Suggestions for improving the level of gasification technology

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Suggestions for Improving the Level of Gasification Technology Author/Source: Wang Shoujian, Chief Engineer of China National Chemical Engineering Corporation Date: 2016-08-03 Views: 17 Although powder coal gasification technology, water-coal slurry gasification technology, and crushed coal pressurized gasification technology have undergone nearly 15 years, 30 years, and 40 years of research, implementation, and improvement respectively, and their technical levels have reached a fairly high standard, gasification remains the process in coal chemical projects that requires the largest investment and consumes the most energy, while also presenting the most problems. Given the inherent characteristics of gasification technology and the problems that arise during operation, there is still much room for improvement in this technology. Further improve the level of localization and reduce equipment investment. Compared with the imported Shell, GSP, GE, and Collin gasification technologies, similar domestic gasification technologies require much higher investment. The main reasons for this are: first, high costs related to software (patent fees + PDP fees + technical service fees); second, the need to purchase certain patented or key equipment; third, the overly complex or unreasonable design of some processes and control systems, which also contributes to the high investment costs. Therefore, when introducing foreign gasification technologies, we need to further reduce the purchase of patented or key equipment (i.e., equipment from designated manufacturers), and further optimize processes and controls in order to lower investment costs. Increasing the gasification pressure and raising the coal feed rate per furnace: Among the various gasification processes currently in use, only water-coal slurry gasification achieves a gasification pressure of 6.5–8.7 MPa; the pressures for other gasification processes range from 1.0 to 4.0 MPa. This not only results in larger equipment and higher investment costs, but it is also a major factor contributing to the high energy consumption of coal chemical projects. Therefore, there is still room to increase the gasification pressure; for example, Lurgi has raised the gasification pressure of its MK+ syngas gasification process to 6.0 MPa (though it has not yet been put into full industrial use). The low gasification capacity per furnace is also a major reason for the high investment and large land footprint required in coal chemical projects. At present, aside from the Shell powder coal gasification and multi-nozzle opposed water-coal slurry gasification processes, which allow a maximum coal feeding rate of 3000 t/d per furnace, the maximum coal feeding rate for other gasification processes remains between 1000 and 2200 t/d. Although some processes can achieve 3000 t/d, there is still no industrial practice for them. Comprehensively reducing investment and improving energy efficiency: Looking at the coal chemical projects that are currently in operation or under construction, it seems that coal gasification has fallen into a paradox – either the investment in such facilities is high and their energy efficiency is also high, or the investment is low and their energy efficiency is low. For example, THE SHELL gasification process is equipped with a syngas cooler that can produce medium-pressure steam at 5.2 MPa; it offers a higher energy potential, but the investment required is also higher ； For example, gasification processes such as GSP, space furnace, and Corin have lower investment costs compared to Shell gasification. However, they produce low-pressure saturated steam from the water wall, resulting in low energy content that is difficult to utilize. The high-temperature gas exiting the gasifier at 1350°C is cooled directly, causing the gas to contain a large amount of water vapor. This approach is suitable for coal chemical projects that require extensive CO conversion (such as ammonia synthesis projects), but for those that do not require such extensive conversion (coal-to-gas, coal-to-oil), the excess water vapor in the gas not only leads to a decrease in energy content and energy losses but also requires additional heat exchangers and separators to recover energy. This in turn prolongs the process, increases resistance, and raises investment costs ； Slurry coal gasification almost always employs a quenching process, and this problem also exists there ； Coal pulverization pressurized gasification and CO conversion also generate large amounts of low-pressure saturated steam, which is difficult to utilize. Therefore, the selection of the gasification process should be considered in conjunction with the overall coal chemical project, aiming to reduce costs while increasing the energy value, so as to make the entire project reasonable and efficient. Reducing energy and water consumption: Coal chemical projects are those with high energy and water consumption; it is therefore essential to further reduce such consumption. There is also room for improvement in the field of coal gasification. For example, the Shell gasification process uses compressed cycle gas to cool the syngas down to around 900°C, which also results in energy waste. Could other media or methods be used to achieve cooling and dust removal? Domestic engineering companies are already trying to develop improvements in this regard ； The Shell gasification process uses dry dust removal, which requires a large amount of purge gas and thus leads to increased consumption; improvements can also be considered. Currently, large-scale coal chemical projects consider combining several different types of gasification processes in order to address the issue of the proportion of raw material coal powder and crushed coal; it is possible to explore the use of a coupled approach to address the various problems associated with different gasification processes simultaneously. For example, in current large-scale coal-to-natural gas projects, most adopt a combination of pressurized coal gasification and fluidized-bed gasification (slurry coal gasification or pulverized coal gasification). In a gasification process that combines pressurized coal gasification with slurry coal gasification, the large amount of harmful wastewater generated by pressurized coal gasification can be used in the slurry coal gasification process. This not only saves water usage but also reduces the investment and costs associated with wastewater treatment, thereby minimizing environmental pollution. Optimize the ash water treatment system to reduce dust emissions. Some gasification units currently have high levels of dust in the gas, which leads to blockages in the heat exchangers of the CO conversion units and deactivation of the catalysts located at the top of the first conversion furnace. There are also issues related to high amounts of wastewater generated and large volumes of ash water that need to be recycled; it is therefore necessary to further explore ways to optimize the gas washing process and the ash water recycling process.