(821b) Effects of Temperature and Oxygen Concentration On Torrefaction of Oil Palm Kernel Shell

Lignocellulosic biomass is one of the most abundant biomass resources on the earth and can be used as a feed-stock for preparing fuels and chemicals. Some of these technologies are still under development. Due to its availability in Malaysia, oil palm waste is considered to be the best biomass waste. In 2010, Malaysia was the second largest producer of palm oil, producing 17.8 million tones, or 39% of the total world supply. Indonesia was the world’s largest producer of palm oil, producing 22.2 million tonnes of oil, or 48% of the total world supply. In 2010, productive oil palm plantations in Malaysia covered 4.9 million hectares, a 3.3 % increase from 2009, when productive oil palm plantations covered 4.7 million hectares. Types of biomass produced by the oil palm industry include empty fruit bunches (EFB), mesocarp fiber, kernel shells, fronds and trunks. EFB, mesocarp fiber and kernel shells are used or discarded at palm oil mills, while fronds and trunks are used or discarded at the plantations. The total amount of the biomass residues from the oil palm industry amounts 18.2 Mtoe (million ton oil equivalent). Since the current primary energy supply in Malaysia is roughly 70 Mtoe (million tons of oil equivalent), the total oil palm biomass energy potential of 18.2 Mtoe may contribute considerably to decreasing the consumption of fossil fuels (natural gas, coal and oil). In order to use biomass waste for energy efficiently, the following drawbacks, compared to fossil fuels, must be overcome:(1) Higher energy consumption during collection, (2) Heterogeneous and uneven composition, (3) Lower calorific value, (4) Quality decay by biodegradation, (5) Difficult to transport, (6) High ash content. A number of options exist to reduce these drawbacks. The most common techniques are pelletization, liquefaction and gasification of the biomass. Pelletization involves drying, chipping, grinding and pelletizing lignocellulosic biomass. Pelletization is the cheapest option, but has some disadvantages that include lower heat values and quality deterioration by moisture (pellet disintegration, moss growth or bioorganic decomposition). Recently, treatment of biomass at a low temperature, around 473-573 K, under an inert atmosphere was found to be effective for improving the energy density and shelf life of the biomass. This treatment was called ‘torrefaction’ and has been widely applied to wood and grass biomass over the past few years. Torrefaction studies have largely been conducted on wood and grass biomass, including wood dusts, beech, eucalyptus, willow, larch, bamboo and canary grass. Few reports on the torrefaction of agricultural, lignocellulosic waste species, like straw, were found, despite these waste species being among the most promising renewable resources, particularly in Southern Asia. The authors have previously reported on the torrefaction behavior of three types of oil palm waste under inert condition (Fuel, 90, 2585-2591 (2011)). Although torrefaction is one of the most promising methods to improve lignocellulosic solid fuels, the procedure requires thermal energy and an inert atmosphere. If it was possible to use flue gas from burners, inert gas and a considerable quantity of energy could be saved, making the process more economically viable. Little data are available that demonstrates whether torrefaction can be effectively carried out in the presence of oxygen. This information is necessary since flue gas contains varying quantities of oxygen. In this study, torrefaction of a lignocellulosic waste was carried out in a fixed-bed tubular reactor in the presence of oxygen ranging from 0-15 mol%. The effects of oxygen concentration, temperature (493-573 K), time (30-90 min) and biomass size (unground and 0.25-0.50 mm) on the torrefaction behavior (mass and energy yields) were investigated. The lignocellulosic biomass waste used was oil palm kernel shell (PKS) in Malaysia. Since oxygen concentration in flue gas from a variety of furnaces ranges from 6 to 14 mol%, the oxygen concentration of 0 to 15 mol% was selected in this study. Torrefaction time did not give significant impact as much as temperature. This means the decomposition of lignocellulosic biomass was practically terminated within the shortest torrefaction time applied (30 min). Lignin in PKS was highly resistant to the temperature, so that approximately 50 % of the biomass solid was still retained after being treated with severe torrefaction at 573 K. When the concentration of oxygen was varied from 0 to 15 mol%, the solid yield was lowered by only 7 % compared to the inert torrefaction (control). This insignificant decrease may be an indicator that torrefaction in the presence of oxygen (0-15 mol%) is rational, since the torrefaction performance is not much affected by the existence of oxygen up to 15 mol%.