Category Archives: Technologies for Converting Biomass to Useful Energy

CHEMICAL LOOPING COMBUSTION

Chemical looping is a special kind of combustion technology which is based on the splitting of global chemical reactions into two or more sub-reactions which take place in different reactors. Intermediate products are reacted and regenerated when sub-reactions occur.

Since intermediate products are produced in separate reactors, chemical looping allows sep­aration of more easily unwanted components which are formed during the combustion process. A particular interest for this kind of combustion/separation technology is for cleaning up the flue gases from pollutants or from greenhouse gases, thus allowing having efficient and cost-effective reduced emissions combustion. Chemical looping is also interesting to split a chemical reaction with high irreversibility into more reactions which have a global lower entropy generation.

The basic feature of chemical looping is to use a medium which can participate in the reactions and facilitate the reaction of the reactants and the production of the products. Such a medium should be highly reactive at process temperature and pressure conditions and physically and chemically stable, should allow an easy separation from the reactants and the products and have moderate exothermic to endothermic heat of reactions. A medium with these features should behave as a fluid, either a liquid or a fluidized solid, being the latter more often employed.

Chemical looping combustion is a technology that can be used with any kind of fuel, either gaseous or liquid or solid, but it is more interesting with solid fuels where conventional combustion systems do not allow easy separation of intermediate products and chemical reactors are less easy to be built and operated as efficiently as for liquid and gaseous fuels.

SYNGAS AND BIOGAS COMBUSTION AND EMISSIONS

Syngas can be produced using a variety of feedstocks and conversion processes, particularly using gasification, as discussed in the preceding section. On the other hand, biogas is generally produced by anaerobic digestion or fermentation of biodegradable materials in an oxygen-free environment (http://en. wikipedia. org/wiki/Biogas). There is significant potential for using syngas and biogas fuels for transportation and power generation. Both of these fuels represent a clean and renewable energy source, and offer great flexibility in their production and utilization. The next two sections provide an overview of the fundamental and applied research dealing with these fuels.

2.1.2 Syngas combustion and emissions

Syngas is a renewable energy source with wide flexibility in feedstock and conversion processes. Most of the harmful contaminants and pollutants can be removed in the post-gasification process prior to combustion. Moreover, technologies for its production and utilization are fairly developed, as several IGCC plants are currently operational around the world. There is also significant interest in using syngas as a transportation fuel. In addition, the use of syngas in fuel cells, such as solid oxide fuel cells, through the reforming of hydrocarbons and other routes is also being explored (Kee et al., 2005; 2008).

Considerable work has been reported on syngas combustion and emissions (Lieuwen, 2009; Cheng et al., 2009). Fundamental studies have focused on various aspects, including the devel­opment of thermo-transport and kinetic models, and examining the ignition and combustion characteristics. A major challenge identified in these studies is due to a substantial variation in its composition and heating value. This requires that the syngas combustion and emission behavior be analyzed for a wide range of composition. Thus, properties such as adiabatic flame tempera­ture, laminar burning velocity, flammability limits, flame stability, extinction, and blowout need to be determined for a wide range of syngas composition. This presents challenges while design­ing syngas combustors, requiring optimization for locally available fuels. As indicated in Table

2.1 (Kee et al., 2005), the main components in syngas are H2 and CO, with varying amounts of diluents, such as CO2, H2O, and N2, as well as CH4 in small amounts. Consequently, previous studies on syngas combustion have considered several representative compositions. Table 2.2 lists an average syngas composition, based on the values in Table 2.1.

Fundamental combustion properties can be analyzed by starting with the stoichiometric mass balance for a syngas-air mixture as:

xCO + (1 — x)H2 + a(O2 + 3.76N2) ^ xCO2 + (1 — x)H2O + dO2 + 3.76aN2

Table 2.1. Representative compositions (in terms of percentage of mole fractions) and related properties of syngas utilized in various IGCC plants; from Kee et al. (2005).

Syngas

PSI

Tampa

El

Dorado

Pernis

Sierra

Pacific

ILVA

Schwarze

Pumpe

Sarlux

Fife

Exxon

Singapore

Motiva

Delaware

PIEMSA

Tonghua

H2

24.8

37.2

35.4

34.4

14.5

8.6

61.9

22.7

34.4

44.5

32.00

42.30

10.3

CO

39.5

46.6

45.0

35.1

23.6

26.2

26.2

30.6

55.4

35.4

49.50

47.77

22.3

CH4

1.5

0.1

0.0

0.3

1.3

8.2

6.9

0.2

5.1

0.5

0.10

0.08

3.8

CO2

9.3

13.3

17.1

30.0

5.6

14.0

2.8

5.6

1.6

17.9

15.80

8.01

14.5

N2 + Ar

2.3

2.5

2.1

0.2

49.3

42.5

1.8

1.1

3.1

1.4

2.15

2.05

48.2

H2O

22.7

0.3

0.4

5.7

39.8

0.1

0.44

0.15

0.9

LHV [(Btu/ft3]

209

253

242

210

128

183

317

163

319

241

248

270.4

134.6

LHV [kJ/m3]

8224

9962

9528

8274

5024

7191

12492

6403

12568

9477

9768

10655

5304

Tfuel F/C

570/300

700/371

250/121

200/98

1000/538

400/204

100/38

392/200

100/38

350/177

570/299

338/170

H2/CO ratio

0.63

0.8

0.79

0.98

0.61

0.33

2.36

0.74

0.62

1.26

0.65

0.89

0.46

Diluent

Steam

n2

N2/Steam

Steam

Steam

Steam

Moisture

H2O

Steam

h2o/n

n2

n/a

Equivalent LHV [Btu/ft3]

150

118

113*

198

110

200

*

116

150

129

134.6

Equivalent LHV [kJ/m3]

5910

4649

4452

7801

4334

7880

4600

5910

5083

5304

*Always co-fired with 50% natural gas.

Table 2.2. Average composition and standard deviation based on syngas mixtures listed in Table 2.1.

Syngas constituent

Average [% vol]

Standard deviation [% vol]

H2

31.0

14.9

CO

37.2

11.0

CH4

2.2

2.9

CO2

12.0

7.7

N2 + Ar

12.2

19.7

H2O

7.8

14.1

Table 2.3.

Heating values and adiabatic flame temperatures of various syngas mixtures.

H2 mole fraction

CO mole fraction

Mol. weight [kg/kmol]

Heating value [kJ/kg]

Heating value [kJ/kmol]

Adiabatic flame temp

(Ф = 10) [K]

0

1

28.0

10100.5

282814.0

2394.2

0.1

0.9

25.4

11145.3

283090.6

2385.1

0.2

0.8

22.8

12428.3

283365.2

2381.6

0.3

0.7

20.2

14041.3

283634.3

2379.3

0.4

0.6

17.6

16130.2

283891.5

2377.8

0.5

0.5

15.0

18942.6

284139.0

2376.9

0.6

0.4

12.4

22932.9

284368.0

2377.8

0.7

0.3

9.8

29036.9

284561.6

2379.3

0.8

0.2

7.2

39539.4

284683.7

2381.6

0.9

0.1

4.6

61871.7

284609.8

2385.1

1

0

2.0

141794.1

283588.2

2386.7

Here x defines syngas composition in terms of the mole fraction of CO, a, is related to the equiv­alence ratio Ф (Ф = AFstoichimetric/AFactual withAF = m Air/m Fuel = air to fuel ratio) through the relation a = 1/(2Ф), and d represents the excess O2 (for Ф < 1.0), given by d = (1 — Ф)/(2Ф). Air is assumed to contain 21% O2 and 79% N2 by volume. The above equation can easily be modified to include the presence of diluents in syngas. The syngas heating value can be deter­mined from the standard enthalpies of formation of reactant and product species (Turns, 2011). Table 2.3 lists the heating values of various syngas mixtures. For comparison, the heating val­ues of methane (representative of natural gas) on mass and volume basis are 55,500 kJ/kg and 888,000 kJ/kmol, respectively. Thus, the volumetric heat release rate from syngas combustion is low compared to those for methane. There are other such differences between the chemical and physical properties of syngas and natural gas. This presents challenges in replacing natural gas by syngas in existing combustion devices. Table 2.3 also lists the adiabatic flame temperatures (Tad) of various syngas-air mixtures at Ф = 1.0.

The variation of Tad with Ф for different syngas mixtures is plotted in Figure 2.5. The equi­librium temperature (Tad) was computed using the EQUILIBRIUM algorithm in CHEMKIN software (Chemkin, 2007). The algorithm is based on the application of the first and second laws of thermodynamics. As indicated in Figure 2.5, Tad is nearly independent of the CO fraction in syngas. However, diluents, such as CO2, H2O, and N2, can be used to modify its value.

Ignition of a fuel-air mixture is often characterized in terms of ignition delay time (tign), which has been measured using a variety of devices, including shock tube (Petersen et al, 2007), rapid compression machine (RCM) (Walton et al, 2007) and constant volume (or constant pressure) combustor.

The ignition delay also represents an important target for the development and validation of reaction mechanisms. The computations of tign are often performed using a homogeneous reactor

image007

Figure 2.5. Computed adiabatic flame temperature versus equivalence ratio (Ф) for three different syngas mixtures.

model (Aggarwal, 2011). Davis et al. (2005), Li et al. (2007) and others have reported such mechanisms for syngas oxidation. The GRI-3.0 mechanism (Smith web-link), which includes the oxidation chemistry of C1-C3 species, has also been employed. The homogeneous reactor model is based on the mass and energy conservation equations for a transient, spatially homogeneous system containing a gaseous reacting mixture. Figure 2.6 from Dryer (2008) summarizes the measured and predicted ignition delay data for different syngas mixtures reported by various researchers.

Laminar flame speed or burning velocity represents another fundamental property of a fuel — air mixture. It is of critical importance with regards to flame spread, stabilization, flashback, and blowout in practical systems. In IGCC premixed burners, the problem of flashback and combustion instability represents a major challenge to the designer, especially due to the wide variation in fuel composition. Similarly, it is an important parameter for designing and optimizing the syngas-powered spark ignition (SI) engines, where backfire and inadequate mixing time due to rapid flame propagation represent important issues. The laminar flame speed and its response to stretch are also fundamental to the analysis of premixed turbulent flames. In this context, turbulent flame speed (ST) is another important property for the combustor design, as it has direct influence on important operational issues, such as flame blow-off, flashback, and combustion instability.

Numerous studies have been reported concerning laminar premixed syngas flames. The pri­mary objective of these studies is to determine the effects of various parameters, such as syngas composition, diluents, temperature, and pressure, on the laminar flame speed, flame stability, and emissions. Laminar burning velocities for H2-CO mixtures have been measured using different systems, including flat flame burner (Yan et al., 2011), bunsen burner (Natarajan et al., 2007), counter flow burner (Vagelopoulos et al., 1998), and spherically expanding flames (Prathap et al., 2008). Simulations have often been performed by considering a one-dimensional con­figuration and employing the PREMIX algorithm (Kee et al., 1993) in CHEMKIN software. Multi-dimensional flame simulations have also been performed using various algorithms (Briones et al., 2008). The computations are based on the solution of mass, momentum, species, and energy conservation equations, along with appropriate models for thermodynamic and transport proper­ties. Such properties include standard enthalpy of formation, viscosity, thermal conductivity, and diffusivity of each species. The number of species depends upon the particular kinetic mechanism employed to model the fuel oxidation chemistry. The above set of equations is closed by using

image008

Figure 2.6. Ignition delay times of various syngas and hydrogen mixtures under different pressure and temperature conditions. Filled and open circles correspond to strong and weak ignition events, respectively. All experimental data have been normalized to 20 atm assuming p-1 proportionality. Lines correspond to ignition delay predictions using the Li et al. mechanism at 20 atm; the solid line corresponds to the syngas mixture used in shock tube experiments (Li etal., 2007).

an appropriate equation of state. The numerical algorithms used for solving these equations have employed different approaches, such as finite-difference and finite-volume schemes. An adaptive grid refining of the computational mesh is often used, based on the first and second derivatives of the dependent variables. Further details can be found in the FLUENT user’s guide (2005). Important results from these studies are summarized below: •

Flame a; 50% со [1] 50% Fij

Подпись: 210
image010
image011
Подпись: Equivalence Ratio
Подпись: McLean et al. [1994]
Подпись: so
Подпись: 100
Подпись: Fiamo B! w5%CO • 5% H2

image017Vo ume Percent of CO in Fue

Figure 2.7. Measured and predicted laminar burning velocities for syngas-air mixtures. Variation of laminar flame speed with equivalence ratio Ф for Flames A and B (a), and with CO fraction in syngas at Ф = 2.0 (b) (Mclean etal., 1994).

extinction, turbulent flame propagation, flame stabilization, blowout, and transition to deto­nation. The classical approach yields the following relationship between the stretched flame speed and stretch rate (Mueller et al., 1999):

SL = SL — LaK

Here SL and S£ are the stretched and unstretched flame speeds, respectively, K the stretch rate, and La the Markstein length. Note that S°L corresponds to the burning velocity of a freely propagating planar flame discussed earlier. The flame stretch refers to the rate of change of

image018

Figure 2.8. Measured and predicted laminar flame speeds for various H2/CO spherically expanding premixed flames (Bouvet et al., 2011).

the flame surface area, which may be due to flame curvature, unsteadiness, and flow non­uniformity or hydrodynamic stretch (Bouvet et al., 2011). By determining SL as a function of K through measurements or computations, both SL and La can be obtained. Figures 2.7 and 2.8 from Bouvet et al. (2011) present such data for spherically expanding H2/CO flames. Results in Figure 2.7 for the unstretched flame speed are consistent with those presented earlier. The variation of Markstein length with Ф (Fig. 2.8) indicates that these flames are prone to thermo-diffusive instability under lean conditions, since La becomes negative for Ф < 1.

As discussed by Kishore et al. (2011), this instability is related to the non-unity Lewis number (Le) for stretched flames, with Le > 1 and Le < 1 corresponding to stable and unsta­ble situations, respectively. Similar behavior has been observed by Pratap et al. (2008) and Kishore et al. (2011). Further, previous studies have shown that the presence of H2 in syn­gas increases the flame propensity for instability, while that of CO has the opposite effect. However, the overall instability is predominantly determined by H2 rather than by CO.

• Since syngas typically contains significant amounts of CO2 and H2O, and N2, it is important to examine the effects of these diluents on syngas combustion and emissions. Moreover, dilution is often used to lower the flame temperature and thereby limit NOX emissions. The effects of various diluents on laminar flame speed and stability have been reported by several researchers (Das et al., 2011; Sun et al., 2007; Law, 2006; Burke et al., 2006; Pratap et al., 2008; Kishore et al., 2011). A general observation is that the addition of these diluents decreases the laminar burning velocity due to the increase in heat capacity and the decrease in heat release rate. For a given amount of dilution, the effect is more pronounced with CO2 and H2O dilution compared to that with N2 dilution, mainly due to different heat capacities. The addition of a diluent also shifts the location of peak laminar burning velocity to leaner mixtures (Kishore et al., 2011). Some studies have also observed that the CO2 and H2O addition can affect the combustion chemistry and modify the syngas combustion characteristics (Das et al., 2011). For example, Das et al. (2011) observed that the laminar flame speed varies non-monotonically with H2O addition for CO rich mixtures, but decreases monotonically with H2O for H2-rich mixtures.

• Laminar burning velocity and cellular stability of flames burning other biomass-derived gaseous (BDG) fuels have also been investigated (Burbano et al., 2011). Such studies have con­sidered BDG fuels consisting of varying amounts of H2, CO, CH4, CO2 andN2.Yanetal. (2011) determined unstretched laminar burning velocities for four different BDG mixtures using a per­forated flat flame burner. Vu et al. (2011) reported laminar burning velocities and Markstein lengths for spherically expanding flames for three different BDG mixtures. The PREMIX 1- D algorithm was used for computing the corresponding burning velocities in these studies.

image019

Figure 2.9a. Measured Markstein Lb length versus equivalence ratio Ф for 50/50% H2/CO spherically expanding premixed flames (Bouvet et al., 2011).

A representative result from Vu et al. (2011) is depicted in Figure 2.9a and Figure 2.9b, which plots the unstretched burning velocity versus Ф for three BDG-air mixtures.

As indicated, the revised GRI-3.0 mechanism provides much closer agreement with mea­surements, especially under rich conditions. In the revised mechanism, rate constants of key reactions were modified based on the data in Davis et al. (2005) and Li et al. (2007). The Markstein lengths extracted from measurements for the three BDG-air flames were found to be negative, indicating a propensity for cellular instability. In addition, it was observed that the propensity increases and decreases with H2 and CH4 addition, respectively, and remains essentially unchanged with CO addition.

• There have been relatively few investigations on emissions from premixed syngas flames, although extensive data have been reported for the hydrocarbon flames. While it is impor­tant to consider both soot and NOX emissions from hydrocarbon flames, only NOX formation is relevant in syngas flames. NOX formation in hydrocarbon flames is essentially due to four mech­anisms, namely the thermal (Zeldovich), the prompt (Fenimore), N2O, and NNH mechanisms (Das et al., 2011; Vu et al., 2011; Briones et al., 2007). Thermal NO involves the following reactions: O + N2 ^ N + NO, andN + O2 + NO + O, andN + OH ^ NO + H. Here the first reaction is the rate limiting step, and becomes significant at high temperatures due to its high activation energy. Prompt NO formation is initiated through the reaction CH + N2 ^ NCN (or HCN) + H (or N). Thus the prompt mechanism is absent in syngas flames, since it is directly linked to hydrocarbon combustion chemistry, which produces a CH radical from acetylene. The prompt NO, however, may be important for syngas mixtures containing CH4. The N2O — intermediate mechanism involves N2 + O + M ^ N2O + M as the initiating reaction, with subsequent NO formation occurring through reactions such as N2O + H ^ NO + NH and N2O + O ^ NO + NO. This route is found to become important for lean mixtures and high pressures. Finally, the NO formation through NNH route involves reactions: N2 + H ^ NNH and NNH + O ^ NO + NH (Guo et al, 2007). Ding et al. (2011) investigated the extinc­tion and emission behavior of lean premixed syngas flames in a counter-flow configuration. Numerical simulations were performed using the OPPDIF algorithm in CHEMKIN and the Davis Mechanism (Davis, 2005). It was observed that the NO in these flames was formed predominantly through the NNH and N2O intermediate routes. The contribution of thermal NO was small due to the low flame temperatures. In addition, increasing the CO fraction in syngas was found to increase the amount of NO formed.

LOW NOX BURNERS (LNB)

Most of the utility boilers do not use reburn with natural gas due to high cost of natural gas and development of conventional low NOX burners where the difference between total air and primary air is split into swirling secondary air and tertiary air; i. e. air is introduced in stages to reduce O2 availability thus reducing NOX. A 30kWt LNB facility has been built and tested for cofiring coal:DB blends; the reader is referred to Lawrence et al. (2012) and Lawrence (2013) for more details on facility, experiments and results from cofiring in LNB. However this LNB facility used overfire air as tertiary air. Dairy biomass is evaluated as a cofiring fuel with Wyoming Powder River Basin subbituminous coal in a small scale 30kWt burner boiler facility equipped with air staged combustion for low NOX control. The cofiring experiments were performed with 90:10 (by mass percent) coal: dairy biomass blended fuels as well as pure coal. Standard emissions from solid fuel combustion (O2, CO2, CO, NOX, and SO2) were measured in addition to the temperature profile along the axial length of the furnace. In addition to these emissions measurements, NOX on a heat basis (g/GJ) was calculated. Figure 3.42 shows the preliminary results on variation in NOX emission with % tertiary air; when compared to PRB, only about 12% reduction in NOX was obtained.

Fiscal incentives and market regulation measures

The Chinese government follows the “market-based” principle, and puts “market measures” as a main tool to encourage and guide the development of biomass gasification technology. A series of fiscal incentives, such as financial assistance, tax breaks, investment subsidies, interest-free, subsidized loans etc, have also been developed and implemented, to encourage businesses to use biomass gasification technology. In addition, the Chinese Ministry of Finance, National Development and Reform Commission jointly issued “On the development of bio-energy and bio-chemical and taxation policies to support the implementation of views” in 2006 to propose tax incentives: “the state will give tax incentives to some bio-energy and bio-chemical companies who really need support to enhance the competitiveness of enterprises.” Appropriate guidance from public policy can make some companies, whose motivation is pursuing the most interest, put the biomass gasification technology as their “rational choice”.

4.4 CONCLUSIONS

4.4.1 Co-combustion

The development of renewable energy is supported strongly by the Chinese government. Biomass co-firing technology is one of the key technologies supported. But it still has not resulted in corresponding economic incentive policy for biomass co-firing. There are a large number of small and medium-sized coal fired generators forced out and closed, which instead could be combined with abundant biomass energy resources. Concerning biomass co-firing technology, there are certain project barriers: less project experience, lack of resources, lack of support system and technical system support, uncertain project factors, more difficult financing. Biomass co-firing projects need more support and motivation.

4.4.2 Gasification

Gasification is a versatile thermo-chemical conversion process which produces a gas mixture of CH4, CO, and H2, the proportions of which are determined by the use of air, oxygen or steam as the gasification medium. Biomass gasification has broad prospects in China. Biomass gasification technology will make great progress, and the heating value of gas produced will be higher with the advanced technology. Biomass gasification now used in poly-generation becomes more and more popular, and can give energy at a maximum level. Using the technology, energy utilization efficiency could be increased.

Downdraft gasifers

The downdraft gasifier was originally developed for gasifying high volatile fuels, for example wood and biomass. The reactor is schematically shown in Figure 6.6b. The fuel enters the gasifier at the top of the reactor and its main feature is the concurrent flow of gases through a slowly downwards descending packed bed of solids.

The bed is supported by a constriction in the middle section known as the throat, where most of the gasification reactions occur. It is in this section that the air or steam/oxygen is added. The reaction products are intimately mixed in the turbulent high-temperature region around the throat. Most of the tar cracking takes place in this high-temperature region. Some tar cracking also occurs below the throat on the residual charcoal bed, where the gasification process is completed. In order to minimize radial temperature gradients, it is of importance that the oxidizing agent is distributed homogeneously throughout the whole cross-section of the throat. In the bottom, there is a system to recover the ash and other non-volatile components.

Fixed bed downdraft gasification is relatively simple, reliable and proven and it results in a high conversion of the pyrolysis intermediates and hence giving a relatively clean gas. It is suitable for dry fuels (up to 25 wt% moisture), blocks or lumps with a low ash content (<6 wt%) and containing a low proportion of fine and coarse particles, preferably between 10 and 300 mm (in the longest dimension). Owing to the low content of tars in the gas, this configuration is generally favored for small-scale electricity generation with an internal combustion (IC) engine. The physical limitations of the throat diameter and particle size relation result in that there is a practical upper limit to the capacity of this configuration of about 500kg/h or 500 kWe. The temperature of the gas is approximately 700°C.

FUEL PROPERTIES

The overall purpose of this chapter is to summarize the fuel properties and current state of the art in developing environmentally benign thermal conversion or non-biological technologies (gasification, co-firing, and reburn, etc.) and they are presented in order to show how animal wastes can serve as a fossil fuel supplement, reduce GHG, dispose of the waste, and reduce NOX emissions to convert low-value inventories of animal wastes into renewable energy.

The emissions (NOX, SOX), flame temperatures, and thermal energy outputs from burning biomass are required to determine the effectiveness and profitability of biomass energy conversion systems. However, in order to estimate these outputs, fuel properties of both coal, a widely used fossil fuel, and biomass must be known.

CO-COMBUSTION IN CHINA

1.2.1 Introduction

Biomass is one of the most important renewable energy resources. The amount of agriculture waste is about 700 million tonnes per year in China. It was assumed that about 50% of the

image156

Figure 4.1. Six different co-combustion methods.

agriculture waste can be used as energy, for power generation, heat supply and cooking. It is scheduled that biomass power generation capacity in China will reach 3000 MW in 2020.

Biomass thermal conversion technology is commonly classified as combustion (direct com­bustion, co-combustion), gasification, pyrolysis, carbonization, and so on. Biomass utilization is regarded as a CO2-neutral process. It is beneficial for continuous supply of energy and environmental protection.

Pyrolysis and devolatilization

Once all the moisture is evaporated and particle temperature has reached the pyrolysis threshold (about 280°C, as above mentioned) the devolatilization/pyrolysis process begins. The pyrolysis reaction can be generally represented by the following equation:

Biomass + heat ^ H2O + CO2 + H2 + CO + CH4 + C2H6 + CH2O +••• + tar + char

(5.16)

During the devolatilization phase a wide range of gaseous products are released through the decomposition of fuel (Fig. 5.6). The gaseous products most commonly produced by hemicellulose (composed by pentoses — such as xylan — and hexoses — d-glucose, d-galactose etc.) are: acetic acid, formaldehyde, carbon monoxide, hydrogen, but also furfural and furan. The first step in cellulose pyrolysis is the production of active cellulose (Browne, 1958; Sullivan and Ball, 2012; Liao and Ma, 2004), then if reaction temperature is low, activated cellulose will produce charcoal by dehydration.

As temperature increases, active cellulose will produce mainly levoglucosan (LG) and its isomeric anhydrosugar by cracking of glucosidic bonds at the same time the opening of acetal structural rings and cracking of internal carbon-carbon bond in pyroanoid rings will bring into the formation of hydroxyl-acetaldehyde (HAA), acetol, furfural, CO and other compounds. If secondary reactions happen then anhydrosugar will undergo a reaction similar to the opening and reforming of pyroanoid ring, producing small molecule gas and secondary char.

Lignin pyrolysis produces aromatic compounds and char. The production of char is higher if compared to cellulose. The initial breakdown during pyrolysis affects the straight chain links

image211
Подпись: CH4. C,He. C,H4. etc.

Подпись:

image214
Подпись: LIGNIN

Подпись:

image217
image218
Подпись: 3H,0
Подпись: CELLULOSE

image221Tempwelve [KJ

Figure 5.6. Pyrolysis and devolatilization mechanism.

Table 5.3. Pyrolysis kinetic constants for main biomass components (Anca-Couce et al., 2012).

Component

E [kJ/mol]

Log A [log s 1]

n [-]

Wood

107

6.50

0.91

Cellulose

146

9.71

0.59

Hemi-cellulose

116

8.07

1

Lignin

167

11.3

2.78

which connect aromatic units such as: vanillyl, syringyl, guaiacols, cresols and catechols. The aromatic chains produce phenols, xylenols, guaiacols, cresols and catechols. The straight-chain links produce carbon dioxide, hydrocarbons, formic acid, acetic acid, higher fatty acids and methanol.

The development of biomass pyrolysis depends both on chemical and physical processes, so pyrolysis modeling is based on two approaches taking into account both kinetics rates and heat transfer rates (Di Blasi, 2008, Slopiecka etal., 2012). Some examples of pyrolysis kinetic values are proposed in Table 5.3.

Alkali metal catalysts

Alkali metal catalysts for product gas tar reduction have also been studied. However, most of the work carried out in this field focuses on the effect of the catalysts on the gasification and pyrolysis reactions. The alkali metal catalysts are often used as primary catalysts, added directly to the biomass by dry mixing or wet impregnation. When directly added to the gasifier bed, these catalysts tend to increase the rate of gasification, reduce the tar content (sometimes significantly) and reduce the methane content of the product gas (Sutton, 2001). However, they tend also to be rapidly deactivated and the recovery of the catalysts is difficult and costly.

Inherent in the ash of several biomass types are the high concentrations of alkali and these ashes are effective catalysts for tar decomposition (Hauserman, 1994).

Alkali metal catalysts are also active as secondary catalysts. For example, potassium carbonate supported on alumina is more resistant to carbon deposition than nickel, although it is not as active.

COFIRING

More recently, animal biomass has also been considered as a possible feedstock for smaller, on-the-farm combustion systems designed to properly dispose of animal solids and wastewater. Using commercially available equipment like solid separators, augers, and dryers, DB can be pre­pared for smaller combustion processes (Carlin etal., 2007). A study by (Rodriguez etal., 1998) investigated the effect of drying on biomass heating values. Moreover, additional information on biomass fuel properties and heating values can be found in Annamalai and Puri (2007) and Annamalai et al. (1987). In the following sections of this chapter, the facilities and experimental work being conducted at the TAMU Department of Mechanical Engineering will be discussed.

It has been proposed that cattle biomass (CB) and litter biomass (LB) can be used as a fuel source for power generation. The CB includes both FB and DB. Previous attempts to use CB as a sole fuel source in gasifiers or direct combustion have resulted in limited technical suc­cess due to the high moisture content, high ash content, and/or low heating value of manure (Sweeten et al., 2003). These properties which are found in most animal manure-based biofuels cause flame stability problems, and the high ash/soil content can clog conventional combustion devices and accelerate boiler tube erosion and corrosion. The more the volatile matter and lower the temperature at which volatiles are released, the better the flame stability in the boiler burner. Thus, co-firing biomass with coal may improve flame stability. FB, DB or LB could be used as a fuel by mixing it with coal and firing it in an existing coal suspension fired combustion system. This technique is known as co-firing. The high temperatures produced by the coal will allow the biomass to be completely combusted. If only 10% by mass LB is used, the fuel proper­ties will not change radically, and few adjustments will have to be made to existing combustion system equipment. Previous boiler co-firing experiments involving biomass with pulverized coal have included: wood waste (Gold etal., 1996), switch-grass (Aerts et al., 1997), straw (Hansen et al., 1998), sewage sludge, tire-derived refuse (Abbas et al., 1994), or grass (Spliethoff et al.,

1998) . Chapter 5 of this book deals with combustion of biomass fuels and various combus­tors used for energy conversion: pile, grate, fluidized beds, suspension burners (Desidiri and Fantozi, 2013).

The use of FB as a co-firing fuel was previously investigated by Frazzita et al. (1999) using a small-scale boiler burner to co-fire FB and coal under transient conditions. A lack of adequate insulation and steel combustor walls allowed the experiments to obtain only transient results at low temperatures. Additional co-firing experiments are summarized by Sami etal. (2001).

Figure 3.19 presents an overview of a number of co-firing plants in Europe. All together, there have been around 100 co-firing units in Europe. Co-firing plants in the Netherlands, Denmark, Finland and Sweden are mostly operating on a commercial basis while many of the plants in the UK are in trials or demonstrations. Positive experience in co-firing has been made mainly with woody biomass (EUBIA). There are different biomass employment methods for co-combustion and three main co-firing combustion methods (direct, indirect, parallel). Co-firing can be accomplished by different technologies like atmospheric or pressurized flu­idized bed combustors, pulverized or grate combustors (EUBIA). The majority of the European co-fired power plants operate in direct co-combustion with circulated fluid bed boilers (EUBIA).

It is apparent from a literature review that there are no prior data on the effect of co-firing low quality and high nitrogen DB on the combustion and emission characteristics.