Discussion on the reasons for the abnormal increase in the oxygen-coal ratio in shell gasifiers

The oxygen-to-coal ratio was artificially adjusted by several dozen points in order to maintain the normal operating parameters of the gasifier, such as the heat load, steam production, and methane output, among others. The change in coal quality is the cause of this phenomenon; I wonder what other possible factors there might be.

What you said about intentionally increasing the oxygen-to-coal ratio in order to maintain the heat load, steam production, and methane output of the gasifier might not be correct. 1. In the case of Shell, due to the gasification temperature of over 1400 degrees, the methane content in the gas is extremely low, at around 0.1%. This is because the methanation reactions of CO and CO2 (CO + 3H2 = CH4 + H2O or CO2 + 4H2 = CH4 + 2H2O) are both exothermic reactions; at Shell’s gasification temperature, thermodynamic equilibrium is reached quite quickly. From a thermodynamic perspective, high temperatures are not favorable for exothermic reactions, which results in a very low CH4 percentage when thermodynamic equilibrium is attained. In other words, it is Shell gasification; methane is not the target product – the goal is to produce CO and H2 as feed gases for synthesizing chemical products. 2. Steam: It is a by-product of Shell’s gasification process; it is not the desired product. The desired product is a synthesis gas rich in CO and H2. The waste heat from gas is recovered through steam generation, thereby improving the thermal efficiency of the system. 3. Heat load: In your question, you mentioned the oxygen-coal ratio. Within a certain range, if the amount of coal increases, the amount of oxygen can increase approximately in proportion; that is, the oxygen-coal ratio should remain relatively constant. The possible reasons for your problem are: 1. The melting point of the ash increases, so it is necessary to raise the gasification temperature to ensure proper slag formation and removal. Increasing the oxygen-to-coal ratio raises the temperature of the gasifier ； 2. The steam-to-coal ratio needs to be increased. The gasification reaction between steam and carbon is an endothermic process, and steam must absorb heat in order to reach the gasification temperature once it enters the gasifier. To maintain this temperature, it is necessary to increase the oxygen-to-coal ratio, thereby increasing the heat generated by combustion to compensate for the heat absorbed ； 3. The moisture content in coal increases, in a manner similar to the increase seen with steam-coal. However, once water enters the gasification furnace, there is latent heat of vaporization as well as the need to raise its temperature to that of the furnace; therefore, the heat absorbed per unit mass of water is greater than that of steam, which requires an increased oxygen-to-coal ratio. To maintain the gasification temperature ； 4. The coal fed into the gasification furnace has an increased carbon content and a decreased oxygen content, requiring more oxygen for gasification ； 5. The combined result of the above reasons.

Our gasifier uses downward water quenching; there is no need to supply steam inside the gasifier, probably because of the poor quality of the raw coal (lignite), which already has a sufficient moisture content. Currently, the furnace has a low heat load, and the level of methane is likely a reflection of the furnace temperature. Compared to previous operations, the methane level has decreased from 200 ppm to less than 100 ppm. The company attributes this to the deterioration in the quality of the raw coal, with an increase in ash content. So I’d like to know if there are any other influencing factors. Another recent phenomenon is that the water vapor deviation rarely reaches negative values; it is generally positive, which doesn’t seem to be a good sign.

The conditions for your question are not complete; for example: 1. What was the type of coal you used before? Is it lignite now? Coal quality data before and after, including the industrial analysis of coal (content of moisture, ash, fixed carbon, and volatile matter), elemental analysis data (C, H, O, N, S, etc.), coal calorific value (LHV), and coal ash melting point, especially FT. 2. What was the previous oxygen-to-coal ratio? What is the current oxygen-to-coal ratio? 3. What was the previous vaporization temperature? What’s the time now? 4. What were the previous gas compositions (CO%, H2%, CO2%, CH4%, N2%)? What was it later? 5. What was the previous water vapor anomaly you mentioned? How much is it now? Now that you know this, let everyone help you analyze the possible reasons; otherwise, with so many factors lacking, it’s impossible to identify them.