Discussion on methanol carbon supplementation

Since methanol production from coke oven gas is similar to that from natural gas, the feed gas contains more hydrogen and less carbon; therefore, CO2 needs to be added to the feed gas to maintain an appropriate hydrogen-to-carbon ratio. CO2 in flue gas can be recovered to supply carbon.

It is better to produce methanol using a mixture gas in combination with coal gasification, as this is better than producing methanol solely from coke oven gas. Adding CO2 is significantly less effective than adding CO; the conversion gas produced from coke oven gas already contains a high level of CO2, so adding more CO2 does not yield very good results. First, replenishing CO2 consumes more available hydrogen; second, the crude methanol has a high water content; third, too high a CO2 level (above 8%) can affect the proper operation of methanol synthesis.

1. The use of coke oven gas for methanol production with carbon supplementation is based on the fact that there is more hydrogen than carbon; carbon is thus supplemented in order to make full use of the hydrogen in the feed gas, maximize methanol production, and fully utilize the plant’s capacity. 2. Sources of carbon supplementation: (1) If there is an ammonia synthesis plant, using the excess CO2 from that ammonia synthesis system for carbon supplementation is the most economical and environmentally friendly approach ； (2) If CO2 from boilers or lime kilns is to be used for carbon supplementation, purification, refinement, and pressurization are required; this represents a viable option for facilities in the vicinity that do not produce ammonia synthetically. 3. To supply CO, there needs to be an excess source of CO gas from another device. This post was last edited by FEIJING on 2009-4-1 09:25.]

I wonder if there is a mature carbon supplementation process available in China?

Do you mean the carbon dioxide content in the converted gas, or in the syngas after it has been mixed with the recycle gas?

My brief opinion: 1. The H2/C ratio after the conversion of coke oven gas is around 2.6–2.7; as a feed gas for methanol production, there is an obvious excess of H2, which provides the possibility for carbon supplementation； 2. There are several possible ways to supply carbon, namely CO and CO2; in practice, supplying CO is a more reasonable and feasible option ； 3. The specific methods of operation can be tailored to the actual conditions of the unit; if ammonia synthesis is carried out, water gas can be added as a supplement ； 4. Those without the necessary conditions can install several new batch gas generators as auxiliary equipment for methanol production, thereby achieving low investment and quick results.

After the conversion of coke oven gas, decarburization is not carried out; as a result, the CO2 content in the fresh gas is high, and the CO2 content in the recycled gas used in synthesis also becomes high. Since methanol production from coke oven gas involves a high hydrogen content and low carbon content, while methanol production from gasification involves a high carbon content and low hydrogen content, mixing these two types of feed gases in a certain proportion can achieve an appropriate hydrogen-to-carbon ratio. But this requires much larger investment; the process for producing methanol from coal is long and involves many pieces of equipment.

1. Coke oven gas contains more hydrogen and less carbon, but it depends on how much less carbon there is. Although hydrogen makes up the majority of coke oven gas, compared to the process of producing methanol from natural gas, hydrogen is not particularly abundant (as can be seen from the conversion reaction equations). Whether it is worthwhile for coking plants to install carbon recovery devices should be determined based on cost considerations; this is different if there is an inexpensive source of carbon available in the vicinity. 2. Carbon dioxide can be added before the conversion process, as there is a shift reaction at the bottom of the converter, so there is no need to worry about too high levels of carbon dioxide in the syngas. But there is also less hydrogen. CH4+H2O=CO+3H2 CO+H2O=CO2+H2+Q CH4+CO2=2CO+2H2 These are just my humble opinions; please feel free to correct me.

Are there any mature processes that can be used as a reference?

If another gas generator is installed, many more public facilities will need to be added, so it’s not a good solution. Is there any simple process that could be used as a reference?

Flue gas carbon dioxide recovery technology is well-developed, and many manufacturers use carbon dioxide recovery processes for carbon supplementation. As for producing carbon monoxide and water gas, corresponding facilities are required, which involves significant investment.

Replying to floor 6, I’m referring to conversion gas. Additionally, the CO needed can be produced using the recovered CO2 + O2 fixed-bed gasification technology, and the investment required for producing 200,000 tons of methanol is low.

I agree with what yangzijiang on floor 9 said; this can help avoid the problem of excessive CO2 levels.

The source of carbon needed for carbon supplementation must be carefully considered; in the case of newly built gas furnaces, it is necessary to assess the economic viability； If it is to recycle flue gas, it needs to be purified and pressurized, and the investment required is also quite substantial ； Considering the existing energy sources, to increase production capacity, it is more convenient to feed the vent gas into the coke oven gas stream, thereby replacing more coke oven gas with it for use in production.

Question: The carbon dioxide content in the converted gas should not exceed 8%. So what is the typical carbon dioxide content in the mixture of fresh gas and recycled gas that enters the synthesis tower?

Comrade on the 15th floor, we are indeed using the method you mentioned, but I wonder if there are any other methods as well

There is no basis for the claim that carbon dioxide in the converted gas should not exceed 8%. In our plant, natural gas is used to produce gas; in the converted gas, the ratio of carbon monoxide to carbon dioxide is roughly 1:1. In the syngas, carbon dioxide levels are slightly higher than those of carbon monoxide, but the yield is significantly lower. If it is a new plant, carbon compensation can be considered, with the synthesis tower designed to be slightly larger (that is, taking into account a lower yield). For the renovation of an existing plant, a thorough analysis is required; it needs to be determined whether carbon compensation should be applied before or after the conversion process. It’s not just one sentence: carbon dioxide consumes a lot of hydrogen, the gases are heavy, the reactions proceed slowly, and water is produced as a byproduct – these factors need to be carefully considered.

To increase methanol production by adding carbon to coke oven gas, it is possible to incorporate an additional heat exchange section in the converter, thereby reducing the load on the pure oxygen conversion process, lowering oxygen consumption, and increasing the hydrogen content in the converted gas. This makes it advantageous to add carbon after conversion, justifying the investment. With such a modification, a small synthetic tower might also be necessary for synthesis. This can result in a significant increase in production, making it an approach worth considering. Regarding the question of carbon supplementation before or after conversion, I personally believe it should be done after conversion, as the consumption during conversion is too high, and the reverse conversion is not significant.