Emission reduction - gas engines

In some areas an efficient multi-component emission reduction is required. Wärtsilä’s solution for such cases, especially for bigger power generation units, is the combined SCR and oxidation catalyst system.

The combined SCR and oxidation catalyst unit can be designed to reduce NO_x, CO and hydrocarbon emissions to the lowest practical level. The unit is typically used for big gas or dual-fuel fired engine plants with strict emission requirements. NO_x emissions are typically reduced by 90-95 % in the SCR process and carbon monoxide and hydrocarbon emissions effectively oxidized by means of strong oxidation catalyst.

The combined catalyst system comprises of a selective catalytic reduction for NO_x emissions and oxidation catalyst for CO and hydrocarbon emissions. Similarly to the normal SCR unit for gas engines the NO_x emissions can be lowered to single digit levels but concurrently low levels of CO, unburned compounds and combustion intermediate products can be achieved.

Reducing agent, ammonia or urea, of appropriate quality is required for the SCR phase.

The combined catalyst system also includes a thorough optimisation of engine operation in different conditions in order to obtain the best overall performance.

Dozens of Wärtsilä power plants are equipped with the combined SCR and oxidation catalyst unit.

Features

• Applicable with Wärtsilä gas and dual-fuel engines
• Concurrent reduction of NO_x, CO and hydrocarbon emissions
• NO_x reduction typically 90-95 %
• Oxidation stage to be designed according to the project specific data
• No solid or liquid end-product generated

The use of primary methods is often recommended, as these are effective ways of eliminating the need for additional investments and possible additional environmental loads.

In Wärtsilä power plants, the first priority in emissions reduction is to use dry primary methods, which in practice means optimizing the engine, while at the same time guiding the customer in choosing the right quality of fuel.

Modern Otto gas engines use the lean-burn combustion method. In the lean-burn process natural gas and air are pre-mixed before being introduced into the cylinders. This low fuel/air ratio – lean-burn – reduces NO_x emissions efficiently due to a lower combustion temperature.

The drawback in reducing NO_x emission formation is often an increase of other emissions, such as CO.

Gas fired lean-burn engines have sufficiently low NO_x emissions to comply with most national/local regulations using lean-burn primary methods only.

Features

• Lean-burn
• Low combustion temperature

Benefits

• Reduced NO_x emissions

Wärtsilä IOXI (Integrated Oxidation Catalyst) is a compact and cost efficient oxidation catalyst that is applicable for moderate carbon monoxide (CO) and formaldehyde (CH₂O) abatement of gas engines.

Wärtsilä 34SG gas engines together with IOXI are designed to meet German TA-luft 2002 limits for CO, CH₂O and NO_x.

IOXI consisting of a metallic catalyst element that is inserted inside the exhaust gas duct next to a bellow. In this way there is no need of a separate catalyst housing, extra flanges, or extra support structures. Already, dozens of IOXI’s have been delivered and are successfully operating.

Features

• Moderate carbon monoxide and formaldehyde abatement
• Applicable with gas engines; standardised for W34SG engines
• Compact and cost efficient
• No consumables and does not produce waste effluents nor by-products
• Virtually maintenance free
• No space requirements
• No extra flanges or extra support structures

Oxidation catalyst oxidizes unburned components with the help of the residual oxygen in the flue gas and form carbon dioxide and water as end products.

Wärtsilä gas engines can be equipped with oxidation catalysts to reduce carbon monoxide and hydrocarbon emissions if e.g the national regulations or local pollution levels require.

Performance of oxidation catalyst depends considerably on the flue gas temperature, amount and type of active material, catalyst volume and the components to be removed. Significant methane and ethane reduction cannot be accomplished with an oxidation catalyst. Oxidation catalyst does not require any consumables and does not generate any waste effluent or by-products.

Oxidation catalyst consists of a reactor and catalyst elements. In some areas efficient multi-component emissions reduction is required. In this kind of combined catalyst systems oxidation catalyst elements are placed after the SCR elements in a common reactor. If only moderate carbon monoxide and formaldehyde reduction is needed integrated oxidation catalyst (IOXI) is a compact and cost efficient solution.

More than 2500 MW of Wärtsilä power plants are equipped with oxidation catalysts (including both diesel and gas engines).

Features

• No consumables and does not produce waste effluents nor by-products
• Virtually maintenance free
• Standardized units for low efficiency models (IOXI), but bigger oxidation catalysts are dimensioned case by case

Reference: Ringkøbing
One Wärtsilä 20V34SG generating set is situated in the Rindum plant in Denmark and has a net output of 8 MWe and 9.6 MW^th. This power plant delivers heat to the town’s 3,500 consumers and electricity to the local utility company RAH. The emission requirements set by the Danish environmental regulations are fulfilled by applying a compact oxidation catalyst for CO removal; otherwise the control of emissions is taking place by primary means. Read more on our  References pages.

The SCR unit reduces NO_x emissions to harmless nitrogen and water vapour, typically by 80-90%. Even up to 95 % reductions are achievable with more sophisticated control and bigger reactors.

Wärtsilä’s gas power plants can be equipped with SCR to reduce NO_x emissions e.g. if the national regulations or local air pollution levels require.

The SCR technique requires an additional reagent (reducing agent) of appropriate quality. Urea or ammonia is used for this purpose and is injected in aqueous form into the flue gas. Typically solutions of 25 wt-% ammonia or 40 wt-% urea are applied. Urea can be delivered to the site also as granulates and mixed with water on-site. SCR does not generate any solid or liquid end-product.

In some areas efficient multi-component emissions reduction is required. This kind of combined catalyst systems SCR elements are placed before the oxidation catalyst elements in a common reactor.

More than 2000 MW of Wärtsilä power plants are equipped with SCR (including both diesel and gas engines).

Features

• Applicable with Wärtsilä gas and dual-fuel engines
• NO_x reduction typically 80-90 %, but even 95% reductions are achievable
• No solid or liquid end-product generated