Naked Science Forum
Non Life Sciences => Chemistry => Topic started by: Mohamed Abdo on 17/05/2018 16:28:06
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What is the effect of cooling diesel exhaust gas temperature on emissions concentration? in another word, is there is a relation between the exhaust gas temperature and its emissions and pm concentration at a certain point (load)? please consider your answers apart from EGR and Catalytic converters.
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If you cool any gas enough, it will liquefy or solidify. However if you burn the fuel in an internal combustion engine at a lower temperature, it won't generate as much power. Particulates represent incomplete combustion: ideally you want to run the engine at a constant speed with maximum possible oxidation of the fuel. Ships do this by operating very large pistons, very slowly, with a "stratified charge" where the fuel is injected in stages as the piston moves so it is completely burned at each point. This is very efficient for a 10-day cruise through the Pacific but not suitable for city traffic! My preference would be to burn the fuel in a gas turbine that charges a battery.
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If you cool any gas enough, it will liquefy or solidify. However if you burn the fuel in an internal combustion engine at a lower temperature, it won't generate as much power. Particulates represent incomplete combustion: ideally you want to run the engine at a constant speed with maximum possible oxidation of the fuel. Ships do this by operating very large pistons, very slowly, with a "stratified charge" where the fuel is injected in stages as the piston moves so it is completely burned at each point. This is very efficient for a 10-day cruise through the Pacific but not suitable for city traffic! My preference would be to burn the fuel in a gas turbine that charges a battery.
Yes, Well said!. my point here is to ask if we imagine that we have exhaust gas temperatures passing through somehow a heat exchanger to exchange its heat with another substance (without mixing) the temperature of the exhaust flue gases decreases as a result of the heat transfer occured in the heat exchanger. does it reflect somehow on the emission concentration?
i've read a scientific article once, that while decreasing the exhaust gas temperatures, the mass concentration of PM increased while the gaseous emissions decreased as a result of condensation of flue gases. but i could not identify if this is a real story or not.
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This depends on what you mean by "emissions," what temperatures are actually involved, and what other mechanisms might be employed before or after the cooling?
For example: cooling the exhaust to 10 °C (somehow) is likely to condense water vapor, any unconsumed fuel, and some of the products of partial combustion. Do these all count as emissions? Does it condense into an aerosol, or does it drip out the tailpipe as a liquid, or is it captured by the vehicle?
Catalytic converters require certain (hot) temperatures for efficient conversion of NOx and CO, so cooling the exhaust before the converter is likely to make problems worse.
As far as I understand, the amount (and type) of particulate matter produced is determined by the composition of the fuel, the fuel/air ratio, and the temperature and pressure profiles during the combustion. I don't believe that the pm concentrations change that much after the combustion, but I could be wrong.
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This depends on what you mean by "emissions," what temperatures are actually involved, and what other mechanisms might be employed before or after the cooling?
Emissions here refers to all gaseous components come out from the combustion chamber of a diesel engine such as (CO, CO2, HC, SO2, NOx, ...) - the temperature of the exhaust gases would be nearly 120-degree centigrade while after heat transfer it reaches to 100-degree centigrade. the preliminary results of the experiments resulted in a small variation of emission readings.
For example: cooling the exhaust to 10 °C (somehow) is likely to condense water vapor, any unconsumed fuel, and some of the products of partial combustion. Do these all count as emissions? Does it condense into an aerosol, or does it drip out the tailpipe as a liquid, or is it captured by the vehicle?
The exhaust was cooled above the saturation temperature, as there is no condensable water or liquids noticed.
Catalytic converters require certain (hot) temperatures for efficient conversion of NOx and CO, so cooling the exhaust before the converter is likely to make problems worse.
Yes, well said. as a result, I asked that the answers i seek should not be related to converters or exhaust gas recirculation.
As far as I understand, the amount (and type) of particulate matter produced is determined by the composition of the fuel, the fuel/air ratio, and the temperature and pressure profiles during the combustion. I don't believe that the pm concentrations change that much after the combustion, but I could be wrong.
(Ning, Cheung et al. 2004) investigated experimentally the effect of cooling the exhaust gas on diesel particulate which is one of those emissions come out from diesel exhaust. As cooling the exhaust temperature can result in condensation of volatile materials and coagulation of particulates; they used a water cooler to adjust the exhaust temperature and studied the influences on mass concentration, composition, size distribution, and number average diameter of the particulates under different operating conditions. They concluded that diesel particulate transformation mainly depends on the level of cooling, the concentration of volatile materials, and the time of being in the exhaust pipe. By cooling the exhaust gas they concluded that the number of particulates decreased while the number-average diameter of them increased while gaseous hydrocarbon concentration decreased. They recorded the results using Aerosol Size Analyzer. Their experiment was set up to determine the total particulate mass concentration and particulate size distribution as a function of the temperature of the cooled exhaust gas under different operating conditions. The experiment occurred on a 4-stroke engine-cylinder, naturally aspirated and water cooled diesel engine with a maximum power of 8.8 kW where the exhaust gas cooler was a two pass shell and tube heat exchanger where the exhaust gas temperature was being adjusted from 40 ⁰C to 400 ⁰C with adjusting the flow rate of cooling water. They concluded that the concentration of particulates is increased in proportional to the engine load and this increasement was not obvious in low and medium loads but was significant at high loads. The resulted in the relation between the cooled exhaust gas and mass concentration of particulates. They found that the relative change in mass concentration - which is the percentage increase in the mass concentration of the particulates upon cooling compared to no cooling – were high in both low and high load conditions as when the gas cooled down to 50 ⁰C the relative change in mass concentration of particulates was 40% for low load and 25% for high load but only 15% for medium load condition. They concluded that the mass concentration of particulates mainly increases when the exhaust gas was cooled down below 150 ⁰C or below as when exhaust gas cooled down to 200 ⁰C or above; there was a slight increase. They stated that the main reason for increasing the mass concentration while cooling the exhaust gas is that the volatile organic/inorganic substances were nucleated or condensed during the cooling process and results in decreasing other gaseous emissions where the driving force for gas-particulate conversion process is the saturation ratio which is a function of exhaust gas temperature.
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(Ning, Cheung et al. 2004) investigated experimentally the effect of cooling the exhaust gas on diesel particulate which is one of those emissions come out from diesel exhaust. As cooling the exhaust temperature can result in condensation of volatile materials and coagulation of particulates; they used a water cooler to adjust the exhaust temperature and studied the influences on mass concentration, composition, size distribution, and number average diameter of the particulates under different operating conditions. They concluded that diesel particulate transformation mainly depends on the level of cooling, the concentration of volatile materials, and the time of being in the exhaust pipe. By cooling the exhaust gas they concluded that the number of particulates decreased while the number-average diameter of them increased while gaseous hydrocarbon concentration decreased. They recorded the results using Aerosol Size Analyzer. Their experiment was set up to determine the total particulate mass concentration and particulate size distribution as a function of the temperature of the cooled exhaust gas under different operating conditions. The experiment occurred on a 4-stroke engine-cylinder, naturally aspirated and water cooled diesel engine with a maximum power of 8.8 kW where the exhaust gas cooler was a two pass shell and tube heat exchanger where the exhaust gas temperature was being adjusted from 40 ⁰C to 400 ⁰C with adjusting the flow rate of cooling water. They concluded that the concentration of particulates is increased in proportional to the engine load and this increasement was not obvious in low and medium loads but was significant at high loads. The resulted in the relation between the cooled exhaust gas and mass concentration of particulates. They found that the relative change in mass concentration - which is the percentage increase in the mass concentration of the particulates upon cooling compared to no cooling – were high in both low and high load conditions as when the gas cooled down to 50 ⁰C the relative change in mass concentration of particulates was 40% for low load and 25% for high load but only 15% for medium load condition. They concluded that the mass concentration of particulates mainly increases when the exhaust gas was cooled down below 150 ⁰C or below as when exhaust gas cooled down to 200 ⁰C or above; there was a slight increase. They stated that the main reason for increasing the mass concentration while cooling the exhaust gas is that the volatile organic/inorganic substances were nucleated or condensed during the cooling process and results in decreasing other gaseous emissions where the driving force for gas-particulate conversion process is the saturation ratio which is a function of exhaust gas temperature.
I stand gladly corrected. Thank you for updating my rudimentary understanding of the parameters influencing pm distribution.
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I stand gladly corrected. Thank you for updating my rudimentary understanding of the parameters influencing pm distribution.
You are welcome. it is a part of my literature reviews about this section of the thesis which supports the specific point of view.
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Fact is that all the carbon, sulfur etc in the fuel has to come out in the exhaust, plus any compounds of nitrogen and oxygen that might be formed during combustion. Whether they are exhausted as gas, liquid or solid depends on how you treat the exhaust after combustion.