Article pubs.acs.org/EF
Thermochemical Reactions of Blue Gum and Fossil Wood with CO/ H2O: Some Mechanistic Comments Dirgarini J. N. Subagyono,†,‡ Marc Marshall,† W. Roy Jackson,† Yi Fei,† and Alan L. Chaffee*,† †
School of Chemistry, Monash University, Clayton, Victoria 3800, Australia Chemistry Department, Mulawarman University, Samarinda 75123, Indonesia
‡
ABSTRACT: Our previous work suggested that liquid product yields from wood, unlike those from other biomass types, were not increased by CO/H2O reaction. Reactions of blue gum (BG) and fossil wood (FW) using high-temperature, high-pressure reactions have been investigated to define more precisely the biomass types for which CO/H2O is beneficial in liquefaction and to help understand the mechanism of the reaction. BG contains 25% lignin and 45% cellulose, whereas FW consists almost entirely of lignin derivatives with negligible cellulose derivatives. The effects of gas, water, and alkali have been investigated separately. Reactions of BG gave similar results under N2, H2, or CO with or without added alkali, and improved yields were obtained with an increase in the water-to-biomass ratio. These results are in agreement with the reactivity of BG being mainly associated with its carbohydrate content. In contrast, the product yield from FW was enhanced by the use of CO and further enhanced by the addition of a strong base, sodium aluminate. Some of the effect of alkali addition is associated with the extraction of humic materials from the much greater amount of lignin present in FW. The beneficial effects of CO and alkali are both consistent with the greater phenolic content of FW. Surprisingly, increasing the water-to-biomass ratio for FW led to a dramatic decrease in conversion to liquid products. The highest-quality products in terms of lower oxygen content were obtained from reactions of both BG and FW with CO/H2O/alkali. Pinus radiata,7 and separated cellulose.10 For isolated lignin, one study found a favorable effect of CO/H2O11 and two did not.12,13 To understand how and why the reaction of some substrates is favored by CO/H2O relative to an inert atmosphere or H2 whereas that of other substrates is not, reactions of a hardwood species, blue gum (Eucalyptus globulus) (BG), were compared with those of a fossil wood (FW) consisting mainly of lignin derivatives. The FW was about 11− 16 million years old,14 and the carbohydrate material had been destroyed. FW is a special form of brown coal that has not been subjected to changes such as gelification that complicate the structure and for which the coalification process has been very mild.15 The advantage of using FW as a lignin source in a mechanistic study is that changes in structure due to different methods of separating lignin from wood, which may contribute to the variation in the effect of CO/H2O on lignin liquefaction, can be avoided. Solid-state 13C NMR spectra of FW collected from the Yallourn Open Cut Mine (Victoria, Australia) showed that most of the assignable peaks in the spectra were due to lignin units with only a small contribution, if any, from cellulose,16 justifying the use of FW as a lignin source in this study. Nevertheless, some changes due to the lengthy burial period would be expected to occur, for example, enrichment in long-aliphatic-chain hydrocarbons,17 perhaps derived from waxes, the precursors of which might have migrated into the wood over geological time. The lignin and cellulose contents of E. globulus have been reported to be 25% and 45%,18
1. INTRODUCTION High-temperature reaction of a wide range of renewable organic substrates is a promising approach for obtaining liquid fuels. Many of the studies in this area have been carried out under an inert atmosphere. However, a method of increasing the liquid yield, particularly for low-rank coals, is to substitute CO/H2O for an inert atmosphere. 1−3 Other potential advantages of the CO/H2O system are that hydrogen, which can be used for upgrading reactions, is produced by the watergas shift reaction and that drying of the feedstock is not required. Explanation of the effects of CO/H2O has usually been given in terms of the mechanism suggested originally by Ross,4 which involves two pathways that are parallel and thus competitive. Initially, CO reacts with hydroxide ions and generates reactive intermediates, possibly formate ions. Then the intermediates can react either with water to form H2 and CO2 (the water-gas shift reaction) (pathway I) or with the feedstock to produce soluble products (pathway II). Alkaline additives can be used to influence the formation of the reactive intermediates and/or the pathway by which the intermediates react.4 In addition, strong alkalis, such as NaOH and NaAlO2, “dissolve” the humic materials to a significant extent.5 Our previous work demonstrated that CO/H2O can enhance liquid yields, particularly in the presence of additives, for a wide range of organic renewables, including algae6 and grape marc.7 Other workers have also noted the advantages of CO/H2O in this connection; recently the production of transport fuel precursors from lignocellulosic biomass under CO/H2O has been patented.8 However, there is a large class of woody materials for which CO/H2O appears to be of little benefit in high-temperature liquefaction. These include aspen poplar,9 © 2015 American Chemical Society
Received: October 8, 2015 Revised: December 3, 2015 Published: December 4, 2015 1039
DOI: 10.1021/acs.energyfuels.5b02358 Energy Fuels 2016, 30, 1039−1049
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Energy & Fuels respectively, which are similar to the values of 30% and 40%, respectively, reported for P. radiata.19 To obtain a better understanding of the mechanism of CO/ H2O reactions in these materials, this study compares the effects of the reactant gas (CO/H2O, N2, H2), pressure, waterto-biomass ratio, and alkaline additives (Na2CO3, NaAlO2) on the liquid and gas product yields for the two substrates. Particularly for CO/H2O reactions, comparison of the compositions of the original feedstocks and the products is attempted. Most of the experiments for comparing the effect of CO as a reactant gas were conducted using dry N2 or H2, but experiments were also carried out under N2/H2O or H2/H2O to establish whether the addition of water had any significant effect on the yields using these gases.
Table 1. Physical and Chemical Characteristics of Blue Gum and Fossil Wooda blue gumb moisture content (wt %) 7.5 ± 0.0 Proximate Analysis (wt % db) volatile matterc 89.8 ± 0.7 fixed carbonc 9.7 ± 0.7 ash yieldd 0.5 ± 0.0 Elemental Analysis (wt % db) carbon 47.7 ± 0.2 hydrogen 6.1 ± 0.1 nitrogen
