FROM WASTE TO VALUABLE FUEL: HOW MICROWAVE-HEATED PYROLYSIS CAN RECYCLE WASTE AUTOMOTIVE ENGINE OIL Su Shiung Lam 1,2 , Alan D. Russell 1 , Howard A. Chase 1 1 Department of Chemical Engineering and Biotechnology, University of Cambridge, New Museums Site, Pembroke Street, Cambridge CB2 3RA, United Kingdom 2 Department of Engineering Science, University Malaysia Terengganu, 21030 Kuala Terengganu, Terengganu, Malaysia Introduction The production of waste automotive engine oil (WO) is estimated at 24 million tons each year throughout the world, posing a significant treatment and disposal problem for modern society. WO, containing a mixture of low and high molecular weight aliphatic and aromatic hydrocarbons, also represents a potential source of high- value fuel and chemical feedstock. The preferred disposal option in most countries is incineration and combustion for energy recovery, though vacuum distillation and hydro-treatment have been researched to recycle this waste [1]. However, these disposal routes are becoming increasingly impracticable as concerns over environmental pollution, and additional cost, sludge and wastewater disposal are recognized due to the undesirable contaminants present in WO [1, 2]. As part of the growing interest in waste recycling, alternative treatments have been investigated with the aim of recovering both the energetic and chemical value of the WO. Pyrolysis techniques have recently shown great promise as an economic and environmentally disposal method for WO [3-5] – the waste material is thermally cracked and decomposed in an inert atmosphere, with the resulting pyrolysis oils and gases able to be used as a fuel or chemical feedstock, and the char produced used as a substitute for activated carbon, though the use of this technology is not widespread as yet. The pyrolysis-oil is of particular interest due to its easy storage and transportation as a liquid fuel or chemical feedstock. The oil can be treated and catalytically upgraded to transport-grade fuels, or added to petroleum refinery feedstocks for further processing and upgrading [3]. While most WO pyrolysis studies have been focused on conventional electric-resistance-heated and electric-arc-heated pyrolysis [4, 6], there are very few studies about the pyrolysis oil generated during microwave-heated pyrolysis of WO. Microwave- heated pyrolysis has recently shown promise as a route for the treatment and recycling of the WO [3]; the advantages of microwave- heated-pyrolysis have been elaborated in previous work [5] and will not be duplicated here. In this process, WO is mixed with a highly microwave-absorbent material such as particulate carbon; as a result of microwave heating the oil is thermally cracked in the absence of oxygen into shorter hydrocarbon chains. The resulting gaseous products are subsequently recondensed into pyrolysis oils of different composition depending on the characteristics of the input substances and reaction conditions. This study investigates the characteristics of the pyrolysis oils produced from microwave-heated-pyrolysis of WO collected from different sources (unleaded gasoline and diesel automobile engines, and mixtures from a service station), with a focus on their elemental and hydrocarbon composition, and potential fuel properties. The evaluation of the influence of the nature of the different waste oils is important to assess the technical feasibility and applicability of using the pyrolysis process as a route to energy recovery/feedstock recycling from WO. There have been no reports on the composition of oils resulting from microwave-heated pyrolysis of WO, although a few studies have been performed on biomass pyrolysis [7, 8], and conventional-electric-heated pyrolysis of WO [9, 10]. Experimental Materials. Shell 10W/40 motor oil was used throughout the experiments. The WO was collected from three sources: the crankcase of both a gasoline and a diesel engine run on unleaded fuel, and mixtures of used automotive oils sampled from a garage/waste recycling centre. Before pyrolysis, the oil samples were filtered such that the size of any remaining particulates (e.g. metal particles, carbon soot, other impurities etc) was less than 0.45 μm; volatiles and water were eliminated by heating at 110ºC; samples were examined for hydrocarbon composition by Gas Chromatography – Mass Spectrometry (GC-MS); the C, H, N, S, O content were obtained by elemental analysis; calorific value was determined by Differential Scanning Calorimeter (DSC). Particulate carbon (TIMREX FC250 Coke, TIMCAL Ltd, Bodio, Switzerland) was used as a microwave absorbent to heat the WO; this was pre- heated to 800ºC for 50 minutes to remove any water and sulphur- containing compounds). Experimental details. The experimental apparatus and method developed and used during this investigation have been described in detail in previous work [5], though modifications were made to the apparatus to enhance the collection and quality of pyrolysis-oils and noncondensable-gases generated from the pyrolysis. Improvements were achieved by installing a mixed-cellulose-ester-membrane filter (ME-filter, 0.45 m ME25, Schlecher & Schuell, Germany) to remove any metallic solid residues trapped in the pyrolysis-volatiles before they pass through the condensation system. The refined experimental apparatus is shown in Figure 1. Figure 1. Schematic layout of microwave-heated pyrolysis system Briefly, microwave-heated pyrolysis of WO was performed in a bell-shaped quartz reactor (180x180x180 mm) filled with 1 kg of particulate carbon, which is stirred and heated by a 5 kW microwave oven over a range of pyrolysis temperatures (250 to 700ºC) and purge gas flows (0.1 to 0.75 L/min) to understand the influence of these process conditions on the final pyrolysis oils obtained; N 2 purge-gas was vented through the system to maintain the apparatus in an inert nitrogen atmosphere. WO sample was continuously added to the reactor at a constant feeding rate of about 1 kg/h over a period of about 2 hours as soon as the target pyrolysis temperature was achieved. Gases, solids, and vapors generated in the pyrolysis reaction, termed generally as pyrolysis volatiles (consisting of a hydrocarbon mixture of gases, liquids, and solids existing in a vapor phase), leave the reactor, and either condense into pyrolysis oil or are sampled as noncondensable-gases before vented from the system. The yield of residue material, pyrolysis oil, and noncondensable gases were determined and analyzed by GC-MS (liquid and gas fractions only), elemental analyzer (liquid only), and DSC (liquid only) to identify their chemical composition. The data recorded is the average of the results obtained from three valid repeated runs performed under identical conditions. Temperature measurement and analytical methods. The temperature measurement and its limitations, and the relevant 1) Microwave oven 2) Reactor 3) Motor stirring system 4) Injecting vessel and pump 5) ME filter 6) Condenser (60ºC) 7) Condenser (25ºC) 8) Condenser (0ºC) 9) Cold trap (about -70ºC) 10) Main collecting vessel 11) 2 nd collecting vessel 12) Cotton wool filter 13) Milipore 7015 pump 14) Gas bag 15) Pressure gauge 16) Pressure relief valve. Prep. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2011, 56 (1), 19 Proceedings Published 2011 by the American Chemical Society