Pyrolysis is a term used to describe the effect of heat on a substance such that no or little external oxidation or hydrolysis occurs. In this way, the pyrolysis can occur in an ambient environment provided combustion does not occur. Recently, the term pyrolysis has been associated with the development of an alternative means to recover energy from organic materials and is one of several possible strategies to develop energy sources from renewable sources rather than fossil fuels. There are biochemical methods for producing bioethanol as an energy source and these are based on either direct fermentation techniques for sugar rich crops or fermentation combined with chemical treatments for more woody and low-sugar plants.18 This traditional sort of fermentation process to produce bioethanol is often described as a first generation biofuel technology. Other first generation biofuel technologies include transesterification (e. g. the reaction of vegetable oil with an alcohol) and biological anaerobic digestion of biomass. Despite the modern policy driven trends towards the use of bioethanol as a fuel or fuel additive19 there is a conflict between the use of what are essentially edible crops and food security.20 The use of lignocellulosic materials as a potential biofuel source would do much to prevent fears on shortages of food that might arise as arable land is used to generate fuels rather than foods. Despite the clear need to develop low quality crop fermentation, the science is not facile and chemical/bio pre-treatments and designer yeasts are being developed to allow this technology to be delivered.21

Pyrolysis is one of a related series of thermo-chemical methods to extract energy from organics that rely on heating them and effecting a conversion of the materials. The most direct thermo-chemical method is combustion where the materials are heated in excess oxygen to form carbon dioxide, water and heat (from the highly exothermic reaction). The combustion reaction may be catalysed to maintain lower flame temperatures thereby limiting oxidation of nitrogen and the production of nitrous oxide pollutants.22-24 Energy can be recovered from the exothermicity of the combustion reaction in several ways. These include heat to drive turbines, the volume expansion deriving from the liquid expansion to gas as well as from high pressure steam raised in the combustion. Partial oxidation, when the oxygen used is significantly below that needed for complete oxidation, is described as gasification as it yields light fuel gases and CO and H2 known as syn(thesis) gas (from its use in the industrial synthesis of methanol). Catalysts and conditions (flow, pressure, contact times) can be used to control the gas products that result from this controlled oxidation.25

Pyrolysis is the heating of the organic materials (biomass, waste food, waste plastic, etc.) in the absence (or at partial pressures and/or temperatures where reduction rather than oxidation is favoured) of oxygen. Pyrolysis produces a range of products including solids (char), which is charcoal like and can be used for solid combustion systems, liquid (tar) and gaseous products both of which can also be used for energy storage, generation and transportation although the char usually requires further upgrading for optimum use.26-29 The volatile, but readily condensable, components are sometimes described by the term bio-oil or pyrolysis-oil. These terms are used because they reflect the possible use of the product as a replacement for petroleum in automotive and energy applications. The mechanisms involved in pyrolysis are exceedingly complex involving free — radical reactions.30 They are briefly summarised below.

As detailed above, pyrolysis is a complex process with products varying considerably according to the temperature and pressure used. This is because the reaction involves several different chemical reactions in both the gas and condensed phases and there are further heat and mass transport limitations which prevent an accurate representation by equilibria.31 One of the challenges in delivering cost-effective commercial technologies is modelling these complex kinetic processes which are necessary for the design of efficient plant.32 Because of this, the reaction products depend not only on the temperature and pressure but also on the rate of increase of temperature and the residence time in a reactor.

The main product of a pyrolysis reaction can be written as:

Organic feedstock ^ char + volatiles

The organic feedstock (see below) can be anything from biomass (plants and other vegetation), vegetable oils, food waste, waste polymers, animal fats, etc.

The char is a carbon-rich, low hydrogen, ash containing solid.33 It has many applications; as the name suggests as a coke substitute,34 an advanced adsorbent35 and as a soil additive to increase fertility.36 The char can also be gasified by suitable oxidants and catalysts to provide a route to syn gas.37,38 The volatiles are essentially a mixture of compounds that are condensable (in usual conditions) or non-condensable. The non-condensables are the basic components of syn gas, H2, CO, CO2 together with methane and are normally referred to as gas or syn gas.39 In general, the relative amount of the gas component compared to the condensable and solid (char) content increases with temperature.18,40,41 The gas component is, possibly, an important part of the strategy in a move towards a ‘hydrogen economy’ but there are many competitive processes to be considered.42 One of the more promising methods is the combination of pyrolysis with water gasification.43

In most cases, the most sought after component of a pyrolysis reaction is generally the condensable or liquid fraction because of its potential as a possible fuel in power stations or internal combustion engines.44,45 The liquid fraction (or bio-oil) that results from pyrolysis is a complex mixture of chemicals and contains a range of molecular weights from light hydrocarbons through to molecular weights of 200+.46 The elemental composition of the liquid (which is often described as a biofuel because of the uses outlined above) is close to that of the biomass (or other material) feedstock.47 The bio-oil is a mixture of an aqueous phase containing a variety of low molecular weight oxygenated organic compounds (methanol, ethanol, acetic acid, acetone, etc.) and a hydrophobic non-aqueous component consisting of heavier molecular weight oxygenates (e. g. alcohols, phenols, cresols), aromatics (e. g. benzene, toluene, indene) and polycyclic aromatic hydrocarbons (PAHs — e. g. naphthalenes, anthracenes).48 The bio-oil, as-produced, can be burned in engines and turbines directly. However, it is relatively unstable, acidic (and, therefore, corrosive), of relatively low calorific value compared to petroleum oils and viscous. This ensures that it has limited application for direct use in turbines or engines49 and much work has been carried out into developing methods whereby the product oil can be upgraded for practical use and this is an important application of catalytic pyrolysis.50

As briefly mentioned above, the temperature at which pyrolysis takes place plays an important role in the product distribution obtained from the pyrolysis reaction.51,52 At lower temperatures (<600 K), formation of char is favoured whilst at higher temperatures (>800 K), increased reaction rates and the breakdown of C-C bonds lead to gas formation. Intermediate temperatures favour oil production. Experimentally, much time is spent varying temperatures and conditions in order to define conditions for optimum product distribution. As well as temperature, ‘residence time’ plays a major role in defining the product distribution. In a simple batch reactor, residence time has little meaning being simply the time over which the reaction is run. Simple batch reactors are of little practical use and, instead, flow-through reactors are used for most of the research being carried out both academically and industrially. In flow-through or continuous reactors, great care is required to control residence time and this is most important in reactions of this type where the products are kinetically not equilibrium defined. Whilst full discussion of this subject is beyond the scope of this review, it is generally found: that high residence times at lower temperatures favour char production, high residence times at higher temperature favour gas synthesis whilst low residence times at higher temperatures favour liquid production.26 The requirement to control product distribution coupled to a need to generate technologies that can process industrially significant amounts of feedstock has led to several forms of pyrolysis which are often differentiated in the literature. They are differentiated by the heating rate used and the residence time in the reactor chamber. In a basic reactor for industrial use, the aim will be for the feedstock to pass through a heated zone, complete reaction and subsequently pass through separators and secondary reactors such as reformers and crackers to upgrade products.50 The residence time of the reactor will be defined by the time feedstock spends passing through the pyrolysis chamber heated zone. The heating rate is defined as the time taken to reach maximum temperature although equilibrium with the chamber temperature is not always possible and effectively the feedstock maximum temperature may be less than the actual chamber temperature. The products and unused feedstock will be separated via cooling and condensation with an appropriate recycle if needed. A complete description of reactor design is given elsewhere.53