How gas engine exhaust waste heat boilers work
Gas engine waste heat boilers recover heat from spark-ignition engines running on natural gas, biogas, landfill gas or other process gases in cogeneration and trigeneration plants. Exhaust leaves the turbocharger at about 400 to 550°C, hotter than most diesel engines, and passes through a boiler or exhaust gas heat exchanger, often followed by a secondary hot water heat exchanger that cools it to about 100 to 120°C. Natural gas exhaust is clean. Biogas and landfill gas bring SO₂ from hydrogen sulphide and silica from siloxanes, which deposit on hot surfaces and form acid where the gas cools. The engine drives the exhaust through the boiler within its back pressure limit, and most installations have no ID fan.
Where the ID fan sits
- Engine turbocharger outlet
- Catalyst (if fitted)
- Waste heat boiler
- Hot water heat exchanger
- ID (booster) fan
- Stack
Where fitted, the fan sits at the cold end, after the boiler and any second-stage hot water heat exchanger, so it handles gas at about 100 to 180°C. It offloads the added resistance so that pressure at the turbocharger outlet stays inside the engine maker's limit. It is controlled from pressure at the engine outlet. On biogas, the fan sits where the gas may be close to its acid dew point, which drives its materials.
Gas at the ID fan
| Gas temperature, normal | 100 to 180°C after the boiler and heat exchanger |
| Maximum or upset | Up to 450 to 550°C if the boiler and exchanger are bypassed through the fan route |
| Dust at the fan | Negligible on natural gas; fine silica from siloxanes on biogas and landfill gas |
| Moisture and dew point | About 10 to 12% water vapour, water dew point near 50°C; acid dew point higher with biogas |
| Corrosive species | SO₂ and some SO₃ from hydrogen sulphide in biogas |
| Density at the design temperature | About 0.84 kg/m³ at 150°C, 70% of standard air. Pressure developed and power drawn scale with density, so the fan is selected at this condition and the motor is checked at the coldest operating or start-up condition. |
ID Fan selection for gas engine exhaust waste heat boilers
| Blade form | Backward curved, low pressure rise, for clean or lightly loaded gas |
| Material of construction | Mild steel for natural gas; stainless steel such as SS 316 for biogas and landfill gas exhaust near dew point |
| Drive and arrangement | Direct coupled, arrangement 8 |
| Bearings and cooling | Pedestal bearings with a shaft cooling disc |
| Flow control | Variable speed drive on engine outlet pressure |
| Also specified | Casing drain, insulation, and interlock with the bypass damper on fan trip |
What this ID fan has to cope with
Acid condensate on biogas
Biogas exhaust carries SO₂, and the second-stage exchanger may cool it near or below its acid dew point. A fan at that location collects acidic condensate in the casing. Stainless steel wetted parts, a drain at the low point and insulation prevent it eating through a mild steel casing.
Siloxane silica deposits
Siloxanes in landfill gas and some biogas burn to fine silica that sticks to hot surfaces and to fan blades as a hard white layer. Gas pre-treatment is the real cure; a fan blade profile that sheds deposits and access for cleaning keep the fan balanced meanwhile.
Keeping inside the back pressure allowance
Gas engines are sensitive to exhaust back pressure, which affects output and exhaust temperature. A booster fan must take away only the excess resistance. Too much suction draws air through duct joints and upsets the oxygen measurement that some engines use for control. Control from outlet pressure prevents this.
Energy and control
Gas engines in cogeneration often run long hours near full load, so exhaust flow is steady. A booster fan, where fitted, can run on speed control from engine outlet pressure and needs little power. In most cases the better investment is a low-resistance boiler and exchanger that make the fan unnecessary.
Questions buyers ask
Is an ID fan required on a gas engine waste heat boiler?
Normally not. The engine's exhaust pressure pushes the gas through the boiler. An ID or booster fan is only needed when the total exhaust system resistance, including any catalyst and second-stage heat exchanger, exceeds the back pressure the engine maker allows. Designing the train for low resistance is usually cheaper than adding a fan.
Why does the exhaust fan corrode on a biogas engine?
Biogas contains hydrogen sulphide, which burns to SO₂. When the exhaust is cooled to recover more heat, acid can condense on cool surfaces in the fan casing. Stainless steel wetted parts and good insulation prevent it. Running the fan on after the engine stops, until the casing is dry, also helps. Mild steel casings at this duty can perforate within a few years.
What is the white deposit on the fan blades of a landfill gas engine?
Usually silica from siloxanes in the gas, formed in combustion and deposited on hot surfaces downstream. It is hard and builds up unevenly. Removing siloxanes in gas treatment solves the cause; cleaning and balancing deal with the effect on the fan. The deposit should be removed before rebalancing.
Information needed to quote an ID fan for gas engine exhaust waste heat boilers
- Duty point. Gas volume in m³/h or Am³/h at the fan inlet and the static pressure the fan must develop, with the margin you want on each.
- Gas condition. Normal and maximum gas temperature at the fan, dust loading after the collector, moisture and any acid gases such as SO₂ or HCl.
- System. Boiler or furnace capacity and fuel, collector type and its pressure drop, and the existing fan curve or nameplate if this is a replacement.
- Mechanical. Drive arrangement, flow control (damper, inlet vanes or variable speed drive), rotation and discharge orientation, and a layout drawing or site photograph.
- Engine and fuel. Engine model, fuel type, and hydrogen sulphide and siloxane content for biogas or landfill gas.
- Heat recovery train. Boiler, catalyst and secondary exchanger pressure drops, and the engine back pressure limit.
Built and tested in Chennai
Adroit Fantech Engineers has designed and manufactured industrial fans at Ayanambakkam, Chennai since 2004. Each impeller, casing and shaft is built for the gas and temperature it will see, performance tested as per IS 4894 or BS 848 Part I, and balanced to grade G 6.3 or G 2.5 as per ISO 21940-11, with the records supplied with the fan. We also replace rotors and retrofit existing fans of any make.