and the UGI furnace, as shown in Table 1. As can be seen from Table 1, the energy consumption per thousand cubic meters of gas produced by atmospheric pressure ash fusion gasification is 2,242,563.12 kJ lower than that of the UGI furnace; among the various components, the oxygen production unit has a high energy consumption. Overall, the energy consumption per thousand cubic meters of gas for atmospheric pressure ash fusion gasification is slightly lower than that of the UGI furnace. 2.4 Energy consumption in pressurized ash fusion fluidized bed powder coal gasification technology The advantages of the pressurized ash fusion fluidized bed powder coal gasification technology over the atmospheric pressure version are: (1) increased gasification efficiency ； (2) The gasification capacity per furnace increases ； (3) Save compressed air energy consumption ； (4) Reduce carryaway loss. It can be seen from this that developing pressurized ash fusion fluidized bed powder gasification technology is of great significance for development. The energy consumption per thousand cubic meters of gas in a pressurized 1.0 MPa gray fusion gasification furnace is listed below, as shown in Table 2. As shown in Table 2, the ash pyrolysis gasifier operating at 1.0 MPa has a lower energy consumption per thousand cubic meters of gas produced compared to UGI by 4,631,108.36 kJ, and it also has a lower energy consumption per thousand cubic meters of gas produced compared to the ash pyrolysis gasifier operating at atmospheric pressure by 2,388,545.24 kJ. By comparison, it can be seen that the pressurized ash fusion fluidized bed pulverized coal gasification technology has greater advantages in terms of energy savings. 2.5 Advantages of the industrial application of oxygen-enriched ash fusion fluidized bed powder coal gasification technology: For ammonia synthesis plants, the ash fusion fluidized bed powder coal gasifier can also be used for oxygen-enriched gas production. Compared to pure oxygen oxidation technology, the oxygen-enriched gas production technology reduces investment costs, simplifies associated processes, and saves on related expenses. The advantages of oxygen-enriched oxidation technology are mainly reflected in the following aspects: (1) The scale of the air separation unit required is smaller, resulting in a significant reduction in investment costs for air separation ； (2) The composition of the oxygen-enriched gas can be easily adjusted to (CO+H2)/N2≈3.1; no modification is required for the original compressor, and a nitrogen compressor can be omitted. (3) The CO2 content in gas produced by pure oxygen oxidation is 23%–25%, whereas it is 12% in gas produced by oxygen-enriched oxidation; the methane content also decreases. While maintaining a certain scale of ammonia synthesis, the gas decarburization load required by oxygen-enriched oxidation technology is much lower than that of pure oxygen oxidation ； (4) The oxygen-enriched oxidation retrofit UGI system shares the same characteristics as pure oxygen oxidation, enabling local sourcing of raw materials, increasing gas production per furnace, and saving energy and reducing consumption. 3 Conclusion At present, in China’s nitrogen fertilizer industry, particularly among medium and small-sized nitrogen fertilizer enterprises, the batch gas production process used not only results in high raw material costs and a short effective gas production time as well as low gasification capacity per furnace, but also leads to the discharge of large amounts of flammable gas, causing severe environmental pollution. Therefore, enterprises are in urgent need of a new type of gasification technology that enables local sourcing of raw materials, has an appropriate coal gas production capacity, allows for the use of domestically produced equipment, results in lower production costs, and is environmentally friendly. Practice has shown that the ash fusion fluidized bed powder coal gasification technology holds certain advantages in the local transformation of raw coal used by medium and small nitrogen fertilizer enterprises in China.