What are the shortcomings of fixed-bed gasification?
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When it comes to current gasification technologies, what comes to mind first is the fluidized bed technology (slurry coal, dry coal powder), and people instinctively consider fixed-bed technology to be outdated and inferior to fluidized bed technology. But what exactly is wrong with fixed-bed gasification? In terms of energy consumption: the coal consumption per unit is almost the same as that in fluidized bed systems ； Its oxygen consumption per unit is nearly half that of water-coal slurry, and 1/3 lower than that of dry coal powder ； In terms of power consumption, it is mainly because fixed-bed reactors operate at atmospheric pressure; however, with the new types of pressurized fixed-bed reactors, the power required for gasification is much lower than that in fluidized-bed reactors, and the power consumption per unit of product is now roughly on par with that of fluidized-bed reactors.... In terms of steam consumption, water gas is produced by burning water with oxygen to turn it into steam; dry coal powder also requires steam. The new type of pressurized fixed-bed systems can generate their own steam, which can even be supplied externally. It can also achieve the moisture ratio required for the gasified coal gas to meet the conversion requirements. Thus, in terms of energy consumption, the fixed-bed reactor does not necessarily perform worse than the fluidized-bed reactor. In terms of environmental protection: Regarding exhaust gas emissions, the pressurized fixed-bed reactor also operates on a continuous basis, and like the fluidized bed reactor, it produces virtually no exhaust gases ； Regarding wastewater discharge, fixed-bed reactors present significant problems in this regard. The Lu Strange furnace uses a gas outlet cooling process; as a result, all pressurized gasification furnaces that use bituminous coal face the serious issue of large amounts of organic wastewater. Even when anthracite is used, the amount of inorganic substances carried away by the gas is greatly reduced, but the large volume of gas condensate still generates a substantial amount of difficult-to-treat wastewater. This is also a major reason why the industry considers fixed beds to be inferior. Unfortunately, a gasification research company has now developed a solution that effectively addresses the problem of excessive wastewater generation in pressurized fixed-bed reactors, enabling these reactors to produce only small amounts of wastewater that are easy to treat, just like fluidized-bed reactors. Regarding solid waste, the ash from pressurized fixed-bed systems can be recycled, just like that from fluidized bed systems. Additionally, since water-coal slurry uses refractory bricks, its ash contains heavy metal ions, whereas fixed-bed systems do not have this problem. Therefore, based on the above comparison, the new type of pressurized fixed-bed reactor performs no worse than the fluidized-bed reactor in terms of environmental protection. In terms of investment: The huge costs associated with fluidized-bed gasification units, which can reach hundreds of millions, are also astonishing. For a gasification unit with a capacity of 300,000 tons, the investment for a pressurized fixed-bed system is less than 200 million, while the cost for coal-water slurry systems is over 300 million; for dry coal powder systems, the investment ranges from 350 million to 400 million. The pressure-fixed bed has the lowest investment cost. Through this simple comparison, it is not evident that the fixed-bed system is inferior to the fluidized-bed system; taking into account the maintenance costs and operational difficulties associated with the fluidized-bed system, the fixed-bed system actually seems to be better. Let’s all discuss it together as well
The fixed-bed furnace is primarily the Ruhr process furnace. Apart from the difficulty in sewage treatment mentioned by the original poster, the use of coal in such furnaces is also a drawback; Ruhr process furnaces require lump coal, whereas lump coal accounts for only 30% of the coal produced in mines using comprehensive mining methods. This can easily lead to an imbalance between lump coal and pulverized coal, and moreover, lump coal is more expensive. Furthermore, and most importantly, the type of coal and the target product determine the type of furnace. Pulverized coal pressure gasification is generally used by coal-to-gas companies, as the gasification temperature is low, the methane content in the raw gas is high, and the subsequent processes and supporting facilities required are fewer compared to other gasification methods. Overall, the Ruhr process is the only one for coal-to-gas production that achieves an energy efficiency of 56%. With the increasing pressure to protect the environment in recent years, the costs of wastewater treatment have risen steadily. To date, no system has been able to achieve zero wastewater discharge; on the contrary, incidents of wastewater buildup and environmental contamination due to wastewater have occurred frequently in coal-to-gas plants, which has led subsequent companies to consider using other types of boilers. Some companies have also begun to consider using two types of furnaces in combination in order to address the issue of an imbalance between lump coal and pulverized coal.
Please explain your fixed-bed technology in detail!
I think that in addition to the points regarding fixed-bed reactors mentioned by the original poster, when making investments, one should also consider 1. the comparison of investment costs for gasification per unit of productive gas. 2. Gasification investments should include environmental protection investments (such as wastewater treatment using fixed-bed systems). 3. If the target product is not natural gas, then the investment required for subsequent methane separation also needs to be considered.
The gas production rate of a single fixed-bed gasifier indeed cannot compare to that of a fluidized-bed gasifier; even pressurized fixed-bed systems fall far short in this regard. The highest gas production rate achievable with current fixed-bed systems is only around 50,000 Nm3/h. Therefore, fixed-bed technology is not suitable for large-scale and ultra-large-scale projects. Over the past decade, driven by the domestic and international economic environment, coal chemical enterprises that are part of large state-owned and central-state enterprises have essentially completed their upgrading using fluidized bed gasification technology. However, 75% of the current production capacity still relies on atmospheric-pressure batch fixed-bed reactors, which are mainly found in small and medium-sized fertilizer manufacturers from certain eras, especially those under the Jinmei Group. For these small and medium-sized fertilizer manufacturers, the fixed-bed reactor remains the preferred option due to its lower investment costs and shorter construction time. However, the mention of fixed-bed reactors brings to mind the Lurgi reactor, as well as its serious environmental drawbacks; as a result, they are forced to abandon the fixed-bed approach and consider fluidized-bed reactors instead. If a fixed-bed reactor uses bituminous coal as raw material, it will contain large amounts of organic substances such as tar, and no process based on the Lurgi furnace can solve the problem of organic wastewater. If anthracite is used, there will be much less tar and other organic substances, and it may even be possible to do without the devices used in the Roche process to handle these organic substances. So here comes the question: bituminous coal has a low fixed carbon content and a high volatile matter content, but its price is low; anthracite, on the other hand, has a low volatile matter content and a high fixed carbon content, and its price is high as well. Currently, the price difference between bituminous coal and anthracite is significant, mainly due to special circumstances related to the main sources of anthracite and the companies involved. While other types of anthracite are seeing price reductions, these companies insist on maintaining their prices unchanged. This is also one of the reasons why many small and medium-sized fertilizer companies give up fixed-bed reactors. With the advancement of technology, new methods have also been developed to address the issue of high methane content in fixed-bed gas, enabling the decomposition of most of the methane in the gas into useful gases. The validation of this technology is currently ready to proceed with pilot testing; if the pilot testing is successful, the concern regarding the high methane content in the gas produced under pressure in fixed-bed systems will be resolved. Even when bituminous coal is used as raw material, the problems associated with tar and other organic substances in the gas, as well as those related to organic wastewater, can also be addressed.
Fixed-bed reactors have relatively strict requirements regarding coal quality. Moreover, they operate at low gasification temperatures, resulting in syngas with a high methane content as well as large amounts of tar; the wastewater contains high levels of phenolic organic compounds, making wastewater treatment difficult. In addition, the carbon conversion rate of fixed-bed reactors is relatively low, and there is a high amount of residue
Please discuss the advantages and disadvantages of fluidized beds compared to fixed-bed and pneumatic beds Is there a future for fluidized beds after overcoming the problems of high residual carbon in fly ash and its difficulty in treatment?