Thermophysical and Thermochemical Behaviour of Coal and Biomass during Chemical Looping Combustion

Abstract

Thermal power plants burning fossil fuels are the major anthropogenic sources of carbon dioxide emissions into the atmosphere. Chemical Looping Combustion (CLC) is a promising fuel conversion technology for inherent carbon capture with a low energy penalty. The present way of using pulverized coals in a fluidized bed (FB)-CLC has drawbacks like loss of unconverted char and gaseous combustibles. The utilization of large solid fuel particles (in mm-sizes) potentially overcomes these problems and also reduces the energy involved in size reduction. The thermophysical and thermochemical changes involved during the conversion of these large-sized particles are large in magnitude and of greater significance. Thus, they along with the fuels’ thermochemical changes become critical inputs for the effective design of process equipment. This study is aimed at (i) gaining a qualitative understanding of the progressive thermophysical and thermochemical changes during fuel conversion and (ii) quantifying the influence of various operating parameters on the same. Thermochemical changes, namely devolatilization, char conversion, carbon transformations, and the thermophysical behaviour in terms of primary and secondary fragmentation, shrinkage, and microstructural changes are studied using single-particle experiments in fluidized bed insitu-gasification CLC conditions. Two types of Indian coals, one type of Indonesian coal and one type of carbon-neutral biomass (fuel wood), of three different sizes in the range of +8-25 mm are used in this study. Natural hematite is the oxygen carrier bed material used (in the size range of +250-425 μm), with steam as the fluidization-cum-gasification agent at 2.5 times the minimum fluidization velocity. The experiments are conducted at three different bed temperatures of 800, 875 and 950 oC. This work is comprised of six different experimental programs, viz. (i) development of a new method to determine devolatilization time in flameless FB-CLC conditions, called ‘Colour Indistinction Method (CIM)’, (ii) devolatilization and char yield experiments, (iii) experiments of primary fragmentation during devolatilization (iv) char conversion and char fragmentation (secondary) experiments, (v) char reactivity experiments using thermogravimetry, and (vi) char structural analysis using instrumental techniques. CIM is developed based on the observation of particle disappearance in the bed at the end of devolatilization and validated using standard diagnostic methods such as residual-volatile measurements and particle-centre temperature profilometry. CIM produced reliable results within the error range of -7.57 to +3.70 %. The devolatilization experiments revealed that the larger particles have a relatively lower amount of volatile release. However, increasing the bed temperature enhances the volatile release rate as well as the quantity of release (up to 12% in coals; 30% in biomass). With the decrease in sphericity (seen in flake coal particles), a maximum of 56% reduction in devolatilization time is noticed. A correlation for determining devolatilization time under the CLC environment is developed, with a coefficient of determination of 0.95. Char yield is found to be strongly influenced by operating bed temperature, but it is a weak function of particle size and shape. Shrinkage in biomass is witnessed for all sizes, with an effective reduction of 31-52% in initial particle volume. Char conversion times of fuels increase by 60 to 170% when particle size is increased by 2 to 2.5- folds, while an increase in bed temperature by 150 oC caused a reduction of 42 to 86%. It is also understood that if the fixed carbon content is higher than the ash content in fuel, intensive fragmentation occurs and brings down the char conversion time. Primary and secondary fragmentation phenomena are quantified using various indicators such as probability of fragmentation events, frequency and timing of fragmentation, number of fragments, fragmentation index and particle size distribution of fragments at different residence times. The intensity of primary fragmentation increases with the increase in particle size and bed temperature, while it decreases with the increase in compressive strength. Only a maximum of 60% of the tested particles undergo fragmentation, irrespective of fuels. High-volatile Indian coal and biomass, respectively, are the most and least susceptible fuels to primary fragmentation irrespective of particle size and bed temperature. Indian coals are found to fragment in the earlier stages of conversion, thus becoming a dominant factor in shortening the overall fuel conversion time. Unlike during devolatilization, the largest sized particles of all the tested fuels experience secondary fragmentation. Among the different bed temperatures studied, 950 oC is found to be the most favourable for char conversion and fragmentation. Regardless of fuel type and feed size, the inception of char fragmentation is noticed in the very first quarter of conversion time, indicating its substantial effect on the char conversion time, and therefore, it becomes necessary to carefully incorporate this size reduction with respect to time in the char conversion models. Percolative mode of fragmentation is noticed in the final quarter of char conversion, except for high-ash Indian coal particles. A minimum critical char size exists below which char weakening does not yield breakage, whose values vary between 4.4 and 14.2 mm, depending on fuel type and feed size. Fuel type is found to be the prime influencer of fuel conversion and comminution phenomena, followed by particle size and operating bed temperature. This study establishes that large fuel particles up to 25 mm can be used in CLC systems without any prior size reduction, except in the case of high-ash Indian coal. Isothermal char reactivity studies using TGA show that samples exhibit high reactivity if char preparation is done at low temperatures for high-volatile fuels and at high temperatures for low-volatile fuels. Peak reactivity is noticed during the initial stages of char conversion regime for all coals and in later stages for biomass samples. Char micrographs show mesoporous char formation with pore size of about 2-4 nm in all fuels, during the course of char conversion. Electron dispersive studies indicate that the high volatile Indian coal retains Ca throughout the conversion period, whereas biomass chars retain the catalytic species like K and Ca. Raman spectroscopic analyses show that graphitic carbon structures are selective towards the steam atmosphere, while defective carbon structures are relatively more selective towards CO2.