Co-pyrolysis of coal and raw/torrefied biomass: A review on chemistry, kinetics and implementation

https://doi.org/10.1016/j.rser.2020.110189Get rights and content

Highlights

  • Less heat and mass transfer limitations during co-pyrolysis with torrefied biomass.

  • Torrefied biomass surface plays a catalytic role on volatiles during co-pyrolysis.

  • Co-pyrolysis reactivity is predicted from kinetic parameters of individual fuels.

  • Most efficient heat transfer in co-pyrolysis reactors is by conduction through sand.

  • Rotating cone reactor is a promising design for commercialization of co-pyrolysis.

Abstract

Thermochemical conversion via co-pyrolysis has the potential to be an efficient route for converting biomass to bio-energy and bio-refinery products. In this review, the implementation of co-pyrolysis of torrefied biomass and coal was critically assessed against co-pyrolysis of raw biomass and coal from both a fundamental and engineering perspective. This evaluation showed fundamental advantages for torrefaction of biomass prior to co-pyrolysis such as a decrease in mass and heat transfer limitations due to an increase in permeability and thermal conductivity of biomass. Co-pyrolysis volatiles may also be upgraded through the catalytic activity of the torrefied biomass surface, producing higher quality oil. Due to properties more similar to coal, torrefied biomass requires less energy for milling (lower operating costs) and can be more easily blended with coal in reactor feeding systems. A state-of-the-art research on co-pyrolysis kinetics revealed that reactivities of blends may be predicted from kinetic parameters of individual feedstocks using an additive approach. To conclude on the preferred reactor design for this process, different reactors were evaluated based on heat transfer mode, operation and product formation. Although both the fluidized bed and rotating cone reactor provide high oil yields, the rotating cone has been more successful commercially. This design shows great promise for specifically co-pyrolysis due to the intimate contact that may be achieved between fuels to maximize synergy. The co-pyrolysis of torrefied biomass and coal may be encouraged from a scientific point of view, however further research is recommended on the effective integration of torrefaction and co-pyrolysis technologies.

Introduction

The production of affordable, sustainable and clean energy is a cornerstone for global socio-economic growth [1]. The total world energy consumption is expected to rise a further 28% by 2040 [2]. Fossil fuels are a widely available energy source, however this source of energy production is associated with large amounts of greenhouse gas (GHG) emissions, a driver of climate change [3]. At COP25 2019, the urgency for countries to improve their emission reduction strategies was highlighted.

Considering the need that faces governmental agencies to decrease GHG emissions, quick implementation of mature green technologies is required. Technologies utilizing fossil fuels such as coal are well established, however one of the important contributions to GHG reduction from industry is the shift towards renewable feedstocks [4]. Patel and co-workers [3] recently reviewed the techno-economic and life cycle assessment of thermochemical conversion technologies and suggested that the implementation of biomass primarily depends on the cost competitiveness of biomass-based energy and chemicals compared to those derived from fossil fuels. They concluded that the cost of bioenergy-based technologies remains higher.

To stimulate the transition to bioenergy-based technologies, a reduction in process costs is required. This may be achieved by improving the efficiency of these technologies regarding energy and chemical production and GHG abatement [5]. To improve the existing industrial technology, a thorough understanding of thermochemical conversion processes is required. The pyrolysis process is the starting point of all these technologies, however stand-alone pyrolysis technology has also attracted wide attention [6]. This technology is being extensively developed in the bioenergy area for significant potential to co-generate energy and chemicals [7].

The scientific and industrial community’s interest in the pyrolysis process has increased significantly over the past few decades. The amount of scientific documents published on pyrolysis using either coal or biomass as feedstock for different years is shown in Fig. 1. It can be observed that research in both coal and biomass pyrolysis processes has increased significantly since the 1960s. The oil crises in the 1970s have intensified development in these areas, however since the late 1990s an exponential shift towards biomass pyrolysis research is evident with the signatures of conventions and protocols (Fig. 2). The increased awareness of environmental issues related to fossil fuel usage in the twenty first century and resulting calls for clean and renewable energy sources is the main reason for the observed shift in scientific interest [8]. Research in coal pyrolysis has become less popular compared to biomass pyrolysis in the twenty first century, however an increase is still evident in countries with high coal reserves such as China [9].

A viable option for industrial thermochemical conversion technologies to transition to renewable feedstocks is the co-utilization of lignocellulosic biomasses with coal as feedstock in existing coal-based processes [11,12]. For pyrolysis technologies, in particular, the co-utilization of biomass and coal has become an attractive option not merely due to a reduction in the carbon footprint of the overall process [13], but also due to the potential of producing higher oil yields with improved quality (composition closer resembling crude-oil) [14]. During co-pyrolysis, the hydrogen released from biomass stabilizes the large radicals produced from coal resulting in improved oil quality and yields [15] (See Section 3 for further details). Several authors have reviewed co-pyrolysis of biomass and coal along with other feedstocks such as waste plastics and tyres [6,[16], [17], [18]]. The thermal decomposition of materials was broadly discussed in these reviews, but no information on co-pyrolysis kinetic studies were reported. Abnisa and co-workers [6] concluded that the success of the co-pyrolysis process mainly lies with the synergistic effect observed during the reaction between different materials which increases the yield and quality of the oils. They suggested that the pyrolysis reactor configuration is important for achieving synergistic/antagonist effects; however, the review lacked a detailed comparison between different technologies.

Although the co-pyrolysis of biomass and coal is favourable to upgrade the quality of the oil products through synergistic effects, large amounts of oxygenated species mostly derived from the biomass are present in the oil [19]. Due to its high oxygen content, bio-oil has an acidic nature and a high chemical reactivity, which results in phase separation during storage [20].

Biomass pre-treatment techniques enable the optimization of pyrolysis product yields and composition, and limit the formation of undesired products [21]. Pre-treatment techniques have been comprehensively reviewed (see Section 4.1). Among these pre-treatment methods torrefaction is considered one of the most promising [22]. This is evident from the increasing trend in scientific publications on torrefaction (Fig. 1). This mild pyrolysis treatment is performed at temperatures of 200–300 °C resulting in moisture removal, the decomposition of hemicelluloses and partial depolymerisation of lignin and cellulose [23]. The physical and chemical properties of biomass as fuel are improved by increasing its energy density, lowering O/C and H/C ratios and inverting its hydrophilic nature [24].

The use of torrefied biomass as feedstock for the pyrolysis process also improves the quality of the resulting bio-oil by reducing the moisture, oxygen and acid content, and increasing the carbon content [20]. This process has developed rapidly but has only been reviewed recently [25,26]. Both these reviews focused on the effects of torrefaction on the quality of the pyrolysis products, and demonstrated the potential to upgrade bio-oil quality but at the cost of bio-oil yield. Dai and co-workers [26] analysed the integrated process of torrefaction and pyrolysis and concluded that the process is cost-effective with good economic potential. It was suggested that the integration of torrefaction with advanced pyrolysis techniques such as co-pyrolysis can result in an increased competitiveness of commercial bio-oil.

The co-pyrolysis of torrefied biomass and coal combines the advantages of co-utilization of biomass and coal with the advantages of torrefied biomass as feedstock in pyrolysis technologies. Reviews on co-pyrolysis studies by Abnisa et al. [6], Quan and Gao [16], Hassan et al. [17] and Mushtaq et al. [18] have mainly focused on summarizing general trends, which do not convey clear conclusions on the origins of antagonist/synergistic events due to a disjoint approach to evaluate the different scales of pyrolysis. The novelty of this review is the fundamental evaluation of chemical and physical aspects of the co-pyrolysis process, which has been neglected in other reviews. To the authors’ knowledge, this is also the first review providing insights into fundamental differences in the co-pyrolysis of torrefied biomass and coal compared to co-pyrolysis of raw biomass and coal. For co-pyrolysis studies using raw/torrefied biomass and coal, only 3% of studies have included torrefied material. The recent progress made in the understanding of chemistry and physics covering mainly mechanistic pyrolysis aspects (Sections 3.1 Yields and product distribution, 3.2 Chemistry and physics of co-pyrolysis products) are first reviewed followed by a state-of-the-art on kinetics of co-pyrolysis (Section 3.3). The engineering applications of this process are then discussed in Section 4. Finally, the conclusions and prospects are summarized in Section 5.

Section snippets

Feedstocks: coal and biomass

The composition of the most popular studied types of biomass and coal are shown in Table 1 and reveal important chemical differences. Lignocellulose is described as a polymeric structure made up by three main constituents (i.e. cellulose, hemicelluloses, lignin). The chemical and structural properties of a specific biomass mainly depend on the content and nature of these biopolymers [27]. On the other hand, coal consists of small “nuclei” of aromatic and naphthenic rings, which are linked to

Fundamentals of co-pyrolysis

Co-pyrolysis of biomass and coal cannot be dissociated from the concept of synergistic/antagonist effects because it is expected to draw several advantages in terms of emissions, energy savings and enhancing product quality. Here, we would like to reflect on the current usage of the word ‘synergistic' that is often mentioned when discussing chemical mechanisms in co-pyrolysis. The semantic of the word confirms that ‘synergy’ is used when the interaction of components when combined produce a

Engineering applications

After reviewing the fundamentals and kinetics of co-pyrolysis, the application of this process is now discussed. Co-pyrolysis has different applications such as the production of bioenergy or chemical products (biorefinery) and the successful design and integration of these processes is vital for its commercialization. Co-pyrolysis is a fractionation technology and is coupled to a series of engineered sections: the feeding system, heated reactor, gas/liquid/solid separation and downstream

Conclusion

The co-pyrolysis of torrefied biomass and coal is an attractive process for the thermochemical conversion industry’s transition to green energy and products. This review showed how useful it is to understand the effects of operating conditions on the fundamental physico-chemical changes that occur during co-pyrolysis. By studying these changes, it was possible to predict how torrefied biomass might behave differently to raw biomass during co-pyrolysis with coal. The following are key take-home

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Research Foundation (NRF) [Coal Research Chair Grant No. 86880] and Sasol. Opinions, findings and conclusions or recommendations expressed in any publication generated by the NRF supported research are that of the author(s) alone, and that the NRF accepts no liability whatsoever in this regard.

The authors also acknowledges the French scientific program MOPGA (reference ANR-18-MPGA-0013) managed by the National Research Agency and financially

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