In China Industrial Competitive Intelligence Research, Wuhan city circle is a base for grain, oil and cotton, which contains Wuhan, Huangshi, Ezhou, Xiaogan, Huanggang, Xianning, Xiantao, Tianmen, Qianjiang, where agriculture is developed and arich source of biomass energy. Accord­ing to the essential characteristic of village biomass and its distribution, combining the status of biomass utilization, two important comprehensive utilization projects of two main biomass sources are put forward, which are the polygeneration scheme of cotton stalk system (Fig. 4.10 and Fig. 4.11) and the rice husk system (Fig. 4.12).

It is found that the straw enrichment area can be classified into two categories. In the first category, there are cotton stalks in the field, and other resources are classified according to the production of straw or wheat-straw. In the second category, there are no cotton stalks in the fields and they have plenty of straw. The two types of drawing the all-round systems are shown in Figure 4.10 and 4.11, respectively.

The utilization of straw in the polygeneration system is effective. Without external basic energy, subsystems are coordinated to eliminate secondary pollutants, and overcome the problem that single technology of using biomass suffers from with poor economic returns, and by generation of secondary pollution.

As is shown in Figure 4.12, the rice husk poly-generation system connect the rice processing and rice husk utilization, making full use of the energy in the rice husk, and solving pollution problem of secondary ash accumulation. This system consists of some subsystems, such as rice husk gasification power generation, rice processing, rice drying, and rice coke burning systems. The rice husk poly-generation systems decreases the cost of producing rice because it needs to consume not any external energy. At the same time, the system supplies power for its own operation, while transferring the dump power to resident’s homes. The ash can be used for steel works and white carbon black production. It shows a great potential economic value.

The temperature is one of the most important gasification parameter influencing the product gas composition in the gasification process. A higher temperature will result in higher conversion efficiency and thereby increase the product gas yield and in a decrease in tar content (Kumar, 2009; Narvaez, 1996). The contents of H2, CO, CO2 and CH4 are affected due to a simultaneous interplay of the reactions (6.3) to (6.6), when the temperature is increased. At temperatures above 750-800°C, the H2 and the CH4 content will increase and decrease, respectively, as a result of the endothermic nature of the H2 production reactions (6.4), the water gas reaction and (6.5), the steam reforming as well as a reverse of the exothermic water gas shift reaction (6.6). If the temperature is further increased to temperatures above 850-900°C, also the reverse Boudouard reaction (6.3) plays a substantial role together with the reactions (6.4) and (6.5).

3.2.2 Digestion

Anaerobic digestion is a biological process, which produces biogas. It is performed on the waste (at T = 30-35°CoratT = 50-55°C) by two types ofbacteria and it involves two steps: (i) breakdown of complex organics in the waste by acid-forming bacteria: into simpler compounds, including volatile acids (e. g. such as acetic and propionic); and (ii) the conversion of these acids by methane — producing bacteria into CO2 and CH4 called “biogas”. Typically both steps are performed in a single tank (GTI reports) and biogas contains mainly CH4 (~60%), CO2 (~35%) a mixture of H2, N2, NH3, CO and H2S (~5%). The heat value of biogas is about 22350kJ/m3 for a mixture of CH4:CO2:inerts = 60:35:5. The investigations on digester-based energy conversion systems involving high moisture and/or high ash (HA) animal biomass (typically collected with soil, Fig. 3b) have mostly dealt with capturing biogas from biological systems such as anaerobic digesters. The percentage of CH4 may be reasonably predicted using atom conservation equations for the reaction between digestible solids and H2O (Annamalai and Puri, 2007):

CH1.98O0.83N0.086S0.0084 (s) + 0.09 H2O (£) ^ 0.54 CH4 (g) + 0.46 CO2 (g) + N0.086S0.0084 (s)

(3.1)

Even though very little water is consumed (0.09 kmoles of H2O per unique carbon atom in fuel (or empirical kmole of fuel), the bacteria can survive only in dilute slurry of water and digestible or volatile solids (VS). Element conservation yields 54% CH4 and 46% CO2. One SI tonne of liquid manure with 5% dry matter (DM) produces about 20 m3 biogas (Gregersen, 2009). There were only 40 operational systems in this country as of June 2004 (USEPA, 2004). Further discussions of biological energy conversion of manure-based biomass solids can be found in Meyer (2003), Matthews et al. (2003) and Schmidt et al. (2000). The digester efficiency is defined as the ratio of volatile solids (VS) converted into gas to VS fed in. For onion waste the digester efficiency is about 54% (Romano, etal., 2004; Gunaseelan, 1997, 2004).

Up to 162 digestion systems were operating in the USA as of 2010 generating 450 million kWh (402 million as electricity and others as supplemental fuel, mixed with natural gas; 2.8 million barrels or equivalent to 25,000 homes per year i. e. average power consumption per home 20.5 kW) with 15 new digesters every year (Agstar Bulletin 2011; also see GTI reports). Digesters pro­duce a renewable fuel in the form of CH4 which also has a higher global warming potential when compared to CO2 (20 times the global warming potential (GWP) compared to CO2). Thus approx­imately 246,000 tonnes of CO2 avoided; by capturing CH4 and CO2 in digester (as opposed to releasing CH4 and CO2 during atmospheric natural digestion), 1.1 million tonnes of CO2 equiva­lent is destroyed. The majority of the plug flow reactors operate at mesophilic temperatures of about 35-40°C; others include covered lagoons; and about 25% of them co-digest with other organic wastes (food waste, agricultural wastes, cheese whey, etc.). Typical yields are as listed in Table 3.1.

Agricultural residues mainly refer to straw, stalks and husk of crops. In China, the main crops include rice, wheat, corn, beans, tubers, sorghum, coarse grains, oil bearing crops, cotton and sugarcane (Li et al, 2005). Presently, the usage of agricultural residues include cooking and heating in rural households, fertilizer, forage and the raw material of paper (Li et al, 2005). The forest residues are usually categorized into this type especially in agricultural areas. They come from fuel wood and waste of forest industries which are widely available in rural China but with unbalanced distribution (Li et al, 2001).

Agricultural residues can be identified as two types. Primary residues are the biomass generated during the harvest (e. g. rice straw, sugar cane tops) which are usually used as fertilizer or animal feed and are hard to collect (Bhattacharya et al, 2005). Comparably, secondary residues refer to the co-produced residues during the further processing after harvest such as rice husk and bagasse. Relatively large quantities of secondary residues are easy to get at the processing site without further transportation and handling cost (Li et al, 2005) and thereby are considered as a suitable biomass resource for commercial purpose of energy generation. Energy potential from agriculture is expected to be 5.31 EJ in 2010 (Bhattacharya et al, 2005). However, few

data are available about distribution between primary and secondary residues and further work is required here.

In general, agricultural residues are widely available in China but with unbalanced distribution among regions (Li et al., 2001). East and middle-south China have the largest portion of total production (in order of 10,000 tonnes/y) while Northeast China has the highest per capita pro­duction (in order of 100kg/y), which is shown in Table 4.1. For energy purposes, a three-stage calculation model has been developed by the National Development and Reform Commission (NDRC, 2008), the total production, the accessible amount and the energy potential. According to NDCR’s data for 2005, 0.3 billion tonnes of agricultural residues can be used for energy purposes which is equal to 0.15 billion tonnes standard coal.

This parameter is essential for the optimization of the energy conversion process, because inad­equate particle size can cause the following problems: clogging or system damage in conveying and transportation, bridging in storage and conveying systems, increasing resistance to air flow in aeration and drying, inhibition of particle spreading on fire beds, dust formation during trans­portation, combustion efficiency and emissions control. There are three methods available to determine biofuel size: oscillating screen, vibrating screen and rotating screen method; how­ever, they all measure the quantity of biomass, which is sieved through screens of varying dimensions.

The methods to remove tars from product gas are in principle thermal, by for example by partial oxidation or direct thermal exposure or catalytic. In these processes, tars decompose to form additional product gas. Tar destruction can be accomplished thermally only at above about 1200°C or with catalysts at moderate temperatures of 750-900°C. These approaches have the potential to increase conversion efficiencies while simultaneously eliminate the need for collection and disposal of tars. The catalytic cracking of tars can be very effective, up to >99%.

6.4.2.2.1 Thermal processes for tar destruction

While catalysts facilitate tar destruction at intermediate temperatures, tars can also be cracked thermally at higher temperatures, typically above 1000°C. The minimum temperature required for efficient tar destruction is not well characterized; it depends on the types of tars formed in the gasifier. Thus, thermal destruction of the oxygenated tars from updraft gasifiers might be treated at lower temperatures than the refractory ones from high temperature reactors. Apart from economical and material problems, thermal decomposition at high temperatures can lead to soot formation, which can be even more troublesome than the aromatics. The difficulties of achieving complete thermal cracking, in parallel with operational and economic considerations, often make thermal cracking less attractive in large-scale gasification systems.

6.4.2.2.2 Catalytic processes for tar destruction

This approach has the potential advantage that tars can be rapidly destroyed as they are formed or just thereafter, eliminating downstream problems. The tars are cracked to smaller molecules on the catalyst surface, and the mechanisms of tar destruction are reasonably well documented. The ideal catalyst may be characterized as follows: effective in the removal of tars, resistant to deactivation, easily regenerated, strong and inexpensive. Additionally, they may be capable of reforming methane thus providing a suitable syngas ratio (if the desired product is syngas) (Sutton, 2001).

Tar decomposition catalysts used in biomass conversion can be divided into two different types depending where in process the catalyst performs; primary and secondary catalysts. The primary catalysts are added directly into the biomass prior to gasification whilst the secondary catalysts are placed in a secondary reactor downstream from the gasifier.

The primary catalysts are added directly to the biomass prior to its gasification. The addition can be done either by wet impregnation of the biomass material or by dry mixing of the catalyst. These catalysts may catalyze the gasification reactions, but their main purpose is to reduce the tar content of the product gas. They operate at the same conditions as the gasifier, they are usually non-renewable and consist of low-cost disposable materials. Normally, they have little effect on the methane and C2-3 hydrocarbon conversion. It has been noticed that the primary catalyst alternative tend to be more problematic with respect catalyst deactivation, erosion and elutriation. Therefore, more emphasis has been put on studying and investigating the separate secondary catalytic bed alternatives.

The secondary catalysts are placed in a secondary reactor downstream the gasifier. Since the catalytic bed operates independently of the gasifier, these catalysts can operate under different process conditions than those of the gasification unit. This type of catalyst is also active in hydrocarbon reforming and often in methane reforming.

Additional advantages with this alternative are that the pyrolysis reactions do not continue in the catalytic reactor and that it is easier to arrange for a good catalyst-gas contact in a separate catalytic reactor. Particles and other impurities may also be removed before catalyst exposure

Figure 6.12.

and the temperature and other process conditions in the catalyst reactor may as mentioned, be controlled separately from those of the gasifier.

Several different materials have been investigated and classified as potential catalyst. Fig­ure 6.12 illustrates the different types of catalytic materials (Abu El-Rub, 2004). The catalytic materials most comprehensively studied are dolomites, both as primary and secondary catalysts, nickel-based, mainly as secondary catalyst and alkali metals, mainly as primary catalyst.

The biomass consists of several components including hemicellulose, cellulose and lignin. Since the activation energy is related to the bond energy and bond energy varies widely within biomass fuels having multiple components, it can be assumed that the pyrolysis process consists of an infinite number of reactions proceeding in parallel with E ranging from 0 to infinity (Anthony et al., 1973). The parallel reaction model (PRM), can also be called the distributed activation energy model with a large number of reactions in order to avoid confusion with the two or three single reactions proceeding in parallel. If Smv E is the mass change within a short period of time dt having activation energy between E and E + dE, the rate of liberation of volatiles for a first order pyrolysis can be written as:

where the specific reaction constant kE [1/s] is given by the Arrhenius expression:

where B and E are the pre-exponential factor and activation energy, respectively.

Assuming a Gaussian distribution, the fraction of initial total volatiles mass having activation energy betweenE and E + dE can be expressed as:

and:

J f (E)dE = 1

0

where Em is the mean activation energy, and a is the standard deviation of activation energy. The Gaussian distribution indicates that 1% mass has activation energy within E < Em — 2.3a; these E values refer to low activation energy components of the volatiles. Similarly, 1% of mass corresponds to high activation energy components with E > Em + 2.3a. Thus 98% of mass is covered for Em — 2.3a < E < Em + 2.3a while about 99.9% of the mass is located for E such that Em — 3a < E < Em + 3a. Assuming pre-exponential factor B is the same for all volatiles having activation energy 0 < E < ж and equal to B and integrating over all possible positive values of E will provide the volatile fraction:

(3.30)

With further rearranging, equation (3.30) becomes:

Note: the limits of integration have been changed from 0, <x to Em ± 3a that covers 99.9% of total mass. Defining:

G can be represented as a 2D matrix for values of E between Em — 3a and Em + 3a and values of T between T q and T n, where T 0 corresponds to temperature at the beginning of pyrolysis (99% VM remaining) and Tn corresponds to temperature at the end of pyrolysis (1% VM remaining), respectively (Martin, 2006; Chen, 2012b). With equation (3.32) in equation (3.31):

(3.33)

The equation (3.33) can be broken down into:

G(Em — 3a, Tq) G(Em — 3a, Tq + AT)

G(Em — 3a + AE, Tq)

G(E, T)

G(Em + 3a, Tq + nAT)

(3.34)

where T = Tq, Tq + AT, Tq + 2AT, T0 + 3AT,…, Tn = T0 + nAT, Em — 3a < E < Em + 3a.

Note that total number of terms in the G matrix will increase as the temperature T is increased or as AT is reduced. The value forB was set at 1.67 x 1013 [1/s] from transition state theory (Anthony et al., 1973). Assuming Em and a, volatile mass fraction {mv(T)/mv0} can now be calculated at a selected T by using G (E, T) in equation (3.33). Let the error between the calculated and measured mv(T)/mv0 fromTGAbe Sj at selected T = T j. The values for Em and a were calculated by minimizing the summed squared error Ejej at all selected data points within the domain of pyrolysis.

A spread sheet program was developed to determine the values of Em and a for the minimum most Ejs2. In the spread sheet, first the value of Em is fixed and a is varied. For a fixed value of Em, there is a value of sigma that will produce the minimum amount of error Ejs2. Then Em is varied and the combination of Em and a that produces the minimum most error can be determined.

According to the report named Post-evaluation by Case Study of Various Technical Solutions to Biomass (crop straw) Power Generation in Jiangsu, two models of co-combustion are introduced in the book. They are Fengxian XinYuan Biomass CHP Thermo Power Co., Ltd and Baoying Xiexin Biomass Power Co., Ltd.

Fengxian XinYuan Biomass CHP Thermo Power Co., Ltd in Xuzhou, was put into operation in 2003. The total investment of the project is 250 million Yuan, of which registered capital is 66 millionYuan. The project construction started in March 18, 2003. Scale in stage 1 covers an area of 220 mu (1 mu = 614 m2), using three 75 t/h sub-high temperature and sub-high pressure circulat­ing fluidized bed boilers produced by Jinan boiler factory and two 15 MW extraction condensing turbo generator units and related auxiliary equipments produced by Nanjing turbine factories, in which units 1 and 2 respectively went into operation in October 17, 2003 and November 26, 2003 in way of co-generation.

Fengxian XinYuan uses two different kinds of fuels, biomass (renewable) and coal (non­renewable). Types of coal include coal, coal gangue and slime and so on. Biomass is mainly from rice husk, loose sawdust and compression molding sawdust. In addition, crushed poplar branches, fruit tree branches (Fengxian is rich in fruits such as apples and peaches), bark as well as forestry residues also can be used.

At present, the mixing ratio of biomass is approximately 20% (mass ratio) in this plant, where coal is still the main fuel. The co-combustion model establishes a good foundation for steady operation and reduces the risk of discontinuous supply of biomass fuels.

Biomass mixed to burn in Fengxian XinYuan is mainly rice husk and sawdust, which are particle fuels and can be directly put into furnace without pre-treatment.

The feeding process of the case is as follows: Biomass is delivered to the hopper first, then transported into the pipeline from the bottom of the hopper and next sent to the biomass and coal mixture room by “V”-shaped belt conveyor. The coal is from pipelines in the vertical direction of the biomass transportation pipeline. After mixing in the hopper, coal and biomass fall on the belt conveyor and then are sent to the storage bin and furnace.

The power plant adopts Combined Heat and Power (CHP) and supply heating mainly, power secondly, which is a thermal power plant of half-public welfare. Heat output of the plant in 2006 is

289,0 tonnes, which is mainly supplied to 15 to 16 enterprises of the industrial park in Fengxian, with price of steam 110 Yuan per tonne on average. The total generation capacity is 210 million kWh, of which net generation to grid is 189 million kWh (electricity consumption in the plant is 10%).

The mixed ratio of biomass in Fengxian XinYuan is 20% (mass ratio), which fails to meet the requirements of the country about the electricity subsidy enjoyed by co-combustion power

plant (ratio of biomass accounted in total heating value of power generation consumption exceeds 80%), so they are unable to enjoy the subsidy of electricity price which is 0.25 Yuan/kWh.

The basic principle of a fluidized bed combustor consists of distributing air homogeneously across a combustion area where a bed of fuel and inert mineral particles is fluidized, meaning that the air velocity achieves the mass suspension of the bed particles. This is accomplished by blowing air at relatively high pressure, (around 0.15 bar relative with respect to grate firing where five times lower pressures are considered) through a distribution plate on top of which the bed of fuel and inert is positioned. A fluidized bed burner is therefore a relatively high cylindrical or square sectioned vessel that hosts on the bottom part a perforated plate positioned to divide the furnace from a pressurized air-box.

The inert material, which usually represents up to 98% of the bed material, acts as a grate because its supports the fuel which floats in it while also contributing to abrasion of the fuel particles and also acting as a refractory by radiating heat and providing conduction thanks to the continuous turbulent collision with fuel particles. The high turbulence optimizes combustion hence low excess air is required (from 10 to 30%) and the mixing effect may be used to reduce emissions as formed by injecting reactants such as urea for NOX control and limestone for acid gases abatement, directly into the bed.

The plate and holes diameter and geometry is of primary importance to achieve an efficient air distribution which will prevent short-circuiting through a portion of the bed (channeling) and avoid the ejection of the bed (plugging) for missed fluidization. Temperature control is also particularly critical to avoid ash melting and slagging within the bed which would cause the inert particles to agglomerate and the bed to collapse. Typical combustion temperatures therefore will be kept in the 650-900°C range (Obernberger, 1998).

During startup of a fluidized bed combustor air velocity increases until it reaches the terminal velocity where the aerodynamic drag force on the particles equals its buoyancy and weight. The particles within the agglomerated bed start to disengage and the bed swells, increasing its height within the reactor, accordingly the pressure drop across the bed increases until a maximum value is achieved, corresponding to the maximum bed height. Further increases in the air velocity (within a certain range) from this value, named minimum fluidization velocity, will not modify significantly either the bed height or the pressure drop across it.

A fluidized bed working slightly above the minimum fluidization velocity is called a stationary fluidized bed or Bubbling Fluidized Bed (BFB, Fig. 5.20) because the air flowing in the bed forms bubbles that are dragged by the air flow and create typical movements and ruptures of the bed surface, when they reach the top layer, that resemble a boiling liquid. The bed material is usually silica sand of about 0.5-1.0mm in diameter; the fluidization velocity of the air varies between

1.0 and 2.0 m/s. The secondary air is introduced through several inlets in the form of groups of horizontally arranged nozzles at the beginning of the upper part of the furnace (called the freeboard). BFB furnaces are usually considered for plants with a nominal boiler capacity of over 10-20 MWt.

If the air velocity is further increased up to 5-10 m/s, and smaller particles of sand are used (0.2-0.4 mm in diameter), a point is reached when pressure drops across the bed plunge dramati­cally and pneumatic transport of bed particles is achieved. This working condition is characterized by mass extraction from the bed which must be recuperated from the flue gases, through solid separation devices such as cyclones, and continuously returned to the bed. A burner working in this manner is called Circulating Fluidized Bed (CFB, Fig. 5.21).

Figure 5.20. Schematic of a Bubbling Fluidized Bed (BFB).

The sand particles are carried with the flue gas, separated in a hot cyclone and fed back into the combustion chamber. The bed temperature normally is comprised between 800-900°C and it is controlled through an internal heat exchanger. The fuel amounts only to 1-2% of the bed material and the bed has to be heated before the fuel is introduced.

Biomass pyrolysis refers to thermal decomposition in the presence of little or no oxygen, while biomass gasification involves pyrolysis and partial oxidation in a well-controlled oxidizing envi­ronment. Depending on the process variables, such as the reactor temperature and residence time, the biomass pyrolysis yields various amounts of gaseous, liquid, and solid products of varying compositions. For instance, conventional pyrolysis, which has been utilized for thousands of years, involves lower temperatures and longer residence times with the principal product being the solid char. In contrast, fast pyrolysis involves moderately high temperatures (~500°C) and short residence times (~2 s), with the main product being a dark brown liquid or bio-oil along with other gaseous, liquid and solid products, including char. This process is much more com­monly used at present compared to conventional pyrolysis. While most agricultural and forestry residues can be used in fast pyrolysis, most work has focused on wood-based feedstock, including hemicellulose, cellulose and lignin. The pyrolysis process generally requires about 15% of the energy available in the feed, which can be provided by the combustion of char or a combination of char gasification and combustion of resulting producer gas. Note that char and gas are the two main by-products of pyrolysis, which typically contain about 25 and 5% of the energy in the

feed, respectively. Other means of supplying the required energy may include the combustion of bio-oil, fresh biomass, or fossil fuel, depending upon the reactor design and regional conditions.

Bridgwater (2011) and Mohan et al. (2006) provide reviews on fast pyrolysis and the properties of bio-oils generated from this process. The effects of various process parameters on the overall reaction rate, volatile yields and products formed are extensively discussed in these reviews. Such parameters include the biomass composition and structure, reactor temperature, heating rate, and residence time. Various gaseous and liquid fuels produced from bio-oils are also discussed. As stated earlier, fast pyrolysis in general involves high heating rates with a reaction temperature of around 500°C, rapid cooling of the pyrolysis vapors to yield bio-oil, which is the main product, and a rapid removal of product char to minimize cracking of vapors. It is characterized by the strongly coupled processes of heat and mass transport, phase change, and chemical kinetics. As discussed by Bridgwater (2011), a critical factor is to bring the reacting biomass particles to an optimum temperature and minimize their exposure to lower temperatures that favor the formation of charcoal. While there have been studies on the kinetic and thermal decomposition mechanisms for the pyrolysis of plant biomass, various processes associated with fast pyrolysis are generally not well understood.

The major product of pyrolysis is a dark brown liquid or bio-oil, which has approximately the same elemental composition as the original biomass. It consists of a complex mixture of oxygenated hydrocarbons with a varying but appreciable amount of water from both the original moisture and reaction product. Note that the presence of water makes bio-oils immiscible with petroleum-derived fuels. The physical properties of bio-oils are discussed in Czernik (2004). Proximate analysis of the bio-oil gives a chemical formula of CH19O07. The typical heating value of bio-oils is about 17MJ/kg, which is about 40-45% of that of hydrocarbon fuels. Figure 2.3a from Bridgwater (2011) shows typical organics yields from different feedstocks and their variation with temperature, while Figure 2.3b shows the temperature dependence of the four main products, namely organics, char, gas, and water, from a typical feedstock.

In addition, Bridgwater (2012) lists the physical properties of a representative wood-derived bio-oil. The pyrolysis chemistry of different biomass feed stocks is discussed inBridgwater (2012). Bio-oils can be utilized in several different ways to produce energy, fuels, and chemicals. They have been used directly as fuels in stationary applications, especially for electricity generation. A more sustainable approach is to produce conventional fuels for transportation and power generation using either an integrated facility or a decentralized operation. Such fuels include diesel, gasoline, kerosene, methane, liquefied petroleum gas, and others. An integrated facility involves a refinery­like operation with biomass pyrolysis followed by preprocessing, deoxygenation, and refining of

bio-oils. A schematic of such a facility is depicted in Figure 2.4 from Mohan (2006). As discussed in this reference, the decentralized operation has received much interest in recent years. In such an operation, bio-oils or bio-oil-char slurries produced from biomass pyrolysis are transported to a central processing plant for gasification and synthesis of hydrocarbon transport fuels, such as Fischer-Tropsch (FT) fuels and alcohols. While there is some energy penalty associated with transportation and additional bio-oil gasification, it may be compensated by the economy of scale that can be achieved in a gasification and fuel synthesis plant on a commercial scale.

The modeling of pyrolysis processes is extremely complex (Niksa, 2000) due to the wide variation in biomass composition and the amount and number of products formed. Most previous work has focused on developing empirical or global kinetic models for predicting the rate of production of various species, including char, bio-oil, liquids, and other liquid and gas species, formed during pyrolysis. Varhegyi et al. (2011) performed thermo-gravimetric experiments to examine the pyrolysis of different feedstocks, and reported a distributed activation energy model using three pools of reactants. Brown et al. (2001a, b) studied experimentally and numerically the chemistry of biomass and cellulose pyrolysis in a laminar entrained-flow reactor using a molecular — beam mass spectrometer. Computational fluid dynamics (CFD) simulations were performed to model the transport and chemical processes in the reactor. It was observed that the primary cellulose pyrolysis products underwent subsequent secondary reactions. A rate law was developed to describe the thermal conversion of these products. While such studies have provided valuable information on the overall pyrolysis kinetics, there is scope for more fundamental research using surrogate mixtures to examine the transport and thermochemical processes associated with the biomass pyrolysis and subsequent conversion of bio-oil to fuels. Significant research is also needed on catalytic processes for the production of various gaseous and liquid fuels.

In summary, the potential of using biomass pyrolysis and subsequent refining of bio-oils to produce second-generation biofuels is increasingly being recognized. Similar to a petroleum refinery, a biorefinery concept may provide a sustainable and value-added approach for the use of biomass to produce energy, fuels and chemicals. This concept is particularly attractive for biomass because of its chemical heterogeneity and regional variability. However, the chemical composi­tion of biomass, approximately (CH2O)„, is quite different from that of petroleum, (CH2)„, and, therefore, the range of primary chemicals derived from biomass and petroleum will be different. In (Bridgwater, 2011) a schematic of an integrated pyrolysis-based biorefinery concept is shown. It indicates that bio-oils produced from pyrolysis can be processed to provide various gaseous and liquid fuels. These fuels are mostly compatible with conventional fuels, but are cleaner.

Consequently, they can be deployed without significant changes to existing infrastructure. More­over, as discussed in the next section, the biomass gasification can be used to make syngas, a mixture of H2 and CO, for subsequent synthesis of hydrocarbons, alcohols and other chemicals. However, this route may be quite energy intensive, and its cost effectiveness and environmental benefits need to be examined. It may be more economical to use syngas directly for electricity generation.