Current status of oil shale combustion technology

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Current status of oil shale combustion technology: The combustion technology for oil shale has gone through four stages: layered combustion, suspended combustion, fluidized bed combustion, and circulating fluidized bed combustion. In the 1930s, the former Soviet Union tested Estonian oil shale in small medium-pressure boilers with a rated output of 40 t/h ； Starting in the 1950s, suspended combustion was adopted for burning oil shale, leading to the construction of medium-pressure power plants with unit capacities of 12–15 MW and boilers with output rates of 65–85 t/h. Subsequently, two large coal-fired power plants with a total capacity of 1600 MW each were built in Estonia to burn oil shale. Between 1958 and 1968, 8 coal-fired boilers with a capacity of 220 t/h and 4 boilers with a capacity of 320 t/h were installed at the Baltic Thermal Power Plant in Novava to burn Estonian oil shale. In the 1970s, pulverized coal boilers for power plants burning oil shale with a capacity of 670 t/h were built. In the 1980s, Romania built three 330MW oil shale pulverized coal boilers, which used low-quality oil shale as fuel. After the boilers were put into operation, it was necessary to burn 15-30% heavy oil, resulting in high losses due to incomplete mechanical combustion and poor economic efficiency; therefore, Romania decided not to develop more oil shale power plants. The Fushun Power Plant in Liaoning Province, China, as well as Fushun Oil Plants No. 1 and No. 2, once generated electricity by mixing 10-30% oil shale into their coal-fired boilers. Prymian oil shale testing experiments were conducted in 1979 at the Doicesti power plant in Romania. Using a suspended combustion method, the boiler has a maximum output of 100 t/h. However, to maintain normal operation, 10-15% heavy oil must be blended in, resulting in poor economic efficiency. Research results from both domestic and international sources indicate that when oil shale is burned using high-temperature suspension combustion, several problems arise: the oil shale grinding system is large in size and prone to **, resulting in unreliable operation ； Requires auxiliary desulfurization equipment, and has high NOx emissions ； The high temperature of the flue gases causes the sublimation of alkali metals in the fuel, leading to severe corrosion of the heated surfaces inside the furnace and reducing its service life ； The heating surface at the tail end is severely soot-covered and blocked, reducing the boiler’s efficiency ； When the calorific value of oil shale is generally low, below 8000 kJ/kg, fuel injection is required to maintain stable combustion, resulting in high operating costs. The former Soviet Union developed fluidized bed boilers for oil shale for the first time at the Ahhtmu Central Thermal Power Plant in Estonia. This power plant converted a BK3-75-39ΦCЛ type oil shale coal powder boiler into a fluidized bed boiler, and it was put into operation in 1981; a similar conversion was carried out on the boilers at the Baltic Sea Coast Power Plant in 1987. It operates well after the modification, but its thermal efficiency is not high. Israel began research on the utilization of oil shale for combustion in 1981, and a 55 t/h circulating fluidized bed demonstration power plant was built in 1993. The operation of this demonstration power plant has led to some progress in the utilization of oil shale energy, but the oil shale circulating fluidized bed boilers have exhibited problems such as low thermal efficiency, blockage and caking in the superheaters, and ash accumulation on the rear heating surfaces. China began experimental research on the suspended combustion and fluidized combustion of oil shale in 1957. Between 1964 and 1966, low-pressure fluidized-bed boilers with a heating capacity of 14.5 t/h and medium-pressure suspended-combustion boilers with a heating capacity of 35 t/h were built to carry out industrial-scale experiments. In the 1970s, a 35 t/h oil shale fluidized-bed power plant boiler designed by Northeast Electric Power University and manufactured by Jiangxi Boiler Factory was put into operation at Maoming Petroleum Company in Guangdong, delivering good performance. In August 1996, three 65 t/h low-ratio circulating fluidized bed oil shale power plant boilers were installed at the heat power plant in Huadian City, Jilin Province, and have been operating safely for 12 years to date. The furnace height of this boiler is 17 meters; the hot-state fluidization wind speed in the boiler is 4.5–6.5 m/s, the circulation ratio is 6, the operating bed temperature is maintained at around 850°C, and the combustion efficiency of the boiler is as high as 99%. In 2007, the Suixi Thermal Power Plant in Zhanjiang, Guangdong, successfully transformed a 75t/h coal-fired pulverized coal boiler into a 65t/h circulating fluidized bed power plant boiler that burns Maoming oil shale by using the high-low differential velocity bed method. In 1993, Eesti Energia and Foster Wheeler collaborated to initiate combustion experiments using circulating fluidized bed boilers for Estonian oil shale. Utilizing Foster Wheeler’s circulating fluidized bed boiler technology, an 0.8MW experimental unit was built, and experiments were conducted using fluidized bed, circulating fluidized bed, and rapid bed combustion methods. These experiments aimed to examine combustion stability, SO2 and NOX emissions, as well as emissions of heavy metals and ultra-fine particles, in addition to the ash accumulation and slag formation characteristics of the boilers. Based on experimental research, in 2004 Estonia built a oil shale circulating fluidized bed boiler power plant with a capacity of 215 MW, utilizing Foster Wheeler technology; this marked the beginning of the development of large-scale oil shale circulating fluidized bed combustion for power generation.