Microwave and slow pyrolysis biochar—Comparison of physical and functional properties

Abstract

This article reports work that compares slow pyrolysis and MW pyrolysis of two different feedstock (willow chips and straw), with particular focus on physical properties of resulting chars and their relation to biochar soil function. In these experiments, slow pyrolysis laboratory units at the University of Edinburgh and the MW pyrolysis units at the University of York were used to produce biochar from identical feedstock under a range of temperatures. Physical properties and stability of thus produced biochar from both systems were then analysed and compared.

The results showed that using MW, pyrolysis can occur even at temperatures of around 200 °C, while in case of conventional heating a higher temperature and residence time was required to obtain similar results. This article presents new data not only on the comparison of biochar from microwave and slow pyrolysis in terms of physical properties, but also in respect to their carbon sequestration potential, i.e. stability.

Introduction

Biochar is a carbon-rich solid product of thermal stabilisation of organic matter created for safe and potentially beneficial storage in soil. It differs from other solid products of thermochemical conversion in that long-term carbon storage is the primary objective, rather than creation of feedstock for processing industries or fuels such as charcoal, coke and activated carbon. Due to this distinct function, and often a combination of several functions, e.g. soil improvement or remediation, the requirements on biochar are different to those other uses of solid carbonaceous residues. In particular it is necessary to ensure that biochar produced from a particular feedstock by any given technology is at least environmentally benign, or even has positive effects (e.g., on plant growth, soil structure, water management, etc.). The current state-of-the-art knowledge on biochar and its interaction with the environment has recently been reviewed by Sohi et al. [1] and Lehmann and Joseph [2]. In addition to its environmental impact, biochar must also be highly stable to ensure long-term carbon sequestration. The global potential for sustainable global biochar deployment has been recently analysed by Woolf et al. [3], and the potential benefits and risks of biochar were assessed in a report to the UK Department of Energy and Climate Change (DECC) [4]. This report, besides analysing the potential for biochar deployment in the UK also discusses benefits and issues of biochar deployment. It particularly highlights the need to better understand the economics of “pyrolysis biochar systems” (PBS) and the long-term stability of biochar.

The distinctly new use of the material (biochar) presents a number of requirements and challenges that are different from its other (more traditional) uses, such as combustion or activated carbon. As a result, new, or modified traditional thermochemical processes are being proposed that target the specifics of biochar production. This offers the opportunity to produce and test a wide range of biochar and assess its suitability for application under different environmental, economic and agricultural scenarios. Yet, to date there are only very few studies attempting to compare biochar produced from the same feedstock by alternative technologies [5], [6]. This is why we decided to study and compare biochar produced by a novel technology of low temperature microwave (MW) pyrolysis with biochar produced under similar thermal regime by slow pyrolysis (relatively established technology suitable for biochar production).

Microwave heating offers several advantages over conventional heating, as it is often more controllable [7], [8] energy [9], [10], [11] and cost [12] efficient and therefore in many cases may offer a potentially attractive alternative to “conventional” pyrolysis systems. Microwave processing has been shown to be effective at both pilot scale [13] and at industrial scale for the production of plant material extracts of outstanding stability and purity [14]. Efficiency of microwave treatment for pyrolysis of biomass has been proved in a number of publications [15], [16], [17]. Furthermore, several researchers looked at comparing MW pyrolysis with conventional pyrolysis and identified considerable differences between the two methods [18], [19], [20]. These articles emphasise the key differences between the different pyrolysis methods as being temperature of decomposition, heating rates and requirement for feedstock pre-processing (e.g. shredding or drying). However, only few studies looked at MW biochar production [21], [22] and to our best knowledge, direct comparison of biochar properties obtained by conventional pyrolysis and MW pyrolysis have never been reported.

In this study we focussed on low temperature thermochemical conversion (up to 350 °C), as this is the operating range of the new promising low temperature MW pyrolysis technology and we compared the solid products with those produced by slow pyrolysis/torrefaction in the same temperature range. This article presents results from our experimental investigation of the impact of production conditions, i.e. pyrolysis temperature and heating method on the biochar product, its properties and stability.