JPS6142163B2 - - Google Patents

Description

[Detailed description of the invention]

(Purpose and Background) This invention relates to an air-fuel ratio control method for a combustion furnace, in particular, a combustion furnace that contains hydrogen, carbon monoxide, methane, and other light hydrocarbons as main exothermic components, and whose content varies over time. The present invention relates to a control method for maintaining an optimal air-fuel ratio in a combustion furnace that uses gas. Examples of gases that contain hydrogen, carbon monoxide, methane, and other light hydrocarbons as their main exothermic components, and whose contents vary over time include refinery offgas and PSA (Pressure Swing).
adsorption) off-gas, etc. When such a fuel is used for combustion at a constant air-fuel ratio, in some cases there is a lack of oxygen relative to the fuel, resulting in incomplete combustion, and in other cases, there is an excess of oxygen relative to the fuel. In both cases, the energy utilization efficiency of the fuel is reduced and harmful gases such as carbon monoxide or nitrogen oxide are more likely to be generated. In order to maintain the optimal fuel condition using fuel and air with stable composition, it is sufficient to measure the residual oxygen content in the combustion exhaust gas and control the air-fuel ratio to keep it at a constant value. If there are significant changes over time, do the opposite of what is actually required, for example, increase the amount of oxygen supplied because the amount of oxygen consumed by the fuel during combustion is high and the residual oxygen content in the exhaust gas has decreased. Alternatively, even though an operation has been performed to reduce the amount of fuel supplied, the composition of the supplied fuel near the flow rate regulating valve has already changed and the required amount of oxygen may have decreased. In such cases, a commonly considered control method is to constantly measure the content of fuel gas components, such as H ₂ , CO, methane, and other hydrocarbons, calculate the required amount of oxygen, and check the oxygen-containing gas composition. If the oxygen concentration changes, the oxygen concentration is also constantly measured, the optimum air-fuel ratio is calculated, and the supply amount of the fuel gas or oxygen-containing gas is changed based on the optimum air-fuel ratio. This method provides ideal control, but this method uses various expensive analytical equipment such as infrared analyzers and high-performance computers to quickly analyze the fuel composition. It is necessary to process the data and operate it without time lag. The present invention provides a method of controlling the air-to-air ratio to maintain optimal combustion conditions without using such expensive equipment. (Structure) That is, the present invention provides a combustion furnace that uses gas as fuel that contains hydrogen, carbon monoxide, methane, and other light hydrocarbons as main exothermic components, and the content of each of them changes over time. An air-fuel ratio control method for a combustion furnace, which comprises controlling the amount of oxygen-containing gas supplied to the combustion furnace based on the methane content in the gas generated by diverting a part of the gas to perform a methanation reaction. be. More specifically, the present invention calculates the amount of oxygen necessary for combustion of a gas that contains hydrogen, carbon monoxide, methane, and other light hydrocarbons as main exothermic components, and whose contents vary over time. The principle is to perform a methanation reaction on a portion of the gas, thereby approximately replacing the methane in the generated gas with the amount of oxygen required to burn it. By controlling the air-fuel ratio based on the results, it is possible to prevent excess or deficiency of oxygen in the combustion furnace. First, the principle will be explained below. It is expected that the following reaction will proceed in the methanation reactor. 3H ₂ +CO→CH ₄ +H ₂ O (1) 4H ₂ +CO ₂ →CH ₄ +2H ₂ O (2) (2n−m/2)H ₂ +CnHm→nCH ₄ (3) The composition on the left side of each equation and the right side The number of moles of oxygen required to burn the compositions of is the same. Left side of equation (1) 3H ₂ +3/2・O ₂ →3H ₂ O CO+1/2・O ₂ →CO ₂ total 2O ₂ Right side of equation (1) CH ₄ + 2O ₂ →CO ₂ +2H ₂ O Left side of equation (2) 4H ₂ + 2O ₂ →4H ₂ O Right side of equation (2) CH ₄ + 2O ₂ →CO ₂ +2H ₂ O Left side of equation (3) (2n-m/2)H ₂ + (n-m/4)O ₂ → (2n− m/2)H ₂ O CnHm+(n+m/4)O ₂ →nCO ₂ + (m/2)H ₂ O Total 2nO ₂ Right side of equation (3) nCH ₄ + 2nO ₂ →nCO ₂ +2nH ₂ O or more As shown in Equations (1), (2), and (3), if the reaction proceeds completely to the right side, the amount of oxygen required for the generated methane can be expressed as the amount of oxygen required for the fuel gas. can. However, in an actual methanation reaction, a chemical equilibrium exists in each of the above equations, and hydrogen, carbon monoxide, and the like do not completely turn into methane. However, even if a small amount of hydrogen or carbon monoxide remains, the amount of oxygen that corresponds to that amount is sufficiently covered by the oxygen excess rate in the range of 5% to 20%, which is usually adopted when performing combustion. Ru. ``Based on the methane content in the gas produced by the methanation reaction'' means that the required amount of oxygen is not determined stoichiometrically from the methane content, but based on the chemical equilibrium as described above. This means determining the required amount of oxygen by considering the safety factor, including problems with the methanation catalyst's performance. In order to reduce the time lag between the gas that is diverted and methanized and its methane content measured and the gas that is combusted in the combustion furnace, the pipe line from the diversion point to the methanation reactor for measurement should be as short as possible. It is better to specify the split flow ratio at the design stage, and it is sufficient to be able to obtain a sample of approximately 50 ml/min to 10/min as the split flow rate, regardless of the capacity of the combustion furnace main body. . The fuel gas thus divided is introduced into a methanation reactor for measurement and reacted. The reactor is filled with a methanation catalyst. As the catalyst, any known catalyst may be used. After removing moisture from the measuring reactor outlet gas, the methane content in the produced gas is measured. The amount of gas before and after methanation is also measured. Based on this value, the amount of oxygen-containing gas supplied to the combustion furnace is increased or decreased, or the amount of fuel gas is increased or decreased, taking into account the safety factor as described above. This operation can be performed in real time by combining a computer that processes data obtained by a methane analyzer and a flow meter, and a flow rate regulating valve that is operated based on the calculation results. Although the method for controlling the air-fuel ratio within a certain range in a combustion furnace has been described above, there are cases where the furnace temperature and the temperature of the heated object cannot be kept constant by this method alone. This is because when the fuel calorific value (KCal/Nm ³ ) changes, the temperature reached by the combustion gas changes, and the heat transfer rate also changes, resulting in a change in the temperature of the heated body. Naturally, fluctuations in operating load also lead to changes in the temperature of the heated object. Therefore, consideration should be given to making corrections for this purpose using a computer program. Basically, this is done by supplying gas with a certain amount of methane equivalent to the combustion furnace based on the methane content of the methanated gas, and controlling the temperature of the gas at the exit of the combustion furnace, the temperature of the heated object, etc. It can be controlled by inputting it as modification information. Example 1 A gas having the composition shown in Table 1 was obtained as a PSA off-gas in a hydrogen production process. The amount of oxygen theoretically required to burn 100m ³ of this gas was 39.9m ³ (NTP).

【table】

[Table] When this gas was methanized at 0.3Kg/cm ² G and 300℃ to remove moisture, the gas had the composition shown in Table 2: 67.1
Got ^m3 . The amount of oxygen theoretically required to burn methane in ^67.1m3 of the gas obtained in this way is
^39.45m3 (NTP), 98.9% of the required amount of oxygen
is displayed, and the oxygen excess rate commonly used is 5%.
There is still a 4% margin even if the plant operates at Example 2

[Table] COG with the composition shown in Table 3 is applied to PSA to recover hydrogen and the off-gas is used as fuel. This off gas
Results of comparing the amount of oxygen required to burn 100 ml of gas that has been methanized at 300℃ at 0.3Kg/cmG (A) and the amount of oxygen required to burn only methane (B). are shown in Table 4. Even when the difference is maximum (when COG is used as fuel), the ratio between the two is 92.6%, which is within the normal recovery rate range (70%).
~80%), the amount of oxygen (B) required to burn only methane after methanation is approximately 99% of the truly necessary amount of oxygen (A). That is, it can be seen that the air-fuel ratio can be controlled according to this method.

[Table] (Effects) As detailed above, according to the method of the present invention, by simply measuring the methane content of the diverted and methanized gas, the composition changes over time to maintain the optimal combustion state. It is possible to control the air-fuel ratio of the combustion furnace using the fuel. Further, if necessary, the temperature of the combustion exhaust gas is further measured and the amount of fuel supplied is adjusted accordingly, thereby making it possible to maintain the calorific value of the combustion furnace constant. Furthermore, the temperature of the heated object can be controlled to a predetermined value using the operating load and the difference between the temperature of the heated object and the target temperature as information.

Claims (1)

[Claims] 1. In a combustion furnace that uses gas as fuel that contains hydrogen, carbon monoxide, methane, and other light hydrocarbons as main exothermic components, and whose contents vary over time, part of the fuel gas is diverted. A method for controlling the air-fuel ratio of a combustion furnace, which comprises controlling the amount of oxygen-containing gas supplied to the combustion furnace based on the methane content in the gas produced by performing a methanation reaction.