Diesel Particulate Filter Regeneration

The diesel particulate filter (DPF) has a high filtration efficiency for soot particles, which can reach 60% to 90%. During filtration, particulate matter accumulates in the particle filter, which will cause the diesel engine exhaust back pressure to increase. When the exhaust back pressure reaches 16 kPa to 20 kPa, the diesel engine performance begins to deteriorate. Therefore, the particles must be removed regularly to restore the particle filter to its original working state, that is, regeneration. The biggest challenge facing DPF is the regeneration problem. The method of regenerating the particle filter is mainly particle oxidation, and the elements of particle oxidation are high temperature, oxygen enrichment and oxidation time. In fact, the exhaust temperature of diesel engines is generally less than 500°C, especially the exhaust temperature of some buses running in urban conditions is even below 300°C. The key issue of particle filter regeneration is to reduce the equilibrium point temperature, at which the speed of particle formation and oxidation is equal, the back pressure is relatively constant, and the system is in equilibrium. The equilibrium point temperature is related to the flow rate, particle composition, NOx content, sulfur concentration, soot formation, and engine and fuel parameters. The regeneration methods of DPF include active regeneration and passive regeneration. Common active regeneration methods include burner fuel injection heating regeneration, electric heating regeneration, microwave heating regeneration and infrared heating regeneration. Burner heating regeneration is to set a burner at the entrance of the particle filter, spray diesel and secondary air, and ignite the soot particles in the particle filter after combustion for regeneration. This method requires the provision of additional fuel and a constant burner temperature, so a set of automatic adjustment control system for adjusting the fuel and secondary air supply is required. In addition, this burner has certain requirements for vehicle driving, such as not accelerating sharply during regeneration. If the regeneration is accelerated sharply, the airflow will extinguish the regenerated flame, so that the fuel is not completely burned, causing the vehicle to emit white smoke. Electric heating regeneration is to heat the particle filter by electric heating to promote the ignition of soot particles. Although this regeneration method will not cause any smoke, the electric heating has relatively high requirements for the control system, and the problem of high power consumption needs to be solved during vehicle use. It is not easy for small and medium-power diesel engines to meet the power requirements. In addition, the uneven heating will cause uneven regeneration of the filter body, and the filter body is prone to local overheating and damage. Microwave heating regeneration uses the unique selective heating and volumetric heating characteristics of microwaves to form a spatially distributed heat source inside the filter body, heat the soot particles deposited on the filter body, and heat and ignite the soot particles in situ. Realizing filter body regeneration Microwave heating refers to heating an object at a frequency of 300 MHz to 300 GHz. It is different from general heating methods. It is a volume heating caused by dielectric loss in the electromagnetic field. Its energy is transmitted in the form of electromagnetic waves through space or medium. The heating process is closely related to the polarization of molecules inside the substance. The key technology of microwave heating regeneration lies in how to excite as many modes as possible in the resonant cavity and control the combustion temperature of soot particles in the cavity to prevent the filter body from being damaged due to excessive temperature. Compared with other heating methods, infrared radiation heating regeneration has the characteristics of selective heating, can directly heat the heated body, shorten the time it takes for the heated body to heat to the required temperature, and reduce energy consumption; infrared regeneration technology has high regeneration efficiency; the maximum regeneration temperature is suitable, and the filter body temperature gradient is small, which is very suitable for the regeneration of honeycomb ceramics. These advantages of infrared radiation heating are closely related to its heating mechanism. From the perspective of infrared radiation heating mechanism, when the radiation wavelength of the radiation source is consistent with the absorption wavelength of the irradiated object, the latter absorbs a large amount of infrared energy, thereby changing and intensifying the movement of its molecules, achieving the effect of heating and heating. Common passive regeneration methods include fuel additive catalytic regeneration filter system, CRT (continuous regeneration filter) system and CCRT system. The fuel additive of the fuel additive catalytic regeneration filter system is a catalyst dissolved in fuel. After combustion in the engine, it enters the exhaust and is captured by the filter together with the particles, which makes the particles and the catalytic active components in close contact, which is equivalent to adding a catalyst to the particles to reduce the regeneration temperature of the particles. The disadvantage is that fuel additives will endanger human health when entering the atmosphere with the exhaust of motor vehicles. CRT consists of an oxidizer (DOC) and a particulate filter (DPF without catalytic coating). When the exhaust of a diesel engine passes through the oxidation catalytic converter, at a temperature of 200 ℃ to 600 ℃, CO and HC are first almost completely oxidized into CO2 and H2O, and NO is converted into NO2. Then NO2 is oxidized to generate CO2 from the particles in the particulate filter, and NO2 is reduced to NO, thereby achieving the purpose of regenerating the particles. It should be noted that this system can only be used on low-sulfur diesel engines, because the oxidation catalyst can oxidize sulfur and increase the number of particles. At the same time, passive regeneration requires a suitable exhaust temperature. It is not good if the temperature is too high or too low. If the temperature is too low, the oxidation rate of NO2 is too slow; if the temperature is too high, the decomposition of NO2 is accelerated and insufficient NO2 can be formed. The regeneration temperature range of CRT is 240 ℃ ~ 450 ℃, and the regeneration requires the NOx/PM ratio in the exhaust to be greater than 20:1. The CCRT system is also composed of an oxidizer (DOC) and a particulate filter (DPF), but the particulate filter carrier (DPF) of CCRT is coated with a catalyst, so that the NO generated in the oxidizer can continue to generate NO2 in the particulate filter to oxidize the particulate matter, so that the system can more completely remove the particulate matter from the filter. The advantage of the CCRT system is that it has more relaxed conditions for regeneration, and a lower exhaust temperature and a lower NOx/PM ratio in the exhaust can regenerate the system.
However, in the above regeneration process, only the carbon deposits inside the DPF are removed, and the ash substances without carbon deposits cannot be reduced and decomposed at all. These ashes mainly come from the components in the engine oil and fuel. Since the ash cannot be burned, the effective volume of the particle filter is reduced, and the function of the particle filter will deteriorate after a certain period of time. Therefore, for diesel vehicles with DPF, low-ash engine oil should be selected first and the DPF should be cleaned and regenerated regularly to ensure the actual conversion efficiency of the DPF, reduce the emission of pollutants, extend the service life of the DPF, reduce the later maintenance costs, reduce the engine power loss and reduce fuel consumption. For regular cleaning of DPF, you can choose the German Liqui Moly DPF original imported cleaning agent products - LM-5169 diesel particle filter cleaner and LM-5171 diesel particle filter flushing liquid. LM-5169 is a very effective cleaning dispersant that can quickly dissolve substances such as carbon smoke and ash attached to the surface of the DPF. LM-5171 is an efficient flushing fluid. When used after using Liqui Moly Diesel Particle Cleaner, it can distribute the particles dissolved during the cleaning process around the inner wall of the DPF and burn them off. Therefore, regular use of this product can greatly extend the service life of the DPF, reduce the cost of later maintenance and replacement, and effectively reduce particulate emissions.