1. Introduction
Coal, one of the primary fossil fuel sources, plays an important role in China, owing to the abundant reserves and its competitively low price compared to natural gas and oil [
1]. Direct combustion as a main manner of the utilization of coal, which suffers a high energy penalty for carbon capture and a low energy efficiency [
2], requires further improvement. Recently, pulverized coal has been widely used in steel and power plants for improving the burnout of coal, although this technology suffers from separation, crushing, grinding from the raw coal, and pipe blockage issues during the transportation of the pulverized coal [
3,
4,
5,
6,
7]. Moreover, the need for reduced greenhouse gas emissions has spurred the development of clean coal technology, such as chemical looping combustion (CLC) [
8], integrated gasification combined cycle (IGCC) [
9], multi-stage coal gasification (MSCG) [
10], and so on. These innovative technologies based on the pyrolysis and gasification of coal have been applied in the manufacturing industry. However, pyrolysis, or devolatilization, as the primary process during coal gasification and combustion, plays a key role in determining the gaseous production and carbon structure, which causes an inevitable effect on subsequent processes [
11,
12]. Li et al. [
13] reported the transformation of aggregate structure for low-rank coal by in-situ X-ray diffraction (XRD) and thermogravimetric analysis-mass spectrometry (TG-MS). They found that the changes of structure parameters were associated with the release characteristics of gaseous products. Nassini et al. [
14] showed that the layer structure was observed clearly in high pyrolysis temperature, compared with parental coal, in which the structure seems more disordered. Wen et al. [
15] claimed that the decrease in specific capacity was related to the evolution of volatile matter. Lee et al. [
16] examined the pore structure variation of coal char during pyrolysis and concluded the higher surface area and better dispersion of ash phase leads to a higher combustion reactivity of the Shievee Ovoo coal (SOC) char. More recently, Bhoi et al. [
17] and Gao et al. [
18] adopted ReaxFF molecular dynamics simulations to investigate coal pyrolysis at a microscopic view. Meanwhile, the pyrolysis, as part of the coal conversion technology, can extract high-value parts in coal to the gaseous fuel, liquids, and the coke. Therefore, it is essential to comprehensively understand the behaviors of pyrolysis.
A few models have been proposed for simulating the pyrolysis of large coal particles, with a particular focus on heat transfer processes coupled with the devolatilization kinetics. Agarwal et al. [
19] studied extensively on devolatilization models for coal combustion processes. Fu et al. [
20] developed a general devolatilization model for large coal particles, and they concluded that the kinetics parameters of coal only depended on the final temperature of coal, rather than the coal type. This model was later employed by Wan et al. [
21] and the devolatilization characteristic of coal, biomass, and coal–biomass blends were obtained. However, these models mentioned above usually assumed the intraparticle heat transfer to be the rate-controlling mechanism without considering the effect of mass transfer.
Researchers have made a lot of effects on the kinetic numerical model of coal pyrolysis. A single-equation kinetic model was developed by Badzioch et al. [
22], and then a two-parallel reaction kinetic model was put forward by Conesa et al. [
23]. More recently, Samuele et al. [
24] described coal devolatilization with a multi-step kinetic model, which refers to about 30 reactions and lumped species. Chern and Hayhurst [
25,
26] studied the pyrolysis of large particle coal and small granular coal with an improved nuclear condensation reaction model and first order reaction model, respectively. In addition, some sophisticated models, such as the functional group-depolymerization vaporization crosslinking (FG-DVC) model [
27], the chemical percolation devolatilization (CPD) model [
28,
29] and the distributed activation energy model (DAEM) [
30,
31] were proposed to describe the precipitation of gas products and the formation of tar. For both the FG-DVC model and the CPD model, the relationship between coal category and model parameters is difficult to determine. In contrast to the FG-DVC model and the CPD model, the DAEM model is relatively simpler, and it is also commonly used to predict the pyrolysis behavior of large coal particles [
32]. The DAEM model assumes that the pyrolysis is a combination of a series of parallel chemical reactions, and the activation energy of the reaction is expressed by the Gauss distribution function. The relevant experimental data provided by Solomon et al. [
33] and Sadhukhane et al. [
7,
34] showed that this model is in good accordance with the practical process.
Previous studies [
20,
21,
35] have accurately predicted the temperature distribution and volatile matter release during coal pyrolysis, with the particles size ranging from 3 mm to 16 mm. In respect to large particle sizes, e.g, more than 20 mm, which can be accepted by pyrolysis or gasification in the fixed/moving bed [
36] and Lurgi–Spuelgas (L–S) gasifier, they are still under investigation. Wu et al. [
37,
38] have reported the rotary hearth furnace used in the pyrolysis technology with the particles sized between 10 mm and 100 mm. Although much work has been carried out on coal pyrolysis behaviors, so far there are only a few studies focusing the interaction between heat transfer and volatile yield during pyrolysis of large coal particles. For large coal particles, heat transfer is crucial to the coal pyrolysis. Due to the large particle size and low thermal conductivity of coal, a larger temperature gradient exists inside the particle, and heat conduction is the limiting factor for coal pyrolysis [
26]. In our previous studies, we have developed a comprehensive model for coal pyrolysis coupled with heat transfer inside the particle, considering the heat effect of pyrolysis reaction and convective heat transfer due to volatile matters release [
39,
40]. However, the volatile matters were regarded as a substance for simplification. The composition of volatile matter is complicated, which is dependent on the original coal sample, heating rate, and final decomposition temperature. In order to describe the kinetics of volatile components release, Merrick [
41] proposed a model that included a system of parallel first-order reactions, where the composition of the volatile matter was defined in terms of the following nine species: CH
4, C
2H
6, CO, CO
2, tar, H
2, H
2O, H
2S, and NH
3. However, Merrick’s model omitted the heat transfer inside the coal particles and the model is usually applied to small particles.
The aim of this study is to extend our previous research work [
39,
40] and develop an improved comprehensive model, combining the kinetics model of volatile composition release and the heat transfer model, and investigate the interaction between volatile yield and heat transfer during pyrolysis of large coal particles in the moving bed. In the improved model, the volatile matters were assumed to be a mixture of nine components. The pyrolysis was simulated for large coal particles, up to 60 mm, during heating for an hour, and the effects of particle size on the heat transfer, as well as volatile component yields, were investigated and discussed.
4. Conclusions
In this article, an improved comprehensive model, combining the kinetics model of volatile composition release with the heat transfer model, was developed. In the proposed model, the volatile matters were assumed to comprise nine components. The pyrolysis of large coal particles (Maltby coal particles with diameters ranging from 20 mm to 60 mm) during heating for an hour (temperature linearly increasing from 150 °C up to 750 °C, and heating for an hour at the rate of 10 °C/min) was simulated. The effects of particle size on the pyrolysis and temperature variation were examined. Major conclusions can be drawn as follows.
(1) Due to the low thermal conductivity of coal, a large temperature difference (up to 67 °C for the particle with a diameter of 20 mm) exists between the external layer and core at the initial stage, while the temperature is more uniformly distributed in the inner region near the core.
(2) The volatile yield of the particle increases after the coal temperature rises higher than 350 °C, and has a sharp gain within the temperature range of 450–520 °C. The peak of volatile release rate corresponds to about 485 °C, which is mainly attributed to the fast release of tar and H2O. When the temperature rises up to 550 °C, the tar is almost completely released approaching the final yield of 12.49%.
(3) With the increase of particle size, more time is needed to raise the inner temperature of the large particle. For the particle with a diameter of 60 mm, the temperature difference between the surface and core increases up to 20 °C at the end of the heating for an hour, and the yield difference is 0.6%. Additionally, the volatile matter is released much later and achieves a slightly lower yield. To achieve a higher volatile yield, the heating time needs to be adjusted to larger coal particles.