Coal can be called the "Gate of Life" of China's energy. Although China’s coal reserves are abundant, low-rank coals with a low degree of coalification account for almost half of the country’s total. In recent years, with the continuous escalation of energy and environmental pressures, how to “eat and clean up†coal has become one of the most pressing tasks for Chinese coal technicians. The Circulating Fluidized Bed Laboratory of the Institute of Engineering Thermophysics, Chinese Academy of Sciences, is a leading force in China's coal science and technology. Focused on coal burning for more than 30 years, recently, they finally burned out the "Great Hall of Fame."
Recently, the team's research and development of preheating combustion technology has enabled semi-coke and carbon residue to burn efficiently and emit low nitrogen oxides, successfully breaking through the bottleneck of key technologies that restrict the grading and conversion of China's low-grade coal.
Eat dry squeeze the general trend
China's low-rank coal reserves are close to 500 billion tons, accounting for about 42% of proven coal reserves. The coal with low degree of coalification has high volatile content and high moisture content, so it has many restrictions on development and utilization, and energy efficiency and economic efficiency are relatively low.
Low-rank coal is rich in oil and gas components, and direct combustion cannot fully utilize its resource value. Under such a tense national energy situation, this is indeed a huge waste.
In fact, the country has already realized this. In 2016, the “Energy Technology Revolutionary Innovation Action Plan 2016-2030†issued jointly by the National Development and Reform Commission and the Energy Bureau, has classified “coal classification and quality conversion†as the strategic direction for the technical innovation of clean and efficient use of coal; meanwhile, the Ministry of Science and Technology has also It is included in the national key research and development program "coal clean and efficient use of new energy-saving technologies," a special implementation plan.
“The use of low-rank coal is necessary for grading.†Lu Qinggang, deputy director of the Institute of Engineering Thermophysics of the Chinese Academy of Sciences, told the journal of the “China Science Journal†that “hierarchical conversion technology is to extract high-value oil and gas from low rank coal first, and the remaining half The coke or carbon residue is used as fuel to continue to generate electricity, which maximizes the value of coal."
It seems that along this line of thinking, coal can be “squeezed outâ€. However, at the operational level, everything is not as simple as it seems.
Low-rank coal pyrolysis or gasification by-products, semi-coke and residue carbon, are "ultra-low volatile carbon-based fuels," meaning that the volatile content is much lower than conventional coal. When burning with traditional techniques, it will encounter the difficulties of stable combustion and low burnout rate.
"In order to overcome these problems, industries often adopt measures to increase the temperature of combustion, but doing so has also brought about high emissions of pollutants."
Lu Qinggang's pollutants mainly refer to nitrogen oxides. Existing studies have confirmed that nitrogen oxide is one of the important factors causing haze weather, but it is not easy to handle.
Burning is not good, or burning is not clean, efficient and clean, has become a difficult contradiction.
How do you have both fish and bear's paw?
A figure shows that the low-rank coal pyrolysis has a semi-coke yield of about 50 to 70% of the raw coal, and the gasification residual carbon yield is about 20 to 30% of the raw coal. With the development of China's low-grade coal classification and transformation industry, coal gasification, coking and other industries will generate hundreds of millions of tons of semi-coke and carbon residue each year.
"If we can not make good use of these ultra-low-volatile fuels, it will seriously restrict the development of China's low-grade coal grading and transformation industry," said Li Shiyuan, deputy director of the Circulating Fluidized Bed Laboratory of the Institute of Engineering Thermophysics, Chinese Academy of Sciences.
Since the traditional combustion technology has failed, since 2004, the R&D team has started “starting a new life†and explore a new combustion model. They originally proposed a pre-combustion technology for ultra-low volatility carbon-based fuels to form a new type of preheating burner, which is different from conventional DC and swirling burners.
“We have found that after the fuel is preheated to reach the combustible temperature, the reactivity will be significantly improved, and controlling the temperature distribution in the combustion chamber through the staged air distribution can effectively reduce the conversion rate of nitrogen oxides.†Associate Researcher, Institute of Engineering Thermophysics, Chinese Academy of Sciences Zhu Jianguo said, "This guarantees stable, efficient, fast and clean combustion."
From 2013 to 2016, under the auspices of the “Strong Key Technologies and Demonstration of Low-rank Coal Cleaner and Efficient Cascade Utilization†funded by the strategic pilot technology project of the Chinese Academy of Sciences, the team combined the preheating combustion technology with the utilization of ultra-low volatile fuels such as semi-coke and carbon residue. , achieved significant results.
In March 2017, after 36 hours of continuous testing, the team achieved 98.65% of Shenmu's semi-coke combustion efficiency, 63mg/Nm3 of nitrogen oxides' initial emission concentration, and 87.6% of the gasification residual carbon combustion efficiency on the 2MW preheating combustion pilot plant. The original emission concentration of nitrogen oxides was 83 mg/Nm3. These figures mean that while the combustion efficiency is not reduced, the original emission concentration of nitrogen oxides of both fuels is far lower than the national thermal air pollutant emission standards, and the dry ash-free volatiles of these two fuels are only 8 % and 0.7%.
At this point, the team lasted 13 years and completed the mechanism research, key technology research, and pilot study of the preheating combustion technology, through innovation and integration of fuel self-heating, high temperature modification, direct ignition without ignition, and fuel nitrogen precipitation regulation. A series of key technologies have finally overcome the three major challenges of ignition, burn-out and pollutant emission control of ultra-low volatile carbon-based fuels.
Industrial applications for the future
The successful development of preheating combustion technology has broken the key technology bottleneck of low-grade coal graded quality conversion and provided strong advantages for the last ring of this chain: the efficient and clean combustion of ultra-low volatile carbon-based fuels. Strong technical support.
At present, the team is stepping up the promotion of this technology to industrial demonstration applications. Relying on the experimental research results and combining with the actual boiler engineering experience, the preheating combustion R&D team designed the fuel with gasification residual carbon with a volatile content close to 0 and a calorific value no higher than 3200 kcal/kg, and is carrying out 100 tons/day of gasification in Guangxi Hechi. Carbon residue boiler project demonstration project. The demonstration project has now completed construction and is expected to be officially put into operation in the second half of 2017.
In fact, this technology can go further. Along the national “One Belt and One Road†strategy, there are many countries whose energy structures are similar to coal, and they are equally eager for the clean and efficient use of coal technology and equipment.
Lu Qingang called for: “Using preheating combustion technology to achieve efficient and clean combustion of ultra-low volatility carbon-based fuels will bring enormous economic benefits and environmental benefits. We believe that the application prospects are very broad. It is recommended that the country increase technology research and development and industrialization. Support."
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