From Coal Yard to Steam Output: Inside the Coal-Fired Boiler Process

Introduction

A coal-fired boiler is key equipment that converts the chemical energy into thermal energy, and then generates steam through a water medium. It is widely used in thermal power generation, industrial production and central heating. From coal yard to steam output involves multiple stages, coal storage and treatment, fuel preparation, furnace combustion, heat transfer, steam generation and flue gas treatment and emission control. This article will introduce in detail working principles and the interrelationships of these links to help readers comprehensively understand the operation process of coal-fired boiler.

Coal Storage and Handling

Coal storage is an essential link of coal-fired boiler operation. Coal is usually transported from coal mine to coal yard. The design of a coal yard needs to take into account the type of coal, its reserves and the convenience of loading and unloading. Coal may be affected by natural factors during the storage process, such as weathering, spontaneous combustion. So need to take proper protective measures, such as coverage and watering to reduce loss and decline in quality of coal. At the same time, it is necessary to avoid spontaneous combustion of coal (by stacking in layers and turning it regularly to reduce the stacking temperature), avoid moisture (keep the site dry), and use equipment such as stacker-reclaimers and belt conveyors to reclaim and transfer coal.

Coal handling includes smashing, screening, conveying and other links. Raw coal may contain large lumps of coal or gangue and needs to be crushed to a particle size of 20-50 mm by crushers to facilitate subsequent coal grinding and combustion. Sieving equipment (such as vibrating screens) to separate coal of different particle sizes, remove oversized impurities, and ensure uniform fuel particle size. Finally, coal is transported to fuel storage area of boiler through equipment such as belt conveyors and bucket elevator, preparing for the further combustion. Common feeding equipment includes disc feeders and scraper feeders, etc. They automatically adjust coal supply volume according to boiler load.

Fuel Preparation

Fuel preparation is a key link that ensure boiler combustion efficiency. In the fuel storage area, coal is fed in a quantitative manner according to the operating requirements of the boiler. Common feeding equipment includes disc feeders, scraper feeders, etc. They can automatically adjust the coal feeding amount according to the load of the boiler. The control of coal feeding amount needs to be matched with the combustion conditions of the boiler to ensure the adequacy and economy of combustion.

Combustion in the Furnace

Furnace is the core area of coal combustion. A burner mixes pulverized coal and air and transfers them to furnace, forming flame.During combustion, carbon and hydrogen in coal undergo chemical reactions, releasing a large amount of thermal energy. To ensure sufficient combustion, it is necessary to control configuration of burner, wind volume and wind speed and other parameters. Combustion in the furnace is affected by many factors, such furnace temperature, oxygen concentration and coal powder concentration. If the furnace temperature is too high, it may lead to coking, while if it is too low, it will affect combustion efficiency. Insufficient oxygen concentration will lead to incomplete combustion, producing harmful gases such as carbon monoxide. So, it is necessary to monitor and adjustment these parameters in real time through combustion control system to achieving optimal combustion effect.

Heat Transfer in the Boiler

It includes three ways of heat transfer: radiation heat transfer, convective heat transfer and heat conduction. In the furnace, heat from flame and high-temperature flue gas are mainly transferred to water-cooled wall through radiation heat transfer. After the water-cooled wall absorbs heat, the heat is transferred to the water inside the tube by conduction, causing the water temperature to rise and vaporize. At the same time, when the high-temperature flue gas passes through the convective heating surfaces of the boiler, it mainly transfers heat to the working medium inside the tube through convective heat transfer.

Steam Generation and Conditioning

Steam generation is the ultimate target of boiler operation. After the water-cooled wall absorbs heat, water is gradually vaporized to form a water-steam mixture. Water-steam mixture is separated in steam drum, the saturated steam enter superheater for further heat, ultimately forming overheated steam output. Steam drum is an important component, it plays a role of separating steam from water, storage water volume and buffering pressure fluctuations.

Flue Gas Treatment and Emissions Control

The flue gas during combustion process contain a large amount of pollutants, such as sulfur dioxide, nitrogen oxides, particulate matter, etc. If these pollutants are directly discharged into the atmosphere, they can cause serious environmental pollution. So, flue gas treatment and emissions control are an indispensable part.

Conclusion    

From coal yard to steam output is a complete and precise systematic process, including coal storage and treatment, fuel preparation, combustion in the furnace, heat transfer, steam generation and adjustment, flue gas treatment and emission control and other links. Any problem in any of these links affects the normal operation and efficiency of the boiler.

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