Lean Burn Engine Reduces Nox Emissions MovEnergy 25kwh

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The MovEnergy Engine work with an excess air-to-fuel ratio of 60:1

Internal combustion engine-powered generator sets fueled by natural gas are commanding more attention these days as interest grows in on-site power production equipment that is both efficient and environmentally friendly.

In response to this interest, manufacturers have introduced natural gas engine-powered generator sets that feature “lean-burn” technology. The combustion is considered “lean” when excess air is introduced into the engine along with the fuel.

Lean-burn means pretty much what it says. It is a lean amount of fuel supplied to and burned in an engine’s combustion chamber. Normal air-to-fuel ratio is on the order of 15:1 (15 parts air to 1 part fuel). True lean-burn can go as high as 23:1.

Lean-burn engines (both gasoline and diesel) enjoy higher fuel economy and cleaner emissions than conventionally tuned engines. By nature they use less fuel and emit fewer unburned hydrocarbons and greenhouse gases while producing equivalent power of a like-sized “normal” combustion engine. They achieve lean-burn status by employing higher combustion chamber compression ratios (higher cylinder pressure), significant air intake swirl and precise lean-metered direct fuel injection.

Optimizing a engine efficiency is important to minimize fuel consumption and unwanted excess to the environment

 To ensure complete combustion of the fuel used, combustion chambers are supplied with excess air. Excess air increase the amount of oxygen and the probability of combustion of all fuel.
  • when fuel and oxygen in the air are in perfectly balance – the combustion is said to be stoichiometric

The combustion efficiency will increase with increased excess air, until the heat loss in the excess air is larger than than the heat provided by more efficient combustion.

combustion excess air CO fuel

Typical excess air to achieve highest efficiency for different fuels are

  • 5 – 10% for natural gas
  • 5 – 20% for fuel oil
  • 15 – 60% for coal

Carbon dioxide – CO2 – is a product of the combustion and the content in the flue gas is an important indication of the combustion efficiency.

combustion excess air

An optimal content of carbon dioxide – CO2 – after combustion is approximately 10% for natural gas and approximately 13% for lighter oils.

Normal combustion efficiencies for natural gas at different amounts of excess air and flue gas temperatures are indicated  below:

Combustion Efficiency (%)
Excess % Net Stack Temperature1) (oF)
Air Oxygen 200 300 400 500 600
9.5 2.0 85.4 83.1 80.8 78.4 76.0
15 3.0 85.2 82.8 80.4 77.9 75.4
28.1 5.0 84.7 82.1 79.5 76.7 74.0
44.9 7.0 84.1 81.2 78.2 75.2 72.1
81.6 10.0 82.8 79.3 75.6 71.9 68.2

1) The “net stack temperature” is the difference between the flue gas inside the chimney and the room temperature outside the burner.

Flue Gas Loss Combustion Oil

The relationship between temperature difference flue gas and supply air, CO2 concentration in the flue gas, and the efficiency loss in the flue gas combustion oil, is expressed in the diagram below.

flue gas efficiency

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