A Brief Discussion on the Selection of Brown Coal Gasification Technologies

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Lignite reserves account for 20% of the world’s total coal reserves, while in China, lignite makes up 17% of the total coal reserves. In the past, lignite was not utilized sufficiently. With the increasingly strained global energy situation, how to make full, effective, and rational use of lignite has become a global issue that requires further research. **The National Development and Reform Commission and the Ministry of Science and Technology have listed “the development of lignite gasification technology and the use of lignite to produce chemical products such as methanol” as one of the key projects in the ‘Outline of China’s Energy Conservation Technology Policies’ issued in 2007. Simply in terms of the feasibility of lignite gasification, the existing typical lump/coal briquette moving-bed gasification technologies (such as Lurgi and BGL), coal pulverization fluidized-bed gasification technologies (such as the Endler furnace, Winkler, and HTW), and pulverized coal fluidized-bed gasification technologies (such as GE’s single-nozzle water-coal slurry gasification, the four-nozzle water-coal slurry gasification developed by East China University of Science and Technology, Shell’s multi-nozzle dry coal powder gasification, and GSP’s single-nozzle dry coal powder gasification) can all be used for gasifying lignite, but each has its own characteristics in terms of applicability. 1 Considering the characteristics of the coal type, the selected gasification technology must be suitable for the intended lignite type and should enable the easy gasification of lignite. The industrial analysis, elemental analysis, ash composition analysis, ash melting point, and chemical reactivity data of a certain lignite, as determined through experiments conducted by the Northwest Research Institute of Chemical Engineering, are shown in Tables 1 to 5 respectively. Tests determined that the grindability index HGI of this lignite is 86, its calorific value Qb.ad is 16910 kJ/kg, and the maximum slurry concentration of the raw coal is 44%. Furthermore, no tests on the adhesiveness of coal and ash were conducted for this coal type ； For reference, data obtained from coal mine exploration show that the quality of this coal is unstable, with significant variations of 2 to 3 times in indicators such as ash content and calorific value. It can be seen that this type of coal has a high moisture content, high ash content, high volatile matter content, low fixed carbon content, low calorific value, poor thermal stability, good reactivity, low mechanical strength, good grindability, low slurry concentration, low ash fusion point, and unstable coal quality. 1.1 Moisture, fixed carbon, and calorific value: As undesirable components, the moisture content in lignite has varying effects on different coal gasification technologies. For water-coal slurry gasification, the higher the moisture content in the coal, especially the internal moisture, the worse the slurry-forming properties. Under the same coal slurry concentration, the apparent viscosity of the slurry is high, resulting in poor fluidity. To achieve better fluidity, the coal slurry concentration is generally kept low. The higher the moisture content in coal, the more likely slurry overflow occurs during coal grinding, the narrower the range of available additives, and the greater the consumption of additives. Furthermore, due to the low fixed carbon content and calorific value of this lignite, not only is the coal consumption per unit of useful gas high, but excessive heat is also consumed by the evaporation of moisture, making it difficult to achieve thermal balance within the gasifier. Currently, the highest slurry concentration of this lignite raw coal is only 44%, which is clearly too low. If effective measures cannot be taken to raise the pulp concentration to a certain level (at least 55%–57% or even higher, in order to ensure the proper progression of oxidation and gasification reactions in the gasifier as well as to maintain heat balance), then coal water slurry gasification is not a suitable option. For dry coal powder fluidized beds and crushed coal fluidized beds, high moisture content increases the thermal energy consumption during the gasification process; it also makes the transportation of dry coal powder and crushed coal more difficult, necessitating the pre-drying of the coal fed into the furnace to a certain extent. For moving bed systems, since the coal entering the furnace exchanges heat with the rising hot gas in the drying layer inside the furnace, the moisture in the coal evaporates as a result of this heat, so pre-drying is generally not required. This lignite has a low fixed carbon content and low calorific value, which are disadvantages for various coal gasification technologies. 1.2 Ash and ash melting point: Ash does not participate directly in the gasification reaction, but it consumes the heat generated by the oxidation reaction of coal. This type of coal has a high ash content in lignite, which relatively increases the specific coal consumption and specific oxygen consumption in gasification furnaces; it is therefore a disadvantageous factor for various existing coal gasification technologies. In gasification technologies such as GE’s coal slurry fluidized bed gasification process, which makes use of the most experienced and widely adopted quenching method in China, the ash formed after gasification is sent to an ash treatment system. Hard particles in the ash, such as SiO2 and Al2O3, can cause excessive erosion of pipes, valves, and equipment, leading to leaks. Alkaline components in the ash, like CaO and Fe2O3, along with fine ash present in the ash water, tend to deposit and cause fouling in certain parts of pipes and heat exchangers [1][2] ； The high ash content increases the solid content in the slag water, thereby increasing the load on the slag treatment system and raising its operational difficulties; energy consumption also rises as a result. In Shell’s dry coal powder fluidized bed gasification technology, which is already an industrialized process, part of the ash settles as slag within the gasifier, while another portion of the ash rises with the raw gas; after being quenched and solidified, it enters the waste pot and ceramic filter, where the majority of the ash is separated out. The amount of ash processed by the slag-water treatment system is small, and this system is relatively simple as well ； Regarding ash removal, scaling and damage issues in the ceramic filters, quench gas compressors, and waste boilers of the Shell technology affect the proper operation of Shell furnaces. The GSP dry coal powder fluidized bed gasification technology currently employs a quenching process, with all ash being sent to the slag treatment system via quenching chambers and washers, which is somewhat similar to GE’s gasification technology using a quenching process. When lignite with a high ash content is gasified using fixed-bed or fluidized-bed reactors, apart from affecting the gasification efficiency and economic benefits, it generally does not have any other significant adverse effects. This lignite has a low ash melting point, which is an advantage for all fluidized bed gasification processes; however, the advantage of dry coal powder fluidized beds with water-cooled walls, such as those used by Shell and GSP, in gasifying coals with high ash melting points has not been fully utilized. A low ash melting point is an unfavorable factor for fluidized beds, as the gasification process in such beds takes place below the ash melting point in order to prevent ash slugging and disrupt the normal fluidization of the material within the bed. For moving-bed reactors, a low ash fusion point is suitable for BGL furnaces with liquid slag discharge, but it is not advantageous for Lurgi furnaces with solid slag discharge, as the latter are prone to the formation of air pockets after slagging occurs during the gasification process. 1.3 Volatiles and Reactivity Generally speaking, high reactivity is a favorable factor for various coal gasification technologies. A high volatile matter content means that, in a moving bed system, the hot gas flow from bottom to top has to pass through a relatively cooler dry distillation section. The volatiles in coal – such as tar, phenols, ammonia, methane, etc. – are carried away with the gas. Tar can clog pipes and valves, while tar, phenols, and ammonia complicate the purification of the gas. Due to the high volatile matter content, the methane content in the gas after gasification is high; the advantages and disadvantages of this depend on the intended use of the gas. In fluidized bed gasification, due to the high reaction temperature, the aforementioned components are virtually absent in the gas; the volatiles in the coal undergo high-temperature pyrolysis to form active components. The fluidized bed itself requires coal types with good gasification reactivity, and high volatiles are also an advantage for the fluidized bed. 1.4 Mechanical strength, grindability, and thermal stability: Lignite has low mechanical strength and poor thermal stability. For moving-bed systems, lump coal/briquetted coal tends to break into smaller pieces or coal powder as it enters the furnace and moves downward within it; this increases the resistance inside the furnace, reduces the gasification efficiency, and leads to an increased amount of unreacted coal powder being carried away in the gas. It has low mechanical strength and poor thermal stability, but this has no adverse effect on fluidized bed gasification. The low mechanical strength contributes to good grindability, which is an advantage for the processing of coal powder fed into fluidized bed gasifiers. The poor thermal stability of lignite has some adverse effects on fluidized bed operation, such as the need to enhance ash discharge [3]. 1.5 Coal quality stability: Fluidized bed systems have high requirements regarding coal quality stability; otherwise, it can lead to peroxide formation or slag blockages. The fluidized bed is highly sensitive to the properties of the feed coal; changes in the coal’s cohesion, thermal stability, moisture content, and ash fusion point can easily lead to abnormal operation. The moving bed has slightly lower requirements regarding coal quality stability. 1.6 Adhesiveness: In the moving-bed gasification technology, gas flows from bottom to top and comes into countercurrent contact with coal. The reaction temperature ranges from 600 to 1300 °C; it is necessary that coal, ash, and slag exhibit weak adhesiveness over a wide temperature range from high to medium temperatures. Otherwise, it will disrupt the uniform distribution of the gas flow, affect the normal downward movement of the material layer, reduce the gasification efficiency, and make it difficult to remove ash and slag. The operating temperature in fluidized bed gasification is high. In addition to affecting the transportation of water-coal slurry and dry coal powder, viscosity parameters mainly determine the anti-adhesion properties of ash and slag. In the Shell gasification units using the waste boiler process, the raw gas contains a large amount of fly ash; it is a significant challenge for the waste boilers and ceramic filters to ensure that this fly ash solidifies and loses its adhesive properties after cooling. The fluidized bed also has certain requirements regarding the anti-caking properties of coal and ash; otherwise, it affects their fluidization state. 2 From the perspective of gas application requirements, similar to bituminous coal and anthracite, the gas produced by lignite gasification plants has many uses, as shown in Figure 1. Figure 1: Applications of coal gasification. Different coal gasification technologies result in varying concentrations of CO, H2, CH4, tar, phenols, etc., in the raw gas; thus, different compositions of raw gas are suitable for various applications. The conditions of the raw gas produced by several typical coal gasification technologies are shown in Table 6. The data on water-coal slurry and Shell come from relevant domestic operational reports, while the data on fluidized bed and GSP are taken from Reference [4]; the data on BGL come from materials provided by the patent holders. Since the comparison benchmarks for coal types are not on the same platform, the data in the table is for reference only. The crude gas produced by Lurgi furnaces has a lower content of (CO+H2) compared to BGL, and a slightly higher methane content; the levels of impurities such as tar and phenols are similar to those in BGL, and therefore are not listed separately in Table 6. 2.1 Use as fuel for industrial gas, city gas, etc. When used as fuel, gas is required to have a high methane content, high calorific value, and high cold gas efficiency. CO and H2 are also effective components of industrial gas and city gas, but when used in city gas, the CO content should be kept as low as possible. When lignite gas is used as a fuel for industrial gas, city gas, etc., the more suitable gasification technologies are Lurgi and BGL moving-bed systems, followed by fluidized-bed systems such as HTW. 2.2 Use as chemical synthesis gas: The gas produced by coal gasification is used in the production of methanol, synthetic ammonia, and indirect coal-to-oil processes, and is classified as synthesis gas. For these chemical synthesis gases, methane in the gas becomes an impurity, a harmful gas. Chemical synthesis gas does not have high requirements regarding calorific value; the main requirements are related to components such as CO and H2 in the gas. Gasifiers like GE’s coal water slurry gasifiers, as well as Shell’s and GSP’s dry coal powder gasifiers, are suitable for this purpose. Taking the production of methanol synthesis gas as an example, from the perspective of methanol synthesis requirements, the gas produced by the gasification unit should ideally meet the following criteria: its composition should be as close as possible to the ratio required for methanol synthesis (hydrogen-to-carbon ratio of 2.05–2.10) ； Impurities other than the useful gases in the gas, as well as any entrained ash, should be kept to a minimum, so that they can be easily removed through subsequent purification processes ； The pressure of the gas should be as close as possible to the pressure used in methanol synthesis (currently generally 5.0–10.0 MPa, or even higher). Considering the achievable gasification pressure and the need to minimize energy consumption for system compression, coal-water slurry fluidized-bed gasification is the most suitable method for methanol production, followed by dry coal powder fluidized-bed gasification methods such as Shell and GSP. Considering the active components in gas (CO + H2), Shell, GSP, and BGL are most suitable for methanol production, followed by fluidized-bed gasification of coal water slurry. Considering the ratio of active components in gas (H2/CO), coal pulverized fluidized bed and coal-water slurry gas fluidized bed are most suitable for methanol production. If Lurgi or BGL processes are used to produce methanol, in addition to converting the methane present in large amounts in the raw gas, it is also necessary to remove impurities such as tar, phenols, fly ash, and coal dust, which are present in higher amounts in this gas compared to other gasification methods; therefore, it is not suitable as a chemical synthesis gas. The coal powder and fly ash carried away in the raw gas from the coal pulverized flow bed furnace are abundant, the levels of CO2 and methane are high, and the operating pressure is too low. Among the several gasification technologies listed in Table 6, it has the lowest content of useful gas components, making it the least suitable as syngas for large-scale methanol production projects. 2.3 Use in IGCC Integrated Gasification Combined Cycle Power Generation: IGCC refers to the process in which coal is gasified under pressure; the resulting gas is purified before being burned. The high-temperature flue gases drive a gas turbine to generate electricity, while the residual heat from these gases is used to produce high-pressure superheated steam, which in turn drives a steam turbine to generate electricity. The gas used in IGCC requires a certain level of purity with regard to dust and other impurities; it is preferable that the levels of combustible gases such as CH4, CO, and H2 in the gas be high, while no strict requirements are imposed on its calorific value. The gasification process used in conjunction with IGCC typically employs Shell’s dry coal powder fluidized bed gasifier with a waste boiler system, or GE’s coal slurry fluidized bed gasifier with a waste boiler system. The high-quality steam generated by this waste boiler system is advantageous when used in combination with IGCC. In contrast, the medium-pressure and low-pressure steam produced by the quenching process cannot be used directly for combined cycle power generation, which affects the overall efficiency of power production. When using Lurgi or BGL block/briquette moving-bed pressurized gasification reactors, in addition to the inability to produce high-quality steam for IGCC, effective measures must also be taken to remove coal powder and dry ash entrained in the gas.