The key to improving system thermal efficiency after retrofitting a coal-to-oil boiler lies in fully capturing and utilizing waste heat in the flue gas through waste heat recovery devices. These devices typically utilize the principle of cascaded energy utilization, simultaneously recovering the sensible heat in the high-temperature flue gas and the latent heat released by the condensation of steam, converting this heat into heat that can be reused in the boiler system, thereby reducing fuel consumption and exhaust heat loss.
Economizers are common waste heat recovery devices in coal-to-oil boilers. They utilize metal tube bundles arranged within the boiler's tail flue to indirectly exchange heat between the high-temperature flue gas and the boiler feed water. During retrofits, the tube bundle material and layout must be adjusted based on the flue gas characteristics of the oil-fired boiler. For example, corrosion-resistant alloy steel may be selected to mitigate the potential presence of sulfides in the oil-fired flue gas. The elevated feed water temperature after preheating in the economizer significantly reduces the heat required to heat the feed water within the boiler, thereby reducing fuel consumption.
Air preheaters, on the other hand, recover waste heat from the flue gas to heat the combustion air, improving fuel combustion efficiency within the furnace. During the retrofit, the design of the heat transfer components must be optimized, for example, by adopting a rotary structure to enhance heat exchange efficiency. The matching between the air preheater and the burner must also be adjusted to ensure uniform hot air delivery to the furnace. The preheated air promotes fuel atomization and combustion, reducing mechanical incomplete combustion losses and enabling more complete fuel calorific value release.
Condensing heat recovery units are specialized equipment designed to address the high water vapor content in the exhaust flue gas of oil-fired boilers. By lowering the exhaust gas temperature below the dew point, the water vapor condenses and releases its latent heat of vaporization. The recovered heat can be used to heat boiler feed water or domestic hot water. During the retrofit, addressing the corrosive nature of condensate is crucial, such as adopting stainless steel or applying high-temperature corrosion-resistant coatings. A sound drainage system should also be designed to prevent condensate accumulation from impacting equipment operation.
Heat pipe heat exchangers are widely used in coal-to-oil boilers due to their efficient heat transfer and compact design. They achieve rapid heat transfer through phase change within the heat pipes, and the fact that individual heat pipes can be replaced independently reduces maintenance costs. During the retrofit, heat pipes can be flexibly arranged to accommodate boiler flue space, for example, using vertical or horizontal installation to accommodate different structures. Heat pipe spacing can also be optimized to balance heat exchange efficiency and flue gas resistance.
Waste heat boilers are suitable for large-scale coal-to-oil conversion systems. They directly generate steam or hot water through heat exchange between flue gas and a water circulation system for production or heating. During the retrofit, the connection method between the waste heat boiler and the existing boiler must be carefully planned, such as using a parallel or series arrangement. An intelligent control system should be configured to coordinate operating parameters between the two to ensure maximum waste heat utilization without compromising the stability of the existing system.
At the system integration level, a multi-stage waste heat recovery solution should be selected based on the boiler type and operating conditions. For example, large power plant boilers can utilize a combination of air preheaters, economizers, and heat pipe heat exchangers for multi-stage recovery, while small and medium-sized industrial boilers may only require an economizer and a condensing unit. The retrofit also requires optimizing the coordinated control of the waste heat recovery system with the boiler itself and auxiliary equipment. For example, fan frequency can be adjusted to match flue gas flow changes to avoid insufficient ventilation due to increased system resistance.
Regular maintenance is key to ensuring the long-term, efficient operation of waste heat recovery units. A cleaning mechanism is required to remove dust deposits from the economizer tube bundles and air preheater heat transfer elements to prevent reduced energy savings due to decreased heat transfer efficiency. Furthermore, the pH value of condensing unit drainage must be monitored, and acidic condensate must be promptly treated to prevent equipment corrosion. Through scientific design, meticulous modification, and standardized maintenance, waste heat recovery units can significantly improve the system thermal efficiency of coal-to-oil boilers, achieving a win-win situation in both energy utilization and economic benefits.
