Thread Content
Alkane conversion technology for coke oven gas: Typically, the volume fraction of CH4 in coke oven gas is around 23%–27%, while that of CmHn is about 2%–3%. In methanol synthesis, neither CH4 nor CmHn participates in the reaction to produce methanol; they exist as inert gases in the syngas and are recycled repeatedly. One of the key technologies and challenges in converting coke oven gas into methanol is how to transform all the alkanes (CH4 and CmHn), which account for about 30% of the volume of coke oven gas, into the effective components of syngas (H2 + CO), thereby improving the synthesis efficiency. This involves minimizing the amount of gas components that do not participate in the methanol synthesis reaction (CH4, CmHn, N2, Ar), reducing the volume of gas circulated in the methanol synthesis process, and lowering the energy consumption per unit of methanol produced. The main processes for the conversion and reforming of alkanes in coke oven gas currently include steam conversion, non-catalytic partial oxidation conversion using pure oxygen, and catalytic partial oxidation conversion using pure oxygen. (1) Steam conversion process. The steam conversion process of coke oven gas is similar to the furnace conversion mechanism in the two-stage conversion process for producing methanol from natural gas. The main reaction is: CH4 + H2O → CO + 3H2 (16). Reaction (16) is an endothermic reaction, and increasing the temperature facilitates the conversion of methane. In this reaction, heat must be supplied indirectly by burning fuel gas outside the reaction tube; the reaction tube needs to be made of high-temperature resistant nickel-chromium stainless steel. The converter has numerous nozzles, resulting in a complex structure, high manufacturing requirements, and high costs. It is commonly used for the primary conversion of natural gas; the methane content in coke oven gas is only 1/4 that of natural gas, so the steam conversion process is generally not employed. (2) Pure oxygen non-catalytic partial oxidation conversion process. In the pure oxygen uncatalyzed partial oxidation conversion process, the main conversion reactions occur in two stages; the first stage is the exothermic combustion reaction of CH4, H2, and CO ； The second stage is the phase in which methane is converted into H2 and CO; it is an endothermic secondary reaction and represents the control step in the entire conversion process. The reaction equation is: CH4 + H2O → CO + 3H2 (17). When synthesizing methanol, it is required that the volume fraction of CH4 in the fresh syngas be below 0.4%. Since CH4 conversion is an endothermic reaction, constrained by thermodynamic equilibrium, the conversion temperature in the catalytic-free partial oxidation process using pure oxygen must be above 1200°C. The hydrogen-to-carbon ratio in the syngas produced by the pure oxygen non-catalytic partial oxidation process is relatively ideal ； During methanol synthesis, the low content of inert gases in the recycle gas contributes to energy conservation and emission reduction ； In particular, the conversion process does not require a catalyst, eliminating the issue of catalyst poisoning; as a result, the requirements for the feed gas are relaxed. Coke oven gas does not need to undergo extensive desulfurization before conversion, and the precise desulfurization process can be carried out after conversion instead ； For organic sulfur compounds such as thiophenes, sulfides, and thioalcohols in the feed gas, which have complex structures, high chemical stability, and cannot be removed by wet desulfurization, they are completely cracked into H2S and COS at high temperatures of over 1200°C; these compounds can then be easily removed after the conversion process. Compared to dry hydrogenation desulfurization, which is energy-intensive and costly, the non-catalytic partial oxidation conversion process simplifies the desulfurization and purification of coke oven gas. It offers high desulfurization efficiency, low costs for purifying the feed gas, and reduces secondary environmental pollution caused by sulfur compounds emissions; it represents the future direction for the purification and conversion of coke oven gas. The drawback of the uncatalyzed partial oxidation conversion process is that the use of wet desulfurization in the purification of the converted gas necessarily entails carbon removal as well; as a result, there is a severe shortage of carbon available for methanol synthesis. The amount of feed gas required per unit of methanol produced is 30% higher compared to the catalytic conversion process using pure oxygen, and the consumption of pure oxygen is also high ； The conversion temperature is about 200°C higher than that of catalytic oxidation conversion, and the life of the coke oven gas burners at the top of the conversion furnace is short ； To date, there are no precedents for the commercial application of uncatalyzed partial oxidation conversion processes; therefore, a pure oxygen uncatalyzed partial oxidation conversion process is not used. (3) Pure oxygen catalytic partial oxidation conversion process. Since the uncatalyzed partial oxidation conversion process requires alkane conversion reactions at high temperatures of 1300–1400°C, it results in high consumption of feed gas and pure oxygen. Lowering the conversion temperature, adding steam to participate in the alkane conversion, and using a catalyst to accelerate the conversion reaction – this is the pure oxygen catalytic partial oxidation conversion technology. The raw gas subjected to desulfurization using Baijing is mixed with some steam and then fed into the burner of the catalytic partial oxidation converter. Oxygen, after being preheated by steam, is mixed with some steam and also fed into the burner of the converter. Coke oven gas and oxygen mix in the burner and are sprayed out, where they undergo a partial combustion reaction at the upper part of the converter; thereafter, they proceed to the nickel catalyst bed located in the lower part of the converter for further conversion reactions. The gases resulting from these reactions, after heat recovery, are sent to the synthesis section. Its main chemical reaction equations are as follows: 2H2 + O2 → 2H2O (18) CH4 + H2O → CO + 3H2 (19) CH4 + CO2 → 2CO + 2H2 (20) Among these reactions, equation (19) is the controlling step, with the control criterion being that the volume fraction of methane in the syngas after conversion should be ≤ 0.4%. For feed gas with an excessive volume fraction of total sulfur, catalytic partial oxidation can be carried out first, followed by the use of a zinc oxide desulfurization unit, through which the feed gas passes to ensure that the volume fraction of total sulfur in the syngas meets the specified standards. Compared with the uncatalyzed partial oxidation method, the pure oxygen catalytic partial oxidation method requires less fuel gas and oxygen, has a simpler structure for the conversion furnace, is relatively cheaper, and boasts a strong track record of successful large-scale commercial application; it is therefore the widely adopted method for the conversion of naphthenes in coke oven gas. Whether in catalytic or uncatalytic conversion, coke oven gas and pure oxygen must be mixed in a burner; the burner is responsible not only for promoting the mixing of coke oven gas with oxygen but also for creating an appropriate flow pattern in conjunction with the furnace structure, thereby achieving a suitable temperature distribution. The burner is a key device in the converter system; therefore, its design represents the core technology of the conversion process.