Guide: With the acceleration of social and economic development trends and urbanization, the trend of air compound environmental pollution in the central and northern regions of my country is becoming less optimistic.
With the acceleration of socio-economic development and urbanization, the trend of air-complex environmental pollution in the central and northern regions of my country is becoming less optimistic, mainly manifested in the integration of Beijing-Tianjin-Hebei, the Yangtze River Delta, the Pearl River Delta and other large cities with heavy smog Weather conditions are high. According to statistics and analysis of relevant departments, air pollutants discharged from industrial raw coal account for 30% to 40% of the sources and incentives of haze weather. In order to improve the environmental pollution, improve the high efficiency of the ignition of raw coal boilers and reduce the emission of air pollutants, it is urgent.
1. Core technology of pulverized coal boiler
The pulverized coal boiler is composed of pulverized coal storage tower (tank), feeder, burner, heating furnace itself, soot and air pollutant synergistic resin adsorption equipment, ash storage, igniting oil (gas) station, air compression station, rare gas The maintenance station and measurement and control technology consist of 10 sub-systems. The design plan and the assembly of the system software are key to six core technologies, namely coal preparation and transportation, stable storage of pulverized coal, dense phase pulverized coal transportation, pulverized coal burner, heating furnace itself, and low-emission smoke and dust cleaning.
Pulverized coal
There are two key to the production process of pulverized coal: ①Two-step processing technology separated by air drying and grinding; ②One-step processing technology developed separately by grinding and air drying. The former processing technology has low requirements on machinery and equipment, and the system software is easy to manipulate, but its role is decentralized and the processing technology chain is long, so it is only suitable for small-scale taxpayer milling. The latter system software has simple processing technology and is suitable for large-scale milling, but has high requirements for the safety factor of mill equipment and processing technology. Two-step industrial production ball mills are dominated by vertical mills, and one-step process ball mills generally use high-temperature vertical mills. The delivery of pulverized coal is carried out by a special tank truck maintained by the modified N2, which can be seamlessly spliced between the pulverized coal production and processing logistics distribution center and the pulverized coal boiler. Based on the working experience, the economic development of the tanker truck covers a half The maximum is 100km.
No matter what kind of processing technology and machinery and equipment are selected to avoid safety accidents such as explosion accidents, the occurrence of dust on the construction site must be prohibited in the production workshop during all steps of coal preparation. Each machinery and equipment should maintain stable airtightness and cement rotary kiln , The ball mill should be operated under air pressure. The cement rotary kiln is a device for air-drying pulverized coal. The internal temperature reaches 700°C. At this temperature, a part of the coal is evaporated and separated; a small amount of ultrafine powder will be generated during the whole process of basic crushing, which is rotated in the cement rotary kiln When the superfine powder floats in the air, it produces flammable smoke and dust. This comprehensive element has flammable smoke and dust inside the cement rotary kiln, which produces gas-solid flammable explosions. The air-dried pulverized coal has an oxygen content of 12% in soot, and the interior of the cement rotary kiln has three other elements that cause explosion events, so there is an urgent lack of scientific research on its gas-solid flammability explosive features such as the lowest value of fire energy and lower explosion limit. Research, develop and design chain franchise protection equipment to avoid explosions.
Pulverized coal stable storage technology
The important equipment for pulverized coal storage is the pulverized coal storage tower (tank). The storage tank itself is a pressure-resistant vessel used under superheated steam (the working pressure of the design scheme is not less than MPa), so as to avoid statically placing the pulverized coal and tamping the "wall" According to the fluidity characteristics of the pulverized coal, a cone with a half cone of less than 20° is designed under the pulverized coal storage tower (tank), and the arched railway bridge is punched into the cone boundary layer to enter air compression. Considering the safety factor of pulverized coal, the structure of pulverized coal storage tower (tank) and the setting of ancillary equipment generally require a variety of security measures, including electrostatic induction grounding device, housing compression, anti-explosion door explosion, and sensor supervision And rare gas maintenance, etc., the structure is shown in Figure 1. During the entire operation, the closed tank truck from the pulverized coal manufacturing plant or logistics distribution center is quickly connected to the pulverized coal storage tower (tank) and the filling pipe. The pulverized coal in the tanker truck is compressed by the intercepted air or compressed by the customer's spare air to collect dust. The feed gate valve is opened, and the pulverized coal flow is quickly introduced into the tank in a dense phase. The air and powder are separated, the pulverized coal is dropped by weight, and the transport air is filtered according to the bag filter at the top of the pulverized coal storage tank and discharged into the air. If the temperature in the tank is found to rise rapidly and exceeds the warning temperature or the CO concentration value alarm, the rare gas protection device should be activated, that is, first introduce N2 from the bottom of the storage tank, replace O2 in the tank, and then introduce CO2 from the top to cover the surface of the pulverized coal , It has the function of blocking air.
At this stage, most of China's engaged manufacturers lack common sense of pulverized coal storage safety, and there are serious safety risks when designing plans and installing pulverized coal storage towers (tanks), such as placing pulverized coal storage towers (tanks) in a fully enclosed house Inside the building, there is a risk of fire and even explosion. In order to ensure the orderly development trend of the physical and mental health of the pulverized coal boiler manufacturing industry, scientific research should be closely focused on the characteristics of pulverized coal fire and temper tantrums in a confined space, and the theoretical significance for the production of detailed independent safety technical standards for pulverized coal storage tanks .
dense phase pulverized coal transportation technology
The feeder is a more critical machine equipment in the pulverized coal boiler, which has the function of continuously and smoothly feeding the burner without pulsation and high solid-to-gas ratio. The feeding is unstable, and it is very easy to produce safety accidents in the furnace, so as to ensure the smooth operation of the heating furnace, the feeding error cannot exceed ±3%. In recent years, China's independent pulverized coal feeding technology has three key categories: ①Feeding machinery and equipment that imitates power station boiler system software, such as centrifugal impeller powder feeders; ②Pneumatic conveying machinery and equipment, such as (improved) screw conveyors; ③Introduced overseas Technical products, such as gas cone fusion lock valves. The first two types are all dilute phase feeding, the solid (pulverized coal) gas (gas) ratio is generally lower than kg/m3, the original ignition is difficult, and it is not suitable for the pulverized coal boiler industry. Only the third type is dense phase feeding, which has a rapid fire standard and can be used in the pulverized coal boiler industry.
With the gradual advancement of understanding of the characteristics of pulverized coal fluidity, the feeder is upgraded from a spiral structure to an active secondary active type. Ways to actively feed the feeder include the "Star Bird" coupling Venturi tube (Figure 2), the porous structure of the double wheel disc, and the motor rotor scale. The pulverized coal is periodically discharged into the middle buffer bin through the star-shaped unloading valve at the bottom, and the boundary layer of the cone in the middle buffer bin is more ventilated, so that the breathing of the pulverized coal is rapidly activated and the circulation is obviously improved. The relative density of the pulverized coal flow is uniform throughout, and enters the "Star Bird" coupling Venturi valve group through the cone entrance and exit with the help of force. The pulverized coal discharged from the valve continuously and evenly falls into the high-speed water jet switching valve component, and it is transmitted and mixed with the primary air for an instant, resulting in a higher concentration of air and powder flow. The air and powder flow is sent out continuously through the dense phase through the air and powder pipeline with the driving force of the primary centrifugal fan, and enters the middle and downstream burners. The "Xingbird" coupling venturi tube has the characteristics of secondary activity, symmetrical continuous feeding and mixing and pushing with water jet. The experiment shows that the "Xingbird" coupling venturi tube feeding error is ± under the working condition of feeding speed ~ 1%.
Results and proposals
1) In all industrial production steps of pulverized coal preparation, each machine and equipment should maintain stable airtightness. Cement rotary kilns and ball mills should be operated under air pressure to prevent dust on the construction site in the production workshop and prevent "smoke cloud" explosions. Because the cement rotary kiln has three other elements that cause explosions, there is an urgent need to carry out scientific research on its low-value fire energy, lower explosion limit and other characteristics, and develop and design chain franchise protection equipment to avoid malignant explosions.
2) In the whole process of pulverized coal transportation, pulverized coal delivery is carried out by a special tank truck maintained by modified N2. When designing the pulverized coal storage tower, various security measures should be adopted, taking into account the setting of CO, CH4 and other steam Body and its temperature detection and alarm system, equipped with multi-component plasticity protection equipment and force-type anti-blast door equipment.
3) Pulverized coal dense phase (fire) ignition mechanism, stable combustion chamber semi-liquefied gas ignition mechanism, and furnace soft ignition mechanism are the three key technologies that distinguish pulverized coal boilers from power plant boilers. The three key technologies enable the pulverized coal boiler not only to meet customer needs with frequent start-stops and large load changes, but also to maximize resource advantages for air pollutant manipulation.
4) The expensive soot cleaning technology of power station boilers cannot be completely mechanically applied to the pulverized coal boiler. Only the development and design of the low-cost soot cleaning technology that incorporates the characteristics of the pulverized coal boiler. At this stage, mechanical equipment power dust removal equipment and filter type dust removal equipment should be used to remove ash, grade classification low-nitrogen combustion and SNCR flue gas denitrification and semi-dry desulfurization technology.
5) In order to integrate into the rapid development of large cities (towns) and industrial districts in China, the future development trend of pulverized coal boilers is: First, considering the requirements of regional peak shaving boilers, the total volume of more than 100 tons of steam per hour , The high-temperature gas boiler has as much space as possible; second, considering the requirements of kinetic energy step application, gas-fired steam boilers should be designed with medium and high parameters. According to part of the power generation of the drag turbine generator, it can significantly improve the economic benefits of customers; Considering stringent environmental protection regulations, air pollutant discharge operations should be extremely low or nearly zero.
