Phosphorus catalysis in the pyrolysis behaviour of biomass

https://doi.org/10.1016/j.jaap.2008.08.003Get rights and content

Abstract

Phosphorus is a key plant nutrient and as such, is incorporated into growing biomass in small amounts. This paper examines the influence of phosphorus, present in either acid (H3PO4) or salt ((NH4)3PO4) form, on the pyrolysis behaviour of both Miscanthus × giganteus, and its cell wall components, cellulose, hemicellulose (xylan) and lignin (Organosolv). Pyrolysis–gas chromatography–mass spectrometry (PY–GC–MS) is used to examine the pyrolysis products during thermal degradation, and thermogravimetric analysis (TGA) is used to examine the distribution of char and volatiles. Phosphorus salts are seen to catalyse the pyrolysis and modify the yields of products, resulting in a large increase in char yield for all samples, but particularly for cellulose and Miscanthus. The thermal degradation processes of cellulose, xylan and Miscanthus samples occur in one step and the main pyrolysis step is shifted to lower temperature in the presence of phosphorus. A small impact of phosphorus was observed in the case of lignin char yields and the types of pyrolysis decomposition products produced. Levoglucosan is a major component produced in fast pyrolysis of cellulose. Furfural and levoglucosenone become more dominant products upon P-impregnation pointing to new rearrangement and dehydration routes. The P-catalysed xylan decomposition route leads to a much simpler mixture of products, which are dominated by furfural, 3-methyl-2-cyclopenten-1-one and one other unconfirmed product, possibly 3,4-dihydro-2-methoxy-2H-pyran or 4-hydroxy-5,6-dihydro-(2H)-pyran-2-one. Phosphorus-catalysed lignin decomposition also leads to a modified mixture of tar components and desaspidinol as well as other higher molecular weight component become more dominant relative to the methoxyphenyl phenols, dimethoxy phenols and triethoxy benzene. Comparison of the results for Miscanthus lead to the conclusion that the understanding of the fast pyrolysis of biomass can, for the most part, be gained through the study of the individual cell wall components, provided consideration is given to the presence of catalytic components such as phosphorus.

Introduction

Phosphorus is one of the key plant nutrients, and as such, has variable concentrations in biomass and energy crops [1]. It is an interesting element in fuels, since, like potassium, it influences not only the thermal behaviour [2] but also the ash behaviour is combustion systems [3]. A typical concentration of P2O5 in the ash of willow is 11.5% [4], while values are lower for the grasses, of the order of 2–4% [4], [5]. A value of 5.3% P2O5 in the ash has been reported previously for Miscanthus × giganteus [4]. The mobilization and ash behaviour of phosphorus during combustion is a topic generating interest since it impacts not only on slagging, corrosion and emissions, but also on sustainability and the possible beneficial use of ash residues [6], [7].

Phosphorus compounds are well-known flame-retardants, and increase char yields from textiles and woods [8], [9]. They also catalyse dehydration reactions of cellulose and a recent study by Di Blasi et al. [10] report decreasing yields of tar products from phosphorus impregnated fir wood. There has been some previous work examining the mechanism of phosphoric acid catalysed decomposition of biomass, particularly cellulose [11], [12]. Dobele et al. [11] used analytical pyrolysis combined with gas chromatography to study the composition of volatile products of different celluloses impregnated with various amounts of phosphoric acid. The influence on the yields of levoglucosan and levoglucosenone was studied taking into account the supramolecular structure, degree of polymerization, hydrophilic properties and pre-treatment conditions of the celluloses. It was found that levoglucosenone predominates in the volatiles of acid catalysed pyrolysis. However, the relative total amount of both 1,6-anhydrosaccharides varied only in a narrow range of 75–85% regardless of the impregnation and pre-treatment conditions of the celluloses. For pulps with a less ordered cellulosic supramolecular structure, breaking of glycosidic bonds with the formation of levoglucosan besides levoglucosenone occurred and the relative amount of non-dehydrated anhydrosaccharides increased. Dobele et al. [12] also studied the influence of phosphoric acid pre-treatment of biomass materials (beech wood sawdust, recycled Kraft pulp, newsprint, microcrystalline cellulose Thermocell) on the pyrolysis products yields. Birch wood treated with 1% phosphoric acid yields approximately 15% of levoglucosan and 8% levoglucosenone. At higher concentrations of phosphoric acid (2.5%) the formation of more levoglucosenone (17%) was observed. These authors presented a mechanism of acid catalysed splitting of glycosidic bonds in cellulose, showing that the interaction of mineral acids proceeds via protonation of the oxygen atom on glycosidic bonds, stabilisation of the pyranosyl cation by mesomerism, the formation of oxonium ions by water addition, and stabilisation of the hydrogen splitting.

The aims of the present work are to examine the influence of both phosphoric acid and ammonium phosphate on the products from pyrolysis of other cell wall components, hemicellulose (xylan) and lignin (Organosolv). Results are compared with the analogous cellulose decomposition routes. A second aim is to examine how well the pyrolysis of an energy crop, Miscanthus × giganteus, can be described in terms of the pyrolysis of its cell wall components.

Section snippets

Materials

The following cell wall components: cellulose, hemicellulose (oat spelt xylan) and lignin (Organosolv) were used in this study. All compounds were purchased from the Sigma–Aldrich Company Ltd. The biomass sample of Miscanthus × giganteus was obtained from Rothamsted Research (Harpenden, Hertfordshire, UK). The sample was ground and sieved. The fraction 0.15–0.18 mm was used for demineralisation, impregnations and analyses. The cell wall components of Miscanthus × giganteus were determined using a

TGA pyrolysis

The differential thermogravimetric (DTG) results comparing the impact of phosphorus in the TGA pyrolysis experiments of cellulose, xylan and Organosolv lignin are shown in Fig. 1. Pyrolysis yields from these studies are given in Table 1.

The decomposition of untreated cellulose (Fig. 1a) occurs in the temperature region between 448 K and 676 K, with the maximum peak temperature at 642 K. Demineralisation shifts the maximum peak temperature to 631 K, within the same temperature region. As discussed

Conclusions

The main cell wall constituents (cellulose, xylan, lignin) as well as Miscanthus × giganteus sample were subjected to three types of pre-treatment – HCl demineralisation and impregnation of the demineralised samples by ortho-phosphoric acid – H3PO4 – and ammonium phosphate – (NH4)3PO4.

In this study it was observed that the phosphorus salts catalysed the pyrolysis and that the yields of pyrolysis products were modified. The phosphorus-catalysed pyrolytic decomposition resulted in a large increase

Acknowledgement

The authors are grateful to the EPSRC for the financial support of this work through an Advanced Research Fellowship (JMJ) and associated research grant (GR/S49018 and GR/S49025).

References (23)

  • A. Monti et al.

    Biomass Bioenergy

    (2008)
  • D.J. Nowakowski et al.

    Fuel

    (2007)
  • B.M. Jenkins et al.

    Fuel Proc. Technol.

    (1998)
  • J. Beck et al.

    Fuel

    (2006)
  • S. Gaan et al.

    J. Anal. Appl. Pyrolysis

    (2007)
  • R. Stevens et al.

    Polym. Degrad. Stab.

    (2006)
  • C. Di Blasi et al.

    Polym. Degrad. Stab.

    (2008)
  • G. Dobele et al.

    J. Anal. Appl. Pyrolysis

    (2001)
  • G. Dobele et al.

    J. Anal. Appl. Pyrolysis

    (2003)
  • E. Meszaros et al.

    J. Anal. Appl. Pyrolysis

    (2007)
  • J. Rodrigues et al.

    J. Anal. Appl. Pyrolysis

    (1999)
  • Cited by (103)

    • Biofuels production using pyrolysis techniques

      2023, Advances in Biofuels Production, Optimization and Applications
    • Effect of H<inf>3</inf>PO<inf>4</inf> and NaOH additives on the Co-carbonization of cellulose and N-containing compounds to produce N-doped chars

      2023, Journal of Analytical and Applied Pyrolysis
      Citation Excerpt :

      The primary mechanism involves the attack of a proton from acid on the hydroxyl group of cellulose. Finally, the intermediate complex eliminates a water molecule and forms double bonds and carbonyl groups in the heavy molecule, precursors for coke formation [17,37,38]. Fig. 1 shows the TGA and DTG curves of N-containing model compounds and cellulose with and without H3PO4 and NaOH additives.

    View all citing articles on Scopus
    1

    Current Address: Bioenergy Research Group, School of Engineering and Applied Science, Aston University, Birmingham, B47ET, UK.

    View full text