Optimization of multi-stage pyrolysis Adetoyese Olajire Oyedun, Ka Leung Lam, Tesfaldet Gebreegziabher, Chi Wai Hui * Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong article info Article history: Received 23 November 2012 Accepted 22 March 2013 Available online 2 April 2013 Keywords: Pyrolysis Multi-stage pyrolysis Waste-tire Energy Optimization abstract Pyrolysis process is considered as a beneficial option in waste treatment largely due to the products generated and the energy recovery when compared to other methods. In the conventional pyrolysis process, heat is continually supplied to the reactor until the final pyrolysis temperature is attained. The reactor is then maintained isothermally at this temperature until the pyrolysis is completed. This technique does not take into consideration the mechanism of the pyrolysis which involves both exothermic and endothermic reaction and the opportunity of gaining some processing benefits is often ignored. Multi-stage pyrolysis which is an approach to carry out pyrolysis with multiple heating stages in order to gain certain processing benefits has been introduced in our earlier works. 22.5% energy reduction was achieved in our past work with a 100% increase in completion time. This work therefore proposes the optimization of the operating parameters in multi-stage pyrolysis in order to limit the increase in completion time and also reduces the overall energy. This innovative approach can achieve a range of 24.7%e37.9% reduction in energy usage with 37%e50% increase in completion time depending on the heating rate for each heating stages. This approach has also been used for charcoal production. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction One of the thermochemical processes used in waste treatment is the pyrolysis process. This is largely due to the products generated and the opportunity to recover energy when compared to other methods. Pyrolysis process is an overall endothermic process which requires net energy input to complete the process [1]. Yang and Roy further explained that pyrolysis involves series of both exothermic and endothermic processes [2]. During the pyrolysis process, organic matters are cracked down into smaller fractions and this results in the given out of energy (exothermic reactions) [2]. As the temperature increases, secondary products are formed as well as volatile during the vaporization reaction (endothermic reaction) [2,3]. Conventional pyrolysis process does not take into consideration the opportunity to gain certain processing benefits during the heating of the reactor resulting in excess amount of heat being applied into the process. This contributed to the limitation of py- rolysis as an energy intensive process [3]. Furthermore, during pyrolysis, there exists significant heat barrier since pyrolysis is completed earlier at the surface of the particle and some heats are wasted to heat up the pyrolyzed part [4]. Multi-stage pyrolysis however, is a proposed operational strat- egy with multiple heating stages in order to gain certain processing benefits [5]. Multi-stage pyrolysis approach can be applied to solve the critical issue related to conventional pyrolysis. The concept of multi-stage approach in thermochemical methods is not entirely new. Recently, Gómez-Barea et al. [6] proposed a 3-stage (multi- stage) gasification system to increase the conversion of char and tar in fluidized bed gasifiers. They declared their new process as promising since it helped to increase the flexibility and capacity of existing staged gasification developments. The feasibility of the multi-stage process as a method of reducing the overall energy usage in pyrolysis has been demon- strated in previous modeling work [5] and also experimentally in our work on charcoal production via multi-stage pyrolysis [7]. In waste tire multi-stage pyrolysis [5], we proposed four stages: (1) heating (2) adiabatic (3) heating and (4) adiabatic. Only time duration was used as the basis for switching between the different stages and also same heating rate was applied for the two heating stages. An energy reduction of 22.5% was achieved in earlier work [5] but with up to 100% increase in completion time. The switching temperature between different stages is very important in multi-stage pyrolysis and the extent of it can deter- mine both the completion time and the overall heat requirement. For example, in waste tire pyrolysis, if the temperature to start the * Corresponding author. Tel.: þ852 2358 7137; fax: þ852 2358 0054. E-mail addresses: keoyedun@ust.hk (A.O. Oyedun), kaleung.lam@family.ust.hk (K.L. Lam), tgaa@ust.hk (T. Gebreegziabher), kehui@ust.hk (C.W. Hui). Contents lists available at SciVerse ScienceDirect Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng 1359-4311/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2013.03.043 Applied Thermal Engineering 61 (2013) 123e127