Friday, June 23, 2017

Comparison between lean burn and Rich Burn engines

Comparison between lean burn and Rich Burn engines:-
After certain emission calibration levels Lean‐burn gas engines are more economical  and these can even operate at higher loads,
But in rich‐burn engines have lower emission levels with a single after treatment these are more tolerant of broad fuel ranges and ambient conditions, and generally have better transient load capability.

Principle of operation of Rich Burn engines:-
Rich‐burn engines operate at principle of stoichiometric air/fuel ratio (AFR) according to this principle air in exact quantity is supplied to burn all of the fuel.  This will leads to reduction in nitrogen oxides (NOx), carbon monoxide (CO), hydrocarbons (HC), and HAPS (Hazardous Air Pollutants) after certain treatment for all in one i.e. by using catalytic converter used in cars.

Lean burn engines working principle:-
In these engines Lean‐burn engines use a lot of excess air. These engines take up to twice the amount needed in rich burn engines for complete fuel combustion. Excess air used in lean burn effectively cools down the peak combustion temperatures in the cylinder, this will reduces the NOx production and allows low engine‐out emissions without the need for an after treatment system in many applications.
In these engines there are advantages of reducing the detonation probability thus it will leads to higher Brake Mean Effective Pressure loads and an optimized combustion phasing. This results in higher power density and usually produces better fuel efficiency.

Emissions in Rich-burn engines:-
Rich‐burn engines have emissions of 12‐16 g/bhph‐hr NOx i.e. “5,000 ‐ 6,500 mg/Nm3@ 5 percent 02 in the exhaust gas “, in most stoichiometric/AFR exhaust gas composition and the increased exhaust gas temperatures allow the use of a three‐way catalyst.
These engines  have high NOx conversation rates i.e. above 99 percent that significantly reduce all three major types of engine‐out emissions ‐ NOx, CO and HC , Since there are low emissions that will destroy inferior but hazardous pollutants like formaldehyde (CH20).
Rich-burn engines emission are below 50 mg/Nm3 NOx and ultra‐low total hydrocarbon emissions, which will leads to decreased overall greenhouse gas footprint.

Emissions in Lean Burn Engines:-
If we have requirement of High power density and we required highest possible efficiency at moderate emission levels of 500 or 250 mg/Nm3 NOx (@ 5 percent 02 in the exhaust gas) lean burn engines have advantage. These engines at an adequate gas quality they deliver BMEP levels of up to 24 bar with electrical efficiencies up to 46.5 percent  without the need for a NOx or THC after treatment system.

To lower the NOx emissions that are reached by rich‐burn engines with a three‐way‐catalyst, lean-burn engines require selective catalytic converters with urea injection.
Oxidation catalysts perform most of the CO reduction in lean‐burn engines but  the fuel gas must be very pure. These catalysts also can reduce CH20 emissions ‐ again, if the gas is pure ‐ but their low exhaust temperature limits hydrocarbon conversion efficiency.

Operational flexibility
Rich burn engines can operate effectively only at clean fuels such as Natural gases. These will not operate at Biogas, Sewage gas or landfill gases as these will poison the three way catalyst. High combustion temperatures restrict specific output and the BMEP, so there is lower efficiency than with lean‐burn engines operating at higher air/fuel ratios. If lean burn engines are calibrated to operate at extremely low NOx levels (ultra‐lean), their efficiency begins to degrade so that the difference between rich‐burn and lean‐burn fuel consumption is minimized. Since lean‐burn engines have a much higher AFR ‐ with about 10 percent excess oxygen in the exhaust ‐ their engine‐out NOx emissions are only 5 percent to 10 percent of the amount discharged by a rich‐burn engine. Lean‐burn engines require selective catalytic reduction (SCR) treatment to obtain the lowest possible NOx emissions levels in the exhaust gas. SCR injects a controlled amount of urea into the catalyst to convert NOx to nitrogen. Being able to operate at a more optimal AFR with an SCR system makes the lean‐burn engine very efficient and allows high break mean effective pressures.

Oxidation catalysts are used to provide most of the CO and NMHC reduction in lean‐burn engines but, as with other catalytic systems, the fuel gas has to be very pure. These catalysts also can reduce CH20 emissions ‐ again, if the gas is pure ‐ but their low exhaust temperature limits hydrocarbon conversion efficiency