High-efficiency clean combustion and distribution technology for coal powder

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High-efficiency and clean combustion of coal powder and distribution technology Preface In recent years, as countries have placed increasing emphasis on environmental protection, movements to control pollution and safeguard the environment have emerged worldwide. New environmental protection technologies and products keep emerging, while new challenges also arise continuously. Coal powder combustion plays an important role in pollution emissions, and it has always been a key focus as well as a challenge in pollution control. Many **have made it a central task in their efforts to address environmental pollution, and have achieved relatively significant results. It is predicted that from 2000 to 2010, coal will still account for around 70% of China’s demand for primary energy, and this proportion could drop below 50% by 2050; however, the absolute amount of coal consumed will still **increase**. Therefore, for our country**, to control the overall environmental protection indicators, it is first necessary to curb pollution caused by coal combustion; the way forward lies in vigorously developing clean coal technologies aimed at the efficient and clean use of coal. Over the more than decade of continuous development in clean coal technology, many products and complete sets of technologies have been developed both domestically and internationally. There are advanced coal sorting technologies, coal slurry technologies, coal gasification, and coal liquefaction technologies. There are also technologies such as circulating fluidized bed, pressurized circulating fluidized bed, and integrated gasification combined cycle. In addition, there are flue gas purification technologies at various levels of efficiency, as well as technologies for the comprehensive utilization of fly ash. Considering the current situation in our country, high-efficiency and clean coal powder combustion as well as flue gas purification technologies are likely to have broad application prospects in the near future. Current status of high-efficiency and clean coal powder combustion technology and flue gas purification technology. High-efficiency and clean coal powder combustion as well as flue gas purification technologies include high-efficiency combustion technology, low-NOx combustion technology, flue gas desulfurization technology, flue gas denitration technology, dust removal technology, etc. A brief introduction is as follows: Generally speaking, high-efficiency coal powder combustion technology and low-NOx combustion technology are two mutually contradictory techniques. Reducing NOx generation and emissions essentially involves keeping the temperature in the combustion zone from being too high. However, low-temperature combustion affects the combustion rate of coal powder; the goal is to coordinate the use of these two techniques to achieve an optimal overall effect, which in practice requires controlling the entire process of coal powder combustion. It ensures the stability of coal powder ignition, features a lower combustion temperature, and possesses a sufficiently long combustion time at a certain temperature to ensure complete combustion. The more advanced combustion technologies available in the world today take these factors into account to a large extent. Among those that use direct-flow burners, ABB-CE utilizes the inertial separation effect of the primary air elbow; a perforated partition is installed at the outlet of the elbow to divide the coal powder airflow into two streams – one rich at the top and one lean at the bottom – thereby creating upper and lower rich-lean coal powder burners. An axially adjustable V-shaped baffle is also installed at the nozzle, and by properly organizing the secondary air, a combustion process that is stable, efficient, and results in low NOx emissions is achieved ； Japan’s Mitsubishi Heavy Industries (MHI) has developed the PM-type burner, which utilizes the centrifugal force of elbows to divide the primary air into two streams of different concentrations, upper and lower; meanwhile, the use of flue gas recirculation and overall staged combustion within the furnace also yields good results. Those that primarily use swirl burners include FW Company, which utilizes cyclones to increase the concentration of primary air entering the main burner and to reduce the primary air velocity, thereby ensuring the stability of ignition of the coal powder stream and controlling the amount of NOx generated ； Other types that find extensive industrial use include B&W Company’s PAX-type swirl coal burner, Japan’s IHI Company’s swirl coal burner with a wide tuning range, and German company Steimiller’s multi-stage air supply swirl burner, among others. All of the above industrial products can ensure that NOx emissions remain below 400 mg/Nm3, and they feature high combustion efficiency. The low-NOx combustion technologies currently under development abroad can control the NOx generation level at around 200 mg/Nm3, which represents a fairly high level. However, since many advanced countries around the world have set strict standards for NOx emissions, it is difficult to achieve the required NOx control levels merely by improving combustion technology; as a result, some boiler units are equipped with flue gas denitration systems at the rear. In recent years, our country has also developed many types of low-NOx combustion technologies. Representative ones include stratified coal powder burners, such as horizontal stratified burners, vertical stratified DC burners, swirl burners, and controllably stratified swirl coal powder burners. However, due to issues such as the diversity of coal types in our country, these technologies have encountered some problems in practice; even coal-fired power units manufactured using similar foreign technologies face the same issues. Through efforts, a set of combustion technologies for stable combustion at low loads with low NOx emissions, which possesses a certain degree of adaptability to different types of coal, has been developed and put into industrial use for lignite boilers. These technologies enable NOx emissions to be kept below 400 mg/Nm3, while maintaining a combustion efficiency of over 99%, which is quite advanced. Flue gas desulfurization, denitration, and dust removal are the three main aspects of flue gas purification. In flue gas desulfurization, wet desulfurization holds an absolute dominant position, accounting for over 90% of the market in developed countries. Other desulfurization technologies include semi-dry desulfurization, calcium injection in the furnace with tail wetting, flue gas recirculating fluidized bed method, and electron beam ammonia method. From a development perspective, wet flue gas desulfurization remains dominant due to its high performance indicators, while the flue gas circulating fluidized bed desulfurization method is also gradually gaining a share of the market thanks to its comprehensive technical and economic advantages, and it will be increasingly widely used in the future. For our country, it is best to adopt wet flue gas desulfurization technology and equipment from the start for newly built power units ； The modification of the units depends on the sulfur content of the coal type; for high-sulfur coal, the flue gas-circulated fluidized bed method can be used, while for low-sulfur coal, calcium injection into the furnace combined with wetting at the exhaust end can be employed. In particular, the numerous water film dust collectors that are already in use can have their value fully utilized. In terms of flue gas denitration, selective reduction reactors are generally used abroad to remove most of the nitrogen oxides from flue gas, while other methods are still in the stage of development and research. Due to constraints such as funding and site availability in the domestic market, there are no precedents for industrial applications, and the capacity to build demonstration units does not exist at present. In terms of flue gas dust removal, as developed countries impose increasingly strict requirements on dust emission levels and the efficiency of removing ultra-fine particles, many countries have gradually replaced electrostatic precipitators with pulse-jet bag filters, a practice exemplified by countries such as Australia and the United States. Our country plans to replace the cyclone dust collectors or water film dust collectors used in a large number of medium-sized units with electrostatic dust collectors in the near future, with the requirement that the exhaust dust concentration shall not exceed 200 mg/Nm3, and no restrictions apply to floating dust. For the proposed Taishan pressurized circulating fluidized bed boiler in Dalian, the Chinese side plans to use pulse reverse-blow bag filters to achieve higher performance. **Research activities and the application of results during the \"Eighth Five-Year Plan\" and \"Ninth Five-Year Plan\" periods at the Power Plant Combustion Engineering Technology Center; Introduction to the research facilities. Whether it is mechanism research or pilot-scale tests, they can be divided into three categories: aerodynamic characteristic tests, gas-solid two-phase flow characteristic tests, and coal combustion characteristic tests. Therefore, the center has constructed three separate test halls to meet the different requirements of these three types of tests. The aerodynamic characteristics testing hall covers an area of 800 m2 and is equipped with a main air supply system; the total air flow rate is 50,000 Nm3/h. It is divided into four areas: an area for studying the flow characteristics of individual burners, an area for examining flow patterns within the furnace, an area for optimizing structures to reduce energy consumption and drag, and an area for testing measuring devices, dampers, as well as other product technologies. The testing instruments include the Dandy 3D hot wire system, PIV system, and other conventional instruments. To date, more than 30 bench test studies have been completed, and good results have been achieved in numerous field applications. In addition, there are other supporting mechanism research test benches and flow field calculation software, etc. The gas-solid two-phase flow test hall covers an area of 500 m2 and is equipped with a main air supply system. The total air volume is 35,000 Nm3/h, the pressure head is 10,000 Pa, and the powder supply rate can reach 200 kg/h. The available powders include silicon carbide, coal dust, and power plant fly ash. Development of burners with variable combustion intensity, improvements to cyclone separators and dust collectors, research on electrostatic dust removal and bag filters, as well as studies on the flow characteristics of desulfurization towers can be carried out; wear testing studies can also be performed. In addition, there are independent pulverized coal distributor test benches, automatically adjustable cascade louver pulverized coal distributor test benches, and other mechanism test benches. The testing instruments include a laser holography system, a sheet light source imaging system, a Coulter particle size analyzer, an optical fiber concentration analysis system, a flow field pattern reproduction and analysis system, as well as other conventional testing systems for concentration and flow fields. The coal combustion characteristics testing hall covers 700 m2 and can be divided into three main sections. The first major section is the introduction of the Coal Combustion Pilot Test System apparatus (CRF) from Ontario Hydro in Canada. The coal consumption is 20 kg per hour, with a thermal power of 0.2 MW. The device consists of a raw coal drying system, a raw coal crushing system, a powder production system, a combustion system, primary and secondary air systems, a flue gas treatment system, an in-furnace calcium injection system, a flue gas dust removal system, a flue gas composition testing system, and a control system. The testing system and control system utilize the Rosemount online flue gas analysis system as well as other flow, temperature, and pressure sensors; all of these inputs are fed into a computer, which then performs online dynamic adjustments to all variables in order to meet the requirements of the tests. This device can comprehensively and accurately simulate the entire process of raw coal drying, crushing, grinding, powder transportation, combustion, and flue gas treatment. It enables the evaluation and study of the impact of operating parameters and fuel quality on combustion stability, burnout degree, sediment formation, pollutant generation, as well as particle emission and collection. Moreover, it facilitates the development of new low-NOx combustion technologies, in-furnace calcium injection techniques, and other flue gas treatment methods. This system features a high degree of automation, advanced control and testing instruments, as well as good reproducibility in experiments, enabling extensive technical development and research; it is one of the more advanced coal combustion testing systems available internationally. Currently, the center has made further modifications to this testing setup: it has rebuilt the combustion system, increased the combustion power to 0.3 MW, added a humidification and activation device at the exhaust gas outlet, installed a flue gas desulfurization system using the suspension method, and is conducting new research.