Combustion characteristics of ultrafine gasified semi‐char in circulating fluidized bed

Abstract

Coal fluidized bed gasification generates large amounts of semi‐char, which has a high carbon content. Effectively re‐burning semi‐char has become a promising way to improve overall conversion efficiency of coal gasification and use the residue carbon in the semi‐char. However, due to the ultrafine particle size (mean particle size ≈ tens of microns), nearly zero volatile content, and the graphitization structure of the semi‐char, the fluidization and combustion organization of semi‐char in a circulating fluidized bed (CFB) is difficult, leading to poor self‐sustained combustion stability and low combustion efficiency. The physicochemical properties of the semi‐char from an industrial CFB gasifier were studied. The combustion characteristics and activation energy of the semi‐char were tested using thermogravimetric analysis. The occurrence of sulphur and nitrogen in the semi‐char and fly ash after combustion were characterized with X‐ray photoelectron spectroscopy (XPS). A re‐burning experiment was carried out in a 5 t/day CFB pilot facility. The key is to build a balance between sufficient residence time to burnout and a proper fluidization velocity for bed material cycle. The effects of residence time, preheating temperature, and the ratio of preheating air to fuel on the combustion efficiency and pollutant emission was studied. The balance between the preheating time and residence time in the furnace was discussed. Results show that it can reach stable combustion in a specially designed CFB furnace without auxiliary fuel and the combustion efficiency can be higher than 98 %.

Experimental methods in chemical engineering: Thermogravimetric analysis—TGA

“…The keywords in the second and third most cited articles (“Alkanethiolate Gold Cluster Molecules with Core Diameters from 1.5 to 5.2 nm: Core and Monolayer Properties as a Function of Core Size” and “Chemical Oxidation of Multiwalled Carbon Nanotubes”) touch on the red cluster: nanoparticles and nanotubes . In the seven articles published by The Canadian Journal of Chemical Engineering in 2016 and 2017 researchers grafted poly(methyl methacrylate) from the surface of CNCs using aqueous, one‐pot, free radical polymerization (yellow and blue clusters); evaluated thermal behaviour of ground pine chip particles and ground pine pellet particles (yellow cluster); synthesized NiFe 2 O 4 spinel in solid state (green cluster); tested the combustion characteristics and activation energy of the semi‐char from an industrial circulating fluidized bed (yellow cluster); monitored effects of NiO nanosorbants on adsorption and post‐adsorption catalytic thermo‐oxidative decomposition of vacuum residue asphaltenes (green and magenta clusters); evaluated the fuel properties and the reaction characteristics of two different oil shales and sub‐bituminous coal by non‐isothermal thermogravimetric analysis (yellow and green cluster); and analyzed the reaction model and dynamic reaction characteristics of limestone thermogravimetrically (yellow cluster) …”

Experimental Investigation on NOx Generation Characteristic and Burnout Performance of Co-Combustion of Carbon-Based Solid Fuels under Deep-Staged Combustion

“…4−7 Hence, some solutions have been proposed to realize the efficient utilization of such low-volatile fuels by combustion or co-combustion of semicokes. Some scholars have indicated that semi-coke could be burned in a circulating fluidized bed boiler, 8,9 and other researchers have put forward that semi-coke could be preheated before burning in a boiler. 10−12 However, new equipment and/or much reconstruction are necessary for the boiler to fire semi-cokes.…”

“…However, the pyrolyzed and gasified semi-cokes usually have ultra-low volatile content, with high NO x emission and poor combustion performance during combustion. − Hence, some solutions have been proposed to realize the efficient utilization of such low-volatile fuels by combustion or co-combustion of semi-cokes. Some scholars have indicated that semi-coke could be burned in a circulating fluidized bed boiler, , and other researchers have put forward that semi-coke could be preheated before burning in a boiler. − However, new equipment and/or much reconstruction are necessary for the boiler to fire semi-cokes. Besides, the strategy of cofiring gasified and pyrolyzed semi-cokes with bituminous coal in a pulverized coal furnace of the power plant is a promising approach to efficiently and cleanly consume these carbon-based solid fuels with ultra-low volatiles in a large scale. , The pulverized-coal boiler has accounted for the largest proportion in China, and only limited equipment needs to be upgraded for co-combustion.…”

“…The combustion characteristics of semi-cokes have been studied by some researchers. ,,,− Kelebopile et al investigated the fly ash using a thermal gravimetric analyzer (TGA) and compared gasified semi-coke with coal chars produced by the drop tube furnace at various pyrolysis temperatures. The results proved that the reactivity of coal char was higher among two samples.…”

“…The results proved that the reactivity of coal char was higher among two samples. Ren and Bao studied the semi-chars using a TGA and a reburning experiment. According to the results, the combustion efficiency of more than 98% could be obtained without auxiliary fuel.…”