Mathematical modelling of Collie coal pyrolysis considering the effect of steam produced in situ from coal inherent moisture and pyrolytic water

doi:10.1016/j.proci.2008.06.094

Proceedings of the Combustion Institute

Volume 32, Issue 2, 2009, Pages 2675-2683

https://doi.org/10.1016/j.proci.2008.06.094 Get rights and content

Abstract

Low-rank coals have high inherent moisture content, and high yield of pyrolytic water formed from the functional groups in the coal during pyrolysis. Steam produced from coal moisture can lead to significant in-situ char gasification during pyrolysis under conditions where there are strong and prolonged interactions between the char and the in-situ steam. This phenomenon can be encountered in many practical processes yet is often overlooked in pyrolysis modelling. Using Collie coal as an example, this study presents a mathematical model on low-rank coal pyrolysis, considering primary and secondary coal pyrolysis reactions, heat transfer, mass transports, as well as char gasification and volatiles cracking/reforming by steam produced in situ during pyrolysis due to both coal inherent moisture and pyrolytic water. Under conditions where there are strong and prolonged interactions between the pyrolysis products (particularly char) and steam produced in situ from the coal inherent moisture and pyrolytic water, lower char yields from both raw and demineralised coal are predicted, in agreement with the experimental data, under the current experimental conditions. This effect increases with increasing coal inherent moisture content, but decreases with increasing particle size due to the slower heating rate incurred during pyrolysis and the pore diffusion effect during char gasification for the large particles. Volatile steam reforming competes with char gasification for available steam, especially at high temperatures and for large particles. Experiments as well as model prediction also clearly demonstrate that char steam gasification reactivity data should be obtained from experiments carried out on the in-situ basis as the in-situ reactivity is significantly higher than that on the ex-situ basis.

Introduction

Understanding coal pyrolysis is essential for the design and operation of various coal utilisation processes [1], [2]. Naturally, a pyrolysis model is an important step in developing any overall process models. The present contribution aims to develop a predictive model for pyrolysis of Collie coal, a Western Australian sub-bituminous coal, building on recent advances in our research on this coal [3], [4]. It was found that transport effects play an important role in the pyrolysis of large particles [3]. Moisture in coal can cause significant in-situ char steam gasification during pyrolysis [4] under conditions where the coal/char experiences prolonged interactions with the steam, leading to a much lower char yield.

The present model for predicting char yield during Collie coal pyrolysis incorporates both primary and secondary volatile reactions, heat and mass transports, and takes into account the char gasification by the in-situ steam during pyrolysis due to the coal moisture (both inherent moisture and pyrolytic water). The char in-situ steam gasification process is complicated by the volatile cracking and steam reforming in the reactor [5], [6], the latter affecting the steam availability for the char gasification. A coal pyrolysis model is first developed, followed by a char steam gasification model. Both models are then integrated, taking into account the volatile cracking and reforming, to model the in-situ steam gasification of char during pyrolysis. Steam gasification reactivity data should be based on the in-situ gasification of nascent char from pyrolysis. However, most literature data were based on ex-situ experiments, that is, the char is cooled to room temperature after pyrolysis and reheated for gasification [7], [8]. In this study, experiments were carried out to examine the difference between char gasification on both in-situ (the char gasification commences immediately following the pyrolysis) and ex-situ bases, and derive reactivity data as model input.

Section snippets

Pyrolysis model

Coal pyrolysis is modelled by the single-reaction approach similar to previous work on large particle pyrolysis modelling [9], [10], [11]. The ultimate volatile yields are taken from fast-heating pyrolysis experiments of pf-sized (90–106 μm) particles using a drop-tube/fixed-bed reactor [4] having features similar to those used by Hayashi and co-workers [5], where the dried coal was fed in one pulse into the reactor with minimised contact between the char and volatiles.

All gaseous volatile

Experimental

Collie coal typically has proximate analysis of 45.6% fixed carbon, 30.7% volatile matter, 3.7% ash, 20% moisture, and ultimate analysis on dry-ash-free (daf) basis of 76.14% C, 4.56% H, 16.61% O, 1.41% N, 1.28% S. For model validation later, previous results on Collie coal pyrolysis [3], [4] will be cited, together with some additional results from coal char steam gasification in this work.

Experiments were carried out on pf-sized raw and demineralised (demin) coal char at a steam partial

Experimental results as input data to the model

Figure 1 shows the steam gasification results for the pf-sized particles. For raw coal char at 900 °C, the rate of ‘in-situ’ gasification is markedly higher than that of ‘ex-situ’ gasification, probably due to greater thermal annealing as well as some morphological and microstructural changes in the ex-situ char [23]. In both sets of experiments, there was a 1-min holding time following coal feeding before gasification commenced; other experiments were also carried out where the coal was

Conclusions

A model for low-rank coal pyrolysis has been developed and applied to Collie coal, with consideration for char gasification and volatile reforming by in-situ steam from coal moisture (pyrolytic water and inherent moisture). The present model can serve as a framework for low-rank coal pyrolysis in general, where certain parameters (especially char gasification kinetic parameters) may be needed for other low-rank coals studied. The agreements between the model predictions and experimental data on

Acknowledgments

The authors gratefully acknowledge the financial and other support from the Cooperative Research Centre for Coal in Sustainable Development (CCSD), established and supported under the Australian Government’s Cooperative Research Centre program.

References (28)

N.M. Laurendeau
Prog. Energy Combust. Sci.
(1978)
P.R. Solomon et al.
Prog. Energy Combust. Sci.
(1992)
K.M. Bryden et al.
Biomass Bioenergy
(2002)
K.S. Vorres et al.
Fuel
(1992)
I.W. Smith et al.
Fuel
(1972)
B. Bayarsaikhan et al.
Fuel
(2005)
K. Hashimoto et al.
Fuel
(1986)
F.F. Peng et al.
Fuel Process. Technol.
(1995)
P. Samaras et al.
Carbon
(1991)
H. Wu et al.
Fuel
(2002)

X. Li et al.

Fuel

(2004)

H. Wu et al.

Fuel

(2005)

C-Z. Li

Fuel

(2007)

J.B. Howard

Cited by (23)

Effects of FeS<inf>2</inf> on the process of coal spontaneous combustion at low temperatures
2020, Process Safety and Environmental Protection
Citation Excerpt :
The sulfur element in coal is primarily discharged as sulfur dioxide (SO2), hydrogen sulphide (H2S), and carbon sulphide (COS) during combustion. Therefore, if present, not only can FeS2 cause considerable economic losses but also affect the local environment and cause severe ecological harm (Black and Craw, 2001; Yani and Zhang, 2010; Yip et al., 2009; Xue et al., 2020). Pietrzak et al. (2007) reported that FeS2 had a promoting effect on the oxidation rate of coal.
Spontaneous combustion of coal has become an important disaster that threatens the safety of coal mines. FeS₂ is the main component of pyrite, which is suspected to be a major contributor to coal spontaneous combustion (CSC). So, it has important significance to FeS₂ on the characteristics of coal oxidation for prevention and treatment. This study used coal samples mixed with different proportions of FeS₂ (2.0 mass%, 4.0 mass%, and 6.0 mass% mass percentage) were tested to investigate the characteristics of spontaneous combustion, as compared with the fresh sample. The CO and CO₂ production rates, critical temperature, and dry cracking temperature during oxidation were analyzed. The temperature-programmed experiments was conducted to simulate low-temperature oxidation processes realistically. In-situ infrared spectroscopy was used to appraise the evolution of low-temperature (< 200.0 °C) oxidation of aromatic hydrocarbons, aliphatic hydrocarbons, and oxygen-containing functional groups on the surfaces of the samples. Experimental results showed that FeS₂ exhibited a strong influence as the temperature exceeded the dry cracking temperature. Adding 2.0 mass% and 4.0 mass% FeS₂ showed a promoting effect on low-temperature oxidation; however, FeS₂ became an inhibitor which reached 6.0 mass%. Furthermore, adding 2.0 mass% FeS₂ showed the strongest promoting effect. From a microscale perspective, the promoting effect of FeS₂ on coal oxidation was due to thermal release from FeS₂ oxidation, enhancing the reaction between aliphatic hydrocarbons, such as methyl groups and methylene, and oxygen. Due to the reaction of FeS₂, the acidic environment was conducive to the hydrolyzation of lipid structures into highly active alcohol or phenol structures, which in turn promoted the oxidation of the coal sample. These results are crucial for understanding the mechanism underlying the influence of FeS₂ on CSC and mine safety protocols.
Occurrence and characteristics of abundant fine included mineral particles in Collie coal of Western Australia
2018, Fuel
Citation Excerpt :
It is an important cheap energy source for power generation in stationary coal-fired power stations. Over the years, there have been scattered studies on the properties of Collie coal and the reactions of coal organic and inorganic matter during thermochemical processing [18,21–26]. One interesting observation is that abundant fine ash particles are produced from Collie coal combustion, playing important roles in ash formation, deposit initiation and growth in the boilers of WA coal-fired power stations burning pulverised Collie coal [18,22].
Collie coal is the only coal being mined in Western Australia and plays an important role in supplying cheap energy to the State’s energy mix. This study reports a systematic investigation into the occurrence and characteristics of the fine mineral matter in Collie coal that is crushed to the size of <212 µm and density separated. The results show that 94.1 wt% of the mineral matter in the whole Collie coal is present in the three density fractions of 1.2–1.4, 1.4–1.6 and ≥2.0, including 26.8 and 58.6 wt% as included mineral matter in two density fractions with specific gravities of 1.2–1.4 and 1.4–1.6, and 8.7 wt% as excluded mineral matter in the density fraction of ≥2.0. Further analysis of these key Collie coal density fractions using computer-controlled scanning electron microscopy (CCSEM) shows that 54.13 wt% of the total mineral matter in the whole Collie coal are fine mineral particles of sizes <10 μm. These included fine mineral particles (<10 μm) are distributed as 20%:80% in the two coal density fractions of 1.2–1.4 and 1.4–1.6, which contribute to 10.64 and 43.49 wt% of the total mineral matter in the whole coal, respectively. CCSEM analysis also shows that the key minerals in the fine mineral particles are quartz, kaolinite, K-, Ca- or Fe-Al silicates and pyrite-related minerals. However, large proportions of these included fine mineral particles have “unclassified” mineral phases. Such unclassified particles contribute to 43.88 and 36.84 wt% of the fine mineral particles with size <10 µm present in the coal density fractions of 1.2–1.4 and 1.4–1.6 (equivalent to 4.67 and 16.02 wt% of the total mineral matter in the whole coal), respectively. The chemistry data show that these individual fine mineral particles (<10 µm) contain mixed compositions of Si + Al, Fe + Ti and other elements such as Ca, K, S and P, suggesting that one single fine mineral particle contains multiple mineral phases.
Pyrolysis of a lignite briquette – Experimental investigation and 1-dimensional modelling approach
2018, Fuel
Citation Excerpt :
While the previous studies have provided an understanding of the reactions within a small particle [5], little is known about the complicated inter-influences between pyrolysis reactions and physical processes (heat and mass transfer) within a coal briquette. Some research has also been carried out on the pyrolysis of Collie coal briquette in Western Australia, with a focus on the production of metallurgical reductant requiring a mechanical strength of 6.9–30 MPa and a reactivity index of 2.6–28% towards CO2 [6,7]. However, the results are not directly applicable to Victorian brown coal which has a much higher volatile yield and lower strength.
A laboratory-scale shaft furnace was used for the pyrolysis of the lignite briquette together with thermogravimetric analysis to study the intrinsic pyrolysis kinetics under two different heating rates, 10 °C/min and 100 °C/min. Apart from coal conversion rate, the tar yield, quality and radical concentration in char were also measured to explore the difference between lignite briquette and the respective pulverised powder. Additionally, a 1-D model coupled with the chemical percolation devolatilisation (CPD) code was developed to quantitatively understand heat transfer using temperature dependent parameters; product distribution and yields, and the pyrolysis mechanism. It was discovered that heat transfer was the limiting factor for pyrolysis of the lignite briquette, which subsequently lowered the heating rate and led to increased cross-linking and decreased tar production compared to a coal particle. Simultaneously, the primary tar also underwent internal cracking and even deoxygenation to decompose into light aromatics and gases inside the briquette char matrix. Providing a hot gas environment was found to facilitate the cracking of tar species compared to a slow heating rate where tar is released at a lower temperature. The changes in radical concentration in the solid material were linked to the structural changes predicted by the CPD model including bridge-breaking, tar release and cross-linking phenomena.
In-situ steam reforming of biomass tar over sawdust biochar in mild catalytic temperature
2017, Biomass and Bioenergy
Citation Excerpt :
Kongvui et al. [39] suggested that the specific biochar reactivity with a thick char bed is considerably lower than that with a thin char bed, due to the reduced steam partial pressure throughout the char bed. This discrepancy in reactivity appears not to obey the simple m-order power law kinetic equation for steam gasification where the reactivity is a function of (steam concentration)m, with m mostly 0.4–0.6 for various chars [40,41]. From the viewpoint of reaction kinetics, the decreasing tar concentration will weaken the reactivity of in-situ tar H2O reforming over biochar with the thickness of the biochar layer increasing, just as the shown in k shown in Fig. 3.
In order to fully understand the catalytic activities of sawdust biochar on in-situ tar steam reforming in mild catalysis temperatures (650–800 °C), the experiment was carried out in a two-stage fluidized bed/fixed bed reactor. The structural characteristics of biochar were analyzed by the Raman and XPS spectroscopies. The results of in-situ tar steam reforming over biochar were performed by gas chromatograph/mass spectrometer (GC/MS). Kinetic aspects of tar steam reforming in mild temperatures were calculated to evaluate the catalytic effect of sawdust biochar on in-situ tar reforming. The results indicate that sawdust biochar has a positive effect on in-situ tar steam reforming, while the effect is not proportional to biochar amount. During the in-situ tar steam reforming over biochar, the changes of oxygen-containing functional groups on biochar surface are mainly caused by the C-O bonds. The first order kinetic rate constant of sawdust biochar for the heterogeneous reforming of biomass tar at 650–800 °C is found to have an apparent activation energy (E_app) of 35.03 kJ/mol with the apparent pre-exponential factor (k_app) of 1.8 × 10⁴ m³ kg⁻¹h⁻¹. The biochar structures were considerably transformed during the tar steam reforming. According to GC/MS analysis of biomass tar, the biochar seems to promote the cracking of large ring polyaromatic tar compounds to form 1-ring aromatic ones.
Underground in situ coal thermal treatment for synthetic fuels production
2017, Progress in Energy and Combustion Science
Citation Excerpt :
The heat capacity of liquid water is 3-4 times of dry coal [41] and 4 times of dry oil shale [42,43]. Moisture can also participate in pyrolysis reactions and affect the final pyrolysis products [44,45]. Therefore, moisture content is a critical property to be considered in the process design and economic analysis.
Underground coal thermal treatment (UCTT) is a promising concept that was recently proposed for extracting high-value hydrocarbon fuels from deep coal seams, which are economically unattractive for mining. UCTT is essentially an in situ pyrolysis process that converts underground coals into synthetic liquid and gaseous fuels, while leaving most of the carbon underground as a char matrix. The produced synthetic fuels have higher H/C ratios than coals. The remaining char matrix is an ideal reservoir for CO₂ sequestration because pyrolysis significantly increases the surface area of the char. The UCTT concept is relatively new, and there is little research in this area. However, underground oil shale retorting, which is also an in-situ hydrocarbon fuels conversion process, shares key features with UCTT and has gained momentum in demonstration and commercial development. As such, there is a large body of literature available in this area. A review of the studies on underground oil shale retorting that are closely related to UCTT will shed light on the UCTT process. This paper presents a review of the recent literature on underground oil shale retorting that are most relevant to UCTT process. The review provides a background to the reader by comparing the properties of coal with oil shale, with an emphasis on the feasibility of applying oil shale retorting techniques to UCTT process. The review further discusses the coal and oil shale conversion issues and uses the knowledge of the latter as guidance for the development of UCTT. Published data on pyrolysis of large coal blocks at conditions relevant to UCTT process is scarce. Therefore, literature on conventional coal pyrolysis is reviewed for optimization of the UCTT process. Despite the abundant studies on pulverized coal pyrolysis, there are still many open questions on whether they can be directly applied to UCTT. A comparison of the unique environment of UCTT with conditions of conventional pulverized coal pyrolysis clearly shows there are knowledge gaps. Future research needs are then proposed to close these gaps.
Effect of moisture on dehydration and heat transfer characteristics of lignite in low temperature carbonization furnace
2016, Fuel Processing Technology
Dehydration of lignite primarily occurs in two stages, namely, removal of surface moisture and decrease in oxygen-containing functional groups and moisture stage. Moisture on the lignite surface could be removed at temperatures below 100 °C. However, parts of oxygen-containing functional groups were removed first followed by the removal of the adsorbed moisture at temperatures in the range 100–300 °C. Dehydration time increased with an increase in the dehydration load and heat transfer distance. At surface moisture of 7 wt.%, temperature of lignite increased rapidly. Higher temperature of combustion chamber yielded shorter removal times for surface moisture and oxygen-containing functional groups. Based on the rate of increase of lignite temperature and associated heat transfer process, coking chamber could be divided into three zones: the near, middle, and remote zones. The near zone temperature curve was determined to be a convex function $(\frac{\partial^{2} T}{\partial τ^{2}} \leq 0)$ ; however, the middle zone temperature curve was determined to be a concave function $(\frac{\partial^{2} T}{\partial τ^{2}} \geq 0) \cdot \frac{\partial^{2} T}{\partial τ^{2}} = 0$ and $\frac{\partial T}{\partial τ} = k$ were the boundary indices of the middle and remote zones. 100 °C isothermal surface movement rate was found to be affected by the heat transfer capacity and dehydration load. Center temperature distribution of coking chamber was made uniform by steam, and temperature of HM_7% was the most uniform distribution.

View all citing articles on Scopus

¹: Present address: Centre for Petroleum, Fuels and Energy (M050), The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia.

View full text

Mathematical modelling of Collie coal pyrolysis considering the effect of steam produced in situ from coal inherent moisture and pyrolytic water

Abstract

Introduction

Section snippets

Pyrolysis model

Experimental

Experimental results as input data to the model

Conclusions

Acknowledgments

Prog. Energy Combust. Sci.

Prog. Energy Combust. Sci.

Biomass Bioenergy

Fuel

Fuel

Fuel

Fuel

Fuel Process. Technol.

Carbon

Fuel

Fuel

Fuel

Fuel