Swirl concentration and dilution coal powder combustion technology

Due to the variable loads and types of coal used in power plant boilers in our country, the following problems arise during boiler operation: (1) The boilers have poor stability in combustion, especially at low loads, which results in high consumption of oil for combustion support; (2) The combustion efficiency is low, with the efficiency of boilers using low-quality coal being around 85%; (3) Severe air pollution occurs; (4) Slag formation on the water wall and high-temperature corrosion take place. When burning sub-bituminous or bituminous coal, the two most commonly used coal pulverizer burners in power plant boilers are straight-flow burners and swirl burners. DC coal powder burners are typically arranged in a four-corner configuration to achieve tangential combustion, with the coal powder stream being ignited by the tangential flame sphere; whereas swirl coal powder burners use a wall-mounted arrangement, and ignition of the gas flow is accomplished through the suction effect of the recirculation zone formed at the center of the swirling jet, which draws in the high-temperature flue gases. Boilers equipped with swirl coal powder burners in power plants in our country account for about 20% of the total capacity. It was initially believed that when using swirl combustion technology, to ensure timely ignition of the coal powder and flame stability, a higher swirl intensity and a larger recirculation zone should be employed; the factor of coal powder concentration was ignored, which not only prevented stable combustion but also led to severe environmental pollution. The reason for this is that traditional swirl coal-fired burners, whether of the conventional type or the staged combustion type~. 1 Working Principle and Structural Features The new type of swirl concentration/dilution coal powder burner consists of a central tube, a primary air duct (with concentration elements installed inside), and a secondary air duct (swirl or straight-flow type), for a total of 4 ducts. The end of the central duct is equipped with a blunt body, while the end of the DC secondary air duct is fitted with an flare. (1) Basic working principle: By utilizing concentration elements with weak rotation to guide the flow of coal powder gas, the direction of this flow is forced to change. Thanks to the fact that the inertia of particles is much greater than that of air, the coal powder gas stream is concentrated and divided into two streams with appropriate differences in coal powder concentration. The stream located near the central tube contains a higher amount of coal powder, while the stream with less coal powder is injected into the furnace from outside the stream of concentrated coal powder. Secondary air is divided into two types: swirl secondary air and straight-flow secondary air. Swirl secondary air is injected into the furnace through the internal secondary air channel, while straight-flow secondary air is injected into the furnace through the external secondary air channel. In this way, under the combined effect of the swirl secondary air and the blunt body, a central recirculation zone is formed. The stream of concentrated coal powder is injected into this recirculation zone at an appropriate location, thereby creating a region with high temperature and high coal powder concentration. (2) Performance characteristics: Based on the \"three-high zone\" theory and combustion theory, appropriately increasing the coal powder concentration can shorten the ignition time, reduce the ignition heat, and improve flame stability. The lean coal powder airflow and the swirl secondary air mix with the rich primary air in sequence after ignition of the rich coal powder airflow, ensuring the amount of oxygen required during the initial stage of coal powder combustion; the direct-flow secondary air gradually mixes in along its path to assist combustion, providing additional oxygen needed for the coal powder to burn completely. This multi-layered, staged combustion process helps to effectively suppress the formation of NOX. The DC secondary air not only enhances the mixing in the later stages of the combustion jet, separating the reducing atmosphere from the furnace wall and thereby reducing high-temperature corrosion and slag formation on the water wall, but it also plays a role in regulating the shape of the flame. 2 Performance Analysis (1) Low-load stable combustion capability: Stable combustion capability has always been an issue with circular combustion, as in this mode the pulverized coal stream must be ignited by the circular flame. When the load decreases, the reduced combustion intensity in the furnace leads to flame instability. With the development of lean-burn technology, the stable combustion performance of DC burners at low loads has been improved to some extent, but the degree of improvement is not significant. The swirl burner relies on a high-temperature recirculation zone as a stable heat source to enable the coal powder stream to ignite promptly and burn steadily. When the rich-lean combustion technique is employed, a high-temperature, high-coal-powder-concentration zone is created that facilitates flame stability. An appropriate increase in the coal powder concentration within the recirculation zone helps to lower the ignition temperature of the coal powder and advance the timing of ignition. Meanwhile, the swirl secondary air promotes better mixing between the primary and secondary air streams, enhancing combustion and improving the stability of the flame; as a result, its ability to maintain stable combustion at low loads is **enhanced**. The improved low-load stable combustion capability achieved by adopting a new burner to modify traditional swirl burners has been verified. (2) NOX emission control: Compared with the DC burner, where the mixing of primary and secondary air occurs at a later stage due to its four-corner arrangement, traditional swirl coal powder burners achieve early and intense mixing of primary and secondary air, resulting in thorough mixing of coal powder with the airflow and sufficient oxygen supply; this leads to high levels of NOX generation, around 1–1.2 mL/L, whereas tangential combustion results in levels of around 0.6–1 mL/L. There are two main ways to reduce NOX formation: one is to create oxygen-deficient regions in order to minimize the amount of NOX produced; the other is to select the optimal residence time and temperature under fuel-rich conditions, so as to maximize the conversion of N into N2 without affecting combustion efficiency. To achieve these two approaches, low-NOX emission burners generally employ staged air supply and rich-poor combustion methods. The two-channel swirl coal powder burner with staged air distribution and the dual-air-regulation swirl coal powder burner are currently widely used low-NOX swirl coal powder burners. Their structural feature lies in the division of the secondary air channel into two channels: the inner channel supplies swirl air, while the outer channel supplies straight-flow air. Staged combustion is achieved through staged air supply in order to reduce NOX emissions. The high-concentration swirl coal powder burners used in the former Soviet Union involved a coal powder-air mixture with a concentration of 40–50 kg/kg, which was fed through a thin tube into an appropriate position within the primary air stream of the burner. This created a primary air-coal powder flow with a concentration of around 0.9 kg/kg, which then entered the furnace for combustion. Combustion stability was achieved by increasing the coal powder concentration in the backflow ignition zone, thereby reducing NOX emissions. In the new type of swirl burner, these two methods have been organically combined, and the layered injection of rich and lean coal powder streams into the furnace further enhances staged combustion. Operation results show that on the modified boiler, NOX emissions have decreased significantly. (3) Slagging prevention and high-temperature corrosion resistance: The factors affecting coal ash slagging mainly include the characteristics of the coal ash, the temperature level in the burner area, the aerodynamic properties in the burner outlet area, and the atmosphere near the furnace wall. A necessary condition for slag formation inside the furnace is that the ash particles or unburned carbon are in a molten and sticky state. The melting temperature of coal ash is not a constant; it depends on the atmosphere surrounding the coal particles. In an oxidizing atmosphere, the melting temperature of the ash is higher than in a reducing atmosphere. The new type of swirl rich-lean burner utilizes a multi-stage air distribution combustion method, allowing the coal to burn near the axis of the burner outlet. This ensures that an oxidizing atmosphere exists near the burner’s water wall, thereby increasing the ash melting point and preventing slag formation. Another necessary condition for slag formation inside the furnace is the scouring of the furnace walls by a large amount of ash particles and unburned carbon. The new lean-burn mode effectively prevents airflow ‘edge effects’ by using direct-current secondary air, thereby enhancing the oxidizing atmosphere near the water wall and reducing the tendency to slag formation. The high-temperature corrosion of the furnace water wall is also related to a reducing atmosphere and particle erosion of the furnace wall. The stepped burner effectively prevents coal powder particles from scouring the furnace walls, and by creating an oxidizing atmosphere near the water wall, it helps to prevent high-temperature corrosion. (4) Mechanism for improving combustion efficiency: Simply increasing the concentration of the coal powder stream may lead to a decrease in combustion efficiency and burnout rate, as has been confirmed through experiments. Under swirl-rich-poor combustion conditions, the rich pulverized coal flow ensures premature ignition of the coal powder stream, and it involves low-temperature, oxygen-deficient combustion, which is conducive to stable combustion. Subsequently, the stream of diluted coal powder and the swirl secondary air are mixed in at the appropriate time to assist combustion, in line with the principle of supplying air as needed throughout the combustion process, thereby ensuring the amount of oxygen required for coal powder combustion. Furthermore, the swirl secondary air enhances the disturbances in the early stage of combustion, improving flame stability. The ordinary swirl burner is characterized by weak mixing in the later stages of combustion, but the outer ring of the swirl rich-poor burner consists of straight-flow air, which effectively improves mixing during the later stages of combustion and ensures the complete combustion of coke particles; thus, it enhances combustion efficiency.

Cyclone burners typically have circular nozzles; under the action of a swirl generation device, the pulverized coal airflow or secondary air rotates, forming a swirling jet once it enters the furnace. The central area of this swirling jet is a region of negative pressure, and due to this pressure, high-temperature flue gases flow back, creating a high-temperature recirculation zone that facilitates the ignition of the pulverized coal. Since each cyclone can create such a high-temperature recirculation zone and ignite the pulverized coal airflow, the flame from each burner has relative independence. Cyclone burners have a large expansion angle, which results in a large recirculation zone and intense mixing in the early stages. However, an increase in the intensity of the swirl also causes the swirling jet to decay rapidly, leading to weak mixing in later stages and a shorter flame length. In small-capacity boilers, cyclone burners are usually arranged along the front wall, while in larger boilers they are arranged across the front and rear walls or on both sides to create an L-shaped combustion pattern. Depending on their structure, cyclone burners can be classified into types such as volute-type and movable impeller-type.