Combustion characteristics of lignite char in a fluidized bed under O2/N2, O2/CO2 and O2/H2O atmospheres

Abstract

Oxy-fuel combustion, O2/H2O combustion has many advantages over O2/CO2 combustion, and has gradually gained more and more attention recently for carbon capture from coal-fired power plant. The unique physicochemical properties (thermal capacity, diffusivity, reactivity) of H2O will definitely influence the coal combustion process. In this work, the combustion characteristics of lignite char were investigated in a fluidized bed combustor under O2/N2, O2/CO2 and O2/H2O atmospheres with different oxygen concentration (15 %-27 %) and bed temperature (837-937 o C). Results indicated that the average reaction rate (raverage) and the peak reaction rate (rpeak) of lignite char in O2/H2O atmosphere was lower than that in O2/CO2 atmospheres at low oxygen concentrations. However, as the oxygen concentration increased, the rpeak and raverage in O2/H2O atmosphere was significantly improved and exceeded that in O2/CO2 atmosphere. The calculation of the activation energy based on the shrinking core model showed that the order of activation energy under different atmospheres is: O2/CO2 (28.96 kJ/mol) > O2/H2O (26.11 kJ/mol) > O2/N2 (23.31 kJ/mol). Furthermore, with the increase of the bed temperature, the active sites occupied by gasification agent were significantly increased, the active sites occupied by oxygen decreased correspondingly.

Experimental study of a single char particle combustion characteristics in a fluidized bed under O2/H2O condition

“…There is a competitive relationship between oxidation and gasification of char, and the strength of the competition may be related to bed temperature, oxygen concentration and fuel type, etc. [29,32]. Otherwise, interestingly, as the oxygen concentration increases, the T p in O 2 /H 2 O combustion increased more significantly than that in other atmospheres.…”

“…The char was prepared in a mini-bubbling fluidized bed combustor, because the reactor can be heated and cooled at a faster rate and is easy to operate. The detailed description of the reactor can be found elsewhere [28,29]. The bed material was quartz sand (size: 0.3-0.35 mm), and the fluidization number was set to 3.…”

“…It should be pointed out that the Biot number of char in dense phase and freeboard was close to 0.1 (at 800°C, d p = 6 mm, 0.158 and 0.108, respectively), thus, the particle temperature may be considered to be approximately uniform. Herein, the gasification endothermic reaction between gasification agent (H 2 O or CO 2 ) and char will reduce the particle temperature in oxy-fuel combustion, and the effect of gasification is more significant at high oxygen concentrations due to the high particle temperature [29,30]. Also, the specific heat capacities of CO 2 and H 2 O are higher than that of N 2 , which is supposed to bring a higher thermal convection in h total .…”

Effect of O2 concentration on combustion behavior and kinetics of coal gangue during oxy‐fuel combustion and oxy‐steam combustion

“…Furthermore, a zero‐emission combustion technology (oxy‐steam combustion), which is regarded as a promising next generation of oxy‐fuel combustion technology, 8–10 was presented by Canada Centre for Mineral and Energy Technology in 2007. In this case, the fuel is burned in an O 2 /H 2 O atmosphere, 9,11 and due to that the flue gas recycle system is not required, so the energy consumption associated with this option can be further reduced 12 …”

Investigation of Applicability of Polyimide Membranes for Air Separation in Oxy-MILD Zero-Emission Power Plants

“…Among them are CO2, N2, H2O, SOx, and NOx, as well as volatile organic compounds. In addition, particulate matter, formed due to the imperfect oxidation of coal, which is commonly lifted with the above-mentioned gases, should be listed from the outset [1,2,3,4]. Due to their toxic influence on humans and the natural environment, a number of such compounds should be separated from the stream of exhaust gases before its emission to the atmosphere.…”