Fast Pyrolysis as a thermochemical conversion route of biomass particles produces numerous complex compounds as can be deduced from the discussions section 2.1. This section intends to look at the pathways of biomass degradation as a whole.
Chemical engineering applications tend to categorise the products of biomass pyrolysis into three broad groups namely permanent gases, pyrolytic liquid (bio-oil) and chars. These products are a result of the primary decomposition of the biomass particles as well as the secondary decomposition of volatile condensable organic vapour products into low molecular weight gases and char [56].
The very complex nature of the reactions taking place during a fast pyrolysis process means that many homogenous and heterogeneous reactions occur simultaneously and consecutively. These reactions are governed and directed primarily by the reaction conditions [35, 36]. The overall kinetic studies of fast pyrolysis reactions are undertaken as a means of representing them as individual and distinct reaction pathways. This allows for the study of reaction parameters on the yields and compositions of the process.
The complexity of the numerous fast pyrolysis reactions that occur under process conditions makes it a fairly difficult field to study. Consequently, simplifications must be incorporated to kinetic studies.
Fast pyrolysis kinetic studies serve as an important part of the design engineering applications for liquid production. As pointed out by Boukis [5], the study of process kinetics play an important role in
· The development of diagnostic tools in order to evaluate the importance of process parameters on the products
· The prediction of the effect of process parameters such as heating rates, reactor temperature, particle size and residence time on the product yield and characteristics in order to aid the optimisation of the process
· The development of better reactor design techniques
· The optimisation of the parameters for the desired end product.
Three major categorisations in literature exist for kinetic studies of wood pyrolysis. Di Blasi gives a comprehensive explanation of the kinetic models [46, 57]. The major kinetic models of wood pyrolysis are
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Overall decomposition models: Overall decomposition models of wood pyrolysis employ a single reaction to describe the thermal degradation of solid biomass to char and volatiles.
This is achieved by measuring the weight loss of the biomass particles as a function of time and temperature.
One stage multi reaction models: Kinetic Models in this category employ step wise degradation of biomass to primary vapour products, char and gaseous species. These reactions are used to determine rate constants for the formation of the products.
Two stage semi global models: Kinetic models in this category are used when the aim of the study is to determine rate constants for primary pyrolysis reactions and secondary reactions of the evolved volatile products.
Several researchers including Di Blasi, Graham and Peacocke [33, 56, 57] have produced detailed and insightful reviews into the many kinetic studies of wood pyrolysis undertaken over six decades. The individual review of each kinetic study is well beyond the established scope of this thesis. The reader is again referred to their excellent reviews. A brief summary of the biomass kinetic reaction models follow.
2.2.1 Overall decomposition models
The overall decomposition model of thermal degradation of biomass can be represented by the reaction shown in equation 2-1
→ ℎ + Eq 2-1
A common technique used to fit this kinetic model is Thermogravimatric Analysis (TGA).
TGA measures the weight loss of wood samples as a function of time and temperature.
The technique does not allow for the collection of volatile pyrolysis products and as such, only an overall decomposition rate. TGA studies may be carried out in conjunction with Differential Thermal Analysis (DTA) or Differential Scanning Calorimetry (DSC) which can be used to determine the heat of reactions.
Liden [58] points out that though the overall decomposition models are applicable to kinetic studies, the kinetic parameters obtained from them can vary tremendously even for similar feedstocks under similar conditions for a number of reasons which include:
· The use of the steady state temperature as the overall reaction temperature
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· Failure to take into account the feedstock composition (impurities, inorganic compounds, morphology)
· The simplistic nature of the model as it does not take into account secondary reaction pathways
· The failure to incorporate the effects of heating rates and its role in determining the extent of fast and slow pyrolysis reactions. This is especially important when modelling has been carried out using large biomass samples.
Models used to predict weight loss cannot be employed for the prediction of product distribution. Similarly, they assume that the decomposition of particles occur over an infinite period of time [59]. This assumption is particularly flawed because the fast pyrolysis process is limited by the time required for decomposition of biomass particles.
2.2.2 One stage multi reaction models
As cellulose is the most abundant component in most woods and biomass species, many of the early kinetic models relating to biomass fast pyrolysis are based on the decomposition of cellulose. One of the earliest step wise degradation models was proposed by Bradbury [60]. It was a model for the step wise degradation using a three reaction model as shown in Figure 2-11. The model was proposed to account for the variations in product yields that may occur during pyrolysis.
Figure 2-11 Bradbury model for pyrolysis of cellulose [60]
The Bradbury model indicates the initial change of wood to intermediates before formation of final products. A first order initiation reaction occurs that leads to the formation of
‘active cellulose’. The active cellulose as an intermediate has a high activation energy [60].
Diebold theorised further that though the state of the active cellulose is not known, it is considered to be in a liquid like state and has significantly lower degree of polymerisation than the initial cellulose particle.
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The resulting intermediate is further decomposed via two competitive first order reactions whereby one of the reactions yields pyrolysis volatiles and the other permanent gases and char. The concept of Bradbury’s model suggests that the cellulose molecule is not directly converted to low molecular weight vapours, gases and char but undergoes and intermediate physical and chemical transition. The hypothesis of this model is supported by research carried out by Back and Shafizadeh [5, 42, 61].
2.2.3 Two stage semi global models
Models in this category were developed from the need to quantitatively assess the rate of formation of pyrolysis products as a function of time. Though this approach is generally regarded as the most complex of kinetic studies, it is accepted as the one that best fits the fast pyrolysis process because it considers the residence time of the vapours. This is vital in ensuring maximum liquid yields. Other simpler models fail to consider the impact of residence time which makes them less suitable for studying yields during fast pyrolysis [56].
Simpler models also suggest that an increase in temperature should favour the increase in liquid yields. This is however not the case with fast pyrolysis processes as they liquid yield is reduced when temperatures are significantly higher than the optimum reaction temperature of 500 ºC. Researchers are of the opinion that for a model to accurately represent biomass pyrolysis in terms of yields, it must take into account the secondary reactions. These secondary reactions are primarily in the form of cracking and carbon deposition with the conversion of oils to char and gases [56]. This is in line with pyrolysis principles since secondary reactions in the form of vapour cracking are responsible for the reduced liquid yield at higher temperatures.
Liden [58] and Diebold [62] proposed two similar models based on the explanation given.
Their kinetic models take into account the secondary decomposition of primary tars formed. One of the key assumptions of their models was that the primary formation of tars and their subsequent decomposition were all first order reactions. The liquid yield arising from the vapours were estimated as a function of temperature and vapour residence time in the reaction zone [63]. The model proposed by both Diebold and Liden is shown in Figure 2-12.
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Figure 2-12 Diebold and Liden pathways for wood pyrolysis [62]
In the same vein, Gorton and Knight [64] also proposed a kinetic model shown in Figure 2-13 that takes into account the secondary reactions of biomass pyrolysis. The assumptions of the model were the same as that used by Liden and Diebold.
Figure 2-13 Gorton and Knight pathways for wood pyrolysis [64]
Scott et al [43] in related studies successfully used the Liden and Diebold model to predict the liquid yield of products within a temperature range of between 450 and 800 ºC. The results from that study showed that the model agreed with achieved yields within ±10% for a temperature range of between 500 and 700 ºC and a residence time of up to 1s.