I. Boiler Structure Introduction
A pulverized coal boiler is a boiler that uses pulverized coal as fuel. It has the advantages of rapid and complete combustion, large capacity, high efficiency, wide adaptability to coal types, and easy control and adjustment.
The combustion characteristics of a pulverized coal boiler are that the fuel enters the combustion chamber with air and burns in a suspended state. The coal is pre-ground into very fine coal powder, greatly increasing the contact surface area with the air, thus enhancing combustion. The temperature inside the pulverized coal boiler is also higher, so except for coal with very poor quality, various coals can be effectively burned in the pulverized coal boiler, and the combustion is relatively complete, and the combustion efficiency is also relatively high, about 88%-93%. It can fully realize mechanization and automation. Pulverized coal combustion is almost the combustion method for all large coal-fired boilers.
The combustion equipment of a pulverized coal boiler mainly consists of a furnace, burner, and ignition device. The pulverized coal is transported by primary air through the burner into the furnace, and the secondary air is introduced into the furnace through the secondary air annular air duct or secondary air port of the burner.
After the mixture of pulverized coal and air enters the furnace, it undergoes preheating, drying, and volatile analysis, and ignites and burns at a certain distance from the burner outlet. The ash formed after the pulverized coal is completely burned, a small part of which is coarser ash forming slag, falls into the ash hopper, cools into solid slag, and is periodically or continuously removed; the vast majority of the finer ash is carried away by the flue gas, and this part of the ash is usually called fly ash.
The residence time of pulverized coal in the furnace is very short, only 1-2s. To ensure that the pulverized coal is completely burned in the furnace within such a short time, combustion must be intensified, and an appropriate amount of primary and secondary air must be supplied to shorten the combustion preparation stage and create good combustion conditions. The main function of the primary air in the pulverized coal boiler is to transport the pulverized coal to the furnace and ensure the ignition and combustion of volatiles. The function of the secondary air is to supplement the air volume and to mix the pulverized coal and air evenly to ensure complete fuel combustion.
1.1 System Principle  
The pulverized coal industrial boiler system mainly includes a fuel storage and supply system, a pulverized coal combustion system, a boiler system, a fully automatic monitoring and control system, a flue gas purification system, a compressed air system, an inert gas protection system, and an ignition oil (gas) system, etc.
The pulverized coal preparation equipment from this project sends the pulverized coal to the finished pulverized coal storage tank, and through the pulverized coal metering device and the air-powder mixing pipeline, it is sent to the pulverized coal burner and sprayed into the boiler combustion chamber for suspension combustion. The high-temperature flue gas generated by the combustion of pulverized coal in the furnace completes radiation and convection heat exchange and directly enters the tail bag dust collector and flue gas desulfurization device. After sufficient purification, it is discharged into the atmosphere by the induced draft fan through the chimney. The pulverized coal ash collected by the dust collector is discharged through a closed collection system and can be reused. The entire process is fully automatically controlled by a computer.
1.2 Boiler Structure and Overall Layout
This boiler is a single-drum horizontal type loose-packing chamber steam boiler. The furnace is composed of a front wall, left and right side walls, and a rear wall membrane wall. The convection flue is surrounded by a membrane wall structure, and convection tubes are arranged inside the flue. A superheater heating surface is arranged in front of the convection tubes. The heating surfaces of the boiler body are all rising tubes, forming a natural circulation loop with two φ325×18 concentrated descending tubes. The membrane wall structure ensures that the boiler is tightly sealed, ensuring stable, safe, and efficient combustion of the fuel. A three-stage economizer is arranged at the tail end heating surface, and the economizer is a steel tube economizer.
The pulverized coal burner is installed at the top of the front wall of the furnace. After the high-temperature flame radiates heat exchange from top to bottom with the furnace, it enters the lower part of the convection flue from the flue outlet at the bottom of the rear wall of the furnace. The high-temperature flue gas flows upward through the convection flue and performs convection heat exchange, flows out of the boiler from the flue outlet at the top of the convection flue, enters the tail flue, and fully exchanges heat with the tail heating surface-economizer before being discharged. According to the flue gas flow, the convection flue is from bottom to top: furnace outlet convection flue window, convection tube bundle.
Burner interfaces are reserved at the upper part of the front wall, and gravity explosion-proof doors are reserved on the side of the front wall. The total discharge area of the explosion-proof door is large, preventing damage to the boiler caused by explosion during abnormal operation of the boiler.
To facilitate observation of the pulverized coal combustion conditions, three peepholes are arranged on the side wall of the furnace, from top to bottom, along the flame centerline.
To detect the flue gas temperature at the furnace outlet and the negative pressure at the furnace outlet, flue gas temperature and furnace negative pressure measuring points are set at the furnace outlet and at the top of the furnace, respectively.
The furnace wall and protective plate adopt a lightweight furnace wall structure, all using aluminum silicate fiber board, and the aluminum silicate fiber board is fixed with wire mesh. When the ambient temperature is 25℃, the surface temperature of the furnace wall is 40℃.
Platforms and ladders are arranged at the locations where the boiler needs inspection, maintenance, and testing for convenient operation.
1.3 Flue Gas Flow
The pulverized coal burner is arranged at the top of the boiler. After the pulverized coal and air are mixed and burned in the pre-combustion chamber outside the furnace, they are sprayed into the furnace. When they reach the end, they are burned out and turn, and the generated high-temperature flue gas flows through the superheater, convection tube bundle, and economizer in sequence to complete the heat transfer process.
The boiler heating surfaces mainly include: furnace radiation heating surface, superheater heating surface, convection tube bundle heating surface, and economizer heating surface.