Subcritical water liquefaction of oil palm fruit press fiber for the production of bio-oil: Effect of catalysts

Abstract

Decomposition of oil palm fruit press fiber (FPF) to various liquid products in subcritical water was investigated using a high-pressure autoclave reactor with and without the presence of catalyst. When the reaction was carried in the absence of catalyst, the conversion of solid to liquid products increased from 54.9% at 483 K to 75.8% at 603 K. Simultaneously, the liquid yield increased from 28.8% to 39.1%. The liquid products were sub-categorized to bio-oil (benzene soluble, diethylether soluble, acetone soluble) and water soluble. When 10% ZnCl₂ was added, the conversion increased slightly but gaseous products increased significantly. However, when 10% Na₂CO₃ and 10% NaOH were added independently, the solid conversion increased to almost 90%. In the presence of catalyst, the liquid products were mainly bio-oil compounds. Although solid conversion increased at higher reaction temperature, but the liquid yield did not increase at higher temperature.

Introduction

Nowadays, the world is facing multiple problems such as global warming, acid rain and depletion of energy resources. All these problems are directly related to the high dependency of world energy source on non-renewable fossil fuels such as petroleum, natural gas, and coal. Therefore, there is an urgent need to develop alternative, new, cheap, clean, and renewable energy resources, which have the potential to solve the environment problems mentioned above. There are various alternative energy resources available including hydro, biomass, wind, solar, hydrogen and nuclear, however biomass has been receiving considerable attention lately. Biomass is popular because it is widely available in many parts of the world. Using indigenous source of biomass for energy supply can avoid the domination by certain countries. Furthermore, the use of biomass as a substitute for fossil fuels can reduce the carbon content in the atmosphere as carbon dioxide is absorbed by plant via photosynthesis during growth (Demirbas, 2001).

Biomass can generally be divided to four wide categories: (i) forestry waste and residues, including mill wood waste, tree and shrub waste, thinning and urban wood waste, bark and sawdust. (ii) Energy crops, including short rotation woody crops, herbaceous wood crops, grasses and starch crops. (iii) Aquatic biomass such as seaweeds. (iiii) Agricultural residues and waste biomass, such as crops wastes derived from cultivation of crops, municipal solid waste, animal waste, food processing waste and etc. (Demirbas, 2001, Demirbas, 2007, LÉDÉ, 1999, Meng et al., 2006). Malaysia, being a country that actively promotes agricultural activities has abundant biomass wastes. Oil palm is the most important agriculture crop in Malaysia, since Malaysia is one of the main palm oil producing and exporting countries in the world. In year 2008, 4.88 million hectares of land in Malaysia is covered by oil palm cultivation which produces 90.40 million tonnes of fresh fruit bunches (FFB). The amount of oil palm biomass produced by these oil palm plantations in year 2008 is estimated to be about 37.0 million tons, consisting of 22% empty fruit bunch (EFB), 13.5% fruit press fiber (FPF) and 5.5% shell (Malaysia Palm Oil Council (MPOC), 2004 and Malaysia Palm oil Board (MPOB) January, 2009). Indiscriminate disposal of these wastes will cause serious environmental problems. On the other hand, lately, it has been proven that biomass sources can become an economical source of renewable energy. Therefore, developing new technologies for converting oil palm biomass to energy sources (liquid or gas) becomes an attractive research area.

Biomass conversion to energy could be divided into three main approaches: direct combustion processes (e.g. burning wood for heating and cooking, etc.), biochemical processes (such as methane fermentation and alcoholic fermentation, etc.), and thermochemical processes. Thermochemical processes may be further subdivided into pyrolysis, gasification and liquefaction (high-pressure liquefaction, sub to supercritical fluid (SCF)). These processes convert waste biomass into energy rich products. Each technology has its own advantages, depending on the type of biomass source and the form of energy needed (Mirza et al., 2008). For instance, pyrolysis and gasification has the advantage of non requirement of high pressure, but generally biomass feedstock has to be pre-treated through drying to reduce the water content. Apart from that, formation of tar and char is still a major problem for both the processes. On the other hand, liquefaction technology using subcritical water can directly deal with wet biomass and still being able to achieve high liquefaction efficiency as in conventional process such as pyrolysis, and therefore has gained wide attention lately (Demirbas, 2001).

Subcritical water, which is defined as liquid water in the temperature range of boiling point to critical point (373–647 K) or near critical point, is attracting attention as a medium for organic chemistry reactions (Cheng et al., 2009). Subcritical water exhibits properties that are very different from those of ambient liquid water. Subcritical water has a lower dielectric constant, weaker hydrogen bonds, and a higher isothermal compressibility than ambient liquid water. Thus, this medium can be used for various synthesis reactions and some degradation reactions, e.g. biomass liquefaction. It can also support ionic, polar nonionic and free-radical reactions. Since the reaction medium is water, wet biomass can be directly used as feedstock. These properties make subcritical water a very promising reaction medium for conversion of biomass, such as lignocellulosic materials. It has the ability to break the rigid structure of lignocelluloses and consequently, decompose the lignocellulosic materials into smaller components by hydrolysis and further reactions (Khajavi et al., 2005, Cheng et al., 2009, Brunner, 2009). Lately subcritical water liquefaction has attracted more attentions for efficient production of liquid bio-oil and valuable chemicals from waste biomass.

The influence of heat, catalyst, and solvent on decomposition of pure substance and different kinds of biomass using subcritical and supercritical water processes have been studied and reported in the literature (Erzengin and Kucuk, 1998, Demirbas, 1998, Demirbas, 2000, Cağlar and Demirbas, 2001, Sasaki et al., 1998, Wahyudiono et al., 2008). It was found that the sub to supercritical water treatment is more suitable for obtaining high yields of hydrolyzed products from wet biomass. It is used to convert lignocelluloses substance into bio-oil and chemicals in the presence of water as a solvent (Katsunobu and Saka, 2005, Saka et al., 2006). Experimental results have confirmed the good performance of hot and sub to supercritical water as medium for these reactions (Brunner, 2009, Cheng et al., 2009).

The aim of this work is to study the efficiency of using subcritical water to decompose FPF to various liquid and gaseous products, with emphasis on the liquid product. The decomposition of FPF is carried out in a high-pressure batch reactor with and without the presence of catalyst such as ZnCl₂, Na₂CO₃ and NaOH. The effect of catalyst at different reaction temperature on the distribution of liquid products yield, especially bio-oil products, is also reported.