This guide explains what an HFO power plant emits, how limits are usually set, and the control options available. It does not quote limits, because they vary by country, project and lender, and a project must always work to the permit and the lender requirements that apply to it.
The main pollutants
Nitrogen oxides, written NOx, form at high combustion temperatures when nitrogen and oxygen in the air combine. Sulphur oxides, SOx, come from sulphur in the fuel. Particulate matter includes ash, soot and unburnt carbon. Carbon monoxide and unburnt hydrocarbons are products of incomplete combustion, and carbon dioxide follows from the carbon in the fuel.
Each pollutant has a different control approach, and a project usually needs to deal with the first three. Fuel sulphur content and engine tuning are the first levers, followed by add-on equipment.
How limits are set
Limits come from national and local law, the project permit, and lenders. International lenders often apply guidelines such as those of the World Bank Group and the IFC, which set emission and ambient air limits for thermal plants. Where national rules are less strict, lender requirements may govern.
Limits can be expressed as concentrations in the stack gas, as mass per hour or as ambient air quality at the boundary. Understand which applies before choosing equipment.
Fuel choice and treatment
The simplest SOx control is to burn fuel with lower sulphur. HFO grades differ, and some suppliers offer lower sulphur grades at a different cost. Fuel treatment, including settling, separation and filtration, reduces water, sediment and catalytic fines that damage engines and add particulates.
Our article heavy fuel oil compared with diesel sets out the fuel trade-offs in more detail.
Engine-side measures for NOx
Engine tuning, such as adjusting injection timing or using a high-pressure turbocharged design with modern combustion chambers, reduces NOx formation at some cost in fuel use. Some designs use water in the combustion process, and others use charge air conditioning to lower peak temperatures.
These measures have limits. For strict NOx limits, an exhaust treatment system is usually required.
Selective catalytic reduction
Selective catalytic reduction, SCR, injects a reagent such as urea into the hot exhaust, where a catalyst converts NOx into nitrogen and water. It is widely used on engine plants where NOx limits are strict. The reagent must be stored, dosed and controlled with care.
With HFO, sulphur in the exhaust can foul catalysts and form salts, so designers need to choose catalyst materials, operating temperatures and cleaning routines for the fuel. Ask for references from plants burning similar fuel.
Scrubbers for SOx
A flue gas desulphurisation unit, or scrubber, removes SOx from the exhaust. Wet scrubbers use water or a chemical solution, and dry systems use a sorbent. Either way, the process generates a by-product that must be handled and disposed of.
A scrubber adds equipment, water or reagent demand, power use and operating complexity. Many projects compare it with the cost of lower sulphur fuel before deciding.
Particulate control
Good combustion and fuel treatment reduce particulates at the source. Where limits are tight, filters or electrostatic precipitators can capture ash and soot in the exhaust. Access for cleaning and safe disposal of the collected material are part of the design.
Particulate control can interact with the heat recovery equipment, since both sit in the exhaust path.
Monitoring and reporting
Permits typically require continuous monitoring of some pollutants or periodic stack tests, with records kept and reported to the authority. A monitoring system needs calibration, maintenance and trained operators, and it should be designed into the plant, not added afterwards.
Record fuel analysis for each delivery, since fuel quality ties directly to emissions results.
Questions lenders and regulators ask first
Lenders usually ask which emission and ambient air standards the project will meet, what fuel specification is assumed, what the stack height and dispersion modelling show, how emissions will be monitored and reported and what the plan is if limits change in future. A project that can answer these in a short document finds the review easier.
Regulators tend to focus on the permit application, the monitoring plan and community effects. Preparing both sets of answers together avoids contradictions.
Emissions and the long-term plan for the site
Emission rules tend to tighten over the life of a plant. A station built today for HFO may be asked to meet stricter limits in ten years, or may face fuel supply changes. Designing space for future treatment equipment, or for conversion to cleaner fuels, is a low-cost way to keep options open.
Many owners also plan for a transition to gas where supply can be arranged. Our article on moving from HFO to modular gas power describes how a site can change over time.
Planning an HFO project around emissions
Raise emissions early. The decision on fuel quality, engine type and treatment equipment affects layout, cost, water use and operating staff. Early engagement with regulators and communities avoids late-stage redesign.
Explore alternatives as well: a plant designed to run on HFO today may be built to convert to gas later. See how an HFO power plant works to understand the system as a whole.
Who writes this
Permit and lender questions on exhaust emissions are routine for the engineers who design heavy fuel oil stations. Field notes from the USP&E engineering team. USP&E is the contracting party on its EPC and O&M work, not an intermediary, and it has 24+ years in the field, 150+ power projects in 45+ countries and 25,000 MW contracted since 2002. Our power plant EPC projects include environmental design and permitting support.
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Questions about HFO plant emissions
Which emissions matter most for an HFO plant?
Nitrogen oxides, sulphur oxides and particulates, because HFO contains sulphur and ash and burns at high temperatures.
Is selective catalytic reduction needed on every HFO plant?
No. It depends on the NOx limit that applies to the project. Strict limits usually call for it.
Can lower sulphur fuel replace a scrubber?
In some cases, yes. Projects often compare fuel cost against scrubber cost and complexity.
Who sets the limits?
National and local law, the project permit and lenders. The strictest requirement normally governs.
Checking emissions for a planned station?
Send us the site, engine list and permit requirements, and our engineers will outline the controls that fit the project.
An engineer will reply if you email info@uspeglobal.com, call +27 10 822 2324 or fill in the project enquiry form. Engines and sets available now are listed in our diesel and HFO generator inventory.