NOx Formation in a diesel engine

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India is heading toward emission less vehicle on road. For that India has committed for apply BS(VI) norms in 2020 by skipping BS(V). Directly going to BS(VI) after BS(IV) needs technical developments where it is to be ensured that emission should be as minimum as possible. Compared with petrol, diesel is the lower quality product of petroleum family. Diesel particles are larger and heavier than petrol, thus more difficult to pulverize. Imperfect pulverization leads to more unburnt particles, hence more pollutant, lower fuel efficiency and less power.

In atmospheric chemistry, NOx is a generic term for the nitrogen oxides that are most relevant for air pollution, namely nitric oxide (NO) and nitrogen dioxide (NO2). These gases contribute to the formation of smog and acid rain, as well as tropospheric ozone. NOx gases are generally produced from the reaction among nitrogen and oxygen during combustion of fuels, such as hydrocarbons, in air, especially at high temperatures, such as occur in car engines.

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NOx emissions do not form in significant amounts until flame temperatures reach high temperature like 2800 F. Once that threshold is passed, however, any further rise in temperature causes a rapid increase in the rate of NOx formation. Oxygen and nitrogen do not react at ambient temperatures. But at high temperatures, they undergo an endothermic reaction producing various oxides of nitrogen.

Such temperatures arise inside an internal combustion engine or a power station boiler, during the combustion of a mixture of air and fuel, and naturally in a lightning flash. NOx production is highest at fuel-to-air combustion ratios of 5–7% O2 (25–45% excess air). Lower excess air levels starve the reaction for oxygen, and higher excess air levels drive down the flame temperature, slowing the rate of reaction.

NOx reduction is the area of most concern today. First, we must know that how NOx is formed. NOx is formed by three major processes. The three primary and probable sources of NOx in combustion processes: –

  1. Thermal NOx
  2. Fuel NOx
  3. Prompt NOx
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In the first one thermal NOx formation process high temperature combustion of fuels where the temperature is hot enough (above about 1300°C/ 2370°F) to oxidize some of the nitrogen in air to NOx gases. This includes burning hydrogen, as it burns at a very high temperature. Thermal NOx refers to the nitrogen dioxide formation through high-temperature oxidation of the diatomic nitrogen found in combustion air. The formation rate is primarily a function of temperature and the residence time of nitrogen at that temperature. At high temperatures, usually above 1600 °C (2900 °F), molecular nitrogen (N2) and oxygen (O2) in the combustion air disassociate into their atomic states and participate in a series of reactions.

Secondly, Anthropogenic activities are also responsible for nitrogen dioxide formation. The major source of NOx production from nitrogen-bearing fuels such as certain coals and oil, is the conversion of fuel bound nitrogen to NOx during combustion. During combustion, the nitrogen bound in the fuel is released as a free radical and ultimately forms free N2, or NO. Fuel NOx can contribute as much as 50% of total emissions when combusting oil and as much as 80% when combusting coal.

Thirdly prompt NOx is attributed to the reaction of atmospheric nitrogen, N2, with radicals such as C, CH, and CH2 fragments derived from fuel, where this cannot be explained by either the thermal or fuel processes. Occurring in the earliest stage of combustion, this results in the formation of fixed species of nitrogen such as NH (nitrogen monohydride), HCN (hydrogen cyanide), H2CN (dihydrogen cyanide) and CN (cyano radical) which can oxidize to NO. In fuels that contain nitrogen, the incidence of prompt NOx is especially minimal, and it is generally only of interest for the most exacting emission targets.

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The three principal reactions (the extended Zel’dovich mechanism) producing thermal NOx are:

  • N2 + O → NO + N
  • N + O2 → NO + O
  • N + OH → NO + H

There are many gases responsible for emission in a vehicle, but NOx plays a significant role emission formation. Thermally produced NOx is the largest contributor to these types of emissions. Thermal NOx is produced during the combustion process when nitrogen and oxygen are present at elevated temperatures. The two elements combine to form NO or NO2. NOx is generated by many combustion processes other than boiler operation. It combines with other pollutants in the atmosphere and creates O3, a substance known as ground level ozone.

NOx in boiler burners can be reduced with either pre-combustion or post-combustion technology. Post-combustion technology allows NOx to form, then breaks it down in the exhaust gases which process is commonly known as called catalytic reduction. This method is normally confined to larger, utility-size equipment. The pre-combustion method prevents NOx from forming in the first place. Pre-combustion NOx reduction is accomplished by either staging the combustion process or recirculating flue gases into the combustion process.

A comparison of NOx emissions with DI and IDI engines can be done. At light loads, most of the NO may form in the pre-chamber. But, at higher loads, additional NO formation would occur in the main chamber. Although temperatures are higher in the pre-chamber than the main chamber, except at light loads mixture is overall rich and hence, the lower formation of NO. In the DI engines, at the end of premixed combustion higher peak pressures and temperatures are obtained compared to the IDI engines, and NO is formed in near stoichiometric mixtures during mixing controlled phase and post-combustion gases.

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Due to these factors overall, the indirect injection engines emit lower nitrogen dioxide formation. In the DI engines due to low turbulence levels, some of the CO formed in the rich spray regions may not find the required oxygen for complete combustion while the temperatures are still high. It results in higher CO emissions than the IDI engines even though more excess air is present in the DI engines.

Most of the heavy vehicles run on diesel engines. So, emission control in the CI engines, usually called diesel engines are important. Diesel engines emit pollutants in solid (soot), liquid (polyaromatic hydrocarbons, fuel and oil components, Sulphur acids) as well as those in gaseous (CO, HC, NOx) state. Emissions of nitrogen oxides and particulate matter from diesel engines are of main concern. Emission regulations do have limits for CO and HC as well from the CI engines, but the concentration of their emissions is rather small, and these have been relatively easy to control through improved engine and fuel system design.

Nitrogen dioxide formation can be controlled by selection and optimization of many engine design variables e.g., injection timing, injection pressure, boost pressure, etc as a change in some engine variables may although causes reduction in NOx but increases PM and vice versa. Engine design changes to reduce NOx emissions many a times result also in higher brake specific fuel consumption (BSFC). This is important as the emissions of the greenhouse gas, CO2 is also to be reduced.

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