Dynamic injection can be used to cool and burn in a fluidized bed gasifier

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Dynamic injection can be used to cool down the gas flow bed gasifier. Fan Quanrong: The gas flow bed gasifier is the mainstream process for coal gasification, and its gasification mechanism has been a topic of ongoing research in this industry. The gasification mechanism refers to the elementary chemical reactions through which coal powder is converted into syngas, as well as the sequence of conversion and the relative rates. By analyzing the gas flow bed gasification mechanism, it can be seen that coal powder undergoes three processes in the gasifier: heating and pyrolysis, combustion of coal powder, and gasification of coal tar. The main gasification mechanisms are also divided into 3 categories: coal powder pyrolysis constitutes an independent field of study, focusing on issues such as the rate of component formation during pyrolysis ； The combustion mechanism begins with the turbulent mixing of coal powder and oxygen, initiating chemical reactions at the molecular level to form a diffusive combustion flame ； Coal gasification is a chemical reaction between coal coke and a gasifying agent, with the flow field being primarily laminar. In a fluidized-bed gasifier, the pyrolytic volatilization rate of coal powder is very fast, and it overlaps partially with the combustion process. Therefore, the combustion mechanism of the fluidized bed and the coal tar gasification mechanism have become the main subjects of study in gasifiers. The reaction space of the gasifier can be roughly divided into two areas: the combustion area near the nozzle, and the gasification area adjacent to the outlet of the gasifier. The most significant characteristic of the combustion zone is its high flame temperature, which can reach 2500 degrees Celsius; this is unfavorable for the operation of nozzles and vaporization furnaces. Therefore, how to avoid high-temperature combustion flames is a key focus in the development of fluidized-bed gasifiers. Dynamic injection gasification furnaces that utilize flameless dilution combustion technology offer hope for reducing high-temperature combustion in fluidized bed gasification furnaces. Flameless combustion technology is a latest combustion method that has emerged in the field of combustion research over the past 20 years; it is also known as mild low-oxygen dilution technology. This combustion is a volumetric combustion that takes place under conditions of low oxygen levels (2%–15%) and low temperatures (900 degrees Celsius–1200 degrees Celsius). It features a flameless, transparent combustion process, a uniform distribution of heat flow, low combustion noise, and minimal temperature fluctuations. Compared to traditional localized high-temperature flammable combustion, this low-temperature combustion requires much less furnace space; as a result, the average furnace temperature rises, radiation heat transfer is enhanced, and the thermal efficiency improves significantly. And more importantly, its emissions of pollutants (such as nitrogen oxides and carbon monoxide) are virtually zero. The dynamic injection gasification furnace that utilizes flameless combustion technology employs a dynamic feeding method, in which dry coal powder is injected downward from above the dynamic combustion nozzles; the transport of this coal powder is achieved through pneumatic means. The refractory lining consists of water-cooled wall coils, which are cooled using shock water. The gasification temperature ranges from 1300 to 1700 degrees Celsius, while the operating pressure is between 2.0 and 4.0 megapascals. Relevant high-temperature demonstration tests have already begun. Compared with fixed-jet gasification furnaces, the main difference of dynamic-jet gasification furnaces lies in the use of burners with a top-mounted bottom-jet design; they are in a dynamic jet state, creating a dynamic jet zone and a backflow zone. The coal powder is evenly distributed within this dynamic flow field, and the unique feeding pattern alters the shape of the flame and the combustion mechanism. The flame is distributed over a larger combustion area, the peak temperature decreases, thereby reducing the negative effects caused by high-temperature flames. Dynamic injection will also alter the mixing mechanism of oxygen and flue gas; the mixing from the previous fixed-injection suction mixing becomes dynamic-injection impact mixing. Oxygen is rapidly diluted, reducing its concentration percentage, thereby promoting combustion toward dilute combustion and flameless combustion. The combustion nozzle is the soul of the fluidized bed gasifier. As a new type of feeding method, the dynamic nozzle alters the combustion mechanism in the combustion zone, thereby affecting the gasification results in the gasifier. Compared with fixed-jet gasification furnaces, dynamic-jet gasification furnaces feature high-speed jets and relatively stable coal tar gasification. (The author’s institution is Fansmelt Company in Australia)