Post-treatment emission system suitable for methanol engine and cooperative control method

Abstract

The invention discloses a post-treatment emission system suitable for a methanol engine, and relates to post-treatment of the methanol engine, the post-treatment emission system comprises a staged purification module, a first CO sensor, a TWC catalyst, a second CO sensor, a blowdown pipe and an SCR catalyst; a staged purification module, a TWC catalyst and an SCR catalyst are sequentially installed in an exhaust pipe of the methanol engine, a front-end adsorption unit and a rear-end catalysis unit are arranged in the staged purification module, and a first CO sensor and a second CO sensor are installed at the two ends of the TWC catalyst respectively. An oxygen sensor and an air supplementing pipe are installed in the exhaust pipe between the TWC catalyst and the SCR catalyst, and the air supplementing pipe is communicated with the exhaust pipe. The invention further discloses a cooperative control method. According to the method, the maximum combustion consumption level of methanol is obtained on the premise that emission is met, and meanwhile emission of nitric oxide can be effectively reduced.

Description

TECHNICAL FIELD

[0001] The present application relates to the aftertreatment of methanol engines, more particularly, to an aftertreatment emission system and a coordinated control method suitable for methanol engines. BACKGROUND

[0002] Nowadays, with more and more emphasis on the development of new energy in China, various new clean energy has become the focus of research. Among them, with the maturity of methanol production process, the production of methanol has rapidly increased. Compared with gasoline, methanol has a certain cost advantage due to its low price, making methanol the first choice to replace gasoline fuel. Methanol contains oxygen, and under the condition of complete combustion, it generates H2O and CO2, and its combustion emissions are much lower than those of gasoline and diesel. In addition, methanol engines have the characteristics of low cost and low emissions, so they have broad prospects for promotion.

[0003] Currently, the main emission pollutants of methanol engines are formaldehyde, unburned methanol, NO X , etc., which are the main objects of emission reduction in the current aftertreatment system. The aftertreatment system of methanol engine is roughly divided into two categories. One is the aftertreatment system of original gasoline and diesel vehicles. This type of system can effectively reduce particulate matter and nitrogen oxides, but it needs to add a urea solution tank and other devices such as urea solution, which will reduce the effective load of the vehicle. The other type is to add a three-way catalyst to the tail gas for treatment on this basis, which can effectively reduce the emission of formaldehyde, methanol and other pollutants, but methanol is easy to adsorb on the surface of the catalyst, causing the deactivation of active sites.

[0004] In summary, according to the existing technology, the current aftertreatment system of methanol engine has the following shortcomings: First, high concentration of unburned methanol emitted during the cold start stage is easy to adsorb on the surface of the catalyst, causing the deactivation of the catalyst active sites. This not only causes damage to the catalyst and reduces the quality of tail gas emissions, but also greatly increases the maintenance cost.

[0005] Second, when running under high load or multiple working conditions for a long time, the air-fuel ratio in the cylinder cannot reach a good value, resulting in low fuel consumption level, which in turn leads to inefficient conversion of HC and CO, resulting in high content of tail gas pollutants and great threat to the environment. SUMMARY

[0006] The technical problem solved by the present application is to provide a post-processing emission system and a collaborative control method suitable for a methanol engine, which can avoid the poisoning and deactivation of a TWC catalyst by arranging a hierarchical purification module in front of the TWC catalyst, and can control the in-cylinder air-fuel ratio to be within a preset range centered on the theoretical air-fuel ratio by monitoring the CO conversion efficiency. X

[0007] The post-processing emission system suitable for the methanol engine comprises a hierarchical purification module, a first CO sensor, a TWC catalyst, a second CO sensor, a gas supplementing pipe and an SCR catalyst.

[0008] Preferably, the front-end adsorption unit comprises ZSM-5 molecular sieve, and the rear-end catalytic unit comprises a carrier coated with a TiO2 / Al2O3 photocatalytic coating.

[0009] Preferably, the gas supplementing pipe is provided with a control valve.

[0010] The collaborative control method for the post-processing emission system suitable for the methanol engine comprises the following steps: The CO concentrations at the two ends of the TWC catalyst are collected by the first CO sensor and the second CO sensor, and the CO conversion efficiency is calculated according to the CO concentrations.If the current air-fuel ratio deviates from the theoretical air-fuel ratio, the injection pulse width is corrected to maintain the current air-fuel ratio near the theoretical air-fuel ratio. X

[0011] Preferably, the CO conversion efficiency is calculated by the following formula: η= (c1-c2) / c1 ​Wherein, η is the CO conversion efficiency, c1 is the CO concentration collected by the first CO sensor, and c2 is the CO concentration collected by the second CO sensor.

[0012] Preferably, whether the current air-fuel ratio deviates from the air-fuel ratio theoretical value is determined according to the CO conversion efficiency, and specifically: If the CO conversion efficiency is less than a set efficiency threshold, it is determined that the current air-fuel ratio deviates from the air-fuel ratio theoretical value; if the CO conversion efficiency is greater than or equal to the efficiency threshold, it is determined that the current air-fuel ratio does not deviate from the air-fuel ratio theoretical value.

[0013] Preferably, the required corrected fuel injection pulse width is calculated by the following formula: ΔT=K×A / F / ∣Δ(A / F)∣×T0; Wherein, ΔT is the required corrected fuel injection pulse width, K is a working condition correction coefficient, A / F is the air-fuel ratio theoretical value, Δ(A / F) is the difference between the current air-fuel ratio and the air-fuel ratio theoretical value, and T0 is the initial fuel injection pulse width.

[0014] Preferably, the calculation formula for air supplementing of the exhaust gas by the air supplementing pipe is: q a =λ1×1.5×q mf,i / C O2,环境 −q maw,i ×k T +Δq a ; Wherein, q a is the air supplementing amount, q mf,i is the instantaneous methanol consumption amount, C O2,环境 is the oxygen concentration in the ambient air, q maw,i is the instantaneous actual intake air amount, k T is a temperature correction coefficient, and Δq a is a predicted compensation term.

[0015] Preferably, the predicted compensation term is calculated by the following formula: Δq a =k1×dn / dt×q mf,i ; Wherein, k1 is a delay compensation coefficient, and dn / dt is the engine speed change rate.

[0016] Advantages The present application has the following advantages: 1. The present application sets up a hierarchical purification module, which can preliminarily treat high-concentration unburned methanol and formaldehyde, so as to reduce the damage to the TWC catalytic converter as much as possible, improve the problem that high-concentration methanol is easily adsorbed to the surface of the catalyst, causing the deactivation of the active sites of the catalyst, and reduce the burden of the TWC catalytic converter, thereby prolonging the service life of the TWC catalytic converter and improving the overall exhaust purification effect.

[0017] 2. The cylinder air-fuel ratio is controlled by taking the CO conversion efficiency of the TWC catalytic converter as the target, so as to keep the cylinder air-fuel ratio in a preset range centered on the theoretical air-fuel ratio, thereby reducing the generation and emission of combustion by-products and obtaining the best methanol consumption level under the premise of meeting the emission.

[0018] 3. The air supplement mechanism arranged separately in the present application can ensure that the SCR catalytic converter is in an environment with sufficient oxygen during exhaust treatment, and ensure that the nitrogen oxides in the exhaust emitted by the SCR catalytic converter meet the relevant regulations. BRIEF DESCRIPTION OF DRAWINGS

[0019] Figure 1 The present application is a structure schematic diagram of a post-processing emission system suitable for a methanol engine. Figure 2 The present application is a system control method flow schematic diagram.

[0020] Wherein: 1-methanol engine, 2-ECU, 3-hierarchical purification module, 4-first CO sensor, 5-TWC catalytic converter, 6-second CO sensor, 7-oxygen sensor, 8-air supplement pipe, 9-control valve, 10-SCR catalytic converter. DETAILED DESCRIPTION

[0021] The present application will be further described below in combination with examples, but does not constitute any limitation on the present application, and any limited number of modifications made by anyone within the scope of the claims of the present application is still within the scope of the claims of the present application. Referring to Figure 1 The present application is a post-processing emission system suitable for a methanol engine, which comprises a hierarchical purification module 3, a first CO sensor 4, a TWC catalytic converter 5, a second CO sensor 6, an air supplement pipe 8 and an SCR catalytic converter 10. The exhaust pipe of the methanol engine 1 is sequentially provided with the hierarchical purification module 3, the TWC catalytic converter 5 and the SCR catalytic converter 10.

[0022] The methanol engine 1 exhaust gas is pretreated by the hierarchical purification module 3 to reduce the toxic effect of intermediate products such as formaldehyde and formic acid on the subsequent catalyst. Specifically, the hierarchical purification module 3 is provided with a front-end adsorption unit and a rear-end catalytic unit. The front-end adsorption unit includes ZSM-5 molecular sieve; the rear-end catalytic unit includes a carrier made of honeycomb ceramic, and the carrier is coated with a TiO2 / Al2O3 photocatalytic coating, which can maintain stable catalytic activity in the exhaust gas temperature range of 150-450℃, and is suitable for cold start to high load full working condition formaldehyde oxidation requirements.

[0023] The first CO sensor 4 and the second CO sensor 6 are respectively installed at the two ends of the TWC catalyst 5. Specifically, the CO sensors at the inlet and outlet of the TWC catalyst 5 are anti-formaldehyde interference type electrochemical sensors, with a measurement range of 0-5000 ppm and an accuracy of ±5%. When the formaldehyde concentration in the exhaust gas is ≤1500 ppm, there is no cross response, ensuring the accuracy of CO concentration detection.

[0024] The gas converted by the TWC catalyst 5 enters the SCR catalyst 10, and combined with the quantitative injection of reducing agent, the nitrogen oxides are efficiently removed, thereby comprehensively improving the emission control performance of the methanol engine 1 aftertreatment system.

[0025] An oxygen sensor 7 and a supplementary air pipe 8 are installed in the exhaust pipe between the TWC catalyst 5 and the SCR catalyst 10, the supplementary air pipe 8 is connected with the exhaust pipe, and a control valve 9 is installed in the supplementary air pipe 8. The supplementary air pipe 8 and the control valve 9 work together to supply air, which is used to optimize the working efficiency of the SCR catalyst 10.

[0026] In this embodiment, the exhaust gas from the methanol engine 1 first enters the staged purification module 3. The ZSM-5 molecular sieve at the front end rapidly adsorbs unburned methanol, while the TiO2 / Al2O3 photocatalytic coating at the rear converts most of the formaldehyde into CO2 and H2O under the influence of exhaust heat or light. The exhaust gas then enters the TWC catalytic converter 5 for further conversion, and after conversion, the gas is fed into the SCR catalytic converter 10 for further treatment. Therefore, this embodiment, by setting up the staged purification module 3, first pre-treats the high concentration of unburned methanol and formaldehyde, reducing their concentration and improving the overall exhaust gas purification effect. This minimizes damage to the TWC catalytic converter 5, reduces maintenance costs, extends the service life of the after-treatment system, and solves the problems of high unburned methanol concentration and TWC catalytic converter lifespan during cold starts. Furthermore, dedicated CO sensors resistant to formaldehyde interference are installed at the inlet and outlet of the TWC catalytic converter 5. By monitoring the CO conversion efficiency, the in-cylinder air-fuel ratio is controlled to remain within a preset range centered on the theoretical air-fuel ratio, solving the problem of unstable in-cylinder air-fuel ratio causing methanol fuel to not reach optimal consumption levels. To prevent excessive oxygen consumption by the first two stages of the catalytic converter, this embodiment also includes a separate air replenishment mechanism to ensure that the SCR catalytic converter 10 provides sufficient oxygen to the environment in which the exhaust gas is treated, and to ensure that the nitrogen oxides in the exhaust gas emitted by the SCR catalytic converter 10 comply with relevant regulations.

[0027] Example 2 like Figure 2 As shown, a coordinated control method for controlling the above-mentioned after-treatment emission system applicable to methanol engines is described below.

[0028] First, the CO concentration at both ends of the TWC catalyst 5 is collected using the first CO sensor 4 and the second CO sensor 6, and denoted as c1 and c2 respectively. The CO conversion efficiency is then calculated based on the CO concentration. The specific calculation formula is as follows: η = (c1 - c2) / c1.

[0029] The CO conversion efficiency is used to determine whether the current air-fuel ratio deviates from the theoretical air-fuel ratio value. Assuming the air-fuel ratio is at the theoretical value (A / F = 6.45), the CO conversion rate is the efficiency threshold η. max If the CO conversion efficiency is less than the set efficiency threshold η max If the air-fuel ratio is less than the theoretical value, it indicates incomplete methanol combustion and increased CO production, thus deviating from the theoretical air-fuel ratio. If the CO conversion efficiency is greater than or equal to the efficiency threshold η... max If the air-fuel ratio is greater than or equal to the theoretical value, combustion is complete, and the current air-fuel ratio is not deviating from the theoretical value.

[0030] According to the above judgment process, if the judgment result is that the current air-fuel ratio deviates from the air-fuel ratio theoretical value, the injection pulse width is corrected so that the current air-fuel ratio is maintained near the air-fuel ratio theoretical value. Specifically, the injection pulse width required for correction is calculated by the following formula: ΔT = K x A / F / | Δ(A / F) | x T0.

[0031] In the formula, ΔT is the injection pulse width to be corrected; K is a working condition correction coefficient, K = 1.2 at cold start, K = 1.0 at normal driving, and K = 0.8 at high load; A / F is the air-fuel ratio theoretical value; Δ(A / F) is the difference between the current air-fuel ratio and the air-fuel ratio theoretical value, i.e. Δ(A / F) = (A / F) actual - A / F; T0 is the initial injection pulse width, which is read by the ECU.

[0032] To prevent the excessive consumption of oxygen by the first two-stage catalyst, further convert NO X , the current oxygen concentration at the outlet of the TWC catalyst 5 is obtained by the oxygen sensor 7, and whether the oxygen content in the exhaust gas discharged by the TWC catalyst 5 is sufficient is judged according to the current oxygen concentration and a preset oxygen concentration threshold value. If the current oxygen concentration is less than the oxygen concentration threshold value, it indicates that the oxygen content in the exhaust gas is insufficient, and at this time, air is supplied to the exhaust gas through the air supply pipe 8 so that the oxygen content in the SCR catalyst 10 is maintained at a level that enables efficient conversion of NO X .

[0033] Generally, the air supply amount is calculated by the following formula: q a = λ1 x 6.45 x q mf,i - q maw,i .

[0034] In the formula, q a is the air supply amount; λ1 is an oxygen excess coefficient, which is read according to the real-time state of the engine; q mf,i is the instantaneous methanol consumption; and q maw,i is the instantaneous actual intake air amount. q mf,i , q maw,i are all read from the engine information.

[0035] The essence of the above air supply amount calculation formula is "air mass flow balance". However, in the present application, the actual influence on the catalytic reaction is the oxygen mass flow rather than the air mass. Among them, the changes of environmental temperature, air pressure and altitude will all cause significant differences in "oxygen mass corresponding to the same air mass". To solve this problem, the present application proposes to use "oxygen supply amount" to represent "air supply amount", realizing accurate control of oxygen content. The specific representation process is as follows.

[0036] The conversion process of the oxygen content in the air and the air amount is as follows: q a=q a,o2 / C O2,环境 q a,O2 =λ1×m O2,理论 -q maw,02 .

[0037] Where, q a,o2 The required oxygen mass flow rate; m O2,理论 q represents the theoretical oxygen mass flow rate required for the complete combustion and catalytic reaction of methanol. maw,02 C represents the actual oxygen mass flow rate contained in the intake air. O2,环境 q represents the oxygen concentration in ambient air. a This refers to the amount of air mass flow that needs to be replenished, i.e., the amount of replenished air.

[0038] In addition, considering that the actual intake volume is also affected by the ambient temperature, a temperature correction coefficient k is introduced to address this influencing factor. T =273 / (T 环境 +273). The T in this context... 环境 The ambient temperature is given in °C. Furthermore, the air replenishment mechanism suffers from response delay, which can easily lead to a mismatch between the replenished air volume and actual demand under highly dynamic operating conditions. To address the actuator's lag issue, a predictive compensation term Δq is added. a =k1×dn / dt×q mf,i Where k1 is the delay compensation coefficient, typically taken as 0.002-0.005 according to bench calibration; dn / dt is the engine speed change rate. Δq increases with engine speed. a When the value is positive, oxygen is added in advance; when it decreases, it becomes negative, and oxygen addition is reduced. Therefore, the actual mass flow rate of oxygen in the intake air volume is: q maw,o2 =q maw,i ×C O2,环境 ×k T .

[0039] According to the formula 2CH3OH + 3O2 = 2CO2 + 4H2O, the complete combustion of 1 kg of methanol requires 1.5 kg of oxygen. Therefore, in the aftertreatment of the methanol engine 1 of this invention, the final oxygen supply is: q a =λ1×1.5×q mf,i / C O2,环境 -q maw,i ×k T +Δq a .

[0040] Compared with the original technology, the modified air supplement amount calculation formula fully considers the influence of altitude, ambient temperature and other factors on the oxygen intake amount, ensures sufficient oxygen content during air supplement, and solves the problem of delayed response of the air supplement mechanism, so that the air supplement amount is adapted to the actual demand, the air supplement amount is consistent with the actual demand under different working conditions, the principle is upgraded from air mass balance to oxygen mass balance, and the NO X ultra-low emission, so that the NO X emission after the SCR catalyst 10 treatment reaches the regulatory limit value.

[0041] The above only describes the preferred embodiments of the present application, and it should be noted that for those skilled in the art, without departing from the structure of the present application, a number of modifications and improvements can be made, which will not affect the effect of the present application and the practicality of the patent.

Claims

1. An aftertreatment emissions system suitable for use with a methanol engine, characterized in that, The application relates to a methanol engine exhaust purification system, which comprises a staged purification module (3), a first CO sensor (4), a TWC catalyst (5), a second CO sensor (6), a gas supplement pipe (8) and an SCR catalyst (10).

2. An aftertreatment system for a methanol engine according to claim 1, characterized in that, The front-end adsorption unit comprises ZSM-5 molecular sieve, and the rear-end catalytic unit comprises a carrier which is coated with a TiO2 / Al2O3 photocatalytic coating.

3. An aftertreatment system for a methanol engine according to claim 2, characterized in that, The gas supplement pipe (8) is provided with a control valve (9).

4. A coordinated control method for controlling the aftertreatment system for a methanol engine according to any one of claims 1 to 3, characterized in that, The method comprises the following steps: The CO concentrations at both ends of the TWC catalyst (5) are collected by the first CO sensor (4) and the second CO sensor (6), and the CO conversion efficiency is calculated according to the CO concentrations; whether the current air-fuel ratio deviates from the air-fuel ratio theoretical value is judged according to the CO conversion efficiency, if deviates, the fuel injection pulse width is corrected, so that the current air-fuel ratio is maintained near the air-fuel ratio theoretical value, at the same time, the current oxygen concentration at the outlet of the TWC catalyst (5) is obtained through the oxygen sensor (7), whether the oxygen content in the exhaust gas discharged by the TWC catalyst (5) is sufficient is judged according to the current oxygen concentration and the preset oxygen concentration threshold value, if the current oxygen concentration is less than the oxygen concentration threshold value, the exhaust gas is supplemented through the air supplementing pipe (8), so that the oxygen content in the SCR catalyst (10) is maintained at a level that can make NO X efficient conversion level.

5. The method of claim 4, wherein, The CO conversion efficiency is calculated by the following formula: eta = (c1-c2) / c1; In the formula, eta is the CO conversion efficiency, c1 is the CO concentration collected by the first CO sensor (4), and c2 is the CO concentration collected by the second CO sensor (6).

6. The method of claim 4, wherein, According to the CO conversion efficiency, whether the current air-fuel ratio deviates from the air-fuel ratio theoretical value is determined, and the determination is specifically as follows: If the CO conversion efficiency is less than a set efficiency threshold value, it is determined that the current air-fuel ratio deviates from the air-fuel ratio theoretical value; if the CO conversion efficiency is greater than or equal to the efficiency threshold value, it is determined that the current air-fuel ratio does not deviate from the air-fuel ratio theoretical value.

7. The method of claim 4, wherein, The required corrected fuel injection pulse width is calculated by the following formula: Delta T = K * A / F / |Delta (A / F) | * T0; In the formula, Delta T is the required corrected fuel injection pulse width, K is a working condition correction coefficient, A / F is the air-fuel ratio theoretical value, Delta (A / F) is the difference between the current air-fuel ratio and the air-fuel ratio theoretical value, and T0 is an initial fuel injection pulse width.

8. The method of claim 4, wherein, The calculation formula for supplementing the exhaust gas through the gas supplement pipe (8) is as follows: q a = λ1 x 1.5 x q mf,i / C O2,环境 − q maw,i × k T + Δq a ; where q a is the amount of air added, q mf,i is the instantaneous methanol consumption, C O2,环境 is the oxygen concentration in the ambient air, q maw,i is the instantaneous actual intake air amount, k T is the temperature correction coefficient, Δq a is the predicted compensation term.

9. The method of claim 8, wherein, The prediction compensation term is calculated by the following formula: Δq a = k1 x dn / dt x q mf,i ; In the formula, k1 is a delay compensation coefficient, and dn / dt is an engine speed change rate.

Images