The Use Of Energy From Renewable Sources

Published Date: 02 Nov 2017

INTRODUCTION

In accordance with Directive 2009/28/EC on the promotion of the

use of energy from renewable sources, Estonia is obliged to increase

the share of renewable energy sources in the whole of energy

consumption as compared to the reference year of 2005 to 25% by

2020. At the moment, the use of biofuels in Estonia is still low, but

interest in the use of biofuels is constantly increasing. The objective of

the Development Plan for Enhancing the Use of Biomass and

Bioenergy is to create beneficial conditions for the development of

domestic biomass and bioenergy production in order to reduce the

dependence of Estonia on imported resources and fossil fuels and to

decrease the pressure on the natural environment. The objective of the

Development Plan is to reduce the dependence of Estonia on imported

energy resources and to enhance the use of biomass as a raw material

for energy, which coincides with the objective of the Development

Plan of the Energy Sector for ensuring continuous energy supply by

diversification of energy sources and a more even distribution in the

energy balance (National renewable energy action plan Estonia, 2010).

There is around 200 000 hectares of permanent grasslands (grassland

occupation over 5 years) in agricultural production. The active sown

area had changed 11% over the years in the period 2006-2008 and

production of green fodder in tons was from 1.5 to 1.9 Mt in a year

(Agriculture…, 2009).

There is no statistical figure about the unused biomass potential of

permanent grasslands, but researchers at the Estonian University of

Life Sciences have assessed changes in the usage of arable land. The

growing area of forage crops has decreased by 485,000 ha compared to

the year 1990 (Astover et al., 2006). About 283,000 ha of agricultural

land were abandoned and 123,000 ha are no longer in agricultural

registers. Compared to activities of animal husbandry in regions, we

may assume that 40-50% of grasslands are not used for fodder

production, but have been cut for land maintenance once a year

(Roostalu et al., 2008-I).

Kukk and Sammul investigated meadows that are under environmental

protection and estimated that semi-natural meadows cover 130,000 ha

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in Estonia (Kukk and Sammul, 2006). Biomass production in meadows

ranges from 1.7 to 5.7 t/ha yr. Total production from semi natural

meadows is approximately 182,000 t /yr of dry matter (Melts et al.,

2008).

By rough estimation the potential for bio energy from natural

grasslands, unused fodder from grasslands, and abandoned agricultural

land is 6.66, 2.3 and 6.93 PJ respectively, and total potential is then

about 16 PJ annually (Roostalu et al., 2008 - II). Kask estimated the

renewable energy potential of biogas production based on biomass

from abandoned agricultural land and came out with 5 PJ in a year

(Kask, 2008).

Some considerations on the use of agricultural and industry

materials and residues as fuel

Even if there are sufficient residues for briquetting to be considered

feasible, there are other factors that must be considered, and problems

that need to be overcome before the widespread adoption of these

wastes as a fuel substitute becomes possible (K. Mason, 2007).

Residues vary widely in their form and characteristics, which

determines how well they can be used as fuel. In their unprocessed

form, woody residues make the best fuel for stows as they tend to burn

well. Other crop residues are considered to make poorer fuels. For

example, cereal straw and low density stalks are considered to burn too

rapidly in their unprocessed form. Furthermore, their high bulk volume

makes them difficult and uneconomical to transport and store. For

these reasons, although they might be produced in large quantities, in

their unprocessed form they are far from being the fuel of preference.

In order for these residues to become a more attractive alternative fuel,

the residues themselves have to be upgraded to improve their burning

performance. The technology of briquetting is one effective way to

refine materials (Kers et al., 2010).

Final briquette quality depends on basic material composition and

optimal technological parameters. The most important parameters

affecting the briquette quality are fraction size, pressing temperature,

compacting pressure and material humidity. This final briquette quality

was evaluated by measuring briquette density and mechanical strength

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according to recognised European Standards for solid high-grade

biofuels.

Therefore, making briquettes from agricultural and industry waste

residues involves collecting biomass materials that are not normally

considered a useful fuel, due to their low density, and compressing

them into a solid fuel of a convenient size and shape that can be

burned in the same way as wood or charcoal. Briquetting increases the

bulk density of the biomass material, increasing its energy density,

which in turn reduces transport costs and makes it much easier for the

end user. Briquetting has some advantages over straight burning

technologies (Križan et al., 2010).

The simplicity and often accessibility of this technology, with the

benefits it could bring to people’s lives, gives it huge potential as an

alternative energy source which may generate rural employment and

income, eliminate disposal problems sometimes associated with large

quantities of waste agro-residues and provide an alternative to wood

based fuel.

One of the technologies for energy conversion of hay is anaerobic

digestion with manure in an agro-energetic chain. The interest of plant

operators is not very strong because hay is not recognised as one of the

main substrates for biogas plants. It can be used in feedstock, but as it

is lingo-cellulosic material it needs pre-treatment. Many researchers

report that pre-treatment of feedstock can increase biogas production

and volatile solids reduction (Tiehm et al., 2001) and increases

solubilisation (Tanaka et al., 1997). Particle size can affect the rate of

anaerobic digestion as it affects the availability of a substrate (i.e. the

surface area) to hydrolyzing enzymes, and this is particularly true with

plant fibres (Mshandete et al., 2006). Pretreatment of biomass

feedstock such as milling, pulping and steaming increases pore size and

reduces cellulose crystallinity, which is required for bioconversion of

lignocellulosic feedstock (Mandels et al., 1993).

Another possibility for producing briquettes are complex technologies

such as an integrated generation of solid fuel and biogas from biomass

(IFBB) separating materials into liquid and solid fractions (press liquids

and press cake). The development of the IFBB is aimed at increasing

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the efficiency of converting biomass into energy (Wachendorf M. et al.,

2009). Biomass, e.g., from semi natural grasslands which is difficult to

exploit in conventional bioenergy-converting systems, as the chemical

composition is detrimental both for conventional anaerobic digestion

as a result of high fibre concentrations (Prochnow, A. et al., 2009) and

for direct combustion because of high concentrations of elements that

cause corrosion inside the combustion chamber, ash softening and

hazardous emissions (Obernberger, I. et al., 2006). In a similar way

fractionated parts can be considered as a source for bioethanol

production.

It has been estimated that by the year 2020, paper mills will produce

almost 500 million tons of paper and paperboard per year (Wyatt,

2007). Pulp and paper is the third largest industrial polluter to air,

water, and soil and releases well over 100 million kg of toxic pollution

each year (Environment Canada, 1996). Worldwide, the pulp and paper

industry is the fifth largest consumer of energy, accounting for four

percent of the entire world's energy use. The pulp and paper industry

(PPI) uses more water to produce a ton of product than any other

industry (Earth Greetings Co, 2010).

Based on the above mentioned facts, directives and problems with

materials and wastes come from agriculture and industry, which may be

considered as a potential source for fuels.

Due to the availability of resources, but problematic aspects using

materials such as silage and grass, IFBB technology might be a

promising solution for Estonia. Therefore, methods and results for

samples from meadows which are under environmental protection and

compared to the silages produced as cattle feed implementing IFBB

technology are described in this thesis. Energy input, output and other

lucrative changes compared to classical silage digestion or biomass

briquetting are under analysis as well. The influence of lignin inhibition

during the biogasification process and problems related with ash

behaviour characteristics of hay based briquettes are described.

A possibility to use PPI industry lignocellulosic leftovers is to convert it

to bioethanol or biogas. Cellulosic ethanol production is a complex

process compared to first generation grain or sugarcane ethanol

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production. The cellulose in the biomass can be degraded to sugar

monomers only after the lignin seal and hemicellulose sheathing over

the cellulose has been broken and the crystalline structure of the

cellulose has been disrupted. In this thesis the main focus is on the

following PPI leftovers: primary floto sediment or primary sludge

(PFS), sludge of aerobic digestion (SAD), bark and separates from tape

type separator or pulp rejects (PR). PFS, SAD and pulp rejects press

fluids (PRPF) were used to analyze their biomethane potential (BMP)

applying anaerobic digestion (AD). Bark and PR were also used for

briquetting and for determination of bioethanol potential.

Briquetting was under analysis for a wide range of other materials as

well, including industrial wastes, fibre hemp and energy sunflower, and

other materials. Several mechanical parameters which are important for

briquetting were investigated. Additionally economical reasonability

was analyzed, which depends on energy losses during processes like

grinding and briquetting in addition to conventional harvesting,

transportation, and storing.

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1. REVIEW OF THE LITERATURE

This chapter is divided into several sections. In the first two parts, a

review is given about biomass and composition of materials rich in

cellulose. A review is also given about pretreatment possibilities and

some disadvantages and drawbacks of pretreatment. A review about

the IFBB is given under the section of "Pretreatment". The factors that

need to be controlled to manufacture briquettes and the influence of

different fuel properties on combustion behaviour such as ash are also

indentified. There are also several other important indicators regarding

economic aspects and the nutrient circle which are not presented in

this review. The purpose of this review was to identify the main factors

that need to be controlled to manufacture briquettes suitable

(chemically and physically) as stove fuels and for biogas production by

development of IFBB technology.

1.1. Biomass

1.1.1. General information about biomass

The use of biomass as a source of energy is of interest worldwide

because of its environmental advantages (Coll et al., 1998).

Biomass is a renewable resource compared with the fossil energy

resources. By comparison with the other renewable energy resources

such as solar and wind energy, biomass is a storable resource,

inexpensive, and with favourable energetic efficiency (Brokeland and

Groot, 1995; Scholz, V. and Berg, W., 1998).

Biomass materials generally contain a lower percentage of carbon and a

higher percentage of oxygen than fossil fuels. The result is a lower

heating value per unit mass of biomass compared with fossil fuels. This

means that more biomass fuel must be handled and processed to

obtain an equivalent unit of usable energy (Unger, 1994).

The combustion of biomass such as hay, miscanthus, or hemp

generates ashes that can be used as fertilizer. The main nutrients in

these ashes are potassium (K) and phosphorus (P) (Hasler et al., 1998).

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Biomass can be defined as all renewable organic matter including plant

materials, whether grown on land or in water, animal products and

manure, food processing and forestry by-products, and urban wastes

(Kitani and Hall, 1989; The energy educator of Ontario, 1993).

The composition of a wide range of selected biomass fuels is given by

Jenkins et al., (Jenkins B. M. et al., 1998).Compared with other fuels

(such as coal or peat), biomass contains relatively high amounts of

oxygen and hydrogen (Loo S. V. and Koppejan J., 2008).

1.1.2. Materials rich in cellulose

Lignocelluloses

Lignocelluloses (Figure1.1) are composed of cellulose, hemicellulose,

lignin, extractives, and several inorganic materials (Sjostrom, E. 1993).

Cellulose or β-1-4-glucan is a linear polysaccharide polymer of glucose

made of cellobiose units (Delmer, D.P. and Amor, Y., 1995;

Morohoshi, N., 1991). The cellulose chains are packed by hydrogen

bonds in so-called ‘elementary microfibrils’ (Morohoshi, N., 1991).

These fibrils are attached to each other by hemicelluloses, amorphous

polymers of different sugars as well as other polymers such as pectin,

and covered by lignin. The microfibrils are often associated in the form

of bundles or macrofibrils (Delmer, D.P. and Amor, Y., 1995).

Lignin and hemicellulose

The cellulose and hemicellulose are cemented together by lignin. Lignin

is responsible for integrity, structural rigidity, and prevention of

swelling of lignocelluloses. Dissolved lignin is also an inhibitor for

cellulase, xylanase, and glucosidase. Various cellulases differ in their

inhibition by lignin. The reason for an improved rate of hydrolysis by

removal of lignin might be related to a better surface accessibility for

enzymes by increasing the population of pores after removing lignin.

Hemicellulose is a physical barrier which surrounds the cellulose fibres

and can protect the cellulose from enzymatic attack. Many

pretreatment methods are able to remove hemicelluloses and

consequently improve the enzymatic hydrolysis (Table 1.1).

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Crystallinity of cellulose

Several studies have shown a good correlation between the pore

volume or population (accessible surface area for cellulase) and the

enzymatic digestibility of lignocellulosic materials. The main reason for

improvement in enzymatic hydrolysis by removing lignin and

hemicellulose is related to the cellulose accessible surface area. The

effect of this area may correlate with crystallinity or lignin protection or

hemicellulose presentation or all of them. Therefore, many researchers

have not considered the accessible surface area as an individual factor

that affects the enzymatic hydrolysis (Chum, H.L et al., 1985). The

cellulose micro fibrils have both crystalline and amorphous regions. A

major part of cellulose (around 2/3 of the total cellulose) is in the

crystalline form. An enzyme is not so effective in degrading the less

accessible crystalline portion. It is widely accepted that decreasing the

crystallinity increases the digestibility of lignocelluloses. The main

reason for improvement in enzymatic hydrolysis by removing lignin

and hemicellulose is related to the cellulose accessible surface area.

Typically, dry cellulosic fibres have a small size, about 15 to 40 [m, and

therefore they possess a considerable external specific surface area, e.g.

0.6-1.6 m2/g (Fan et al., 1980). Structure of the materials rich in

cellulose and the effect of pretreatment on the accessibility of

degrading enzymes are presented on Fig. 1.1.

Particle size

Particle size can affect the rate of anaerobic digestion as it affects the

availability of a substrate (i.e. the surface area) on hydrolysing enzymes,

and this is particularly true with plant fibres: fibre degradation and

methane yield improve with decreasing particle size (Mshandete et al.,

2006). Maceration of manure to reduce the size of recalcitrant fibres

was found to increase biogas potential by 16% with a fibre size of

2 mm and a 20% increase in biogas potential was observed with a fibre

size of 0.35 mm; no significant difference was found with fibre sizes of

5–20 mm (Angelidaki and Ahring, 2000).

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Figure 1.1. Structure of the materials rich in cellulose and the effect of pretreatment

on the accessibility of degrading enzymes. (Mohammad J. Taherzadeh and Keikhosro

Karimi Pretreatment of Lignocellulosic Wastes to Improve Ethanol and Biogas

Production: A Review; Int. J. Mol. Sci. 2008, 9, 1621-1651; DOI:

10.3390/ijms9091621).

1.2. Pretreatment

Examination of methods and their lucrative value

To access the energy potential of lignocelluloses (biomass) under

consideration as pre- treatment procedures are: 1. Mechanical,

2. Physical 3. Chemical, 4. Biological and 5. Combinations.

Pretreatment of feedstock (biomass feedstock rich in cellulose or

lignin) increases biogas production, reduces volatile solids, increases

solubility, and breaks down recalcitrant polymers (Tanaka et al., 1997,

Tiehm et al., 2001; Alastair J., 2008). The main pretreatment methods

for processing materials rich in cellulose are described in Table 1.1

which is able to remove hemicelluloses and consequently improve the

enzymatic hydrolysis.

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Pre-treatment problems: economy, toxicity and drawbacks

Pre-treatment and additive additional cost must always be balanced

against resultant improvements in efficiency. Alkali treatments,

however, are not without problems, due to toxic compounds generated

during the saponification reaction (Mouneimne et al., 2003). The major

drawback of some pretreatment methods, particularly at low pH is the

formation of different types of inhibitors such as carboxylic acids,

furans and phenolic compounds. These chemicals may not affect the

enzymatic hydrolyses, but they usually inhibit the microbial growth and

fermentation, which results in less yield and productivity of ethanol or

biogas. Therefore, the pretreatment at low pH should be selected

properly in order to avoid or at least reduce the formation of these

inhibitors. There are also some disadvantages of milling as ball milling

involves significant energy costs and its inability to remove the lignin

which restricts the access of the enzymes to cellulose and inhibit

celluloses (Henley, R.G. et al., 1980; Berlin, A. et al., 2006).

An effective and economical pretreatment

Avoiding destruction of hemicelluloses and cellulose, avoiding

formation of possible inhibitors for hydrolytic enzymes and fermenting

microorganisms, minimizing the energy demand, reducing the cost of

size reduction for feedstock, reducing the cost of material for

construction of pretreatment reactors, producing less residues,

consumption of little or no chemical and using a cheap chemical.

(Mohammad J. Taherzadeh and Keikhosro Karimi., 2008).

Table 1.1. Pretreatment processes of lignocellulosic materials (Mohammad J. Taherzadeh and Keikhosro Karimi

Pretreatment of Lignocellulosic Wastes to Improve Ethanol and Biogas Production: A Review; Int. J. Mol. Sci. 2008, 9,

1621-1651; DOI: 10.3390/ijms 9091621)

Pretreatment

method

Processes

Studied

application

Possible changes

in biomass

Notable remarks

1 2 3 4 5

Physical

Pre-treatments

Milling:

- Ball milling

- Two-roll milling

- Hammer milling

- Colloid milling

- Vibro energy milling

Irradiation:

- Gamma-ray irradiation

- Electron-beam irradiation

- Microwave Irradiation

Others:

- Hydrothermal

- High pressure steaming

- Expansion

- Extrusion

- Pyrolysis

Ethanol

and biogas

Ethanol

and biogas

Ethanol

and biogas

- Increase in

accessible surface

area and pore size

- Decrease in

cellulose

crystallinity

- Decrease in degrees of

polymerization

- Most of the methods are

high-energy demanding

- Most of them cannot

remove the lignin

- It is preferable not to use

these methods for

industrial applications

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Continued Table 1.1

1 2 3 4 5

Chemical and

Physico-chemical

Pre-treatments

Explosion:

- Steam explosion

- Ammonia fibre

explosion (AFEX)

- CO2 explosion

- SO2 explosion

Alkali:

- Sodium hydroxide

- Ammonia

- Ammonium Sulphite

Acid:

- Sulphuric acid

- Hydrochloric acid

- Phosphoric acid

Gas:

- Chlorine dioxide

- Nitrogen dioxide

- Sulphur dioxide

Ethanol

and biogas

Ethanol

and biogas

Ethanol

and biogas

Ethanol

and biogas

- Increase in

accessible surface area

- Partial or nearly

complete delignification

- Decrease in cellulose

crystallinity

- Decrease in degrees of

polymerization

-No chemicals are

generally required for

these methods

- These methods are

among the most effective

and include the most

promising processes for

industrial applications

- Usually rapid

treatment rate

- Typically need harsh

Conditions

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Continued Table 1.1

1 2 3 4 5

Oxidizing agents:

- Hydrogen peroxide

- Wet oxidation

- Ozone

Solvent extraction

of lignin:

- Ethanol-water extraction

- Benzene-water extraction

- Ethylene glycol extraction

- Butanol-water extraction

- Swelling agents

Ethanol and

biogas

Ethanol

- Partial or complete

hydrolysis of hemicelluloses

- There are chemical

requirements

Biological

Pretreatments

Fungi and

actinomycetes

Ethanol

and biogas

- Delignification

- Reduction in degree of

polymerization of cellulose

- Partial hydrolysis of

hemi-cellulose

- Low energy requirement

- No chemical requirement

- Mild environ-mental

conditions

- Very low treatment rate

- Did not consider for

commercial application

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1.3. Fractionation of the materials and integrated generation of

solid fuel and biogas from biomass (IFBB)

Besides to other above described effective treatment methods there is

one more method which is the fractionation of the materials by

dewatering presses. These kinds of technologies may meet the best

requirements of effective and economical pretreatment described

previously. The development of the integrated generation of solid fuel

and biogas from biomass (IFBB) is aimed at increasing the efficiency of

converting biomass into energy (Wachendorf M. et al., 2009) Biomass,

e.g. from semi-natural grasslands which is difficult to exploit in

conventional bioenergy-converting systems, as the chemical

composition is detrimental both for conventional anaerobic digestion

as a result of high fibre concentrations (Prochnow, A. et al., 2009) and

for direct combustion because of high concentrations of elements that

cause corrosion inside the combustion chamber, ash softening and

hazardous emissions (Obernberger, I. et al.; 2006). Separation

technologies recover, isolate, and purify products in virtually every

industrial process. Pervasive throughout industrial operations,

conventional separation processes are energy intensive and costly.

Separation processes represent 40 to 70 percent of both capital and

operating costs in the industry (Humphrey, J. L. and Keller, G. E.,

1997).

Combustion characteristics can be improved by water mashing and

subsequent dehydration of the biomass, whereas the remaining liquid is

a suitable substrate for biogas production. Evaluation of energy

production from semi-natural grasslands is given by Buhle according to

a recently suggested technique (integrated generation of solid fuel and

biogas from biomass, IFBB). During mechanical dehydration, 0.80 of

the dry matter was transferred into the press cake. Combustion relevant

nutrients like potassium and chlorine were extracted from the parent

material by 0.78 and 0.84 respectively. Methane yields from press fluid

digestion ranged between 272 and 333 l CH4/kg VS (volatile solids)

(Buhle L. et al., 2011). Some of the mean values of mass flows of dry

matter, ash and chemical constituents are presented in Fig. 1.2.

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Figure 1.2. Mean values of mass flows of dry matter, ashes, nitrogen, sulphur,

potassium, magnesium, calcium and chlorine into the press cake and the press fluid

during mechanical separation of 18 semi-natural grasslands in Germany, Wales and

Estonia (Buhle L. et al., 2011).

1.4. Briquetting and briquettes

1.4.1. General info about briquetting and briquettes

Although there are crops with both higher and lower residue yields, it is

reasonable to assume that about 25% of any dry agricultural feedstock

consists of residues. These residues are not properly collected or

utilized efficiently. The major limitation in utilizing them is their low

bulk densities and irregular size, making transportation, handling and

storage costs enormous. These limitations can be overcome by

compacting and converting the residues into a high density form

(FAO, 1990).

Water in raw materials will prevent the compression of briquettes, and

the steam that evaporates will reduce the density. If the briquette

absorbs humidity from the air, the briquette will swell and the density

will also decrease. This process can lead to the total disintegration of

briquettes (Claus, 2002).

At present two main high pressure technologies: ram or piston press

and screw extrusion machines, are used for briquetting. While the

briquettes produced by a piston press are completely solid, screw press

briquettes on the other hand have a concentric hole which provides

better combustion characteristics due to a larger specific area. The

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screw press briquettes are also homogeneous and do not disintegrate

easily. Having a high combustion rate, these can substitute for coal in

most applications and in boilers (Grover and Mishra, 1997).

The piston press is one of the main high press technologies used for

briquetting. The compressed material is heated by frictional forces as it

is pushed through the die. The lignin contained in all woody-cellulose

materials begins to flow and acts as a natural glue to bind the

compressed material. When the cylinder of material emerges from the

die, the lignin solidifies and holds it together, forming cylindrical

briquettes which readily break into pieces about 10-30 cm long. The

briquettes produced by a piston press are completely solid. The

production (Mp) of these machines is between 25-1800 kg/h,

depending on the press canal diameter, the kind of materials pressed,

and their properties (FAO, 1990).

Screw extrusion briquetting technology was invented and developed in

Japan in 1945. A comparison of a screw extruder and a piston press is

presented in the following table, Table 1.2.

Table 1.2. Comparison of a screw extruder and a piston press (Grover and Mishra

1997)

Another type of briquetting machine is the hydraulic piston press. This

is different from the mechanical piston press in that the energy to the

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piston is transmitted from an electric motor via a high pressure

hydraulic oil system. This machine is compact and light. Because of the

slower press cylinder compared to that of the mechanical machine, it

results in lower outputs. Grover and Mishra have pointed out that the

briquettes produced have a bulk density lower than 1000 kg/m3 due to

the fact that pressure is limited to 40-135 kg/h. This machine can

tolerate higher moisture content than the usually accepted 15%

moisture content for mechanical piston presses (Grover and Mishra,

1997).

There is a correlation between the pressure and the briquette density.

There is also a correlation between the pressure and the radial

compressive strength of the briquette. Increasing the pressure will

increase the briquette density digressively and its radial compressive

strength progressively. Stable briquettes (radial compressive strength ≥

0.25 N/mm2 and dry matter density ≥ 0.8 g/cm2) can be produced

with pressure ≥70 MPa depending on the material, particle size,

moisture content and press diameter. The theoretical model developed

by Claus (2002).

Combustion of hay made from this grassland biomass is also affected

by technical constraints due to high proportions of minerals, nitrogen

and sulphur leading to problems with ash melting, corrosion and

increased emissions (Obernberger et al., 2006). Blending peat with

herbaceous biomass leads to forming sulphates in boilers, instead of

chlorides, and high temperature corrosion is avoided (Lensu T., 2005).

For these reasons herbaceous biomass compositions with peat for solid

biofuel production are recommended.

Many researchers have concluded that fat/oil in compressed material

results in lower pellet/briquette durability (Briggs et al., 1999;

Cavalcanti, 2004). This is because fat acts as a lubricant between the

particles. Due to the hydrophobic nature of the fat, fat inhibits the

binding properties of the water-soluble components in the compressed

material such as starch, protein, and fibre (Thomas et al., 1998).

Sometimes the (natural) fat in the cell wall may come out of the cell

and act as a binding component between particles and make solid

bridges, which may positively influence the pellet durability (Thomas

et al., 1998).

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For solid biofuels there is the general standard EVS-EN 14961-1:2010.

This European Standard, Fuel Specifications and Classes ― Part 1:

General requirements, has been produced by TC 335 Solid Biofuels

Working Group "Fuel Specifications, Classes and Quality Assurance".

The objective of this European Standard is to provide unambiguous

and clear classification principles for solid biofuels and to serve as a

tool to enable efficient trading of biofuels and to enable a good

understanding between the seller and buyer as well as a tool for

communication with equipment manufacturers. It will also facilitate

authority permission procedures and reporting. Specified qualitative

indicators by this standard are for solid biofuels and their

determination requirements. Normative references are as follows:

prEN 14588:2009, Solid biofuels ― Terminology, definitions and

descriptions

EN 14774-1, Solid biofuels ― Determination of moisture content ―

Oven dry method – Part 1: Total moisture ― Reference method

EN 14774-2, Solid biofuels ― Determination of moisture content ―

Oven dry method – Part 2: Total moisture ― Simplified method

EN 14775, Solid biofuels ― Determination of ash content

CEN/TS 14778 (all parts), Solid biofuels ― Sampling

CEN/TS 14780, Solid biofuels ― Methods for sample preparation

EN 14918, Solid Biofuels ― Determination of calorific value

EN 15103, Solid Biofuels ― Determination of bulk density

CEN/TS 15104, Solid biofuels ― Determination of the total content

of carbon, hydrogen and nitrogen ― Instrumental methods

CEN/TS 15149-1, Solid biofuels ― Methods for the determination of

particle size distribution ― Part 1: Oscillating screen method using

sieve apertures of 3, 15 mm and above

31

CEN/TS 15149-2, Solid biofuels ― Methods for the determination of

particle size distribution ― Part 2: Vibrating screen method using sieve

apertures of 3, 15 mm and below

CEN/TS 15150, Solid biofuels ― Methods for the determination of

particle density

EN 15210-1, Solid Biofuels ― Determination of mechanical durability

of pellets and briquettes ― Part 1: Pellets

CEN/TS 15210-2, Solid biofuels ― Methods for the determination

of the mechanical durability of pellets and briquettes ― Part 2:

Briquettes

CEN/TS 15234, Solid biofuels ― Fuel quality assurance

CEN/TS 15289, Solid Biofuels ― Determination of the total content

of sulphur and chlorine

CEN/TS 15290, Solid Biofuels ― Determination of major elements

CEN/TS 15296, Solid Biofuels ― Calculation of analyses to different

bases

CEN/TS 15297, Solid Biofuels ― Determination of minor elements

CEN/TS 15370-1, Solid biofuels ― Method for the determination of

ash melting behaviour ― Part 1: Characteristic temperatures method

1.4.2. Moisture

The moisture content is a measure of the amount of water in the fuel.

In solid fuels, moisture can exist in two forms: as free water within the

pores and interstices of the fuel, and as bound water which is part of

the chemical structure of the material (Borman G. L. and

Ragland K. W., 1998). The moisture content can be found by taking a

small pre-weighed sample and oven drying it at 105 oC until consistency

in the sample’s mass is obtained. The change in weight can then be

used to determine the sample’s percentage moisture content. Moisture

32

content is a very important property and can greatly affect the burning

characteristics of the biomass (Y. B. Yang et al., 2005). Moisture

content has a significant influence for both the briquetting process and

combustion. It affects both the internal temperature history within the

solid, due to endothermic evaporation, and the total energy that is

needed to bring the solid up to the pyrolysis temperature (Zaror C. A.

and Pyle P. D., 1982). During combustion, moisture in the biomass will

absorb heat by vaporization and heating of the resulting vapour,

significantly reducing the heating value of a given fuel. Before

briquetting of the waste, pre-conditioning of the material would be

necessary because lower moisture content improves the strength and

quality of the briquette (Kers, J. et al., 2010). William has pointed out

that the amount of moisture a material has effects almost every aspect

of designing a proper handling system. Besides having an effect on the

material density by adding weight, moisture can exponentially increase

the stickiness of the material. Most biomass inherently has a higher

percentage of moisture than other fuel products and this moisture can

often vary seasonally throughout the year (Williams R., 2010). Methods

for the determination of moisture content are specified in the Estonian

standard EVS-EN 14774-2:2010, which consists of the English text of

the European Standard EN 14774-2:2009.

1.4.3. Volatile matter

The volatile matter represents the components of carbon, hydrogen

and oxygen present in the biomass which when heated turn to vapour,

usually a mixture of short and long chain hydrocarbons. It is

determined by heating a dried ground sample of biomass in an oven

and was measured at 550 °C. Biomass generally has a volatile content

of around 70-86% of the weight of the dry biomass (S. V. Loo and

J. Koppejan, 2008). After the volatiles and moisture have been released,

ash and fixed carbon remain. The relative proportion of volatiles,

moisture, fixed carbon and ash are often quoted for biomass fuels. The

percentage of fixed carbon is normally determined by a difference from

the other quantities (A. Demirbas, 1999), and is given as the following

formula:

Fixed Carbon = 100% - (%ash+%moisture+%volatiles).

33

Essentially, the fixed carbon of a fuel is the percentage of carbon

available for combustion. This is not equal to the total amount of

carbon in the fuel (the ultimate carbon) because there is also a

significant amount released as hydrocarbons in the volatiles.

1.4.4. Calorific Value

The calorific value (or heating value) is the standard measure of the

energy content of a fuel. It is defined as the amount of heat evolved

when a unit weight of fuel is completely burnt and the combustion

products are cooled to the specified temperature (25 °C). The last

standard for determination of the calorific value was CEN/TS 14918.

The period of validity of these CEN/TS is initially limited to three

years. Therefore at the moment (05.10.12) a valid standard for the

determination of the calorific value is not available in Estonia.

When the latent heat of condensation of water is included in the

calorific value it is referred to as the gross calorific value (GCV) or the

higher heating value. However, in stoves, any moisture that is

contained in the fuel and which formed in the combustion process is

carried away as water vapour, and therefore its heat is not available. It

is useful to subtract the heat of condensation of this water from the

gross calorific value. The result is known as the net heating (NCV) or

lower heating value. The calorific value is limited by fuel moisture

content, because heat is used to vaporize the water, lowering the heat

released. The heat released is also limited by the ash concentration in a

fuel; approximately every 1% addition of ash translates to a 0.2 MJ/kg

decrease in the heating value. More details are given by Ragland, Aerts

and also by Jenkins et al.

1.4.5. Ash content

Ash is the non-combustible component of biomass and the higher the

fuel’s ash content, the lower its calorific value (Loo S. V. and

Koppejan J., 2008). It is both formed from mineral matter bound in

the carbon structure of the biomass during its combustion

(Ragland K. W. and Aerts D. J., 1991) (the inherent ash), and is present

in the form of particles from dirt and clay introduced into the fuel

during harvest, transport and processing (the entrained ash) (Loo S. V.

34

and Koppejan J., 2008). The ash content is determined by heating a dry

sample of biomass in an open crucible in a furnace at 550 °C. Ash is

known to cause problems in combustion systems, notably because of

slagging and fouling, and its tendency to increase the rate of corrosion

of metal in the system (Loo S. V. and Koppejan J., 2008). There have

been various empirical indices which have been developed to try and

quantify this undesirable behaviour by relating it to the composition of

fuels. Biomass can contain alkali (K and Na), phosphorus, chlorine,

and amorphous silicon that can contribute to ash bonding and system

corrosion. Chlorine can contribute to corrosion problems in boilers

and air pollution control devices (Steve Benson and Margaret Laumb,

2009).

One simple index which has become popular is known as the alkali

index. This expresses the quantity of alkali oxide in the fuel per unit of

energy. Straws and grasses, for example, have relatively high alkali

indices, which is consistent with the high ash content of these fuels.

Although the alkali index does not fully describe the expected fouling

behaviour, it is useful as a general guide (Jenkins B. M. et al., 1998). If

the alkali metals are removed from the biomass, it is known to increase

the fusion temperature of the ash, which is the temperature at which it

conglomerates together. Experiments have shown that this can be

done by washing or soaking the biomass in water to leach the alkali

metals, and this gives significant reductions in the fusion temperature

of ash. In fact this simple technique has been shown to remove more

than 80% of the alkali and most of the chlorine, which has the added

advantage of reducing corrosion and acid gas emissions (Jenkins B. M.

et al., 1996).

Analysis of biomass reveals the principal constituent as carbon, which

comprises between 30 to 60% of the dry matter. After that, typically 30

to 40% is oxygen. Hydrogen is the third main constituent making up

between about 5-6%. Typically nitrogen and sulphur (and chlorine)

normally make up less than 1% of dry biomass.

35

1.4.6. Material density

The material density of biomass can vary enormously, from around

100 kg/m3 for light dry straw to over 2000 kg/m3 for highly

compressed biomass fuels. The higher the density of the fuel, the

greater the energy density. For example, the bulk density of loose

wheat straw is approximately 18 kg/m3 (Preto, 2007). Preto and Clarke

(Preto F. and Clarke S., 2011) described the main advantages of

biomass densification for combustion: simplified mechanical handling

and feeding; uniform combustion in boilers; reduced dust production;

reduced possibility of spontaneous combustion in storage; simplified

storage and handling infrastructure, lowering capital requirements at

the combustion plant; reduced cost of transportation due to increased

energy density. The major disadvantage of biomass densification

technologies is the high cost associated with some of the densification

processes.

Low moisture results in improved density and durability of the fuel

(Shaw and Tabil, 2007) For most biomass densification processes, the

optimum moisture content is in the range of 8%-20% (wet basis)

(Kaliyan and Morey, 2009). Most compaction techniques require a

small amount of moisture to "soften" the biomass for compaction.

Density of briquettes also affects burning properties of the fuels. Yang

has carried out a study on biomass pellets in a packed bed and this

gives a broad understanding of the behaviour to be expected: the

general trend found is a decrease in the burning rate for an increase in

material density. Yang also found (Yang Y. B. et al., 2005) that the

denser a material the thinner the pyrolysis reaction zone, which reduces

the time that the reacting gases are in this reaction zone. In addition,

the fuel briquette’s density will affect its bulk thermal properties: the

thermal conductivity will be reduced as the density is decreased

(increased fuel porosity), but the lower the density, the less heat is

required for a specific volume of fuel to reach the ignition temperature.

Theoretical bases for density calculations may be used and is given by

(Smits and Kronbergs, 2012). Any individual substance density 

may be determined by applying the mass of the substance to its

occupied volume, expressed as follows:

36

V

m

(1.1)

where – density, kg/m3;

m – mass, kg;

V – volume, m3.

To determine the density of the mixture of biomass as a whole, it is

necessary to calculate the coefficient of mass (k):

1 2

1

m m

m

k



(1.2)

where k – coefficient of basic mass;

1 m – mass of basic component, kg;

2 m – mass of impurity components, kg.

The density, which is produced by the setting up of a mixture of equal

size of the particles, is to be expressed:

2

2

1

1

1 2





m m

m m







(1.3)

where 1 – density of basic components, kg/m3;

2 – density of impurity components, kg/m3.

In terms of expression (2) the basic stock mass can be determined

experimentally by the formula (4) if the impurity component of weight

and the impurity factor are known:

1 2 1

m

k

k

m 

−

(1.4)

Reunifying equations (3) and (4) it is obtained that:

37

2

2

1

2

2 2

1

1





m m

k

k

m m

k

k



−



−(1.5)

Simplifying the expression (5) it is obtained that:

2 1

1 2

(1 ) 





−





k k

(1.6)

To certain components of the mixture of density relations:

2

1





C (1.7)

where C – mixture components density relation.

The density of the mixture:

k k C

C

2 2

2

2

(1 ) 





−



(1.8)

Simplifying the expression (8) it is obtained that:

k k C

C

−



(1 )

2 

(1.9)

38

1.4.7. Binding mechanisms

The strength and durability of the densified products depend on the

physical forces that bond the particles together. Understanding the

particle binding mechanisms is important in order to determine which

test should be used to measure the strength and durability of the

densified products. The binding forces that act between the individual

particles in densified products have been categorized into five major

groups (Pietsch 2002 and Rumpf, 1962).

If fine materials which deform under high pressure are pressed, no

binders are required. The strength of such compacts is caused by van

der Waals’ forces, valence forces, or interlocking. Natural components

of the material may be activated by the prevailing high pressure forces

to become binders. Some of the materials need binders even under

high pressure conditions (Grover and Mishra, 1997).

The physical properties are most important in any description of the

binding mechanisms of biomass densification. Densification of

biomass under high pressure brings about mechanical interlocking and

increased adhesion between the particles, forming intermolecular

bonds in the contact area. In the case of biomass the binding

mechanisms under high pressure can be divided into adhesion and

cohesion forces, attractive forces between solid particles, and

interlocking bonds (Pietsch, 1991).

1.4.8. Ash melting

Ash melting is a complex process where also shrinkage, sintering and

swelling can occur. The test methods described in this Technical

Report provide information about fusion and melting behaviour of the

composite inorganic constituents of the fuel ash at high temperatures.

The test methods available are empirical in most cases. The ashes used

for the tests are homogeneous material, prepared from the fuel, and the

determination is performed at a controlled rate of heating in a

controlled atmosphere. In contrast, under full-scale conditions, the

complex processes of combustion and fusion involve heterogeneous

mixtures of particles, variable heating rates and gas compositions. The

methods described in this document should be used dependent of the

39

following aspects and parameters, respectively: repeatability;

reproducibility; reliability; time efforts (rapid test methods); cost

effectiveness; possibilities for automatic testing. The aim of this

document is to provide a common and successful practice for

describing the ash melting behaviour. The terms ash fusibility and ash

softening are synonyms to ash melting. (Estonian Centre for

Standardization CEN/TR 15404:2010).

This Technical Report describes exemplarily methods for the

determination of shrinking, deformation, hemisphere and flow

temperature for characterising the ash melting behaviour of all solid

recovered fuels. The following terms and definitions are specified by

the Estonian Centre for Standardization (Estonian Centre for

Standardization CEN/TR 15404:2010) and are given in prEN

15357:2008 as following:

Shrinking temperature SST

temperature at which shrinking of the test piece occurs, i.e.

when the area of the test piece falls below 95% of the original

test piece area at 550 °C. NOTE Shrinking can be due to

liberation of carbon dioxide, volatile alkali compounds, and/or

sintering and partial melting.

Deformation temperature DT

temperature at which the first signs of rounding of the edges

due to melting of the test piece occur.

Hemisphere temperature HT

temperature at which the test piece approximately forms a

hemisphere, i.e. when the height becomes equal to half the base

diameter.

Flow temperature FT

temperature at which the ash is spread out over the supporting

tile in a layer, the height of which is half the height of the test

piece at the hemisphere temperature.

40

1.5. Biogasification

1.5.1. About biogasification

Biogasification is also called biomethanization. Biogasification is the

process of converting biomass to biogas, which can then be used as a

fuel. One way to do this is anaerobic digestion through decomposition

of biomass into methane by anaerobic bacteria, while another is by

using high temperatures in a gasifier. Biogas (methane) is one of the

widest ranges of fuels for possible use. Chilson has pointed out that

gasification processes give biomass tremendous flexibility in the way it

can be used to produce energy. These combined power & heat

technologies use a variety of organic residuals, agricultural wastes, and

dedicated energy crops to produce a clean fuel gas. A wide range of

energy conversion devices can be applied to utilize this fuel gas to

produce power, including; gas turbines, reciprocating engines, and

hydrogen powered fuel cells (Chilson, S. and Lewis F. M., 2002).

The production of biogas through anaerobic digestion offers significant

advantages over other forms of waste treatment, including:

Less biomass sludge is produced in comparison to aerobic

treatment technologies. Successful in treating wet wastes of

less than 40% dry matter (Mata-Alvarez, 2002).

More effective pathogen removal (Bendixen, 1994; Lund et al.,

1996; Sahlstrom, 2003). This is especially true for multi-stage

digesters (Kunte et al., 2004; Sahlstrom, 2003) or if a

pasteurization step is included in the process.

Minimal odour emissions as 99% of volatile compounds are

oxidatively decomposed upon combustion, e.g. H2S forms SO2

(Smet et al., 1999).

High degree of compliance with many national waste strategies

implemented to reduce the amount of biodegradable waste

entering landfill.

The slurry produced (digestate) is an improved fertiliser in

terms of both its availability to plants (Tafdrup, 1995) and its

rheology (Pain and Hepherd, 1985).

41

A source of carbon neutral energy is produced in the form of

biogas.

By containing the decomposition processes in a sealed

environment, potentially damaging methane is prevented from

entering the atmosphere, and subsequent burning of the gas

will release carbon-neutral carbon dioxide back to the carbon

cycle (Alastair J. et al., 2008).

The energy gained from combustion of methane will displace

fossil fuels, reducing the production of carbon dioxide that is

not part of the recent carbon cycle (Alastair J. et al., 2008).

Chemical characteristics of grasslands from nature conservation areas

have special demands on the technique used for the conversion of this

biomass into usable energy carriers. Conventional conversion

technique like biogas production from digestion of silage is connected

with low gas yields due to the highly senescent biomass (Richter et al.,

2009). Buhle has concluded that methane yields from press fluid

digestion showed that the liquid fraction is a suitable substrate for

biogas production due to its high anaerobic digestibility (Buhle et al.,

2011).

1.5.2. Archaebacteria and energy generation

The archaebacteria are related only distantly to the other bacteria.

Comparison of 16S ribosomal RNA sequences shows that the

archaebacteria are related to each other but not to eubacteria or

eukaryotic cytoplasm. In fact, there is as much genetic distance

between the archaebacteria and the eubacteria ("true bacteria") as

between the eubacteria and the cytoplasmic component of eukaryotic

cells.

The archaebacteria have certain biochemical features in common. In

particular, their lipids do not have ester-linked fatty acids. The

membrane consists of a bilayer of long chain isoprenoid hydrocarbons

joined at the ends by ether linkages to glycerol. The head group may be

phosphate or contain sugars. Some double-length isoprenoid

hydrocarbon chains stretch across the whole membrane. In addition

the cell wall contains no peptidoglycan.

42

Archaebacteria methane producers are obligate anaerobes which are

very sensitive to oxygen. Convert H2 + CO2 → H2O + CH4.

Metabolism is unique - they contain coenzymes found in no other

living organisms. They have no cytochromes, flavins or quinones.

(PHYSIOLOGY & BIOCHEMISTRY of MICROORGANISMS,

SIUC / College of Science / Microbiology / micr425/425Notes/).

The single stage AD process like it is commonly in use for biogas

production and is schematically presented in the following figure.

Angelidaki has explained that fresh inoculum has to be taken as a

working reactor and not be washed as described before in different

papers (Angelidaki et al., 2009).

The most successful AD processes at this time are high-solids,

thermophilic processes that can produce up to 125 standard cubic

meters of biogas per ton of feedstock, at 50-60% methane

concentration (Sterner R., 2012).

The Fig. 1.3 below illustrates how various populations of micro

organisms break down organic wastes in 4 stages. Commercial AD

plants can decrease the cycle time to roughly three weeks.

Besides carbon dioxide and methane small traces of hydrogen sulphide

and water are also always present in the gas produced. Anaerobic

digestion is the formative basis for all biogas production.

43

Figure 1.3. Single stage AD process.

1.5.3. Biogasifications four steps

The steps involved in the digestion are: hydrolysis, acidogenesis

(fermentation), acetogenesis, and methanogenesis. See Fig. 1.3.

The different steps utilize different bacterial cultures; consortia,

between which a balance should be obtained to ensure a satisfactory

environment in the biogas reactor and an acceptable biogas yield. Many

process instabilities are caused by a failure to maintain the balance

44

between the consortia carrying out the acetogenesis and the

methanogenesis (Chen, 2008), as these two consortia differs in their

nutritional needs, growth kinetics and sensitivity to environmental

conditions.

In the hydrolysis step, high-molecular compounds, e.g., proteins, fat,

and polysaccharides are digested to lower molecular compounds;

amino acids, fatty acids, and mono-, di-, tri-, and oligosaccharides. The

degradation is carried out extracellularly typically by excretion of

enzymes; lipases, cellulases, and amylases from hydrolytic bacteria like

Bacteroides, Clostridium, Acetivibrio, and Fibrobacter. The hydrolysis

involves several steps; enzyme production, diffusion, adsorption,

reaction, and enzyme deactivation. This step has been identified as the

rate - limiting step in processes applying high particulate substrates. For

small chain components however, the acidogenesis is carried out

directly (Drapcho et al., 2008; Mata-Alvarez, 2000; Schink, 1997).

In acidogenesis the sugar monomers from the hydrolytic step or

already present in the substrate are converted to puryvate (C3H4O3),

Adenosine triphosphate (ATP), and NADH (electron carrier molecule)

via the glycolysis or pentose phosphate pathway. The pyruvate and

amino acids are subsequently converted to a variety of short chain fatty

acids (acetic, propionic, and butyric acids), alcohols, hydrogen, and

carbon dioxide through various fermentation pathways. Acidogenesis is

performed by many of the microorganisms also responsible for the

hydrolysis; Bacteroides and Clostridium. Organisms like Lactobacillus and

Anaerolineae carry out acidogenesis (Drapcho et al., 2008; Schink, 1997).

The CO2, H2 and other one carbon compounds obtained via

acidogenesis are turned into methane directly by methanogenic

bacteria. Longer chain fatty acids (C>2), alcohols (C>1) and branched

chain and aromatic fatty acids, however, are oxidized to acetate and

hydrogen in acetogenesis. See Table 5.1 for volatile fatty acid oxidation

reactions. Different organisms are active during acetogenesis, e.g.,

different species from the genus Syntrophomonas and Pelatomaculum

(Drapcho et al., 2008). The pathway utilised for oxidation is different

for the different fatty acids, see Table 1.3.

45

Table 1.3. Volatile fatty acid degradations (Schink, 1997)

Substrate Reactions

Propanoic

acid CH3CH2COOH + 2H2O +3H2→3H2 +CO2

Butanoic

acid CH3CH2CH2COOH + 2H2O→2CH3COOH + 2H2

Valeric

acid CH3CH2CH2CH2COOH + 2H2O→2CH3COOH + 2H2

In the final step of the anaerobic digestion (methanogenesis) acetat,

formate and hydrogen is converted to methane and carbon dioxide by

methanogenic archaea, which are specialised in degrading these

substrates. Different methanogenes exist, some of which are able to

utilize several substrates whereas others are able to utilise a single

substrate only (Solomons and Fyhle, 2000). Two dominating routes for

methane production exist. Through the first route carbon dioxide is

reduced to methane by applying hydrogen as an electron donor. This

reaction is carried out by lithotrophic hydrogen oxidizing methanogens

like Methanobrevibacter, Methanobacterium, Methanogenium, and

Methanospirillium. The second route is a fermentation of acetic acid to

methane and carbon dioxide. This fermentation is carried out by

organotrophic acetoclastic methanogens like Methanosaeta and

Methanosarcina. The two species are favoured under low and high

acetate concentrations respectively.

4H2+CO2→CH4+2H2O

CH3COOH→CH4+CO2

Approximately two-thirds of the methane produced originates from the

fermentation of acetic acid (Drapcho et al., 2008; Galand et al., 2005).

See reactions above.

46

2. AIMS OF THE STUDY

The aim of this thesis is to define the effects and key parameters on

using materials on straight and refined ways for fuel production

through different technologies. These can be grouped into subcategories:

Briquetting, materials pretreatment and fractionation, biogasification

and additionally some necessary indicators of cost

components and efficiency. This was achieved by completing the

following objectives:

1. Manufacture of briquettes in a controlled reproducible way,

from a different particular material, wastes and silage press

cake. Investigation of material properties, which includes

briquette density, moisture content, particle size, physicalmechanical

indicators during the briquetting process,

mechanical properties and chemical composition of briquettes.

2. Pretreatment and fractionation of materials for IFBB.

Investigation of the effect of dewatering silo for composition

change analysis. Determination of ash quantity and behaviour

changes during combustion. Chemical composition and fibre

analyzes for different materials (hey, press cake, pulp and paper

industry residues) Evaluation of the distribution of chemical

elements between fractions during the dewatering process.

3. Bio-gasification potential and process activity tests and a

comparison of effects arising from the pretreatment and

implementation of IFBB technology. Chemical composition

and fibre analyzes for different materials (hey, press cake, pulp

and paper industry residues)

4. Analysis of some process cost components and efficiency

which are important to evaluate economic aspects of the

product or technology.

47

The novelty of this research consists firstly in tests with materials

which due to a lack of knowledge or research data is not available with

reference to Estonian local materials. Such materials are biomass (hay,

silage) grown on nature preserves with requirements to make one late

harvest once a year in July and pulp and paper industry residues

without subsequent purpose of use. Pulp and paper industry residues

are PR and PRPF which currently are mainly stored at landfill