Plant engineers are always interested in how solid fuels can provide the exact temperature profile required in the heavy process manufacturing. Steam boilers usually run at high pressure and are prone to water treatment scaling. A pulverized coal thermal fluid heater circumvents this issue by combusting reduced-size coal particles in suspension, and transferring the heat to the circulating oil at low pressures.
Knowing the step-by-step process can enable plant managers to effectively utilize the fuel, avoid tube fouling, and safely operate the plant.
Fuel Preparation: From Raw Coal to Dry Powder
While the combustion sequence commences well before the fuel enters the combustion chamber. The raw coal is fed via an enclosed belt conveyor into the raw coal storage bunker. The coal feeder feeds the fuel into the mill at a rate determined by the variable speed drive. Heavy rollers crush the lumps of coal into fine dust.
Crushing is not sufficient on its own. Wet coal will gum up the transport lines and burn erratically. Hot air from an air preheater is introduced into the mill simultaneously with the coal, to evaporate moisture from the surface of the coal, while also being used as the conveying medium.
The air/fuel mixture is directed into a dynamic classifier or coarse powder separator. Particles that do not meet the mesh requirement are returned to the grinding zone for further size reduction. The air/fuel stream that meets the specification is separated from the recycled stream, and constitutes the primary air mixture, which transports the fuel to the primary burner nozzle.
Combustion Dynamics: Primary Air and Secondary Air
A stable combustion in a thermal oil heater requires controlled air staging. The primary air stream transports the pulverized coal directly into the furnace throat, where it is ignited in the radiant region. Because this stream normally contains only a fraction of the air required to combust the coal, it prevents local flame quenching and suppresses initial NOx formation.
The secondary air supplies the remainder of the oxygen required for complete combustion. A separate stream of preheated air is fed from the air preheater, into secondary nozzles around the burner throat, to impart a high velocity swirl into the furnace chamber.
This secondary air mixes thoroughly with the gases and carbon particles emitted from the burner, as a result of which the volatile gases are fully oxidized. By staged combustion, we mean the prevention of unburned carbon loss and consequent carbon monoxide emissions. Suspended combustion allows for rapid response to load changes, and can be made to respond to load changes in the order of minutes.
Heat Absorption: How Flue Gas Warms the Thermal Fluid
Once the fuel is ignited, radiant heat fills the thermal oil furnace. While traditional power boilers utilize water-cooled tubes to generate saturated steam. A thermal fluid heater utilizes mineral or synthetic heat transfer fluid, which is pumped through tightly coiled continuous tube coils.
The hot gas stream flows past several different heat transfer surfaces:
- Radiant Coils: These are located in the furnace walls, and consist of a continuous length of tubing, which is subjected directly to the radiant heat of the flame, raising the fluid temperature up to 350C without degradation.
- Convection Bank: The gas exiting the radiant chamber flows past the high density of tube bundles, extracting heat from the flue gas by high velocity convection.
- Economizer: The gas then flows over a dedicated fluid preheater, recovering latent heat to preheat the return oil before it is sent back to the radiant section.
- Air Preheater: The final recovery stage, which transfers the remaining heat in the exhaust gas to the incoming combustion air, increasing the flame temperatures and the fuel economy.
The thermal oil remains in a liquid state throughout the entire closed loop. Since there is no change in phase, the system operates at less than 1 MPa hydraulic pressure, eliminating the explosion hazards of high pressure steam headers.
Flue Gas Cleanup and Ash Handling
The coal combustion produces ash, which must be captured to prevent fouling of the heat-exchange surfaces, and to comply with local air quality regulations. Heavy ash particles fall to the bottom of the furnace hopper as slag. Mechanical slag crushers grind up the large slurry deposits, before they are pumped out of the boiler house.
Lighter fly ash remains in suspension in the flue gas until it is extracted from the heat recovery sections. This gas is cleansed of particulate matter, in a multi-stage dust collection system, which is usually a combination of a cyclone separator and pulse-jet fabric baghouse, capturing over 99% of the particulate matter.
An induced draft fan downstream of the dust collector maintains a negative pressure in the furnace casing. This prevents the combustion gases from escaping into the plant. The cleaned gas is then exhausted to the atmosphere, in accordance with local environmental regulations.
Industrial Engineering and System Reliability
Operating a pulverized coal thermal fluid heater requires stringent manufacturing standards to avoid oil overheating and localized tube hotspots. Chinese manufacturer CEEF (Changzhou Energy Equipment Co., Ltd.), established in 1978, authored the national technical standard GB/T 17410 for organic heat transfer material heaters. CEEF employs 100% Non-Destructive Testing (NDT) on all pressure-bearing tube coils to verify weld integrity.
In large-scale operations, systems such as CEEF’s YLL series pulverized coal heaters can provide outputs of up to 35,000 kW (120MM BTU/hr). The YLL-39000MA model earned the China’s Ministry of Industry and Information Technology “Energy Efficiency Star” by surpassing the national first class efficiency benchmarks. Automated soot blowers keep tube surfaces clear of slag buildup during continuous operation.
When looking to replace older grate boilers or considering heavy process heating options, it is important to match fuel characteristics to the heat absorption profiles of the furnace. Plant operators can consult with CEEF’s engineering team to review fuel analysis data, system footprints, and custom coil arrangements.