Elsevier

Energy Conversion and Management

Volume 87, November 2014, Pages 687-696
Energy Conversion and Management

Kinetic scheme of biomass pyrolysis considering secondary charring reactions

https://doi.org/10.1016/j.enconman.2014.07.061Get rights and content

Highlights

  • A mechanistic scheme for biomass pyrolysis from the literature is adapted.

  • Secondary char formation reactions and catalytic effect of alkalis are considered.

  • The adapted scheme is validated with experimental data from the literature.

Abstract

A widely applicable kinetic scheme for pyrolysis is still missing. In this work an adaptation of the mechanistic scheme developed by Ranzi et al. (2008) for pyrolysis of small ash free biomass particles is proposed. The scheme is modified to include secondary char formation reactions, which are relevant for particles of a certain thickness, and sugar formation is avoided due to the catalytic effect of alkali metals in biomass. The predictions of the adapted scheme are compared to experimental data from the literature of pyrolysis in fixed beds of particles with a size of around 1 cm. It is shown that the adaptation improves the prediction of the final char yield and its CHO composition and also the yields of the main groups of volatiles, as carbonyls + alcohols, sugars and water vapor.

Introduction

Biomass, as other renewable energy sources, is expected to play a more important role in the energy mix of the future. Pyrolysis is a promising conversion process by itself to generate liquid bio-fuel and bio-char and also a main sub-process in other thermal conversion processes as gasification, combustion, smouldering or hydrothermal carbonization. To describe these processes in different reactor configurations, where transport phenomena are coupled with chemical reactions [1], accurate kinetics of biomass pyrolysis which can provide the final product composition are required. However, pyrolysis of biomass proceeds via a very complex set of competitive and concurrent reactions. The exact mechanism remains unknown [2] and a widely applicable kinetic scheme for pyrolysis is still missing.

Many of the available pyrolysis schemes were reviewed by Di Blasi [3]. Schemes can be categorized into single component and multi-component. In single-component schemes the final products are usually lumped into 3 categories: solid (char), liquid (tar) and gas, although there is a high heterogeneity in each category. Examples of main liquid components are water, pyrolytic lignin or acetic acid, which have very different properties. Pyrolysis is described as the competition between formation of each lumped product. This scheme attempts to be able to predict the product distribution at a different range of conditions, ranging from slow to fast heating rate, and it is the most commonly employed to describe the kinetics of biomass pyrolysis in single particle and reactor models. In this scheme the primary tar can further react in a secondary exothermic reaction to produce more permanent gases or secondary char.

In the multi-component schemes there are usually 3 components representing the main biomass components: cellulose, hemi-cellulose and lignin. In this scheme usually each component is represented with one reaction and just the mass loss evolution is predicted, without giving information about the product composition. Sometimes fixed yields of products are assumed for each component, based on experimental data [4]. In Miller and Bellan [5] a competitive scheme between char, tar and gas formation is proposed for each biomass component. But none of these single-component or multi-component schemes are mechanistic and provide a detailed product composition.

For individual compounds as cellulose [6], [7] or lignin [8], [9] mechanistic schemes were proposed based on the analysis of the several reaction mechanisms rather than a global approach. A mechanistic scheme was proposed for biomass by Ranzi et al. [10], considering biomass as formed by cellulose, hemi-cellulose and 3 types of lignin and 20 representative species are considered to describe the volatiles. The product composition provided by this scheme was partially validated with data obtained from fast pyrolysis of small ash free biomass particles and the mass loss evolution from micro-TGA experiments, i.e., conditions where secondary reactions are minimized.

However there is no mechanistic scheme available for pyrolysis when secondary reactions are not negligible. Secondary reactions are present in particles of a certain thickness when there is enough residence time of the volatiles during char formation and this is the case for most of the pyrolysis processes in fixed or fluidized beds, and other processes where pyrolysis is present, as combustion or gasification. The presence of inorganics in the feedstock also promotes secondary reactions [3]. The nature of these reactions is explained in detail in Section 2. The focus of this work is to propose an adaptation of the Ranzi et al. [10] scheme based on the literature to improve the predictions of the pyrolysis product composition when secondary charring reactions are relevant. The predictions of the adapted scheme are compared to experimental data for model validation.

Section snippets

Cellulose

The cellulose pyrolysis mechanism from Ranzi et al. [10], schematically summarized in Fig. 1a and detailed in Table 1, is based on the scheme of Piskorz et al. [11]. Cellulose can be converted to active cellulose without appreciable mass loss through R1 or converted to char and water through R4. Reaction R1 is always the prevalent reaction in this competition, although the proportion of R4 is higher at low temperatures (due to lower activation energy), leading to higher char yields at low

Experimental and model results

Experimental data from literature concerning product compositions from fixed-bed pyrolysis with particles of a few mm or cm was collected for model validation. A complete characterization of the product composition is difficult to find, due to the difficulties of measuring the very different products from pyrolysis, as different techniques are required. The works of Branca et al. [25] and Milhe et al. [26] were employed, as they provide a very comprehensive characterization.

The pyrolysis

Conclusions

An adaptation of the mechanistic pyrolysis scheme developed by the group Ranzi for pyrolysis of small ash free biomass particles is proposed. Secondary char formation reactions, which are relevant for particles of a certain thickness, are included. Moreover, the catalytic effect of alkali metals in biomass is considered, which avoids sugar formation. The predictions of the adapted scheme are compared to experimental data from the literature. It is shown that it leads to a significant

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