Gasification and porolysis
Introduction: Gasification and pyrolysis are two thermochemical technologies that have the potential to contribute in this direction and play a key role in the expansion of bioenergy. As shown in Table 1, the former involves the transformation of a carbonaceous feedstock into a gas, usually called syngas or producer gas, by exposure to high temperatures (850-1000ºC) under mildly oxidizing conditions (usually sub stoichiometric oxygen or/and steam).
Gasification was developed in the early 19th century for the transformation of mineral coal into town gas that was used for lighting and domestic energy applications. It was subsequently adapted for the treatment of organic wastes, petroleum fractions and biomass. Syngas can be used with higher energy efficiency than the original feedstock in burners, engines and turbines for the generation of heat and electricity. After necessary processing and upgrading, syngas may also be used as a chemical feedstock for the synthesis of ammonia, methanol, synthetic liquid fuels or purified hydrogen. Biomass gasification has already passed the demonstration stage and can be regarded as a young commercial technology, with plants of varying scale operating around the world. However, it is also true that the penetration of this technology in the energy market is still limited due to technical problems associated with the formation of tars and the fuel quality of the resulting gases (Knoef, 2005; Bandeau et al., 2009).
Pyrolysis is another thermochemical technology: capable of transforming lignocellulose biomass into high value products. The term pyrolysis is very self-explanatory in its root, deriving from the Ancient Greek words pyro (πρ) meaning heat and lysis (λύσις) meaning rupture. Pyrolysis is mainly associated with thermal decomposition of organic compounds, as they are heated in the absence of oxygen or any other reactive element.
The pyrolysis of lignocellulose biomass gives rise to its transformation into a non-condensable gas, condensable oil and a solid char, which can be used for their energy content or as a chemical feedstock. As will be explained throughout this paper, this technology is highly versatile, with product yields and characteristics highly dependent not only on biomass feedstock but also processing conditions. As shown in Table 1, pyrolysis processes are usually classified in two categories depending on the target product: solid charcoal in slow pyrolysis and liquid oils in fast pyrolysis.
Figure. Typical product yields, reaction conditions and enthalpy in the pyrolysis of biomass, compared against combustion and gasification technologies
Centuries in rural societies for the production of charcoal (Lehmann and Joseph, 2009). Charcoal is more stable and has a higher energy density that the original biomass, which allows for storage for longer periods of time and produces higher temperatures upon combustion. Slow pyrolysis involves heating the wood slowly to temperatures around 300- 600ºC in the absence of oxygen over long periods of time. The process results in the thermal decomposition of the feedstock into a volatile fraction and a solid char. The resulting vapors are allowed very long residence times inside the reaction chamber in order to maximize recombination and polymerization reactions, leading to the formation of the solid char (30-40 wt.%). The anoxic conditions were traditionally achieved by covering the piles of wood with turf or moistened clay. At present, the process is conducted in kilns and retorts which achieve higher carbon efficiencies and reduce processing times. The vapors generated in slow pyrolysis processes are usually condensed in order to reduce emissions into the atmosphere. This condensate, called pyro ligneous oil, amounts to 15-25 wt.% of the original biomass Feedstock and consists of a mixture of water, methanol, acetic acid, phenols and other insoluble organic tars (Strezov et al., 2007; Lehmann and Joseph, 2009).
In a gasifier, the carbonaceous material undergoes several different processes:
1. Thedehydration or drying process occurs at around 100°C. Typically the resulting steam is mixed into the gas flow and may be involved with subsequent chemical reactions, notably the water-gas reaction if the temperature is sufficiently high enough .
2. The prolysis (or devolatilization) process occurs at around 200-300°C. Volatiles are released and char is produced, resulting in up to 70% weight loss for coal. The process is dependent on the properties of the carbonaceous material and determines the structure and composition of the char, which will then undergo gasification reactions.
Counter-current fixed bed ("up draft") gasifier: A fixed bed of carbonaceous fuel (e.g. coal or biomass) through which the "gasification agent" (steam, oxygen and/or air) flows in counter-current configuration.The ash is either removed dry or as a slag. The slagging gasifiers have a lower ratio of steam to carbon, achieving temperatures higher than the ash fusion temperature. The nature of the gasifier means that the fuel must have high mechanical strength and must ideally be non-caking so that it will form a permeable bed, although recent developments have reduced these restrictions to some extent. The throughput for this type of gasifier is relatively low. Thermal efficiency is high as the gas exit temperatures are relatively low. However, this means that tar and methane production is significant at typical operation temperatures, so product gas must be extensively cleaned before use. The tar can be recycled to the reactor.
In the gasification of fine, undensified biomass such as rice hulls, it is necessary to force air into the reactor by means of a fan. This creates very high gasification temperatures, at times as high as 1000 C. Above the gasification zone, a bed of fine, hot char is formed, and as the gas is forced through this bed, most complex hydrocarbons are broken down into simple components of hydrogen and carbon monoxide.