Elsevier

Combustion and Flame

Volume 213, March 2020, Pages 156-171
Combustion and Flame

Particle temperature and flue gas emission of a burning single pellet in air and oxy-fuel combustion

https://doi.org/10.1016/j.combustflame.2019.11.034Get rights and content

Abstract

Pelletization of biomass increases the bulk energy density and the uniformity in size and shape with minimized mechanical degradation from transport, storage, and handling. A pellet can be burned in industrial circulating fluidized-bed or fixed-bed furnaces. This experimental study examines the combustion behavior of single pine wood and empty fruit bunch pellets in a laboratory-scale entrained-flow reactor. Individual biomass pellets were oxidized under both N2/O2 (79%/21%) and CO2/O2 (79%/21%) to investigate the burning characteristics during oxy-fuel combustion. The gas temperature was set at 860 K, 970 K, and 1090 K to examine its effect on combustion behavior. The burning pellets were directly observed in a visualization window in the reactor throughout the sequential combustion process. Direct observation, optical pyrometry, and flue gas analysis are synchronized for each pellet to interpret the relation between phenomenological events, particle temperature, and flue gas emission. The experimental results shows an increased homogenous ignition delay and a conical, less sooty flame in the presence of CO2. The particle temperature under CO2/O2 atmosphere is slightly lower than that under N2/O2, especially for the volatile combustion. The CO emission under CO2/O2 is significantly higher and produced from incomplete combustion. These combustion characteristics are attributed to low oxygen diffusion and the high volumetric heat capacity of CO2. The pine wood displayed a fast, homogeneous ignition and a sooty flame with high NO and SO2 emissions compared to the empty fruit bunches.

Introduction

The global average value of atmospheric CO2, as the heat-trapping greenhouse gas, has increased from 316.4 to 413.3 parts per million between 1960 and 2019 [1], as the National Aeronautics and Space Administration reported in 2019. In the Paris agreement, 195 countries agreed to constrain the increase in the average global temperature to less than 2  °C relative to pre-industrial levels. Two technologies of interest for CO2 reduction from electricity generation plants are oxy-fuel combustion and biomass combustion.

Accordingly, the CO2 reduction technologies, namely pre-combustion capture, oxy-fuel combustion, and post-combustion capture, are currently used [2]. Oxy-fuel combustion is a technology that burns the fuel in pure O2 or CO2/O2 (recycled flue gas). Dissimilar combustion characteristics of solid fuel particles are obtained from the CO2 properties because of the higher molar heat capacity, the higher density and the lower oxygen diffusion under a CO2-based inlet gas [3,4].

Meanwhile, biomass combustion is a feasible method for CO2 reduction because biomass is a CO2-neutral energy source of heat and electricity. Also, IPCC reported that Bioenergy and Carbon Capture and Storage (BECCS) has been suggested as a key technology to attain the preferred target of 1.5 °C global temperature increase and of producing carbon-negative power by removing carbon dioxide [5].

However, biomass has varied burning characteristics because of its physical and chemical differences due to its fibrous nature, high compositional variability, low particle density, and irregular shape [6]. The non-uniform gas evolution profile, faster devolatilization rates, and shorter combustion time [7] are specified for the biomass material. Lignin, hemicellulose, and cellulose are generally accepted as the major components of a biomass material. The compositional variability affects combustion, where complicated chemical reactions occur. Furthermore, less sooty flame and shorter char combustion time when burning biomass materials can be attributed to lower radiative heat transfer [8,9].

A fundamental understanding of single biomass pellet combustion is necessary to quantitatively specify its burning characteristic under rapid heating rates. Particles in circulating fluidized-bed boilers can be burned completely in a few seconds to minutes depending on the size and shape. Compared to the low heating rate of a thermogravimetric analyzer, rapid heating rates and high temperature are the norm in practical applications [10].

Oxy-fuel combustion has been studied for its ignition characteristics, volatiles and char combustion behavior, and flue gas emission [11], [12], [13], [14], [15], [16]. The ignition delay under CO2/O2 conditions has attracted much interest. Aria et al. [11] reported that the ignition temperature increased when N2 was replaced by CO2. The ignition temperature increase with O2 was smaller than that under CO2/O2 because of the O2 diffusivity. Khatami et al. [12] observed that the sequential homogenous combustion mode was generally observed under oxy-fuel combustion. Shaddix et al. [13] measured the longer devolatilization duration on pulverized coal under CO2/O2 atmosphere. The conversion rate for Fuel-N to NOx under CO2/O2 was smaller than that under N2/O2 [14]. The increase of the O2 concentration under CO2/O2 promoted NOx emission during both volatile and char combustion [15]. Watanabe et al. [16] previously reported a mechanism for NOx formation and reduction under CO2/O2 conditions. Nevertheless, the integrated analyses of the abovementioned combustion behaviors are limited in explaining the characteristics of oxy-fuel combustion. These characteristics are interconnected with the combustion phenomenon, particle temperature, and flue gas emission.

This experimental study specified single-pellet combustion of pine wood and empty fruit bunches (EFB) under CO2/O2 and N2/O2 conditions in a well-controlled entrained-flow reactor. A direct observation method, which allowed a clear view of the apparent volatile ignition and flame, was used to characterize comparable ignition delay, physical changes of the volatile flame and char structure under CO2/O2. The synchronized method also gave a deeper insight into the burning characteristics such as the sooty flame, the particle temperature and the flue gas emission for the volatile and char combustion.

Section snippets

Preparation of the biomass samples

Figure 1 shows the samples of the raw and pulverized pine wood and EFB prepared for this experimental study. The pine wood was logged in Gyeongsang Province, South Korea, while the EFB (coconut husk) was imported from Malaysia. Table 1 presents the proximate and ultimate analyses of the samples based on an “as received” and “dry, ash-free” basis, respectively, obtained from a TGA-701 thermogravimeter and TruSpect elemental analyzer. The raw pine wood contained a little higher mass fraction of

Phenomenological observation of burning pellets under CO2/O2 and N2/O2 conditions

The burning behaviors of the pine wood and the EFB herein apparently had different flame structures, although they had similar ultimate analyses. Previous research has reported the burning of biomass particles from phenomenological observation [24,25]. Figures 7– and 8 clearly show the combustion phenomena, including ignition, volatile combustion, and char oxidation. The representative images were obtained from each individual burning particle under N2/O2 and CO2/O2 conditions and at three

Conclusions

This experimental study presented the burning behavior of single biomass pellets under CO2/O2 and N2/O2 at varying gas temperatures and rapid heating rates in a laboratory-scale entrained-flow reactor. In relation to uniformity in particle size and density, the in-situ pelletization process was introduced because the physical properties affected the ignition delay and the burnout time. The well-controlled injection and gas temperature also allowed the accurate description of the sequential

Declaration of Competing Interest

None.

Acknowledgments

This work was supported by the International Energy Joint R&D Program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant, funded by the Korea Government Ministry of Trade, Industry, and Energy (Project No. 20161110100090).

References (33)

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