Is a fluidized bed boiler better or a pulverized coal furnace better?

Coal-fired boilers are efficient, while fluidized bed boilers offer wide fuel adaptability and low emissions.

There are now large-tonnage fluidized bed units as well, including those with a capacity of 60W.

This post was last edited by Little Expert on 2017-12-4 at 11:50. The circulating fluidized combustion method in circulating fluidized bed boilers has the following characteristics compared to boilers using other combustion methods. 1. The range of combustible fuels is wide; the furnace of a circulating fluidized bed boiler contains a large amount of bed material composed of solid particles. These hot solid particles can be sand, gravel, limestone, and coal ash. The added fuel accounts for only 1% to 3% of the total bed material by mass fraction. A circulating fluidized bed is a rapid-bed system in which a strong internal circulation is formed in the furnace: the air flow in the central area causes fine particles to move upward, while in the annular zone near the walls the particle clusters sink downward. Together with the external circulation mechanism, in which the high-temperature solid particles that flow out of the furnace with the flue gas are separated and captured before being sent back into the furnace, the heat and mass transfer processes within the furnace are significantly enhanced. The temperature inside the furnace can be maintained evenly at around 850°C; the fuel particles fed into the furnace are quickly heated to this temperature and catch fire. Therefore, circulating fluidized bed boilers can burn various solid fuels without the need for supplementary fuel; they can combust low-volatility anthracite, high-sulfur bituminous coal, and even high-ash coal with ash content as high as 40%–60% satisfactorily. Furthermore, this type of boiler can also burn other solid fuels such as petroleum coke and shale, offering a very wide range of fuel options. 2. High combustion efficiency: In circulating fluidized bed combustion, although the fuel particles are dozens or even hundreds of times larger than coal powder particles, a well-designed and properly operated system can achieve a combustion efficiency on par with that of coal powder boilers. The main reasons why circulating fluidized bed boilers can maintain high combustion efficiency are as follows: First, fresh fuel particles enter the furnace and quickly mix with a large amount of hot bed material, allowing them to ignite and burn immediately; moreover, the gas-solid mixing in the furnace is intense, resulting in a high combustion rate ； Furthermore, in this type of boiler, the combustion area extends throughout the furnace. The unburned particles that flow out of the furnace with the air stream are separated by a cyclone separator and then sent back to the furnace for further combustion. As a result, the fuel combustion time is significantly extended, which facilitates complete combustion of the fuel ； Of course, some fine particles are not collected by the cyclone separator and flow with the flue gas into the flue of the heating surface at the rear of the boiler, resulting in some losses due to incomplete combustion. To reduce this amount of combustion loss, these fine particles can be collected at the bottom of the flue at the rear of the boiler and sent back to the furnace for combustion. 3. Good desulfurization effect. One of the main disadvantages of coal-fired boilers is that the exhaust gas contains large amounts of SO2 gas generated during combustion. Flue gas containing SO2, when released into the atmosphere, will severely pollute the environment. To reduce the SO2 content in flue gas and meet environmental standards, it is often necessary to use expensive flue gas desulfurization equipment, or to add desulfurizing agents (absorbents) during the combustion process. The commonly used desulfurization agents are limestone (CaCO3) and dolomite (CaCO3 • MgCO3). In circulating fluidized bed boilers, the desulfurization agent carries out desulfurization at the optimal reaction temperature within the furnace. The internal circulation of fuel and materials in the furnace, along with the external circulation provided by separation equipment and recirculation devices, allows the desulfurization agent to remain in the furnace for periods of up to several dozen minutes on average. Thus, the desulfurization process can proceed fully. When limestone is used as a desulfurization agent with a Ca/S ratio of 2, the desulfurization efficiency can exceed 90%, and the utilization rate of the desulfurization agent can be over 50%. The SO2 content in the flue gas discharged into the atmosphere (under standard conditions) is less than 200 mg/m3, which meets environmental standards; thus, there is no need to use expensive flue gas desulfurization equipment. 4. Low emissions of nitrogen oxides NOx. Another environment-harming substance in boiler exhaust is nitrogen oxide NOx. Based on their formation mechanisms, NOx in flue gases can be classified into thermal NOx, rapid NOx, and fuel-derived NOx. Thermal NOx is generated by the oxidation of N2 contained in the air used for combustion at high temperatures ； Fast-type NOx is generated as a result of the reaction between intermediate products produced during fuel combustion and N2 ； Fuel-type NOx is generated by the oxidation of organic compounds contained in the fuel during combustion. In coal-fired boilers, the proportion of rapidly formed NOx in the total NOx content is low, generally below 5%; therefore, the nitrogen oxides present in the exhaust gases are mainly thermally formed NOx and fuel-derived NOx. Studies show that both fuel-derived NOx and thermally formed NOx are closely related to the combustion temperature. The higher the combustion temperature, the greater the levels of these two types of NOx; conversely, the lower the temperature, the lower their levels. In particular, thermal NOx is more significantly affected by the combustion temperature. The furnace temperature in a circulating fluidized bed boiler is around 850°C; at this temperature, the generation of thermally induced NOx is relatively low, accounting for generally less than 10% of the total NOx emissions. In addition, the air required for combustion in the circulating fluidized bed boiler is supplied in stages. Primary air is supplied from beneath the air distribution plate in an amount lower than that required for combustion; as a result, the fuel nitrogen released cannot react fully with oxygen to form nitrogen oxides. Secondary air is introduced into the furnace above the reduction zone in the lower part of the furnace; at this point, the nitrogen released from the fuel is already in the form of molecular nitrogen, so it is less likely to form NOx. By organizing the air supply and combustion in a structured manner, the formation of fuel-derived NOx can be effectively reduced. Therefore, the NOx emission level in the flue gas of circulating fluidized bed boilers ranges from (50–150) ppm, which meets the environmental regulations of various countries. 5. The high heat load per unit area of the furnace chamber facilitates the development of large-capacity boilers. The gas flow velocity in the furnace of a circulating fluidized bed boiler is 3 to 5 times that of a bubbling fluidized bed boiler; as a result, mixing within the furnace is excellent and heat transfer is rapid. The heat load per unit area of its furnace chamber is also much higher than that of bubbling fluidized bed boilers, typically ranging from 3 to 5 MW/m2, which can reach levels comparable to those of coal-fired boilers. 6. The boiler has a wide range of output adjustment capabilities and a fast adjustment speed. In circulating fluidized bed boilers, the furnace temperature can be increased by reducing the flow rate of fluid entering the external heat exchanger. In this way, the drop in furnace temperature that occurs at low loads due to a reduction in fuel and air supply can be compensated for, allowing the furnace temperature to remain within the optimal operating range. Thus, it allows for a wide range of boiler output adjustments; the boiler can still operate stably at 25% to 30% of its normal output level. Furthermore, due to the high gas velocity and rapid heat transfer in the furnace, its output adjustment rate is fast, reaching up to 4%/min. 7. The ash can be utilized in various ways. Thanks to low-temperature combustion and high combustion efficiency, the ash discharged from circulating fluidized bed boilers does not undergo a melting process and has a low carbon content. However, due to the use of in-furnace desulfurization technology with desulfurizing agents, the amount of solid ash discharged is generally 1.5 to 2 times that of coal powder boilers of the same capacity. Moreover, the ash contains large amounts of calcium oxide and calcium sulfate, unlike the ash from coal-fired boilers which is primarily composed of silicon oxide. Its ash can be comprehensively utilized in areas such as cement admixtures, building materials, and brick and tile production. A series of characteristics of circulating fluidized bed boilers have enabled them to evolve into coal-fired boilers with a wide range of applicable fuels, high efficiency, and low pollution. Suitable not only for industrial boilers but also for large-scale power plant boilers, they hold broad application prospects and potential for further development.

Coal-fired boilers have high thermal efficiency, virtually no wear issues with the heating surfaces in the furnace, and their boiler technology is mature; however, the system is complex； Circulating fluidized bed boilers have a wide range of fuel adaptability, good environmental performance, and high combustion efficiency. Their boiler systems are simple, and the ash can be utilized effectively. Their performance at low loads is superior to that of coal powder boilers; however, wear of the furnace heating surfaces is a common issue during operation.

Coal-fired boilers take up a lot of space and are also expensive, right? Fluidized bed combustion is said to be a low-nitrogen combustion method, but even the new fluidized bed systems installed these days must be equipped with denitrification systems; otherwise, they fail to meet environmental standards.