Some thoughts on gasification options for methanation products

Among the coal-to-natural gas plants that are currently in operation, with HuiNeng using slurry coal gasification, all the others make use of the SD furnace (fixed-bed MK4) developed by the Second Research Institute of Chemical Engineering. The main reasons for this were that the coal used by these companies was lignite and long-flame coal, which had high levels of moisture, ash, volatiles, and Gekkin index; the MK4 furnace proved to be highly adaptable in such conditions, and it also allowed for the production of a certain amount of oil as a by-product, thereby reducing the cost of producing coal-to-natural gas； Secondly, using coal-to-natural gas as the end product and a fixed-bed MK4 gasifier, the CH4 content in the raw gas is as high as around 10%, which reduces the load and investment required for subsequent facilities. Thirdly, the plants that have been put into operation and use MK4 were mostly designed by Saiding, which at that time promoted MK4 as its main technology; it would still recommend BGL or even powder gasification today. It’s not that the gasifier in coal-to-natural gas plants must necessarily be a fixed-bed MK4. Moreover, by choosing a fixed-bed gasifier alone, aside from the well-known environmental issues that require additional investment to address, such as coal tar separation, phenol and ammonia recovery, biological treatment of phenol-containing wastewater, and VOC treatment (the odor levels and level of automation in fixed-bed gasifier plants are simply not comparable to those in dry powder gasification and water-coal slurry gasification plants; they are not on the same level), there is also the issue of an imbalance between lump coal and pulverized coal. Therefore, when it comes to coal-to-natural gas production and the entire coal chemical industry, the choice of gasification furnace depends on the suitability of the coal type and its degree of comprehensive utilization, as well as on the end products desired. It also takes into account the overall investment required for the main infrastructure and environmental protection measures, as well as the operational stability of the technology. If there is instability or environmental issues, then it’s not possible to assess the economic viability of such a project. If your coal has good slurry-forming properties, and parameters such as ash melting point, ash content, and moisture content meet the requirements of water-coal slurry gasification furnaces, then choosing water-coal slurry is a good option; in fact, some of the phenol-containing wastewater from the first-stage fixed-bed MK4 reactor can be used to prepare the slurry, thereby helping to eliminate part of the biochemical sludge. Going forward, as long as the coal type is suitable, opting for a dual gasification approach is the trend. Moreover, compared with a gasifier using refractory bricks, a water-cooled wall coal slurry gasifier eliminates the need for vulnerable refractory bricks, which may extend its operational cycle; however, the capacity of a water-cooled wall coal slurry gasifier is currently lower than that of a coal slurry hot-wall furnace. As for the impact on crude syngas? Just take a look at the composition of the crude syngas from these gasifiers. Fixed-bed gasifiers have a high methane content, a high H/C ratio in the crude syngas, low loads for shift, decarburization, and methanation, but the crude syngas contains CnHm compounds, making gas purification and wastewater treatment complex. In this regard, meeting current environmental standards requires large investments and presents difficulties in management ； Compared to fixed-bed syngas shift, purification, and synthesis processes, the load and investment requirements for water-coal slurry crude gas are clearly higher; however, its crude gas is relatively clean, free of impurities above the C1 level, and wastewater treatment is relatively simple and well-established, so it has no adverse effects on SNG production. When just calculating production costs, slurry coal is always more expensive. Fortunately, the water-cooled wall coal slurry gasifier is also a domestically produced device, so the investment should be significantly reduced. To determine the most appropriate gasifier, a detailed comparison by the design institute is required. Personally, I think choosing a water-coal slurry gasifier for the second-phase production of SNG is a good option, provided that the coal parameters are suitable; it can work well in combination with the fixed-bed gasifiers used in the first phase. It would be even better if the gasifiers used by Tsinghua or Jinhua can achieve a capacity of over 1,500 tons, or even 2,000 tons. Wish you success!

This post was last edited by lishouqiang_198 on 2017-12-5 at 09:54. First and foremost, I would like to express my gratitude for the comprehensive explanation provided. For the first phase, powder coal gasification will be used, while for the second phase, a water-cooled wall structure based on the Tsinghua 3.0 technology is planned, along with slurry coal gasification. From the perspective of the by-products generated during gasification, these two methods are superior to the Lurgi gasification process. However, the methane content in the useful gas produced by these methods is much lower than that in Lurgi gasification. In my opinion, investing in substantial additional equipment to recover the by-products in order to increase the methane content means that, both in terms of initial investment and long-term operation, Lurgi gasification does not have any advantages in the SNG production process. Although the CH4 content in the useful gas from powder coal gasification or slurry coal gasification is lower, it is sufficient to add more reactors in the methanation stage. For our current project, we have chosen DAVY’s process package. The details of this package are similar to those of several major SNG production processes in China; there are no significant changes in the overall process flow. The only difference is that, due to the lower methane content in the useful gas, one additional reactor has been added. Thus, compared to the Lurgi gasification process, the methanation stage now involves five reactors instead of the traditional four. I hope we can communicate again

The last edit to this post was made by lishouqiang_198 on 2017-12-5 at 10:01. “For coal-to-natural gas production and the entire coal chemical industry, when choosing a gasification furnace, it is necessary to consider the suitability of the coal type and its degree of comprehensive utilization, as well as the end products desired. Additionally, the overall investment in the main infrastructure and environmental protection measures must be taken into account, along with the operational stability of the technology. If there is instability or environmental issues, then it’s not possible to assess the economic viability of such a project.” “ I’m giving your statement three likes; I urge the industry to abandon the Lurgi technology in future SNG projects, as this gasification technique only increases companies’ investment costs and environmental pressures. Moreover, domestic plants currently in operation are facing serious problems related to corrosion of their lining structures, and opting for water-cooled walls without such lining structures is safer and more reliable, while also allowing for the production of a certain amount of steam.

Let me ask Zizhu again: For technical exchanges, Tsinghua Furnace recommends models with a capacity of 2,000 tons. So I would like to know what the disadvantages are of going straight for a model with a capacity of 3,000 tons? For example, just like in China’s traditional coal chemical industry in earlier years, urea production capacity started at 40,000 tons; today, it has reached 300,000 tons, 520,000 tons, 600,000 tons, 800,000 tons, and even 1.2 million tons. What determines such scale levels is economic viability I also ask Senior Zizhu to have in-depth discussions; thank you.

Thanks for agreeing. Regarding the scale of the gasification furnace, it is also necessary to take into account the scale of each production facility; for example, for 300,000–500,000 tons of synthetic ammonia or 300,000–600,000 tons of methanol, a furnace with a nominal capacity of 2,000 tons is sufficient, as any larger size would be unnecessary. Since the coal-fired furnace has been in actual operation for over 300 days, only one coal gasification furnace is available for use. However, given that the maintenance cycle for slurry coal gasification furnaces is generally within 150 days, it is advisable to have a backup furnace; one furnace in operation and one as backup at a capacity of 1,500 tons each will be sufficient to meet production needs. However, modern large-scale coal chemical plants, such as those for methanol production, coal-to-olefins, and coal-to-natural gas, consume tens of millions of tons of raw coal per year. Increasing the capacity of the gasification units helps to reduce the need for complex frame structures, as well as the number of control systems and equipment required; it also reduces the workload associated with operation and maintenance, thereby helping to lower overall investment and operating costs. But there is a limit to this; the methods for scaling up powder gasification mainly involve increasing the capacity and number of burners, as well as the diameter and height of the furnace, in order to ensure a uniform distribution of the material flow and temperature fields within the furnace chamber. This is done to guarantee an appropriate residence time in the reaction zone, as well as to prevent a decrease in carbon conversion rate and the composition of the useful gases. However, the larger the gasification furnace, the more complex it is, and it is harder to control and stabilize; in the market, the performance of Shell 2700–3000 ton gasification furnaces is not as good as that of 2000 ton furnaces. The 3000 furnace in the quenching process still lacks mature operational experience. In terms of scaling up water-coal slurry gasifiers, they have inherent disadvantages compared to dry powder coal gasifiers; thus, the use of multiple nozzles (whether side-mounted or top-mounted) is necessary. This alters the flow and temperature fields, leading to significant changes in the thermal balance within the furnace as well as in the reaction zones, which in turn affects the composition of the raw gas. Although GE, Huali, and Tsinghua University have all theoretically completed the paper design for 3000-size furnaces, there are currently no reports of actual applications of these designs. Based solely on individual, immature experience and intuition, it’s not the case that the larger the vaporization furnace, the better. Generally, a scale of 2,000–2,500 tons is mature and reliable. It's just my personal opinion, not reliable, for reference only. If necessary, you can message me privately