Torrefaction and Densification of Woody Biomass; Torrefactie en densificatie van houtachtige biomassa

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Delft University of Technology

FACULTY MECHANICAL, MARITIME AND MATERIALS ENGINEERING

Department Marine and Transport Technology Mekelweg 2 2628 CD Delft the Netherlands Phone +31 (0)15-2782889 Fax +31 (0)15-2781397 www.mtt.tudelft.nl

Specialization: Transport Engineering and Logistics Report number: 2014.TL.7900

Title: Torrefaction and Densification of Woody Biomass

Author: G. M. Mul

Title (in Dutch) Torrefactie en densificatie van houtachtige biomassa

Assignment: literature Confidential: no

Initiator (university): Dr. ir. D. L. Schott Initiator (company): TU Delft Supervisor: Dr. ir. D. L. Schott

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Delft University of Technology

FACULTY OF MECHANICAL, MARITIME AND MATERIALS ENGINEERING

Department of Marine and Transport Technology Mekelweg 2 2628 CD Delft the Netherlands Phone +31 (0)15-2782889 Fax +31 (0)15-2781397 www.mtt.tudelft.nl

Student: G. M. Mul Assignment type: Literature Supervisor (TUD): Dr. ir. D. L. Schott Creditpoints (EC): 10

Specialization: TEL

Report number: 2014.TL.7900 Confidential: No

Subject: Torrefaction and Densification of Woody Biomass

Globally, the fossil energy resources are continuously reducing in availability. Therefore the suppliers of energy resources are starting to look for possibilities to continue their operations. Also Europees Massagoed- Overslagbedrijf (EMO) B.V. is interested in the new energy resources, mostly in the handling of biomass. They joined Project Bioforce which researches the possibilities of using bioenergy.

Biomass is a growing energy resource which is already often used to deliver 'green' energy. EMO is new to the handling of this type of product therefore the first step is to conduct research. It is known that collected biomass can be used as an energy resource but there are still some disadvantages to this source. The largest problem of the material is that it has a high moisture content [10-50%]. This causes for example high transportation costs.

To reduce the moisture content of the product, multiple techniques are developed. Torrefaction is the method of interest for this research. This method threats the biomass at a temperature of between 200˚C to 300˚C and will vaporize the water and volatile carbon molecules. Next, the biomass is densified into pellets. These two methods help to increase the energy density so the material can be transported to its destination at lower costs.

With this literature study you will contribute to making this link by answering the following main research question:

 Which processes are necessary to produce woody biofuel of a high quality?

In particular, based on a survey of both scientific and more practically oriented articles you will address the following subquestions:

 What is woody biomass?

 What happens during the torrefaction process?

 Which methods are used to densify the woody biomass and at which quality are they made? What determines the quality of the pellets?

 Which variables can be changed in the manufacturing process to optimize the quality of the wood pellet?

 What happens to the biomass when the torrefaction and densification processes are combined? Based on your literature survey, it is expected that you conclude with a recommendation for future research opportunities and potential for more ideas and/or applications. The report must be written in English and must comply with the guidelines of the section. Details can be found on the website. For more information, contact Dr. ir. D. L. Schott (B-4-300; D.L.Schott@tudelft.nl)

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Abstract

This research has been conducted to gain knowledge about biofuels. Europees Massagoed-Overslagbedrijf (EMO) B.V. and TU Delft section Transport Engineering and Logistics (TEL) have joined Project Bioforce to investigate the possibilities of replacing fossil fuels with solid biofuels. This project is interested in the handling of woody biomass as it is easy available and carbon neutral during combustion. The product must be obtained from areas with a high population of wood, for example North America. This requires long distance transportation of the biofuel. Originally wood contains a high amount of moist (up to 50%). This must be removed to reduce the transportation costs as the moist does not add value to the product.

A method which can be used to decrease the moisture content is torrefaction. During a roasting process at temperatures of about 200 ◦C - 300 ◦C the moisture and some volatiles are removed. The biofuel will become more energy dense with help of this process by eliminating particles that contain little energy. The setup of the torrefaction process depends on the desires of the end-product and can be adjusted by temperature, time and heating/cooling rate. A considaration has to be made to determine the optimal result of loosing energy particles compared with increasing energy density.

Next to torrefaction, the biofuel can also be densified. Multiple methods are considered to increase the energy density. Pelletization has been found as the most interesting process as it shows the largest improvement in energy density compared to the other methods. This process presses saw dust through a die under a specific temperature and pressure which enables the lignin to work as a natural binder. When the natural binding is not sufficient, additives can be added in the conditioning phase. Other variables to adjust the properties of the wood pellet are the equipment setup (temperature and pressure) and the cooling conditions.

The woody biomass will be transported from North America, therefore the energy den-sity is a very important quality parameter. Next to this there are other parameters

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which are important to optimize. The mechanical durability provides information about the strength of the pellet. During transfers, it is not allowed that the pellet falls apart. The amount of fines must be reduced to prevent pollution and hazardous working envi-ronment. To optimize the combustion behavior of the pellet, consistent dimensions are desired. Also the ash behavior is an important parameter during combustion.

Torrefaction and pelletization show interesting results considering the quality of the bio-fuel. Combining them will even result further improvements. Experiments have shown that the energy density, strength and water resistant improve significantly. Therefore it is useful to manufacture the pellets with help of both processes. Currently there are still a lot of questions concerning the process variables to achieve the optimal pellet. But the results show that there is potential for using woody biomass as a main energy source. The developments of solid biofuels will be interesting to follow as multiple parties are conducting research about this topic. Standards are updated, Production of Solid Sus-tainable Energy Carriers by Means of Torrefaction (SECTOR) is finalizing their project and Project Bioforce has recently started to investigate biocoals. All in all, it might be possible to generate “green” energy in a large scale in the near future.

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List of Symbols

GHV: Gross Heating Value

HHV: Higher Heating Value

LHV: Lower Heating Value

NHV: Net Heating Value

tc: Solids cooling

tdry: Pre-drying

th,int: Post-drying and intermediate heating

tn: Initial heating

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AC: Ash Content

CCP: Critical Control Point

CEN: European Committee for Standardisation

DB: Dry Based

ECN: Energy research Centre of the Netherlands

EMO: Europees Massagoed- Overslagbedrijf

FAO: Food and Agriculture Organization of the United Nations

ISO: International Organization for Standardization

MC: Moisture Content

RBE: River Basin Energy

SECTOR: Production of Solid Sustainable Energy Carriers by Means of Torrefaction

TC: Technical Committee

TS: Technical Specification

UBET: Unified Bioenergy Termonology

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Chapter 1 Introduction

Globally, the fossil energy resources are reducing in availability. Therefore the suppliers of energy sources are starting to look for other possibilities for continuing their opera-tions. The Port of Rotterdam is interested in bioenergy and wants a steady grow of the biomass throughput in the harbor (Port of Rotterdam, 2013).

Recently a project has started which aims to support a structural industry for biomass. Project Bioforce has started as initiative of River Basin Energy (RBE) in combina-tion with TU Delft seccombina-tion Transport Engineering and Logistics (TEL) and Europees Massagoed- Overslagbedrijf (EMO) B.V. (River Basin Energy, 2014). Their goal is to produce a biofuel which can be used as a economical component of current energy sources. Ideally a product can be created which is completely ‘green’ but has all the characteristics of coal (biocoal) and can be handled with the existing coal infrastructure. This report will be a introduction to this project and will show how the biomass can achieve higher quality.

Biomass is used to generate bioenergy, this is energy that is obtained from biological material (Biomass.net, 2014). This report will be an analysis of this transformation of the raw material (wood) to an efficient energy source. There are different methods to optimize the material towards a more energy efficient product. The utilization of biomass is a growing industry but it is still required to make further improvements. The Food and Agriculture Organization of the United Nations (FAO) has made an overview of terminologies and description in the Unified Bioenergy Termonology (UBET) report (Killmann, 2004). The basic processes that has to be handled before energy is generated from biomass, is shown in figure 1.1. The first step is to produce the biomass as a raw material, this can be a collection from residues/wastes or freshly cut wood. Next, the physical collection of the biomass has to be done, for example chipping the trees or collect the waste. The biomass has to be transported to (and possibly stored temporary at) the plant where a conversion can take place. This conversion can exist of multiple steps to generate the most energy possible. This energy can eventually be used by the client/end user (Faaij, 1997).

This figure shows the typical process steps for a supply chain of biomass. Project Bioforce will aim on transport of woody biomass towards Europe by vessels. The retrieved wood often has a high moisture content [20-50%] which decreases the value of the material (since the moist does not contain energy). This will lead to high transportation costs per energy unit. Therefore the goal is to convert the biomass after collection into a material with a higher energy density. Multiple methods are available, which possibly affect other properties that influence the quality of the product. This has led to the research question which will be answered in this report is:

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Figure 1.1: Production chain of bioenergy (Faaij, 1997).

Before this question can be answered, the first step is to answer five subquestions. These will guide this research towards the desired results:

• What is woody biomass?

• What happens during the torrefaction process?

• Which methods are used to densify the woody biomass? What determines the quality of the densified product?

• Which variables can be changed in the manufacturing process to optimize the quality of the pellet?

• What happens to the biomass when the torrefaction and densification processes are combined?

Outline

The first step of this research will be to analyze biomass. As wood is the most interested product to use, chapter 2 focuses on the woody biomass and the origins of the energy source. In chapter 3 the torrefaction process will be analyzed, this process uses a roasting process of the biomass to evaporate moist and volatiles. This will be followed by a de-tailed research about the densification methods which can be used and how the product can be made (chapters 4 and 5). The research will be finalized by an overview of qual-ity parameters (chapter 6) and the effects of combining torrefaction with densification (chapter 7).

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Chapter 2 Woody biomass

2.1 Introduction

Biomass has its origin from the first people on planet earth who used it as a heating source. Later, the Romans were the first people who were able to have a structured method of using the material. They retrieved wood from the forests around for example Crete, Cyprus and Mycenaean Greece to distribute it over their empire to France, Spain and North Africa. This distribution of biomass has a lot of similarities with the current distribution of fossil fuels. The product is collected, distributed and used at another location. The civilizations of the Romans began to decline after the resources became exhausted (Perlin and Jordan, 1983). This is also about to occur with the fossil fuels therefore precautions has to be taken. Multiple parties are starting to look for alternative energy resources. Project Bioforce is a collaberation of multiple parties which focuses on the development of using woody biomass.

This research must provide information about the current state of the industrial use of woody biomass. But before analyzing any details of the woody biomass it is necessary to gather information about biomass itself. This chapter will therefore head of with some global information about biomass. When this is clear, the research will zoom into woody materials to investigate further details of the product. The main focus will be on the energy content of the material and which characterizations have an influence on this property.

2.2 Biomass

Biomass is a source of natural material that can be converted into bioenergy. There is many materials available that can be used as biomass. This section will explain the basics of using biomass and why it is often called a ‘green’ energy source. Using these materials will lead to a cleaner energy production, but may come in hand with some disadvantages. An overview will be provided of effects of using bioenergy. Next, an overview will be showed about the utilization of biomass all over the world. As mentioned a lot of products can be named as a biomass. To have a guideline, a classification will be provided to understand the deviation of all products.

2.2.1 Carbon neutrality of biomass

Biomass is material which is obtained from living or recently living organisms. This is one of the many differences between biomass and fossil fuels as fossil fuels are descend

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from million years ago. Both of the products are carbon based which means that during combustion carbon dioxide will evaporate to the atmosphere (the main cause of the greenhouse effect). The difference between the products is that carbon of the biomass can be seen as a product within the global environment. On the other hand, the fossil fuels contain carbons that have not been in the global environment for a long time. This means that during combustion these carbons are added to the atmosphere and will add up the greenhouse effect.

The carbon cycles for both products are shown in figure 2.1. It is clear to see that the fossil fuels are gained from a source deep under the surface, created over millions of years. The production is a long-term process, therefore the eco-system does not take its emissions into account and more carbon dioxide will be built up in the air. This causes an unbalanced carbon cycle which will take millions of years to be restored. The biofuel shows a balanced carbon cycle in the figure since it is gained from the (recently) living organisms directly. This means that with combustion no further disturbances in the air will take place (Brodeur-Campbell and Jensen, 2005).

Figure 2.1: The carbon cycle of fossil fuels and biofuels (Brodeur-Campbell and Jensen, 2005).

The figure shows the difference in combustion of carbon between the biofuel and the fossil fuel. A more detailed view of the carbon cycle of the biofuels is shown in figure 2.2. This cycle can continuously be repeated without having a bad influence on the balance of carbon in the atmosphere. The cycle is neutral in the use of carbon since all of the converted carbon will be collected by the threes with help of the photosynthetic proces. To generate bioenergy from the trees without disturbing the environment a program has to be developed and managed which takes care that a balance is kept and the forest does not run out of resources. The biomass has to be collected and converted into bioenergy. This process will return the CO2 of the biomass back into the atmosphere which eventually can be collected by new trees. This completes the carbon cycle of biomass materials (Dickinson, 2013).

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2.2 Biomass 5

Figure 2.2: Detailed view of the carbon cycle of biomass (Dickinson, 2013).

2.2.2 Advantages and disadvantages of biomass

With the previous paragraph in mind it is clear that the use of biomass is carbon neutral. This is one of the advantages of using biomass. But next to this, there are more advantages compared to using fossil fuels. Of course there are also multiple disadvantages about bioenergy. These will all be listed to get an overview of feasibility of using biomass (Kukreja, 2014).

Advantages of biomass

The following list provides advantages of using biomass as an energy resource:

• The emissions produced by the biomass energy do not harm the environment since no ‘new’ carbon dioxide is released in the air.

• The production of biomass will help to become independent to the import of oil. • The production of biomass is more labor intensive and therefore will lead to more

jobs. It is assumed that using biomass as an energy source will generate 20 times more employment then coal and oil.

• The use of biomass can reduce landfills since next to fresh trees also waste can be used as biomass source. For example garbage can be burned into useable bioenergy. In this way useless material can be used to provide useful energy.

• The biomass can originate from many materials. Therefore multiple different prod-ucts can be made, which enables many options for using biomass.

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• The biomass can be produced by human labor, so running out of this resource can be avoided (if managed correctly).

Disadvantages of biomass

The previous list shows that there are several reasons for using biomass as a energy source. Unfortunately, there are also some disadvantages associated with generating energy from biomass:

• The production of biomass is very expensive since it has to be retrieved from (recenty) living organisms. It takes time/money to feed and grow these organisms. • The energy from biomass is low compared to fossil fuels. The energy generated out of the same amount of material is much less therefore larger quantities must be used to achieve the same results.

• The production of lumber will require an enormous amount of land to generate a significant amount of energy.

• The combustion of biomass can in some cases cause pollution by incomplete com-bustion. The combustion will emit CO, NO2 and other gasses which are toxic and cause pollution (Pennise, 2011).

• The development of biomass consumption is still in the early days. Therefore it is necessary to conduct more research to generate bioenergy more efficient and reduce the production costs.

• There are some quantity limits to the utility of many types of biomass. For example projects that use animal waste to produce bioenergy are limited in the supply of animals. This can not be upgraded to a large scale industry.

The lists of the advantages and disadvantages make it hard to state a conclusion about biomass. As shown there are multiple reasons for choosing for biomass. But the disad-vantages can not be neglected. Nevertheless, the major advantage is that biofuels will eventually eliminate the use of fossil fuels. Therefore multiple projects are willing to invest in the research of these programs.

2.2.3 Use of biomass

There is interest in using biomass as an energy resource since it is a renewable energy source and might be a key product in the future. But next to this there are also compa-nies/countries which are forced to use biofuels. This means that a division can be made between the countries (Rosillo-Calle et al., 2007):

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2.2 Biomass 7

• In developed countries fossil fuels are often used to generate energy, since the costs of these sources are currently lower than of biomass. Nevertheless, most of the developed countries starting to realize that the fossil fuels are running out and looking for other methods to generate their energy.

• The developing countries are often not yet able to generate energy from fossil fuels in a large scale. Therefore they generate most of their energy from material that is directly available, which is biomass. This means that biomass is the main source of energy for these countries and still continues to provide the bulk of energy for many developing countries.

The difference between the consumption of biomass of those type of countries is made clear in figure 2.3 (Rekacewicz et al., 2009). This shows the a map of the world with the percentage of the bioenergy from the national energy consumption. It clearly shows that the developing countries (Africa, parts of Latin America and Asia) do not have options to generate energy in other methods and therefore high percentages of bioenergy can be found. Comparing this to the developed countries shows a difference. Most of the developed countries have an indication of ‘not significant’. This is because these countries currently still have options to generate energy with fossil fuels and have recently started to investigate the possibilities of biomass.

Figure 2.3: Solid biomass consumption including woodfuel in the world (Rekacewicz et al., 2009).

2.2.4 Types of biomass

As mentioned, many types of biomass can be used to generate bioenergy. All of them are (recently) living organisms that contain energy which can be obtained by a conver-sion method. To have an overview of all the types the Unified Bioenergy Termonology

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(UBET) assigns all of them into four groups: woody biomass, herbaceous biomass, biomass from fruits and seeds and others. This overview has been provided in table 2.1. Every type of useful material can be divided into categories. The products are divided by how they can be obtained from nature before bioenergy can be retrieved from it (Killmann, 2004).

• Direct: These materials can directly be converted to generate energy (for example wood).

• Indirect: These materials need some treatment before they can be converted to generate energy (for example black liquor)

• Recovered: These materials are leftovers from previous operations, which still can be used for new conversions to generate energy (for example wastes of the kitchen) Table 2.1: Classification of Biofuel sources by different characteristics (Killmann, 2004).

woody biomass herbaceous biomass from others biomass fruits and seeds

Woodfuels Agrofuels

Energy corp

direct

-energy forest trees - energy grass - energy graim - energy plantation - energy whole

trees cereal crops

By-products

- thinning by- crop production by-products: - animal by-products products -straw -stones, shells, - horticultural by-- logging byby-- husks products

products - landscape manage-ment by-products

indirect

- wood processing - fiber crop - food processing -biosludge

industry by-products processing by- industry by- -slaughterhouse by-- black liquor products products products

End use

Materials recovered

- used wood - used fiber - used products of Municipal

by-products fruits and seeds products

- kitchen waste - sewage sludge

The table shows that there are three main type of fuels: Woodfuels, Agrofuels and Municipal by-products. These fuels are currently used to generate bioenergy in power plants. The type of processes will differ between the materials. In order to specify this research, it will focus on woody biomass strictly.

2.3 Cell structure of woody biomass

The woody biomass can be collected from any kind of wood. This means that there is a large variety of possibilities to produce bioenergy from woody biomass. To determine the demands to the product, it will be analyzed in detail. As a start the structure of the cells will be analyzed and which properties have what influences.

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2.3 Cell structure of woody biomass 9

The first step in understanding the physical properties of woody biomass is to look at the structure of the material. All woody biomass have the same base product which is known as lignocellulose and it contains three main sugar-based polymers (Koeneman et al., 2011):

• Cellulose

Cellulose is the main material of woody biomass. It is mainly concentrated in the inner cell walls of the material. The cellulose has a structure of long polymers, this causes the rigid cell walls of wood.

• Hemicellulose

Hemicellulose is less structured than the cellulose. It is embedded in the cell wall and consists of branching polymers within the material.

• Lignin

Lignin does not consist of a linear structure. Together with hemicellulose it forms a rigid structure of the cell itself. The lignin is mostly enclosed at the outside cell wand. It has a high energy content, therefore it will be very useful as energy source.

These three products can be found in any woody or herbaceous biomass. Figure 2.4 shows how they are composited in the cell wall. The secondary forms the basis of the cell and exist purely from lignocellulose, figure 2.4b shows the composition at different positions within the wall.

(a) All layers of the lignocellulose (Hubbe, 2008)

(b) Composition of the lignocellulose (Pan-shin et al., 1964)

Figure 2.4: Distribution of lignocellulose within the three layered secondary wall.

There are many types of vegetation in the world, all of them have different deviations of the components of lignocellulose. For the same type of materials the differences are fairly little. Table 2.2 shows several groups of materials that contain lignocellulose (Dakar, 2013).

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Table 2.2: Composition of Lignocellulose (Dakar, 2013).

Lignocellulosic materials Cellulose [%] Hemicellulose [%] Lignin [%]

Hardwood stems 40-55 24-40 18-25

Softwood stems 45-55 25-35 25-35

Grasses 25-40 35-50 10-30

Corn cobs 45 35 15

Leaves 15-20 80-85 0

2.4 Determination of the energy content of woody biomass

The most interesting property of the woody biomass is the amount of energy that can be obtained from the product. Multiple factors influence the amount of energy available from the combustion of woody biomass. Two of them are the most important, the moisture and ash content. The wood will contain some moist, this is water which does not energy to the material and the amount should be as low as possible. Also the ash content of the product must be known to determine the energy of the material (Kaltschmitt et al., 2003).

2.4.1 Moisture content

The Moisture Content (MC) is a very important property of the wood. This parameter has a huge effect on the energy, density and strength of the product. Moisture in wood can exist in two forms:

• Free water : Liquid water or water vapor in cell lumens and cavities • Bound water : Water which is chemically held within the cell walls

A moist material will exist of weight that does not posses any energy, as the water will vaporize and turn into steam during combustion. Therefore it is desired that the moisture content is as low as possible. But since the material of interest is tree wood, it is inevitable to have moisture in the product. There are two methods to determine the moisture content of a product:

• Dry Based (DB) moisture content is determined with help of equation 2.1. It describes the ratio of moist in the dry material.

M oisturedry basis = 100% ×

_{W etW eight − DryW eight}

DryW eight

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2.4 Determination of the energy content of woody biomass 11

• Wet Based (WB) moisture content is determined with help of equation 2.2. It describes the ratio of the moist in the wet material.

M oisturewet basis= 100% ×

_{W etW eight − DryW eight}

W etW eight

(2.2)

Calculations concerning moisture content will often use a wet basis. Therefore a wet based moisture content will be used during this report when no details are indicated. Trees will have a moisture content from about 30 wt.% to more than 200 wt.% of the weight of wood substance. When the tree is recently cut it is called green wood. At this point the cell walls are completely saturated with water. Next to that, the lumens also contain additional water (U.S. Department of Agriculture, 1999).

This moisture content is fairly high and should be reduced before combustion to optimize the process. The first step is to air dry the material. This will help to reduce the moisture content to about a minimum of 20 wt.% (Simpson et al., 1999). When further decrease in moisture content is desired, other drying steps can be introduced to optimize the quality of the product. More information about drying of biomass will be provided in chapter 5.

2.4.2 Ash content

The Ash Content (AC) is the percentage of ash that remains after combustion of biomass of the total material that is used. This means that this material will not transform into energy. Thus the higher the ash content, the lower the energy value of the product. The ash content will differ for different sources of biomass (Linnæus University, 2012). For example when a material has an ash content of 15 wt.% the total possible energy that can be generated is 85 wt.% from the available energy. When this material also has a dry based moisture content of 15 wt.%, only 70 wt.% of the combustible material can be used.

Next to the decrease in energy content, it will also lead to wastes in the combustion chamber. When the ash content is high, it means that a lot of residues will stay in the combustion chamber and a method has to be used to clean this equipment.

2.4.3 Energy value of biomass

The energy value of biomass is, as mentioned, dependent on the moisture and ash content. But it is mainly dependent on the type of material that is used. From these properties the energy value can be determined of the biomass. There are two main characteristic heat properties for woody biomass (Rosillo-Calle et al., 2007):

• Gross Heating Value (GHV), also called Higher Heating Value (HHV) in [M J_kg ] This is the heat that is generated during oxidation of the completely dry biomass

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divided by its total mass. The water vapor contained within the material is con-densed with help of a conversion device. In this way additional energy will be released. Also energy is generated during the formation and vaporization of water from the hydrocarbon molecules contained in the hydrogen. This energy is all included in the determination of this heating value

• Net Heating Value (NHV), also called Lower Heating Value (LHV) in [M J_kg ] The additional conversion device of the GHV is not used. Therefore less energy will be generated from the same material.

For fossil fuels the difference between these types of energy is fairly little and rarely more than about 10%. The biomass often has a higher moisture content and therefore less energy can be gained from the same amount of material.

The theories to determine the amount of energy can be generated from material has been determined more than 2 decades ago. The research of Channiwala (1992) determined that the GHV is dependent on the molecules within the material, C, H, S, O and N. Next to these molecules also the amount of ash that will be restored is a variable . After many experiments, an atlas of many sample materials has been made (Gaur and Reed, 1995). This research has determined an equation that holds for all materials during gasification (with little error). This theory is purely based on experimental research and is shown in equation 2.3.

GHV = 0.3491 · XC+ 1.1783 · XH+ 0.1005 · XS− 0.0151 · XN− 0.1034 · XO− 0.0211 · Xash

(2.3) Where Xi is the content of the dry based weight percentage of:

• C: Carbon • H: Hydrogen • S: Sulfar • N: Nitrogen • O: Oxigen

• ash: the ash content of the material

With help of this for all materials the GHV can be determined. Table 2.3 shows the for some woody biomass the maximum energy that can be generated from the material. Table 2.2 showed that softwoods have a high amount of lignin. It has been stated that lignin contains the most energy of the materials in lignocellulose. Comparing this to table 2.3, this statement is accepted. The softwood show a higher GHV compared to the hardwoods.

Next to that table 2.3 displays the maximum available energy that can be generated. But as mentioned, the moisture content and ash content will reduce the real heating

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2.4 Determination of the energy content of woody biomass 13 Table 2.3: Energy content and concentrations of untreated woody biomass compared with coal (Kaltschmitt et al., 2003).

C H S N O Ash GHV

Type of biomass in % of dry substance [MJ/kg]

Sprucewood (with bark) 49.8 6.3 0.015 0.13 43.2 0.6 20.3 Beech-wood (with bark) 47.9 6.2 0.015 0.22 45.2 0.5 19.3 Poplar wood (short rotation) 47.5 6.2 0.031 0.42 44.1 1.8 19.3 Willow wood (short rotation) 47.1 6.1 0.045 0.54 44.3 2 19 Bark (softwood) 51.4 5.7 0.085 0.48 38.7 3.8 20.6

Hard coal 72.5 5.6 0.94 1.3 11.1 8.3 30.7

Lignite 65.9 4.9 0.39 0.7 23 5.1 26.3

value (NHV). The NHV that can be obtained is determined by equation 2.4 (van Loo and Koppejan, 2008). N HV = GHV (1 − MC 100) − 2.444 · MC 100 − 2.444 · XH 100 · 8.936 · (1 − MC 100) (2.4) In this equation:

• 2.444 = the enthalpy difference between gaseous and liquid water at 25◦C. • 8.936 =MH2O/MH2; i.e. the molecular mass ratio between H2O and H2. • MC is the moisture content in weight percentage wet based.

• XH is the concentration of hydrogen in weight percentage dry based. – For woody biomass fuels this is around 6 wt.% (see table 2.3). – For herbaceous biomass fuels this is around 5.5 wt.%.

Combining the information provided by table 2.3 and the equations 2.3 and 2.4, the NHV can be determined for all moisture contents. This is very useful to have an overview of the possible energy that can be generated. Figure 2.5 shows a linear declining line of energy that can be generated by the material of a given moisture content (figure created by author of this report, with information of Kaltschmitt et al. (2003)).

The diagram of figure 2.5 can be used to determine the energy from a product under any circumstances. This method can easily be used for any material to determine the NHV. The diagram shows that it is essential to reduce the moisture content as much as possible.

The diagram also clearly shows the benefit of using a fossil fuel like hard coal or lignite. The NHV is much higher for these materials than for woody biomass at the same mois-ture content. Therefore it is necessary to optimize the moismois-ture content of the wood. As mentioned before, after air drying the wood will remain with a minimum moisture content of 20%. When this can be minimized towards for example 6% (optimal results

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Figure 2.5: Net Heating Value of wood depending on the total moisture.

which can be achieved with advanced techniques) it will increase the NHV with 20%. This improvement significantly and important to achieve lower costs for transportation.

2.4.4 Energy density

The energy density is the most important factor of the energy calculations. This deter-mines the amount of space that is required for a certain energy amount. The NHV will be used together with the density of the material, see equation 2.5.

ρenergy = N HV × ρbulk (2.5)

In this equation the ρenergy is the energy density

h

M J m3

i

and ρ_bulk is the bulk density h_kg

m3

i

. The energy density is a very important factor for the transportation of biomass. A high energy density will allow the process to be more cost efficient. As more energy (profit) can be obtained from the same transportation costs. The overall desire when using woody biomass is to increase the energy density as much as possible. Only at that moment it might be possible to compete slightly with the low cost fossil fuels.

2.5 Availability of woody biomass over the world

Woody biomass is an easy accessible product. All over the world the product is available but to generate energy in an interesting amount requires a lot of trees. In Europe the population is fairly high and reduces the space for tree growth.

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2.6 Conclusion 15

Therefore the biomass is often imported from other continents. North America is the main exporter to Europe considering the pellet (see chapter 5) industry as they are able to create the product with less effort and costs (Lamers et al., 2013).

Figure 2.6 shows the quantities of the pellet transport in the world for 2012. Only the countries in Europa can be notices as importers of wood pellets. All other nations use the woody biomass as an export product.

Figure 2.6: Global trade of pellets in 2012 (Lamers et al., 2013).

When using this type of import it is desired that a usable product is delivered to Europe. Therefore this research will examine what is necessary to deliver the product in Europe while remaining at the quality level that can be used in industries. For the remainder of this report it will be assumed that the import of the biomass will have an origin from North America as this demands long transport distances. This might have an impact on the product which will be used.

2.6 Conclusion

The first goal of this research is to determine what is biomass and how can it be used, with the focus on woody biomass. This chapter has made clear that biomass is material that origins from (recently) living organisms which can be used as an energy source. During combustion of the material carbon dioxide will be released but as it consists of carbons from recently living matter, it will not add up to the greenhouse effect.

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Woody biomass consists of lignocellulose, a product built from three components (cellu-lose, hemicellulose and lignin). These materials determine the structure of the cells from the tree. The energy density is the most important property for the transport of woody biomass as it is related to the transport costs. Before being able to determine the energy density, the HHV and NHV have to be calculated. From this the energy density can easily be calculated with the bulk density. Important parameters for the energy density are the moisture content and ash content.

There are not many possibilities of producing woody biomass in Europe, therefore it must be imported. North America can be seen as a major distributor of woody biomass. Therefore during the remainder of this report, it will be taken into account that large transportation has to take place to import the product. This requires a product with a high energy density to minimize the transport costs. How to optimize the densification of the biomass will be analyzed in the following chapters.

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Chapter 3 Torrefaction

3.1 Introduction

Torrefaction is originally a French word and directly translated means roasting. Torrefac-tion has its origins in the coffee and tea industry where it is used to achieve improved flavor. During the torrefaction process, the bean is roasted at a temperature above 200 ◦C to emulsify, caramelize and release natural sugars, fats and starches. Different torrefaction times will lead to different tastes of coffee (Cafe Britt, 2014).

This process is copied to optimize the quality of the woody biomass. As analyzed in the previous chapter showed, the wood has a relative high moisture content when retrieved. The torrefaction process will be used to decrease the moisture content and increase the Net Heating Value (NHV). This will be done within a temperature range between about 200◦C - 300◦C in an inert atmosphere. In this temperature range the moist and other volatiles will evaporate and a more useful product remains (Medic, 2012).

3.2 Torrefaction mechanism

The torrefaction process is a method to increase the energy density. It has its origins in the 1930s when France researchers wanted to modify woody biomass to achieve better combustion. The process needed a lot of development before suitable results could be achieved. At this moment these developments are still going on and further improve-ments are necessary (Bergman et al., 2005b).

The first step to understand the torrefaction process is to explain the basic concept during this section. When the basics are determined a specific analysis of the decomposition mechanism will be made. This will provide an overview about the processes that take place with the biomass. The heating of the torrefaction process will be performed with five typical stages. These stages will be analyzed in the final part of this section.

3.2.1 Concept of torrefaction

The torrefaction process is used to increased the NHV. This is done by roasting the biomass for a certain time to evaporate moist and volatiles. During the torrefaction of the biomass also useful components of the product will evaporate. This means that during the process contents with low energy density will be lost. Figure 3.1 shows the effects of during the process. About 70% of the initial mass (m) will be retained after torrefaction. The content which is lost has a low energy density which results that typically 90% of the energy (E) of the material will be retained. This meas that the NHV

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will increase with the energy densification ratio (equation 3.1). The NHV will increase typically with a ratio of about 1.3, which clearly shows the advantages of torrefaction Bergman et al. (2005b).

Energy densification ratio = E

m =

0.9M J

0.7kg = 1.28

M J

kg (3.1)

Figure 3.1: Mass and energy changes of a feed undergoing torrefaction (Basu, 2013).

3.2.2 Heating conditions

Figure 3.1 suggests that during torrefaction the heat will be generated by fire. This is not necessarily the heating medium. The study of Yan et al. (2009) has been focused on two types heating medium, in a wet or dry environment. There are multiple differences between the processes for the heating mediums:

• Dry torrefaction is usually performed in an inert environment and will result in gases and solid fuel. The temperature range is between 200 - 300◦C.

• Wet torrefaction is performed in hot compressed water and will result in gases, aqueous chemicals and solid fuel. The temperature range is between 200 - 260◦C. As mentioned, dry torrefaction usually takes place in an inert environment. The reason for this is that when oxygen in present during torrefaction, the carbon converts into a flue gas instead of leaving it in a solid form. Also, it will increase the combustion temperature to an undesirable level. These changes will lead to a unsafe and inefficient operations of the torrefaction process, therefore oxygen is not allowed (Nhuchhen et al., 2014).

3.2.3 Decomposition mechanisms

The temperature range of torrefaction has been mentioned before and is between about about 200◦C - 300◦C. The reason for this is the based on the structure of the material.

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3.2 Torrefaction mechanism 19

Section 2.3 described that the woody biomass has been built of a combination of three main polymeric stuctures: hemicellulose, lignin and cellulose, which combined are called lignocellulose.

The three polymeric structures show the same reactions during heating. But the temper-ature range that these reactions take place differ for each structure. Figure 3.1 shows the reactions that take place when the lignocellulose is heated for each polymer individually. The pathways are indicated by five reaction regimes (Bergman et al., 2005b).

Figure 3.2: Typical mass and energy balance of torregaction process (Bergman et al., 2005b).

As shown in the figure, there are five main reaction regimes: (A) Drying: Physical drying of the biomass

(B) Softening: This regime will only occur with the lignin. In this regime the biomass constituent will soften. This is beneficial during the densification of the biomass since softened lignin is a good binder.

(C) Depolymerisation and recondensation: The shortened polymers will start to con-dense within the solid structure

(D) Limited devolatilisation and carbonization: The intact polymers and solid struc-tures which are created in regime C will devolatilize and carbonize limited. (E) Extensive devolatilisation and carbonization: The material which is created is

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The figure shows that the components of the lignocellulose originally all have different reactions to temperature changes. Research has shown that there is no interaction among the three constituents during torrefaction. Hence it can be concluded that the reactions of the materials can be seen independent of each other and no synergetic effect is shown. This enables identification of each polymer separately and examine the effects of torrefaction on the weigth loss (Chen and Kuo, 2011).

Section 2.3 already showed that the woody biomass consists mostly of cellulose. Never-theless this polymer will not devolatilize significant during torrefaction. As hemicellulose and lignin have lower devolatilisation temperatures these temperatures should not be ex-ceeded. Otherwise the devolatilisation will decrease the energy content of the biomass which is not the goal of torrefaction. Figure 3.2 shows that the three polimeric structures will all start to devolatile and carbonize heavily at temperatures above 300 ◦C. This is the reason for the allowable range of temperature during torrefaction (200◦C - 300◦C). The blue line (at 250◦C) in figure 3.2 indicates that there are two torrefaction regimes. The lower regime will mostly affect the devolatilisation and carbonisation of hemicellu-lose. The lignin will have minor decomposition until this temperature and the cellulose will cause no weight loss during low torrefaction. Torrefaction above 250 ◦C will even cause the hemicellulose to decompose extensively and will cause devolatilisation of lignin and cellulose (Bergman et al., 2005b).

The research of Shafizadeh and McGinni (1970) has examined the decomposition rate of the lignocellulosic, see figure 3.3. It shows that the material will start to decompose at temperatures above 200◦C (which can also be seen in figure 3.2. The hemicellulose (mainly exists of xylan) starts to devolatilize at this temperature and will eventually have a peaking rate at 250◦C - 280 ◦C. The lignin shows a much lower decomposition rate in the torrefaction temperature range. The cellulose even starts to have severe devolatilisation at temperatures higher than the torrefaction temperature. Therefore the consideration has to be made which results are desired by the torrefaction process to determine an ideal torrefaction temperature.

3.2.4 Torrefaction products

As mentioned before, the woody biomass consists mainly out of three polymeric con-stituents. This means that they consist of three main elements, Carbon, Hydrogen and Oxygen. When these are torrefied (or burnt), a chemical reaction will take place which will result in multiple reaction products. The reaction products can be in different states, liquid, solid and gaseous (see table 3.1) (Basu, 2013).

3.2.5 Process stages

There are multiple methods to perform the torrefaction process. Figure 3.4 shows the five stages (temperature-time) of a batch-wise reactor where torrefaction takes place1.

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3.2 Torrefaction mechanism 21

Figure 3.3: Decomposition of cotton wood during torrefaction (Bergman et al., 2005a). Table 3.1: Products of torrefaction of biomass (Basu, 2013).

Torrefaction products

Liquid Solid Gas

Water Original and modified H2, CO2, CO, CH4 Organics sugar structures CxHy, tolune, benzene

Lipids New polymeric structures

Ash Char

These consist of the following time steps (Bergman et al., 2005a):

1. Initial heating (tn). The biomass is heated until it has reached the temperature

at which the moist will start to evaporate.

2. Pre-drying (t_dry). The biomass is kept at the drying temperature which enables

the free water to evaporate at a constant rate. The temperature will remain at a constant level until the evaporation rate will start to decrease and the biomass reaches the critical moisture content (about 5-10% wt).

3. Post-drying and intermediate heating (t_h,int). After the biomass has been

dried, it will be heated to a temperature of 200◦C. This will separate the remainder of water content from the biomass. After this step the biomass will practically not contain any moist. During this step the first light organic compounds will start to evaporate, which will lead to mass loss.

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4. Torrefaction (ttor). In this stage the real torrefaction will take place. The stage

starts when the temperature exceeds 200 ◦C and will end when it has returned below 200◦C. This stage will therefore consist of three sections2.

• ttor,h: the heating time from 200◦C till Ttor

• ttor,con: the torrefaction time, the time the temperature will remain at Ttor

• t_tor,c: the cooling time from T_tor till 200◦C

5. Solids cooling (tc). The product will be cooled from 200 ◦C until the desired

end temperature. In this time step no further mass release occurs.

Figure 3.4: Stages of the heating during torrefaction (Bergman et al., 2005a).

The torrefaction time (or reaction time) is the time that is taken to torrefact the biomass and is defined as t_tor,h+t_tor,con(notice that the t_tor,cis not added). During this period the actual torrefaction will take place and energy density increases. The average heating rate is determined by: (Ttor− 200◦C)/ttor,h, normally this value will not exceed 50◦C/min.

3.3 Torrefaction process definitions

The improvement of the torrefaction process must be specified to determine the results. The research of Yan et al. (2009) provides an overview of the parameters that can be calculated to analyze the improvements. The goal of the torrefaction is to increase

2_{Figure 3.4 shows that t}

tor exists of ttor,h, ttor,con and ttor,c. Bergman et al. (2005a) did not define

the time period of constant temperature which causes conflicts in the terminology, therefore ttor,con is

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3.3 Torrefaction process definitions 23

the energy density. This will be achieved by removing volatiles and moisture from the biomass. The volatiles also contain some energy content therefore a suitable process has to be used which agrees with the goals of the product. The biomass must be increased in energy density without torrefacting too much energy.

To determine the improvement of density equation 3.1 can be used. But to achieve this improvement, also some energy will be lost during the process. To determine the amount of energy which is lost, first the mass yield must be determined. This will be done with help of equation 3.2:

Mass yield = Mass of dried pretreated solid

Mass of dried biomass × 100% (3.2) Figure 3.5 shows the mass yields for different circumstances. The study of Prins et al. (2006) tests multiple materials under different residence times and torrefaction temper-atures. The trend shows a decreasing mass yield which could be expected.

Figure 3.5: Mass yield of torrefied wood as a function of temperature and residence time for different biomass types (Prins et al., 2006).

The mass yield is used to determine the amount of material that is reduced during torrefaction. The material which is removed will, as mentioned in section 3.2.3, mostly consist of hemicellulose. This will increase the heating value of the biomass which is used to determine the energy ratio. This is the ratio of the change in Higher Heating Value (HHV) of the product, see equation 3.3.

Energy densification ratio = LHV of dried pretreated solid

LHV of dried biomass (3.3) The energy densifiction ratio has also been analyzed by Prins et al. (2006). The results are shown in figure 3.6. The original Lower Heating Value (LHV) has been provided to

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show the improvement after torrefaction. It is clear that an improvement is achieved for the process and the density will increase.

(a) Willow (17.7 MJ/kg) and larch (18.2 MJ/kg)

(b) Beech (17.0 MJ/kg) and straw (16.1 MJ/kg)

Figure 3.6: Lower heating value of torrefied wood on dry basis as a function of temperature and residence time, between brackets the original LHV before torrefaction (Prins et al., 2006) The two previous equations are combined to specify the energy yield of the torrefaction process. This factor shows the amount of energy that remains. The reaction will cause a loss of energy but as the density increases other goals can be achieved, depending on the specific goal of the torrefaction.

Energy yield = mass yield × energy densification ratio (3.4) Figure 3.7 shows the final results of the research of Prins et al. (2006). The energy yield for each biomass type for different temperatures and residence times. The conclusion that should be stated from this research is that it is hard to state the perfect conditions of torrefaction. It will depend on the parameters which will be used. For starters the process will depend on the material which will be used during torrefaction. But as shown in this research also the residence time and temperature are important variables for torrefaction. Therefore each torrefier must be analyzed properly to determine the optimal setup to achieve the most desirable biofuel.

3.4 Conclusion

Torrefaction is a useful process to increase the energy density of the biomass. This is done in a temperature range of about 200 ◦C - 300 ◦C. In this temperature range the three polymeric structures of the biomass devolatilize at their own specific heat. Hemicellulose is the least heat resistant and will devolatilize at temperatures above 200 ◦_{C. The cellulose and lignin will start to devolatilize at temperatures above 250}◦_{C. The} differences in temperatures of the five regimes for the three polymers causes the biomass to react different for each specific material and temperature. Therefore it is impossible to assign a standard ideal setup for the torrefaction process.

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3.4 Conclusion 25

Figure 3.7: Lower heating value retained in torrefied wood on dry basis as a function of temperature and residence time for different biomass types (Prins et al., 2006).

After torrefaction the NHV has increased but the energy density has not improved significantly and the torrefied biomass has become fairly loose. This can be pressed together to increase the energy density. Multiple methods can be used for this goal, the following chapter will evaluate the different densification procedures.

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Chapter 4 Densification procedures

4.1 Introduction

When the woody biomass has been torrefied, the energy density of the product has increased by removing the moisture and eliminating several volatiles. This helped to improve the quality of the product, but it has become fairly loose during the torrefaction process. This can be improved by pressing the material together and hereby increasing the mass and energy density, in other words: introduce a densification process.

For this procedure there are multiple methods available. This chapter provides an overview of all methods. They all can be used for different applications in the biomass industry. The methods will be compared to determine the right procedures that can be used for large transportation of woody biomass.

4.2 Reason for densification

The bulk density is the amount of mass per volume unit of the biomass material. This property does not determine the level of energy but it combined with the Net Heating Value (NHV) it is used to calculate the energy density. The most agricultural residues have a fairly low bulk density. This means that much more volume is used for the equal amount of mass. This will cause limits for the economical feasible transport region according to Preto (2007). He has found that the allowable transport distance for unprocessed biomass is less than 200 km.

In chapter 2 the energy density has been introduced as the most important factor of the biomass. When a higher energy density of the biomass can be achieved, the transporta-tion costs will be reduced. Without any pretreatment of the biomass the material has a fairly low energy density. Figure 4.1 shows a the amount of volume which is required for (raw) materials. It clearly shows that the biomass uses much more space than the coals, the ratio is 16:4:1 for straw to wood to coal (Clarke and Preto, 2011).

This figure shows the main desire of increasing the energy density. A low density will result in higher costs for handling, transportation, storage and combustion processes. Of course this has to be avoided when possible. Next to an increased energy density, there are more advantages of the densification of biomass:

• A uniform combustion will take place in the boilers when a more consistent product is used.

• Since the biomass will be compressed, less loose material is attached which will result in less dust production.

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Figure 4.1: Equivalent energy content by volume of unprocessed materials (Clarke and Preto, 2011).

• When the biomass has been densified, the risks of spontaneous combustion during storage will decrease.

There are several processes available, which will be discussed during the following section.

4.3 Types of densification processes

There are multiple densification processes which are currently used for different purposes. During this section, the processes will be discussed. Clarke and Preto (2011) shows a clear overview of the processes in the factsheet.

4.3.1 Bales

Bales are used during the collection of biomass from agriculture where straws are densi-fied in round or cubic bales. This procedure will help with the collection of the biomass at the production facility. The material is compressed but will not deliver a large im-provement by the densification. This causes a lack of quality imim-provement to transport it over larger distances. Figure 4.2 shows a stack of bales where it is collected. The bales are often used to gather the biomass before it will be torrefied (and further densified) (Kemmerer and Liu, 2011).

There are two types of bales: round bales and square bales. The round bales have the advantage that they are more easily to produce since the form is easy to compress. But stacking the round bales will cause some space losses and thus reducing the density. With the square bales, such losses would not occur and thus result in a higher bulk density. Since the shaped bales exist of the same material, the square bales have a higher energy density.

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4.3 Types of densification processes 29

Figure 4.2: Stacked bales of woody biomass (Eco2, 2014).

4.3.2 Wood chips

Wood chips are often used in the handling of woody biomass, since wood logs might be inconvenient. When the logs are stacked, they can be transported easily. But handling of the logs can be difficult. Often the wood logs are shredded into small pieces as shown in figure 4.3. In this shape the woody biomass has a more applicable dimension and therefore automated handling and feeding might be possible. In this way it can be possible to create a more uniform fuel delivery to a boiler. Another advantage of wood chips over log wood, is that it has a higher surface area to volume ratio. This will result in better efficiency during combustion or gasification (Biomass Energy Centre, 2011b).

Figure 4.3: Wood chips of woody biomass (The Greenage, 2014).

4.3.3 Pellets

Wood pellets are made by compressing sawdust of small wood particles under high pressure until the lignin softens and binds the material together, see figure 4.4. After this process the moisture content of the biomass is reduced (typically below 10%). This

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will directly increase the volume energy density, see chapter 2. The energy density is often up to 3-4 times higher than the wood chips. The woodchips consist of small particles to enable easier transport and handling, the pellets have the same advantage (Biomass Energy Centre, 2011a).

Figure 4.4: Pellets of woody biomass (Crops, 2013).

The advantages of pellets are very interesting and therefore they are already often used as the carrier of bioenergy. But there are some questions about the product during the handling. Will the lignin be a sufficient binder when handled or will the pellet collapse? This important question will be discussed later in this report.

4.3.4 Briquettes

Briquettes have a lot of similarities with pellets. They are also produced by compressing sawdust and wood particles. The difference between them is the size of the product. Figure 4.5 shows that the briquettes are larger compared to the pellets.

Usually the briquettes are produced under lower pressure, therefore they are easier to produce but this results in lower quality. Since it is easier to produce the briquettes it will also result lower production costs (Chaney et al., 2009).

The briquettes exist in multiple dimensions but always have a diameter above 25 mm according to the standards about biomass (see APPENDIX A). Often a hole is added in the briquette to increase the area to volume ratio of the product which results in a faster combustion and thus more heat can be generated (Chaney et al., 2009). More information about the standards which are used to

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4.3 Types of densification processes 31

Figure 4.5: Comparison of briquettes and pellets of woody biomass (BHS Energy L.L.C., 2011).

4.3.5 Hogfuel

Hogfuel is often a result of waste products. It is a mixture of chips of bark and wood fibers. The product is used as a source for biomass fired power plants as it is a fast burning product (Cloverdale Fuel, 2013).

The product is not very useful for industries over large distances as it is a low dense product and this would result in high costs. For small distances this product can be very useful.

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4.3.6 Firewood

Firewood is gained from tree wood which is cut into smaller parts, see figure 4.7. There might be a small improvement in density but the material will not gain large improve-ment, since no treatment is done. The firewood is the material that is often used by households to heat a stove. For non-industrial use this is a suitable product, but it still has the disadvantages of tree wood. Therefore the firewood will not be useful in an industrial environment.

Figure 4.7: Firewood of woody biomass (Solarfocus, 2014).

4.4 Bulk density after densification

The densified products are all used in the process of collecting biomass. An overview of the bulk densities is provided to compare the results of the densification procedures, see table 4.1. The bulk density is the density of the product when the product is stored. This means that the air holes between the particles will all be taken into account in the determination of the density (Sokhansanj et al., 2006).

Table 4.1: Bulk density for various biomass products (Sokhansanj et al., 2006). Form of biomass Shape and size Bulk density

[kg/bulked m3] Chopped biomass 20 - 40 mm long 60 - 80 Ground particles 1.5 mm pack fill with tapping 200 Briquettes 32 mm diameter x 25 mm thick 350

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4.5 Conclusion 33

The research of Sokhansanj et al. (2006) declares the differences between the bulk den-sities of the products. Switchgrass is used as source to compare the products. The complete product is processed to increase the bulk density. This means that no energy content is removed or added, therefore an increased energy density will be reached. Briquettes and pellets deliver the largest increase of bulk density and thus the energy density of these products will be the highest. For transport over long distances it is re-quired that the energy density is as high as possible. From the provided possibilities only briquettes and pellets show potential. Currently the wood pellets are often used in the industry, nevertheless the briquettes might seem suitable products as the requirements for production are less low. Therefore further research will focus on these two densified products.

The implementation of these products has been notified by the European Committee for Standardisation (CEN). To have a guidance for the use of the product multiple standards have been developed. An extensive declaration about the most important standards is provided in APPENDIX A. During this section the standards will describe how a standardized quality can be achieved for the pellets and briquettes. An additional standard will describe how this quality is remained throughout the complete supply chain. This will ensure that the customer receives a product of the desired quality.

4.5 Conclusion

This chapter provided an overview of all possible densification procedures. Many meth-ods can be used to gather all materials (woody biomass or straws) like bales, hog fuel, log wood and wood chips. These methods only allow transportation over small distance as they have fairly low bulk density.

For further distances it is required to optimize the energy density of the biomass with help of pelletization or briquetting. As this research focuses on the transport of woody biomass over long distances, these methods must be used. Pellets are often used in the industry but briquettes is also an interesting product. To compare both materials further research is required. Therefore the next step is to analyze the fabrication methods of the products.

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Torrefaction and Densification of Woody Biomass; Torrefactie en densificatie van houtachtige biomassa

Abstract

List of Symbols

Table of Contents

Chapter 1

Introduction

Chapter 2

Woody biomass

2.1

Introduction

2.2

Biomass

2.3

Cell structure of woody biomass

2.4

Determination of the energy content of woody biomass

2.5

Availability of woody biomass over the world

2.6

Conclusion

Chapter 3

Torrefaction

3.1

Introduction

3.2

Torrefaction mechanism

3.3

Torrefaction process definitions

3.4

Conclusion

Chapter 4

Densification procedures

4.1

Introduction

4.2

Reason for densification

4.3

Types of densification processes

4.4

Bulk density after densification

4.5

Conclusion