Evaluation of beech for production of bio-char, bio-oil and gaseous materials

Highlights

• Oriental beech wood has subjected to carbonization, liquefaction and pyrolysis products.

• Bio-oil from the beech wood contains specific oxygenated compounds.

• The bio-char obtained from beech wood carbon rich solid bio-fuel.

• The beechnut oil was converted to biodiesel in supercritical methanol.

Abstract

Evaluation of Oriental beech (Fagus orientalis L.) was investigated with aspect of thermo-chemical conversion to obtain bio-char, bio-oil and gaseous. When the pyrolysis temperature increased, the bio-char yield decreased. A high temperature and smaller particles increase the heating rate resulting in a decreased bio-char yield. The bio-char obtained are carbon rich, with high heating value and relatively pollution-free potential solid biofuel. The liquefaction yield sharply increased with increasing the temperature near critical temperature and after that. In the pyrolysis, increases of liquid yields are considerably sharply for all of the samples with increasing of pyrolysis temperature from 690 K to 720 K. The beechnut oil was converted to biodiesel in supercritical methanol without using the catalyst. Experiments have been carried out in an autoclave at 493, 523 and 593 K, and with molar ratios of 1:6–1:40 of the oil to methanol. The yield of alkyl ester increased with increasing the molar ratio of oil to alcohol.

Introduction

The basic structure of all woody biomass consists of three organic polymers: cellulose, hemicelluloses, and lignin in the trunk, foliage, and bark. Three structural components are cellulose, hemicelluloses and lignin which have rough formulae as CH_1.67O_0.83, CH_1.64O_0.78, and C₁₀H₁₁O_3.5, respectively (Demirbas, 2000a, Demirbas, 2001). The woody biomass also consists of extractives and minerals or ash.

The utilization of biomass via thermochemical conversion has received increased attention due to pressure to satisfy the increasing global energy demand and the decrease of environmental pollution (Demirbas, 1998, Tekin and Karagoz, 2013, Tekin et al., 2013). Three main biomass conversion processes are thermal, chemical and biochemical.

Thermal biomass conversion does include a number of possible roots to produce from the initial biorenewable feedstock useful fuels and chemicals. Thermal conversion processes include three sub-categories: combustion, gasification, pyrolysis and liquefaction. Torrefaction is one of the processes as well but can be categorized as a mild form of pyrolysis. Biorenewable feedstocks can be used as a solid fuel, or converted into liquid or gaseous forms for the production of electric power, heat, chemicals, or gaseous and liquid fuels (Gercel and Gercel, 2007, Demirbas, 2008a, Demirbas, 2008b, Demirbas, 2008c, Gonzalez et al., 2008). Fig. 1 shows the biomass thermal conversion processes (Demirbas, 2009).

Liquefaction processes result in liquid product, which can be easily stored and transported and require lower process temperatures (Mohan et al., 2006). The pyrolysis of biomass has been studied with the final objective of recovering a bio-fuel with medium-low calorific power (Frederick et al., 1994, Raveendran and Ganesh, 1996, Bridgwater et al., 1999, Font et al., 1999, Babu and Chaurasia, 2003). Wood and other forms of biomass are some of the main renewable solid energy resources available and provide the only source of liquid, gaseous and solid fuels (Demirbas, 2007).

Oriental beech (Fagus orientalis Lipsky) belongs to the Beech family (Fagaceae) and is closely related to its European counterpart (Fagus sylvatica Lipsky) (Gömöry et al., 1994). It is a large tree, normal mean heights in Turkey are 30 m but it can reach heights of up to 50 m.

Beech fruit is a pyramidal nut (beechnut or mast/beechmast), with a triangular section and about 1.5 cm in length, enclosed in spiky cupules. The cupules open by valves, which are one more than fruit number. The small triangular beechnuts are borne in pairs inside a cup with four prickly brown sides and change from green to brown as they ripen. The endocarp is a thick, smooth and shiny dark brown, hairy inside husk. Its color lightens as the beechnut dries. The husks then blow open by the wind and the nuts fall to the ground.

The fruit is a small, sharply 3-angled nut 10–15 mm long, borne in pairs in soft-spined husks 1.5–2.5 cm long, known as cupules. The nuts have high oil content. Beech oil is the oil expressed from the nuts of the beech tree.

Vegetable oils and their derivatives, especially (m)ethyl esters, commonly referred to as biodiesel. Methyl and ethyl esters of vegetable oils have several outstanding advantages among other new-renewable and clean engine fuel alternatives (Balat, 2005). Biodiesel is generally made of methyl esters of fatty acids produced by the transesterification reaction of triglycerides with methanol in the presence alkali as a catalyst (Clark et al., 1984, Bala, 2005).

Mostly biodiesel is derived from the vegetable oils using NaOH or KOH catalytic transesterification methanol process (Ma and Hanna, 1999). The purpose of the transesterification process is to lower the viscosity of the oil. Transesterification is the reversible reaction of a fat or oil with an alcohol (methanol or ethanol) to form fatty acid alkyl esters and glycerol. It can be alkali-, acid-, or enzyme-catalyzed; however, currently the majority of the commercialized technology resides in transesterification using alkali catalyzed reaction (Ma and Hanna, 1999). The transesterification occurs in supercritical methanol. The reaction can also take place without the use of a catalyst under conditions in which the alcohol is in a supercritical state (Saka and Kusdiana, 2001a, Demirbas, 2002a, Demirbas, 2002b, Demirbas, 2002c). Since supercritical methanol is hydrophobic with a lower dielectric constant, non-polar triglycerides can be well solvated with supercritical methanol to form a single phase oil/alcohol mixture (Saka and Kusdiana, 2001b, Warabi et al., 2004).

Pyrolysis proceeds in three steps. In an initial step, there is moisture and some volatile loss (Eq. (1)). Primary bio-char occurs in the secondary step (Eq. (2)). The last fast step follows by a slower step including some chemical rearrangement of the bio-char. During the third stage, the bio-char decomposes at a very slow rate and carbon-rich residual solid forms. The formation of secondary charring (Eq. (3)) makes the char less reactive.

Biomass → Water + Unreacted solid residue,

Unreacted solid residue → (Volatile + Gases)1 + (Char)1, and

(Char)1 → (Volatile + Gases)2 + (Char)2.

This study is devoted to obtain liquid products from beech wood via alkali glycerol liquefaction and pyrolysis processes. Oil samples obtained from the beechnuts have been converted to their methyl esters (biodiesel) by transesterification in compressed methanol. Some fuel properties of the biodiesel were determined and compared with the biodiesel fuel specifications so as to define whether they were of fuel quality or not.