Plant based biomass pyrplysis and reaction kinetic models in different types of reactor

As for the fast pyrolysis in tubular reactor, first of all, a comparative study of pyrolysis was carried out using both types of biomass: palm kernel cake major non-cellulosic material a

Dissertation for the Degree of Doctor of Philosophy

Plant-Based Biomass Pyrolysis

and Reaction Kinetic Models in Different

Types of Reactor

Thanh-An Ngo

Department of Chemical Engineering

Graduate School Kyung Hee University Seoul, Korea

August, 2010

Dissertation for the Degree of Doctor of Philosophy

Plant-Based Biomass Pyrolysis

and Reaction Kinetic Models in Different

Types of Reactor

Thanh-An Ngo

Department of Chemical Engineering

Plant-Based Biomass Pyrolysis and Reaction Kinetic Models in Different

Types of Reactor

by

Thanh-An Ngo

Supervised by

Prof Jinsoo Kim

Submitted to the Department of Chemical Engineering and the Faculty of the Graduate School of Kyung Hee University in partial fulfillment

of the requirements for degree of Doctor of Philosophy

Dissertation committee

Chairman Prof Sung Hun Ryu …………

Prof Jinsoo Kim ……… Prof Seung-soo Kim ……… Prof Chang Woo Lee ……… Prof Kyesang Yoo …………

ABSTRACT

Plant-Based Biomass Pyrolysis and Reaction Kinetic Models in

Different Types of Reactor

Thanh-An Ngo Department of Chemical Engineering Graduate School of Kyung Hee University

Seoul, Korea

Biomass, a plant-derived material, is currently considered as a renewable resource for producing energy owing to its low cost as well as its abundance It largely contains hemicellulose, cellulose (for wood-based biomass) or cellulose-like compound (for seed-based biomass), and lignin of which the content could change depending on type of biomass

Recently, much attempt has been focused on pyrolysis, a thermochemical technology, as a promising approach for biomass conversion into bio-fuel Although paid much more attention, pyrolysis, especially its reaction pathways, is ambiguously understood because of its complexity In addition, it should be noted

the controlling process would be more simplified and easily operated, resulting in improving technology’s economical feasibility

As a consequence of all previous reasons, in this research, the plant-derived biomass was used as an object for pyrolysis in both closed and open reactor The statistical design of experiment (DOE) was applied to set up for some experiments and then all the model equations would be employed to optimize the process More importantly, the kinetic mechanisms were also explored carefully by using different lumped kinetic models Subsequently, experimental data would be applied for verifying the accordance with proposed models All the detail contents of this research were briefly shown as follows:

For the pyrolysis in closed reactor (micro-tubing reactor), a statistical design of experiment (DOE) was applied to set up for experiment run and then all the model equations would be employed to optimize the process The palm kernel cake was utilized as the feedstock for all experiment using this type of reactor In order to understand more about pyrolysis mechanism in the micro-tubing reactor, through which all the affecting parameters could be clarified, leading to the ability

to upgrade the performance of pyrolysis, the kinetic model was also proposed, and the rate constants were subsequently drawn The results showed that all the primary reactions were much more dominant than the secondary ones and the pyrolysis in closed condition seem to be preferable to generating solid product rather than other products

As for the fast pyrolysis in tubular reactor, first of all, a comparative study

of pyrolysis was carried out using both types of biomass: palm kernel cake (major non-cellulosic material) and pine wood chip (cellulosic material) The effect of biomass feedstock on the distribution and properties of product were also investigated In addition, the kinetic model of fast pyrolysis was also presented and verified From regression calculation, the rate constants were obtained From these constants, it can be found that the fast pyrolysis in tubular reactor was more favorable to occur via the reaction forming tar following by the decomposition of tar to produce gas, rather than reaction forming the mixture product of char and gas

Finally, for the pyrolysis in open reactor (fluidized bed reactor), the central composite rotatable design (CCRD) was employed for investigating the effect of feed rate of feedstock, biomass particle size, pyrolysis temperature and residence time on fast pyrolysis of biomass (palm kernel cake) The activation energy of pyrolysis in the open condition was also determined using thermogravimetric analysis (TGA) Moreover, from TGA data the surveyed temperature range was withdrawn From the data, the mathematical model for liquid product yield was hence obtained and used to reach the optimum of 49.5 wt%, corresponding to the

Table of Contents

Abstract i

List of Figures ix

List of Tables xii

CHAPTER 1 – General Introduction 1.1 Introduction 1

1.1.1 Biomass source 1

1.1.2.Pyrolysis mechanism 3

1.1.3.Reactor design 5

1.1.4 Operating parameters 6

1.2 Motivation 7

1.3 Research objectives 8

1.4 Dissertation overview 10

1.5 References 12

CHAPTER 2 – Literature Review 2.1 Concept of biomass 14

2.1.1 Biomass definition and classification 14

2.1.2 Chemical composition 14

2.1.3 Biomass resource 19

2.1.4 Biomass applications 21

2.1.5 Approaches for biomass conversion into energy 24

2.2 Concept of pyrolysis 26

2.2.1 Definition 26

2.2.2 Classification 28

2.2.3 Pyrolysis mechanisms and pathways 29

2.3 Theory of central composite rotatable design (CCRD) 34

2.3.1 Set up experiment using matrix design 34

2.3.2 Calculating the regression coefficients 35

2.3.3 Verifying the statistical significance of each regression coefficient 36

2.3.4 Verifying the lack of fit of regression equation 36

2.4 References 38

CHAPTER 3 – Effect of Operating Parameters and Kinetics of Pyrolysis in a Tubing Reactor 3.1 Introduction 41

3.3 Results and discussion 46

3.3.1 Characteristics of biomass 46

3.3.2 Thermal decomposition analysis of biomass 48

3.3.3 Optimization of pyrolysis conditions by CCRD 51

3.3.4 Characterization of pyrolysis products 56

3.3.5 Pyrolysis kinetics 62

3.4 Conclusions 70

3.5 References 71

CHAPTER 4 – Comparative Study of Pyrolysis of Palm Kernel Cake and Pine Wood Chip Using an Open Tubular Reactor 4.1 Introduction 73

4.2 Experimental 74

4.2.1 Biomass characteristics 74

4.2.2 Apparatus 74

4.3 Results and discussion 77

4.3.1 Thermogravimetric analysis of biomass 77

4.3.2 Effect of fast pyrolysis conditions on product yield 80

4.4 Conclusions 91

4.5 References 93

CHAPTER 5 – Kinetic Model of Fast Pyrolysis using Palm Kernel Cake in a Closed Tubular Reactor

5.1 Introduction 94

5.2 Experimental 97

5.2.1 Apparatus 97

5.2.2 Calculating product yield 97

5.3 Results and discussion 99

5.3.1 Composition of gas product 99

5.3.2 Pyrolysis kinetics 100

5.4 Conclusions 111

5.5 References 112

CHAPTER 6 – Pyrolysis kinetics and parametric effects on fast pyrolysis of palm kernel cake using thermogravimetric analyzer and fluidized bed reactor 6.1 Introduction 114

6.3 Results and discussion 120

6.3.1 Kinetic parameters of pyrolysis using TGA 120

6.3.2 Fast pyrolysis using fluidized bed reactor 125

6.3.3 Characteristic of pyrolyzing liquid product 136

6.4 Conclusions 140

6.5 References 141

CHAPTER 7 – Conclusions and further researches 7.1 Conclusions 143

7.2 Further researches 146

Acknowledgments 147

List of Figures

Page

Figure 2.5 Some typical applications of biomass 23

Figure 2.8 Pure cellulose pyrolysis pathway: (1): primary pyrolysis; (2) secondary

Figure 2.10 Reaction scheme used by Liden, and Diebold 33

Figure 3.1 Schematic diagram of experimental apparatus 45

Figure 3.2 TGA and DTG curve for pyrolysis of palm kernel cake using TG

Figure 3.7 Proposed pyrolysis model 63

Figure 3.8 Effect of residence time on product distribution at 400°C 68

Figure 3.9 Effect of residence time on product distribution at 430°C 68

Figure 3.10 Effect of residence time on product distribution at 460°C 69

Figure 4.1 Schematic diagram of experimental apparatus 76

Figure 4.2 DTG data of TG analysis for PKC and PWC at heating rate of 20oC/min

79

Figure 4.3 Product yield of fast pyrolysis of palm kernel cake 82

Figure 4.4 Product yield of fast pyrolysis of pine wood chip 82

Figure 4.5 Yield of hydrocarbon and mixture of CO and CO2 in gas products obtaining from fast pyrolysis at different conditions: (1) - 550oC, 20 mL/min; (2) -

550oC, 500 mL/min; (3) - 750oC, 20 mL/min; (4) - 750oC, 500 mL/min 85

Figure 4.6 GC-FID analysis of pyrolyzing gas product of pine wood chip at 750oC,

Figure 5.1 Pyrolysis reaction mechanisms proposed by Liden [1] 96

Figure 5.2 Schematic diagram of experimental apparatus 96

Figure 5.3 Relationship between biomass yield and reaction time described by

Figure 5.4 Product yields of fast pyrolysis obtained from experimental data and

Figure 6.1 Schematic diagram of experimental apparatus 119

Figure 6.2 DTG data from thermogravimetric analysis of palm kernel cake 123

Figure 6.3 The relationship between ln(dX/dt) and 1/T at different iso-conversion

Figure 6.7 Liquid yield at the condition of pyrolysis temperature = 400oC,

Figure 6.8 Liquid yield at the condition of residence time = 0.9 sec, particle size =

List of Tables

Page

Table 3.1 - Sample characteristic of palm kernel cake 47

Table 3.2 Central composite rotatable design 22 + 2x2 + 5 53

Table 3.3 Composition of solid product by Elemental analysis 60

Table 3.4 GC – MS analysis of bio-oil from pyrolysis of palm kernel cake at 460oC

Table 3.5 Reaction rate constants (min-1) of pyrolysis 67

Table 4.1 Bio-oil and water content in liquid product at 550oC 86

Table 4.2 Major bio-oil product analyzed by GC-MS for fast pyrolysis of Palm

Table 4.3 Major bio-oil product analyzed by GC-MS for fast pyrolysis of Pine

Table 6.2 Experimental design matrix and response value 127

Table 6.3 Statistical significance of regression coefficients 130

Table 6.4 GC-MS analysis of bio-oil from pyrolysis of palm kernel cake at 400 and

CHAPTER 1 General Introduction

1.1 Introduction

1.1.1 Biomass source

As commonly known, fossil fuels, namely oil and coal, are limited Oil is estimated to be run out within 40 years and coal within 250 years from now [1] From this issue, many researchers have focused on finding a new source for energy Biomass is one of the most interesting alternative sources, which has been paid much more attention to in recent years, owing to its low cost as well as its abundance [2] The term biomass is used to describe all biologically produced substances World production of biomass is estimated at 146 billion metric tons a year Some farm crops and trees can produce up to 20 metric tons of biomass per acre a year Only algae and grasses may produce 50 metric tons per year [3] The source of biomass is infinite and can be replenished through natural processes, hence it is also considered as a renewable source for producing energy For

According to the International Energy Agency, approximately 11% of the energy is derived from biomass throughout the world [4]

Biomass can stem from timber industry, agricultural crops, forestry residues, household wastes and wood [5] Of all these types of biomass, woody biomass makes up the most major amount Therefore, most researches have now focused on this one This material largely contains hemicellulose, cellulose (for wood-based biomass) or cellulose-like compound (for seed-based biomass), and lignin of which the content could change depending on type of biomass Hemicellulose, a branched biopolymer with a random and amorphous structure, is most favorably decomposed

In contrast, lignin is the most difficult one to be degraded due to its cross-linked three-dimensional structure Cellulose and cellulose-like compound are more thermally stable than hemicellulose because of its crystalline and linear-chain structure Also, the structure of cellulose and cellulose-like compound are rather simple compared to a cross-linked three-dimension structure of lignin, leading to its lower thermal stability [5]

A biomass can be cellulosic or non-cellulosic depending on whether its carbohydrate contains cellulose or not Most recent researches have used cellulosic biomass as a feedstock for conversion into energy Apart from cellulosic biomass, the non-cellulosic one is also cropped with a considerable amount every year For instance, only Malaysia can produce an annual quantity of 1.4 million tons of palm kernel cake, a non-cellulosic biomass, as a by-product in the milling of palm kernel

oil [6] The major composition of carbohydrate building up this biomass structure

is mannan, a cellulose-like compound Although produced with such a large amount, there have been few researches reported about its usage for studying in biomass conversion As can be seen, if the biomass composition changes, it might result in varying the properties and distribution of pyrolytic product Nevertheless, the characteristics of pyrolysis of a non-cellulosic biomass, especially like palm kernel cake, have not been studied yet

1.1.2 Pyrolysis mechanism

As raw biomass is solid, it is difficult to use in many applications without substantial modification Conversion to gaseous and liquid energy carriers has many advantages in handling and application There is a wide range of processes available for converting biomass and wastes into more valuable fuels, but only two general thermochemical and biological processes are considered as feasible solutions [7] The thermochemical conversion technology tends to be grouped in four distinct categories for fuel production: combustion, pyrolysis, liquefaction, and gasification As for biological technology, this refers to approaches involving

Since bio-oil is now received more interest as an alternative for the depleted fossil-based fuel, much effort has been done to find an effective and economically feasible technology for obtaining as high bio-oil yield as possible A pyrolysis process, or a mild depolymerization of biomass producing pyrolytic oil, can satisfy for such a requirement This is the technique of applying high heat to organic matter (lignocellulosic materials) in the absence of oxygen or in reduced air, typically in the range of 400–650°C The process can produce charcoal, condensable organic liquids (pyrolytic fuel oil), and non-condensable gasses [8] Also, it can be adjusted to favor charcoal, pyrolytic oil, or gas depending on process conditions [9] This process is considered as a potential approach for biomass conversion into energy Consequently, understanding pyrolysis kinetics is valuable for the in-depthexploration of process mechanisms However, owing to the very complicated nature of reactions occurring in the process, as well as its unpredictable amount of products, the pyrolysis mechanism has not been carefully understood yet As a consequence, it seems to be impossible for specifying the reaction kinetics if each specific compound is considered

Recently, a general and simple kinetic mechanism named lump model has been employed to investigate the pyrolysis [10, 11] For applying this model, all pyrolytic products should be gathered to form various lumps depending on their phases such as gas, liquid or solid This model is just applied effectively if the content of each lump is clearly determined Specifically, these lumps will include

remaining biomass, gas, liquid and char In fact, the liquid product and char contents can be easily measured and calculated based on the weight balance before and after pyrolysis The problem derives from how to determine the content of pyrolytic gas product In addition, it can not confirm that the biomass feedstock will be completely decomposed during the process As a result, the content of remaining biomass after pyrolysis is also needed to verify If the pyrolyzing residence time is long enough, it is believed that the biomass will be completely decomposed As a result, the remaining biomass will approach zero and the content

of gas product can be easily obtained through the content of liquid and solid by the formula as follows: gas yield = 100% - liquid yield – char yield However, if the pyrolysis is explored in a short time, the biomass is just partially decomposed Hence, it seems impossible to calculate the gas yield as mentioned above Also, there has been no method for quantifying the biomass content in a mixture of solid phase From this information, it is necessary to find out a feasible method in order

to specify the yield of pyrolytic gas and remaining biomass, through which facilitates for applying the lump model in investigating the kinetic mechanism

conventional pyrolysis was for decomposition at a low heating rate and long residence time to produce mainly gas fuel or charcoal The other was fast pyrolysis

at which the pyrolysis occurred with a very high heating rate in a short residence time, largely for achieving high yield of bio-oil In the pyrolysis process, the type

of reactor is very significant to control the bio-oil yield In fact, there have been some researchers trying to apply pyrolysis conducted in several kinds of reactor for biomass conversion into fuel For instance, Li et al [13] or Wei et al [14] utilized different types of biomass for fast pyrolysis in a free fall reactor Lappas et al [15] researched about fast pyrolysis of biomass for liquid fuel in a fluidized-bed reactor

As can be seen for each type of reactor, the reaction might be different, thus leading to changing reaction pathway as well as product yield

1.1.4 Operating parameters

Recently, pyrolysis has been considered as a promising technology, largely due to its economical feasibility as well as its operational simplicity There must be

an appropriate set of operating parameters for each specific reactor applied in the

pyrolysis process For the fast pyrolysis using fluidized bed reactor, these

parameters include pyrolysis temperature, residence time, biomass particle size, and biomass feed rate For pyrolysis in a tubing reactor, some factors affecting on the pyrolysis might be pyrolysis temperature, residence time, and biomass particle size In general, there are many factors needed to be investigated in a pyrolysis

process However, there have been few researches studying fully and systematically about the effect of these operating parameters on pyrolysis performance In fact, if there are many factors influencing on the process, the operation and technology cost will increase proportionally to its number of controlling variables Therefore, once all these operating parameters can be explored carefully, it is possible to eliminate the insignificant ones out of controlling process, thus resulting in further simplifying its operation

1.2 Motivation

Biomass has been paid attention for energy production because of its abundance and diversity Among the types, the cellulosic biomass is employed as a feedstock for studies the most Much effort is now placed on enhancing pyrolysis performance by exploring the operating parameters [15-17] There are just few works involving the effect of biomass composition on properties and distribution product Besides, how biomass is pyrolyzed when its major carbohydrate is cellulose-like has not been mentioned so far For the compensation to the above lack, this dissertation aims to characterize the pyrolysis of the cellulose-like

process can be selectively controlled to accelerate the biomass conversion In fact, there are various researches focusing on pyrolysis mechanism [10-11] Still, it has not been fully understood yet owing to the difficulties in determining product contents for several reasons First, it is impossible to specify the content of too many compounds in bio-oil Second, there is no feasible method for differentiating between char and remaining biomass after pyrolysis process, thus the char yield, or the biomass conversion can not be calculated In order to avoid the first difficulty

as previously mentioned, a lumped kinetic model should be applied to simplify the kinetic calculation However, this model can be used only if all product yields are available Consequently, finding out how to determine the product yields is necessary to facilitate the investigation of pyrolysis mechanism

Finally, the most desirable objective of pyrolysis is to upgrade the bio-oil yield This can be obtained by improving the reactor design to work more stably and reliably, which is another target for this research In addition, in spite of pyrolysis’s widespread application, all of its operating factors have not been systematically studied As a result, the dominant factors on pyrolysis performance have not been defined and hence needed to be carried out in this research

1.3 Research objectives

There are two major objectives for this research as follows: one is to understand the characteristics of pyrolysis of palm kernel cake (a non-cellulosic

feedstock), and the other is to clarify all the effects on both performance and kinetic mechanism of the pyrolysis occurring in various types of reactors

Palm kernel cake, a non-cellulosic biomass, was applied as a feedstock for all experiments Simultaneously, in some experiments, pine wood chip, a cellulosic biomass, was also pyrolyzed similarly to palm kernel cake From this comparative study, the characteristics of palm kernel cake can be clearly understood

In order to carry out pyrolysis, tubing, tubular and fluidized bed reactors were employed for pyrolyzing biomass For each type of reactor, the properties and distribution of products were characterized carefully In addition, appropriate kinetic models are also proposed and then verified From the results, all the rate constants could be investigated clearly, leading to the ability to upgrade the performance of pyrolysis

The detailed objectives are enumerated as follows:

(1) Optimization of operating parameters in pyrolysis of palm kernel cake using a tubing reactor

(2) Kinetic model study of pyrolyzing palm kernel cake using a tubing reactor and a closed tubular reactor

1.4 Dissertation overview

This dissertation consists of seven chapters, wherein the remainder will discuss the following subjects:

In Chapter 2, all the concepts of biomass and pyrolysis were fully expressed

to provide a fundamental background of biomass resources, pyrolysis classification,

as well as pyrolysis kinetics Also, the theory of the central composite rotatable design, a design of experiment, was also described carefully This design is a useful method for organizing the experiments, through which equations depicting the effect of all operating parameters can be obtained easily

Chapter 3 showed the studies about the pyrolysis of palm kernel cake using

a tubing reactor at closed condition The pyrolysis temperature and residence time were designed for experiment following the central composite rotatable design (CCRD) As a result, the model equations describing all the affecting parameters were obtained, leading to approaching the optimal conditions for pyrolysis process

In addition, a kinetic lumped model was also proposed and used for modeling the pyrolysis process in this chapter A non-linear regression method was then applied for experimental data to calculate all the global kinetic parameters Subsequently, a kinetic analysis was performed for better understanding of pyrolysis characteristics

and reaction mechanisms during the pyrolysis of palm kernel cake

In Chapter 4, palm kernel cake (non-cellulosic biomass) and pine wood chip (cellulosic biomass) were used for pyrolysis in an open tubular reactor

Different conditions of pyrolysis temperature and sweeping-gas flow rate were also applied for the pyrolysis of both biomass types From this research, the characteristics of pyrolysis of palm kernel cake (a non-cellulosic biomass) were more clarified

Chapter 5 presented an investigation about the kinetic model of palm kernel cake pyrolysis using a closed tubular reactor A new method for determining the pyrolytic gas content and an innovative procedure for a non-linear regression were described meticulously In this research, the pyrolysis process was modeled based

on the kinetic model proposed by Liden [10] From the obtained kinetic constants, the favorable and unfavorable pathway of pyrolysis can be clarified

In Chapter 6, kinetics study in an open condition using thermogravimetric analysis (TGA) was also carried out From TGA data, the activation energy of biomass decomposition can be obtained In order to explore the fast pyrolysis of palm kernel cake, a new improved fluidized bed reactor was designed and then applied The central composite rotatable design was also employed to set up all experiments and to investigate the effects of operating factors on pyrolysis performance such as pyrolysis temperature, residence time, biomass particle size

va:p:1001t:8099407855520:7bd79b24c1104e5e:4bdde224&scopeid=defLink

[5] http://www1.eere.energy.gov/biomass/feedstock_glossary.html

[6] http://www.jphpk.gov.my/Agronomi/PKC.htm

[7] C.A.C Sequeira, P.S.D Brito, A.F Mota, J.L Carvalho, L.F.F.T.T.G

Rodrigues, D.M.F Santos, D.B Barrio, and D.M Justo, Energy Conversion and

Management, 2007, 48, 2203–2220

[8] A Demirbas, and D Gullu, Energy Education Science and Technology, 1998, 1,

111–1115

[9] A Demirbas, Energy Education Science and Technology, 1998, 2, 23–28

[10] A.G Liden, F Berruti, and D.S Scott, Chemical Engineering

Communications, 1988, 65, 207-221

[11] Y.H Park, J Kim, S.S Kim, and Y.K Park, Bioresource Technology, 2009,

CHAPTER 2 Literature Review

2.1 Concept of biomass

2.1.1 Biomass definition and classification

Biomass is biological material derived from living, or recently living organisms In the context of biomass for energy this is often used to mean plant based material, but biomass can equally apply to both animal and vegetable derived material [1] Biomass is available in a variety of forms and is generally classified according to its source (animal or plant) or according to its phase (solid, liquid or gaseous)

2.1.2 Chemical composition

Biomass is carbon-based substance which consists of a mixture of organic molecules containing hydrogen, usually including atoms of oxygen, often nitrogen and also small quantities of other atoms, including alkali, alkaline earth and heavy metals As for the plant-derived biomass, it can be classified as wood-based biomass and seed-based biomass Basically, the plant-derived biomass contains carbohydrate, lignin, minerals, and remaining resin (wood)/vegetable oil (seed)

For wood-based biomass, the carbohydrate mainly includes hemicellulose and cellulose This wood-based biomass can be named as cellulosic material A typical composition of a wood-based biomass can be enumerated as following:

• Hemicellulose (20–40% of total feedstock dry matter) is a short, highly branched polymer of five-carbon (C5) and six-carbon (C6) sugars Specifically, hemicellulose contains xylose and arabinose (C5 sugars) and galactose, glucose, and mannose (C6 sugars) The structure of hemicellulose is shown in Figure 2.1

• Cellulose (30–50% of total feedstock dry matter) is a glucose polymer linked by ß–1,4 glycosidic bonds The basic building block of this linear polymer is cellubiose, a glucose-glucose dimmer The structure of cellulose

is shown in Figure 2.2

• Lignin (15–25% of total feedstock dry matter), a polyphenolic structural constituent of plants, is the largest non-carbohydrate fraction of lignocellulose Unlike cellulose and hemicellulose, lignin can not be utilized in fermentation processes The structure of lignin is shown in Figure 2.3

• Other compounds present in plant-derived biomass are known as extractives These include resins, fats and fatty acids, phenolics, phytosterols, salts, minerals, and other compounds

For seed-based biomass, the carbohydrate largely consists of hemicellulose, and cellulose-like compound This biomass is classified as non-cellulosic material For instance, the cellulose-like compound existing in palm kernel cake is mannan, with the structure shown in Figure 2-4

Figure 2.1 Structure of hemicellulose

Figure 2.2 Structure of cellulose

Figure 2.3 Structure of lignin

2.1.3 Biomass resources

Biomass can be divided into several biomass categories as following: (1) Pulp and paper industry residues, (2) Forest residues, (3) Agricultural or crop residues, (4) Urban wood wastes, and (5) Energy crops Each of these biomass categories comprises different types of biomass, the main ones being products (harvested biomass) and residues (by-products from cultivation, harvesting and processing)

2.1.3.1 Pulp and paper industry residues

The largest source for energy production from wood is the waste from the pulp and paper industry called black liquor [1, 2, 3] Black liquor is generated in the kraft process Usually, it consists of lignin and pulping chemicals used to separate lignin from the cellulosic fraction of wood Wood processing produces sawdust and a collection of bark, branches and leaves/needles

2.1.3.2 Forest residues

The forest products industry generates large amounts of residual biomass as timber is harvested and manufactured into marketable goods such as lumber and

keep transport costs high, and so it is economical to reduce the biomass density in the forest itself

2.1.3.3 Agricultural or crop residues

Agriculture crop residues include corn stover (stalks and leaves), wheat straw, rice straw, nut hulls etc Corn stover is a major source for bioenergy applications due to the huge areas dedicated to corn cultivation worldwide

2.1.3.4 Urban wood wastes

Such waste consists of lawn and tree trimmings, whole tree trunks, wood pallets and any other construction and demolition wastes made from lumber The rejected woody material can be collected after a construction or demolition project and turned into mulch, compost or used to fuel bioenergy plants

2.1.3.5 Energy crops

Dedicated energy crops are another source of woody biomass for energy These crops are fast-growing plants, trees or other herbaceous biomass which are harvested specifically for energy production Rapidly-growing, pest-tolerant, site and soil-specific crops have been identified by making use of bioengineering

Herbaceous energy crops are harvested annually after taking two to three years to reach full productivity These include grasses such as switchgrass, elephant grass, bamboo, sweet sorghum, wheatgrass etc

Short rotation woody crops are fast growing hardwood trees harvested within five to eight years after planting These include poplar, willow, silver maple, cottonwood, green ash, black walnut, sweetgum, and sycamore

Industrial crops are grown to produce specific industrial chemicals or materials, e.g kenaf and straws for fiber, and castor for ricinoleic acid Agricultural crops include cornstarch and corn oil, soybean oil and meal, wheat starch, other vegetable oils etc Aquatic resources such as algae, giant kelp, seaweed, and microflora also contribute to bioenergy feedstock

2.1.4 Biomass applications

An ideal renewable resource is one that can be replenished over a relatively short timescale or is essentially limitless in supply Resources such as coal, natural gas and crude oil come from carbon dioxide ‘fixed’ by nature through photosynthesis many millions of years ago They are of limited supply, can not be

source of energy, biomass can be used to produce not only energy, but also chemicals and materials [4, 5] The application of biomass was shown in Figure 2.5

The basic concept then of biomass as a renewable energy resource comprises the capture of solar energy and carbon from ambient CO2 in growing biomass, which is converted to other fuels (biofuels, synfuels) or is used directly as

a source of thermal energy or hydrogen One cycle is completed when the biomass

or derived fuel is combusted This is equivalent to releasing the captured solar energy and returning the carbon fixed during photosynthesis to the atmosphere as

CO2 Hydrocarbons identical to those in petroleum or natural gas can be manufactured from biomass feedstocks This means that essentially all of the products manufactured from petroleum and natural gas can be produced from biomass feedstock Alternatively, biomass feedstock can be converted to organic fuels that are not found in petroleum or natural gas

Figure 2.5 Some typical applications of biomass [4, 5]

2.1.5 Approaches for biomass conversion into energy

In recent year, application of renewable material, especially biomass, for producing energy attracted a great attention to scientists as well as governments [3, 6] Some common approaches were presented in Figure 2.6 Biomass, a natural resource mostly including hemicellulose, cellulose and lignin, was a potential material not only due to its abundance but also its high content of hydrocarbon Many types of biomass were used as a subject for researches, such as wood chip, rice husk, straw, sawdust, etc However, depending on the composition of cellulose and lignin, there might be the most appropriate method for converting each biomass into fuel There were some popular methods applying for the biomass with the high content of cellulose For instance, fermentation by enzyme or hydrolysis with support of acid or alkaline was applied widely as presented in literatures [7, 8]

As for the high lignin biomass, due to the firm structure of this component, until now, it seems that there was only the thermal decomposition also known as pyrolysis being most effective and most simple This method can be also applied for the biomass with a broad distribution of various compositions Moreover, owing to its reasonable cost and simple operation, pyrolysis has been more accepted as a feasible approach for converting biomass into energy as well as chemical

Figure 2.6 Energy products and classification [3]