Emission reduction - liquid fuel engines
Nitrogen oxides (NOx), sulphur dioxide (SO2) and particulate matter (PM) emissions are the air emissions of main interest with respect to stationary diesel engines. SO2 and PM emissions are mainly related to the quality of the fuel oil. Diesel engines used in the power industry have low carbon monoxide (CO) and hydrocarbon (HC) emissions.
Wärtsilä’s product development strategy for fuel oil-fired power plants is to meet the stack emission limits set by the World Bank/IFC guidelines “General/Thermal Power Plants EHS Guidelines” for new stationary reciprocating engine installations up to 300 MWth in non-degraded airsheds by using dry primary methods. In practice this means the use of a suitable fuel oil quality and Low-NOx combustion in the engine.
It is generally preferable to apply primary methods for reducing air emissions at the source of formation rather than attempting to remove them from the flue gas.
Secondary flue gas treatment methods are available for installations located in polluted airsheds (degraded airsheds), those subject to stricter national/local emission limits or when only low grade liquid fuels are commercially available.
NOx emissions formation in the combustion process can be minimized using primary or secondary abatement methods.
Dry primary methods
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.
The target of dry primary methods in Wärtsilä engine development process is to achieve the best overall emission performance with a good efficiency. The drawback in reducing NOx emission formation in the combustion process is often a loss in efficiency and a small increase of other emissions, such as CO. This tendency is compensated as far as practicable in the Low-NOx combustion process.
The main elements of the Low-NOx combustion process of Wärtsilä diesel engines are the following:
- Late fuel injection start
- High compression ratio
- Optimized combustion chamber
- Optimized fuel injection rate profile
- Early inlet valve closing (Miller concept)
- High boost pressure
These are the key elements for suppressing the combustion peak temperatures, resulting in reduced NOx formation.
Benefits
- Reduced NOx emissions
Selective catalytic reduction (SCR)
The SCR process reduces NOx 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 diesel engine power plants can be equipped with SCR e.g. if the national regulations or local air pollution levels require. The fuel oil quality is an important design parameter of SCR systems. Wärtsilä applies a robust durable design and efficient soot blowing for applications fired with high ash/sulphur fuel oils.
Catalyst elements with a honeycomb structure are used to ensure a compact structure when running on light or heavy fuel oil with sulphur content below 2%. With lower grade fuel oils, a less dense element structure and efficient soot blowing are required to minimize fouling of the elements. In addition catalyst systems with high sulphur containing fuels are designed to avoid blue haze generation as far as possible.
SCR is an effective method; NOx emissions are typically reduced by 80-90 % by means for SCR techniques. At higher NOx reduction rates the control system is critical due to its operation within a narrow window. At high reduction rates the size of the SCR reactor increases and more complicated premixing and reagent injection systems are needed.
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 is then mixed with water on-site.
More than 2000 MW of Wärtsilä power plants are equipped with SCR (including both diesel and gas engines).
Features:
• NOx reduction typically 80-90 %, but even 95% reductions are achievable
• No solid or liquid end-product produced
Oxidation catalyst
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 liquid fuel engines running with good quality of LFO or LBF can be equipped with oxidation catalysts to reduce carbon monoxide and hydrocarbon emissions if e.g. the national regulations or local air 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. Oxidation catalyst does not require any consumables and does not generate any waste effluent or by-products.
Typically diesel engines equipped with oxidation catalyst are also equipped with SCR to reduce NOx emissions. In this kind of combined catalyst systems oxidation catalyst elements are placed after the SCR elements in a common reactor.
More than 2500 MW of Wärtsilä power plants are equipped with oxidation catalysts (including both gas and diesel engines).
Features
- No consumables and does not produce waste effluents nor by-products
- Virtually maintenance free
- Temperature, active material and volume affects on performance
- Oxidation catalyst for diesel engines are designed case by case
Flue gas desulphurization (FGD)
Flue Gas Desulphurization (FGD) is a method used when sulphur dioxide (SO2) limits cannot be met owing to the sulphur content of the fuels. Two types of FGD systems are used in Wärtsilä power plants: sodium hydroxide (NaOH) and limestone (CaCO3). Both these wet FGD systems are typically capable for removing up to 90 % of the SO2 emissions.
Where flue gas desulphurization is concerned, the investment cost and/or the operating costs generally have a significant financial impact on the plant, and for this reason it is always important first to compare fuel qualities and costs. Plants with FGD systems also face the problems of handling and disposing of the end product, and the dispersion and visibility of the flue gas.
Several FGD types are available for the power plant market. The types can be classified based on the type of reagent and use of water. The most feasible methods in stationary engine plants have shown to be wet NaOH FGD in smaller plants and wet CaCO3 FGD in bigger plants, using fuels with high sulphur content.
Flue gas desulphurization with sodium hydroxide (NaOH)
The FGD process for removing sulphur dioxide can be based on use of sodium hydroxide (NaOH) as alkali.
The NaOH based FGD process is an efficient desulphurization method for diesel engine flue gases. This process is often used when the sulphur content of the fuel oil is relatively low due to its costly reagent. The typical SO2 removal efficiency is 90 %.
In addition to the need of alkali (aqueous NaOH), the wet FGD systems requires large amounts of process water.
A liquid effluent having a high salt content is produced by the NaOH scrubber as end-product. The effluent should be properly treated before discharging and possible environmental impacts of the discharge needs to be studied in advance.
Features
- Applicable for SO2 removal with Wärtsilä diesel engines
- Simple process with relatively low investment cost
- Typically only feasible for small plants fired with low sulphur fuel oils due to high reagent cost
- Efficient SO2 removal; up to 90 %
- Typically 0,6-1,0 m3/MWhel water required, depending on water quality and the degree of heat recovery
- Availability of water and its quality is crucial
- Liquid effluent containing high salt content as end-product
- The treated flue gas is cold and saturated with water
Flue gas desulphurization with limestone (CaCO3)
The traditional wet FGD process based on limestone (CaCO3).
The CaCO3 FGD system is an advanced desulphurization process, of which feasibility appears when cheap high sulphur fuel oils are burned and especially when the power plant is relatively big. The typical SO2 removal efficiency is between 80-90 %.
In addition to the need of alkali (limestone), the wet FGD system requires big amounts of process water of appropriate quality.
Gypsum is produced by the limestone FGD system as end-product. Gypsum disposal and utilization possibilities should be studied carefully in an early project phase.
Features
- Applicable for SO2 removal with Wärtsilä diesel engines
- Advanced process with relatively high investment cost, using low cost reagent (limestone)
- Typically feasible for big plants fired with high sulphur fuel oils
- Efficient SO2 removal; up to 90 % depending on the design
- Typically 0,6-1,0 m3/MWhel water required depending on water quality and the degree of heat recovery
- Availability of water and its quality is crucial
- Appropriate limestone with high enough reactivity and fine size distribution is required
- The process can be designed to be waste water free in favourable operation conditions
- Gypsum as end-product
- The treated flue gas is cold and saturated with water
Electrostatic precipitator (ESP)
A dry Electrostatic precipitator (ESP) unit is used to reduce particulate matter (PM) flue gas emissions when burning high ash fuels. The dry ESP technique provides a stable, low pressure-loss option to reduce PM emissions.
Typically PM levels of 50 mg/m3 (dry at 15 % O2, NTP) (PM measurement method ISO 9096 or other mainly similar method) can be achieved when using as ESP system in an HFO -fuelled power plant. The performance rate depends on the flue gas temperature and fuel quality.
The ESP consumes electricity but no reagents are needed for the process. Small amounts of fly ash are produced as end-product. The fly ash should be utilized or disposed of in an environmentally acceptable way. The composition of the end-product depends on the fuel and lubrication oil used. The disposal and utilization options available for the end-product should be examined in the environmental assessment of the project by considering the existing infrastructure and legislation/regulations.
- Applicable for dry PM emission reduction with Wärtsilä diesel engines
- PM emission target of 50 mg/m3(dry at 15 % O2, NTP)
- Rigid, massive structure
- Fly ash as end-product