Using some of it can reduce fuel use, cut the load on cooling plant or provide process heat to neighbours. This guide explains where the heat comes from, which uses are practical at an HFO station, and the points that decide whether recovery pays off.
Where the heat goes
A heat balance for an engine usually shows four main streams. Exhaust gas carries a large share at high temperature. The jacket water or high-temperature cooling circuit carries another share at a moderate temperature. The charge air cooler and lube oil cooler remove a further amount at lower temperatures, and the rest is radiated from engine surfaces and lost in the exhaust leaving the stack.
Each stream has a different temperature, and temperature decides what the heat can be used for. High-temperature exhaust can make steam. Warm jacket water can heat buildings or fuel. Low-temperature heat is hard to use and is usually rejected.
Use the heat inside the station first
HFO must be heated to be pumped, separated and injected. Fuel storage tanks, transfer lines, separators and day tanks all need heat, and jacket water or exhaust heat can supply much of it. Many stations already do this with a small exhaust boiler or a thermal oil circuit, and it is the simplest recovery with the shortest payback.
Other internal uses include heating the lube oil for separation, heating workshops and offices and preheating engines in standby. These loads are small, but they run all year and need no outside customer.
Exhaust gas boilers
An exhaust gas boiler sits in the exhaust path of an engine or a group of engines and produces hot water or low pressure steam. The key design issue with HFO is fouling: soot and ash deposit on heat transfer surfaces, and sulphur in the fuel can cause acid corrosion if the metal temperature drops below the acid dew point.
Designers therefore keep exhaust outlet temperatures above the dew point, provide soot blowing or cleaning access and choose materials that suit the fuel. Ask the supplier for experience with the same fuel type, and plan for regular cleaning in the maintenance programme.
Steam for process or power
If a customer nearby needs steam, such as a processing plant, a mine plant or a food factory, an HFO station can supply it from exhaust boilers. Steam can also drive a small steam turbine to add electricity without extra fuel, a combined arrangement that raises the overall electrical output of the station.
The economics depend on engine size, the number of engines in service and operating hours. A steam cycle needs additional equipment, water treatment, operators and maintenance, so it suits larger stations that run steadily. Always compare it against simpler uses of heat.
Hot water and building heating
Jacket water heat is useful for district heating, hot water for mine accommodation and similar loads where the demand is steady and close by. The main limit is distance: hot water pipelines are costly, and heat loss grows with length.
In warm climates, demand for heating is limited, and cooling may be a better target for the heat.
Absorption cooling
Absorption chillers turn heat into chilled water. They can use hot water or exhaust gas as the driving heat and are used in some stations to cool buildings, process areas or inlet air in climates where cooling is needed all year.
They are more complex than electric chillers, and they need cooling towers and skilled maintenance. They make sense where the cooling load is large and steady and where electricity or fuel is expensive.
Seasonal and load variations
Heat recovery follows the engine load. When the station runs at partial load, exhaust temperature and heat output fall, and when engines are off, the heat is gone. A recovery system must be designed for the range of conditions, with bypass dampers or dump radiators to protect the engines when the heat user is not taking heat.
Check the real load profile of the station before sizing. A design based on full-load heat output will disappoint if the station seldom runs at full load.
Economics and interfaces
The value of recovered heat is the cost of the energy it replaces, less the cost of the equipment, extra operators and extra maintenance. A project with a steady internal user and a short pipe run tends to pay back, while one that depends on a distant customer with uncertain demand carries more risk.
Agree who owns the heat equipment, who operates it and who is responsible if it constrains engine operation. The best arrangements keep engine availability as the top priority and treat heat recovery as a secondary benefit.
Operations and maintenance of recovery equipment
Boilers and heat exchangers need cleaning, inspection of tubes, water treatment and safety testing of valves and controls. Plan outages for them in step with engine overhauls to avoid extra shutdowns.
Our pages on HFO station auxiliary systems and balance of plant explain how these systems fit together.
Who writes this
Heat balances for engine stations are part of the design package our station engineers prepare. Authors: USP&E engineering team. Company background: not a broker but an EPC and O&M contractor, USP&E reports 24+ years in the field, 25,000 MW contracted since 2002 and 150+ power projects in 45+ countries. Read how we deliver power plant EPC projects including auxiliary and heat systems.
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Questions about HFO station heat recovery
How much of the fuel energy can an HFO station recover as heat?
A large share leaves as heat, but how much can be used depends on temperature, the user and the load profile. Heating fuel inside the station is the most dependable use.
Why is fouling a concern with HFO exhaust boilers?
Soot and ash deposit on surfaces and sulphur can cause acid corrosion if metal temperatures fall too low, so boilers need suitable design and regular cleaning.
Can heat recovery increase electricity output?
Yes, if recovered heat makes steam that drives a steam turbine. That needs extra equipment and suits larger, steadily running stations.
Does heat recovery affect engine reliability?
It should not. Systems need bypasses so that engines keep running safely when the heat user does not take heat.
Looking at heat recovery for a station?
Send us the engine list, running hours and nearby heat users, and our engineers will outline recovery options and what they need from the site.
For a first conversation, use the project enquiry form, ring +27 10 822 2324 or send the details to info@uspeglobal.com. Engines and generators available now are listed in our diesel and HFO generator inventory.