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.
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.
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
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