GSP technology is the best choice for coal-to-syngas (or H2) processes

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GSP technology is the best option for coal-based syngas (or H2) production. Author/Source: Li Dashang (Second Chemical Industry Design Institute of China, Taiyuan 030001). Date: 2005-6-16 -------------------------------------------------------------------------------- GSP refers to the dry pulverized coal pressurized fluidized bed process; it was developed in the 1970s by the former German Fuel Research Institute. Pilot plants with thermal capacities of 3 MW and 5 MW were first built in Freiburg, Germany, where tests were conducted on dozens of different feedstocks from around the world. In 1984, a demonstration unit with a coal feeding capacity of 720 tons per day per furnace was built at the Heihe Pump Gasification Plant. This coal-based plant was in operation until Germany’s reunification in 1991. After Germany's reunification, for various reasons, the GSP technology was not introduced to the market in a timely manner, and the raw materials were changed from coal to tar, oil residues, etc. As GSP technology has just entered the Chinese market, many customers are eager to gain a thorough understanding of its advantages in terms of advancement and reliability, the requirements regarding the raw coal used, the characteristics of the gasification furnace and the associated processes, as well as its technical and economic indicators and investment costs. Therefore, in addition to providing an overview of GSP technology, this article also offers objective analyses and comparisons based on typical data from different processes, to assist customers in making informed decisions. 1 Overview of GSP gasification technology 1.1 Wide range of feedstocks with high adaptability All types of coal, from young mud coal and lignite to older anthracite, as well as petroleum coke, oil sludge, industrial sludge, etc. with an ash content of >1%, can be used as feedstocks for gasification. It requires that the coal fed into the furnace or other raw materials meet basic requirements regarding particle size, moisture, and ash content. Particle size mainly affects the carbon conversion rate, while moisture primarily influences the transportation of coal powder. The ash content must be above 1%; otherwise, it is not possible for the water wall to form a slag layer that constitutes a membrane wall. The requirements for various feed materials entering the furnace in terms of particle size, moisture content, and ash content are shown in Table 1. 1.2 GSP Process 1.2.1 The GSP process includes the preparation and transportation of qualified pulverized coal, utilizing both low-pressure nitrogen slurry transport systems and high-pressure CO2 slurry transport systems; the coal is then fed into the gasification furnace through coal hoppers, pressure feed bins, and nozzles. 1.2.2 Bituminous coal, pure oxygen, and steam (young coal may not be used) are used to convert coal into gas and molten slag at 4 MPa and 1,400°C–1,500°C. Due to the young age of the raw coal and the use of CO2 as a feedstock, with little addition of water vapor, the effective gases (CO+H2) in the gas make up as much as 94.5%, CO2 accounts for 4%, CH4 for 0.02%, N2 for 0.7%, and the rest for 0.78%. 1.2.3 The high-temperature gas is cooled to 220°C by water sprayed in the quench chamber, and then enters the Venturi scrubber, where the volume concentration of dust in the gas is reduced to 1 mg/m3 (dry). The saturated water in the gas is used for shift reaction purposes. 1.2.4 The slag cools and solidifies into granules in the quenching chamber, falling into the water bath at the bottom of the chamber; it is then discharged into the slag tank through a ash hopper, and a slag skimmer is used to lift the slag onto a belt for transport to the slag storage area, from where it is carried by truck to the processing site. 1.2.5 The black water containing about 0.4% dust (slag, carbon) from the quench chamber is sent to the solid separation unit; after preliminary separation of the residual carbon/dust, the water is then pumped back to the quench chamber via filters and storage tanks. The dust-containing black water coming out of the lower part of the separator is subjected to vacuum flash evaporation; thereafter, it undergoes coagulation and sedimentation under the action of flocculants. After concentration and filtration for dewatering, the clean water is pumped back to the gasification cooling chamber for further filtration before being sent outside for treatment. To maintain the salt balance in the cold shock water, wastewater accounting for about 15% of the total black water volume is discharged outside for treatment. 1.2.6 The function of the coal powder lockhopper is to feed coal powder at atmospheric pressure into the pressurized gasification system. The function of the gray lock hopper is to send the slag water from the pressurized gasification system to the atmospheric pressure slag discharge system. They are constantly in a periodic process of alternating between normal pressure and increased pressure, and then back to normal pressure. CO2 is used to pressurize the coal hopper, while water is used to pressurize the ash hopper; this process is controlled by a programmable controller. 1.2.7 The pulverized coal is fed into the pressurized silo via a lockhopper, where the introduced CO2 and coal powder are in a dense-phase fluidized state, and then sent to the combustion nozzle of the gasifier through a feed pipe. 1.2.8 A raw material density meter is installed on the feed pipe from the feed bin to the burner of the gasifier; a mass flow meter is used to measure the amount of coal powder supplied. Together with the oxygen supplied and the pressure differences in the gasifier, gasification chamber, and quenching chamber, these elements form a control system for regulating the operation of the gasifier. 1.3 Structural features of the GSP gasification furnace: The GSP gasification furnace consists of a main burner and a pilot burner, a gasification chamber, a quenching chamber, and a pressure-resistant housing. The gasification chamber is equipped with a water wall, whose primary function is to resist the high temperatures of 1,450°C to 1,500°C as well as the erosion caused by slag. The water wall consists of a water-cooled coil, clamps fixed to the coil, and SiC refractory material, forming a cylindrical membrane wall. There is a gap of about 50 ma between the membrane wall and the pressure-bearing shell, with a small flow of ambient-temperature syngas (or CO2, N2) filling this gap. The water in the water-cooling tubes of the water wall is subjected to a forced closed-loop circulation. Within this circulation system, there is a waste heat boiler that generates low-pressure steam at 0.5 MPa(g) to remove heat, thereby keeping the temperature of the water in the water-cooling tubes within a constant range. The quenching chamber is a pressure-bearing shell with the same outer diameter as the vaporization chamber, and is equipped with several cold water nozzles at its upper part. Here, the gas is suddenly cooled to 220°C. The gas is drawn out from the middle of the quenching chamber; the lower part of this chamber is conical in shape and filled with water. The slag solidifies into particles upon cooling and falls into the water bath, from where it is discharged into the ash hopper. Except for the burners and a small amount of special stainless steel, the rest of the components in the GSP gasifier are made of carbon steel. The service life of the gasifier and its water wall is 10 to 20 years, while the service life of the burner is 10 years (with maintenance at the top portion once a year). 1.4 Analysis of the advancement of GSP gasification technology parameters and raw material energy consumption 1.4.1 Technical parameters 2 Comparison of GSP-based syngas (or H2) production with other typical coal gasification technologies. There are dozens of coal gasification processes, which can be classified into three categories: fixed-bed gasification processes represented by Lurgi ； Fluidized bed gasification processes represented by high-temperature Winkler and gray fusion ； The fluidized bed gasification process, represented by GSP, Shell, and Texaco. Lurgi’s fixed-bed gasification process is mature and reliable; it features high gasification efficiency including with respect to tar, a high carbon conversion rate, and high thermal efficiency of gasification. The oxygen consumption is the lowest among all types of gasification processes, and the preparation of raw materials as well as the handling of slag are simple. The calorific value of gas is the highest among various gasification processes, making it most suitable for producing city gas. If the production of syngas is chosen, the following problems arise: (1) The composition of the gas is complex, with about 16%–18% unwanted CH4 in the syngas; converting this CH4 into H2 and CO requires significant investment and results in high costs. (2) A large amount of condensed wastewater needs to be treated. Wastewater contains large amounts of tar, phenols, ammonia, fatty acids, cyanides, etc.; therefore, tar recovery units, as well as units for the recovery of phenols and ammonia and biochemical treatment systems, are required, which increases investment and raw material consumption. (3) The raw material for Lurgi gasification technology is lump coal ranging from 5mm to 50mm in size. Lump coal is expensive; if raw coal is purchased, 50% to 55% of it consists of pulverized coal that needs to be dealt with. The power generated from this pulverized coal cannot be used entirely by the factory itself, and selling it to the grid at a low price affects the factory’s economic efficiency. The fluidized bed gasification process is more suitable for coal with high volatiles and high reactivity, as well as coal with high ash content and high ash fusion temperature. But the vaporization pressure is low. A fluidized bed for gasification at 3.0 MPa is under development; it features a low gasification temperature of around 1,000°C, low gasification intensity, as well as a simple structure for coal preparation and the gasifier, resulting in lower investment costs. The production of syngas (or H2) in a fluidized bed presents the following problems: (1) The pressure is above 2.0 MPa, and the methane content in the gas is high. (2) The carbon conversion rate and gasification efficiency are relatively low. (3) There is a large amount of fly ash, increasing the difficulty of post-treatment. The commonly recognized gasification technologies for coal-based syngas (or H2) in large-scale plants today are the GSP, Shell, and Texaco fluidized-bed gasification processes. To this end, it is necessary to conduct a more detailed comparison of the three technologies. 2.1 Comparison of GSP syngas (or H2 production) with Shell and Texaco gasification technologies 2.1.1 Comparison basis 2.1.1.1 Analysis of raw coal is shown in Tables 2–4. The grindability index HGI of the raw coal is 73, and its calorific value is Qnet.ar. It is 21,351 kJ/kg. 2.1.1.2 Comparison Scope (1) Coal Powder Preparation and Conveying System ； (2) Gasification unit and slag discharge system ； (3) Gas cold shock washing and dust removal system ； (4) Black water treatment system ； (5) Raw gas shift unit. 2.1.1.3 The pressure of the raw gas exiting the gasification furnace is 4.0 MPa. 2.1.1.4 The production capacity of the comparison unit is 200,000 m3 (CO+H2)/hour, with 300 operating days per year. 2.1.1.5 Prices of raw materials and energy: Coal: 150 yuan per ton ； Oxygen (99.6%): 0.2 yuan/m3 ； Electricity: 0.4 yuan/kW·h ； 4.5 MPa(g), superheated steam at 350°C: 70 yuan/t ； 0.5 MPa(g), saturated steam at 158°C: 50 yuan/t ； Boiler feed water (deoxidized): 10 yuan/t ； Depreciation, major, medium, and minor repair costs: 10% of the investment per year. 2.1.2 Comparison Results The comparison of coal gasification process technologies, investment comparisons, comparisons of coal gasification consumption and technical indicators, and the cost per m3 of (CO+H2) are shown in Tables 5 to 8 respectively. Table 6 Comparison of investment costs for the GSP, Shell, and Texaco gasification processes, in ten thousand yuan. 2.2 Advantages of GSP over Shell and Texaco coal-based syngas processes. 2.2.1 Comparison between GSP and Shell coal-based syngas. 2.2.1.1 Compared to Shell coal-based syngas, GSP features a simpler process design, easier operation, and lower investment costs. When developing the GSP gasification technology, the goal was to produce methanol from coal-based syngas; therefore, a simple design with special water-cooled walls, as well as processes for cooling, washing, and dust removal, were employed. The development goal of Shell’s gasification technology is combined-cycle power generation; therefore, waste heat boilers are used to recover the waste heat from the gas, and dry ceramic filters are employed for dust removal in order to achieve the highest thermal efficiency. However, for the production of syngas, whether for manufacturing methanol, synthetic oils, dimethyl ether, or for producing products such as synthetic ammonia and hydrogen, the H2/CO ratio is between 1 and 2, or all of the CO is converted to H2. Therefore, the large amount of CO in gas needs to be converted into H2 and CO2 together with water vapor under the action of a catalyst. GSP simply cools the gas to around 220°C, at which point the amount of water vapor present in the gas is sufficient to meet the steam requirements for CO conversion ； The steam produced through Shell’s gasification project, which involved an investment of nearly 300 million yuan, is used for conversion in about 70% of cases; this proportion is even higher when it is used to produce pure H2. This is highly unreasonable and uneconomical. 2.2.1.2 Low GSP power consumption: Due to the Shell gasification waste boiler and dry dust removal process, a large volume of CO2 or N2 is required for purging. Apart from ammonia production, N2 cannot be used for other syngas processes; only CO2 can be used. The amount of CO2 used by Shell for coal transportation and purging is 1.5 times higher than that of GSP, and the pressure of CO2 is 1 time higher. Before the gas enters the waste heat boiler, its temperature must be reduced from 1,500°C to 900°C in order to solidify the molten substances present in the gas and prevent them from sticking to the heat exchange tubes of the waste heat boiler. Therefore, a centrifugal compressor with a gas handling capacity of approximately 190,000 m3/h needs to be added to return the gas, which is at 40°C after dust removal, to the cold shock tube at the outlet of the gasification furnace. The power consumption of the GSP gasification unit of the same scale is 60% lower than that of Shell. 2.2.1.3 Wide adaptability to raw materials: The GSP water wall structure requires only a ash content in the raw coal or other feedstocks of >1%, whereas Shell gasification demands an ash content of >8% in the feedstocks. When low-ash coal or other low-ash feedstocks are used, it is necessary to add a certain amount of ash to the feedstocks to raise their ash content above 8%, which increases the complexity of the process. 2.2.1.4 The GSP water wall has a very simple structure; it is of the cylindrical coiled type, with simple water pathways, and is easy to manufacture using carbon steel. The Shell water wall consists of multiple sections of vertical tubes, has complex water pathways, is made of alloy steel, and is more difficult to manufacture. The waste boiler operates in an environment characterized by high temperature and pressure, erosion from high levels of solid particles, and highly corrosive media; therefore, it needs to be regularly purged and tapped to remove ash. Several sets of dry ceramic filters need to be periodically back-blown to remove ash, and the equipment must be imported. In short, this not only **increases investment**, but also raises the difficulty of operational control and the amount of equipment maintenance required, thereby reducing the reliability of the plant’s operation. 2.2.1.5 The problem of transporting large-scale units resulting from the enlargement of gasifiers: A gasifier with a daily coal input of 2,000 tons and an outer diameter of 3.5m–4.0m can be transported by rail in its entirety, whereas a Shell gasifier with the same production capacity has an outer diameter of around 4.5m, requiring on-site processing and assembly, which **increases manufacturing costs and delays delivery times. 2.2.1.6 The low cost of GSP crude gas: The cost of crude gas reflects the advancement and suitability of gasification technology. Upon comparison, it can be seen that the GSP gas cost is 13% lower than that of Shell. 2.2.2 Comparison between GSP and Texaco coal-based syngas production Although both GSP and Texaco coal gasification technologies utilize fluidized bed pressurization and liquid slag discharge methods, as well as processes for cooling and dust removal of the raw gas, there are significant differences between them because GSP uses dry pulverized coal as feedstock, while Texaco uses coal slurry as feedstock. 2.2.2.1 Compared with Texaco, GSP has lower raw material consumption and more advanced gasification parameters. The raw coal consumption per unit of product (CO+H2) for GSP is 12%–13% lower than that of Texaco (including coal drying), and its oxygen consumption is 15% lower. Although GSP’s electricity consumption is higher, its impact is minimal compared to the first two factors. Compared with Texaco technology, GSP has a 10% higher cold gas efficiency, a 1.5% higher carbon conversion rate, and a 4% higher gasification heat efficiency (including steam for shift reaction). For the gaseous components in gas (CO+H2), the GSP is 92%–94%, while that of Texaco is 78%–81%. 2.2.2.2 GSP has wide raw material adaptability; the coal suitable for GSP gasification covers almost all types of coal, from lignite to anthracite. Tar, oil sludge, petroleum coke, etc., with an ash content of >1%, can also be used as raw materials. The restrictions on the ash content and ash fusion temperature of the feedstock are much more lenient compared to the Texaco gasification process. The most important function of the water wall is to automatically adjust the thickness of the slag-coating layer in response to changes in the ash fusion temperature. For coals with a high ash fusion temperature, the water wall can handle such conditions, but this leads to increased consumption and negative impacts on economic efficiency. For Texaco gasification, young lignite, coal with high moisture content, and coal from which the prepared water-coal slurry cannot reach a concentration of over 60% should not be used as raw materials. Coal with a ash fusion temperature greater than 1,500°C and an ash content of over 20% is also unsuitable for use. 2.2.2.3 The GSP gasification process features good reliability and a long operational lifespan. Production using the GSP gasification method has shown that the water wall can last for over 10 years, with an expected lifespan of 20 years. The gasifier has a service life of 20 years, the nozzle body has a service life of 10 years, and the front end requires maintenance once a year; therefore, GSP does not need a spare gasifier. The expensive chromium- and zirconium-containing refractory materials used in Texaco gasifiers can only last for one year, while the nozzles, under normal conditions, need to be replaced and repaired after 60 days; due to their short service life, spare gasifiers are necessary. 2.2.2.4 Regarding the issue of transporting large components resulting from the enlargement of gasifiers: For a gasifier that consumes 2,000 tons of coal per day, the outer diameter is approximately 3.5m to 4.0m for GSP gasifiers, and around 4.5m for Texaco gasifiers. Due to the problems associated with railway transportation of large-scale equipment and the frequent need for maintenance, it is not conducive to enlarging the Texaco gasifiers; otherwise, increased on-site processing and spare parts capabilities would lead to higher investment costs. For a methanol plant with an annual production capacity of 600,000 tons, if standard boilers with a diameter of 2.8 meters are used, the daily coal consumption will be around 600 to 700 tons; 5 to 6 such boilers will be required, which will increase the investment costs and significantly affect the plant’s economic efficiency. 2.2.2.5 GSP gasification operates reliably at low cost; it is dependable in operation with minimal maintenance needs, and the medium used for transporting coal powder is the factory’s waste gas, CO2. For a methanol plant with a capacity of 600,000 tons per year, Texaco gasification would result in additional operating costs of around 12 million yuan per year solely due to the consumption of coal slurry additives, while maintenance costs for refractory materials would increase by 20 million to 25 million yuan. There are also the costs associated with the frequent replacement and maintenance of nozzles. 2.2.2.6 The cost of gas per unit of product for GSP is low; it is approximately 0.21 yuan/m3, while the cost of gas per unit of product for Texaco is about 0.26 yuan/m3. GSP’s gas cost is 20% lower than Texaco’s. 3 Conclusion 3.1 For coal-based syngas, GSP combines the advantages of Shell and Texaco gasification. If gas is used for power generation, the GSP thermal efficiency is 14% to 15% lower than that of Shell gasification, at which point Shell gasification has a significant advantage. A gasification pressure of 4.0 MPa is one of the basic requirements; the gasification pressure for Texaco can be increased to 6.5 MPa, which allows for a slight reduction in energy consumption. However, since the pressure ratio from 4.0 MPa to 6.0 MPa is small, the reduction in energy consumption is quite limited. 3.2 Based on the comprehensive analysis and comparison above, it can be seen that GSP gasification for syngas production has clear advantages in terms of raw material adaptability, gasifier structure and scalability, consumption metrics, operating costs, investment, and the cost per unit of syngas. It is an advanced technology suitable for China’s national conditions, and represents the best choice for coal-based syngas production processes