Transformation of Biomass: Theory to Practice

Preface

Biomass is seen as a key feed material for the energy and material demands of mankind in the future. New businesses and technologies are therefore developing around biomass and its application. This textbook aims to help create an understanding of such processes related to the conversion of biomass into energy, heat and chemical products: processes based on biological or thermal routes.

The education of new generations of engineers, scientists and technicians is important to reach such goals. Therefore, this textbook intends to offer first guidelines to students as well as people transferring from different sectors into the biomass conversion technologies.

The different chapters deal with fundamental details but also recent research and highlight the possible problems and failures if methods are done wrong.

The textbook also carries two programmes for the evaluation of formal kinetic parameters as well as a calculation of business models.

Very often literature does not offer adequate answers to the questions arising from research, for example how to describe the thermal conversion processes of biomass and the evaluation of data to characterise real reactor systems in terms of temperature and residence time. The programmes related to this field will help the reader gain their own understanding. They can also be used to analyse data from lab work and therefore help to reach a better general understanding of the work done.

The business case model aims to enable the reader to compare different markets and their specific sensitivities, such as incentives and green subsidies, feed price and product price impact as well as general economic frame conditions.

Each chapter starts with a general motivation for the topic and at the end of each chapter the reader will find some questions which should help in understanding the background of the chapter and in building up the mind of the reader to understand the material presented in the right way.

No direct answers to the questions will be given by this textbook! The questions should sharpen the understanding and if the reader is unable to give an answer then the chapter should be studied again!

The questions highlight the basics and interdependencies and will improve the ability of the reader to transfer skills within topics.

For a person in charge of new technologies or working at the front end of research and development, such skills are of importance to give the right guidance or to find new pathways to better transform biomass.

The first chapter will give the reader a broad overview of biomass and its composition, conversion routes and products. The following chapters deal with specific technologies, such as anaerobic digestion, pyrolysis and gasification, as well as hydrothermal and supercritical conversion. In addition, chapters for analysis and reactor design help to understand how processes are designed and how analysis helps to understand the sometimes complex composition of the products resulting from biomass. These chapters are very advanced and might be best read at a later stage of the learning curve.

The same advice is given for the chapters on numerical simulation and formal kinetic parameter evaluation. The related programme offers an up-to-date platform for calculations, but the reader will need an already profound understanding to apply them properly.

Finally no product will reach the market if it is not set properly in a business framework.

The final chapter of this book gives you an insight into possible future products based on the solid product from pyrolysis, such as char turned into activated carbon or biochar. The market for biochar particularly is developing all over the world.

I wish you a stimulating time while studying this book!

Best regards

Prof. Dr. Andreas Hornung

Fraunhofer UMSICHT

Institute Branch Sulzbach-Rosenberg

Germany

and

Chair in Bioenergy

School of Chemical Engineering

College of Engineering and Physical Sciences

University of Birmingham

UK

Online Supplementary Material

Programs for the evaluation of formal kinetic parameters, as well as the calculation of business models, can be found online. This software, and PowerPoint slides of all figures from this book, can be found at http://booksupport.wiley.com.

1

Biomass, Conversion Routes and Products – An Overview

K.K. Pant and Pravakar Mohanty

Department of Chemical Engineering, Indian Institute of Technology Delhi, India

1.1 Introduction

The world consumes nearly two barrels of oil for every barrel produced. The depletion of conventional resources and stricter environmental regulations, along with increasing demand for commercial fuels and chemicals, has led to the need to find the alternatives to conventional fuel and chemical sources. Renewable plant materials are considered as one of the most promising alternatives for the production of fuels and chemicals. The conventional sources for fuels and chemicals are fossil fuels, crude oil natural gas, coal and so on, which are dwindling rapidly. With the concept of green chemistry, there is every necessity to produce alternative sources of energy and fuels from renewable biomass. Biomass refers to all organic matter generated through photosynthesis and many other biological processes. The ultimate source of energy this renewable biomass is inexhaustible solar energy, which is captured by plants through photosynthesis. It includes both terrestrial as well as aquatic matter, such as wood, herbaceous plants, algae, aquatic plants; residues such as straw, husks, corncobs, cow dung, sawdust, wood shavings, sawn wood, wood based panels, pulp for paper, paper board, and other wastes like disposable garbage, night soil, sewage solids, industrial refuse and so on [1]. Biomass can provide approximately 25% of our current energy demand, if properly utilized. Taking into account the production of biomass with respect to land and forest area, there are 4033 million ha of forests worldwide, as presented in Table 1.1.

Table 1.1 Forest resources, area (ha), year (2010).

In India 55.2 million ha of waste land is available for a wide range of short period energy crop productions [2]. Tropical and subtropical forests comprise 55% of the world's forests, while temperate and boreal forests account for rest 45% [3]. The average area of forest and wooded land per inhabitant varies regionally. Production and use of wood fuel, industrial round wood, sawn wood, wood-based panels, pulp for paper, paper board (m³) usage and its production are presented in Table 1.2. The total carbon stored in forest biomass is approximately 331 Giga tonnes (GT). About 27% of biomass is used directly as carbon feedstock, for example, sawn wood, wood based panels, pulp for paper, paper and paper board, mainly in developing countries. However, 33% is used as an industrial raw material and the remaining 40% is used as primary or secondary process residues, suitable only for energy production, for example, for production of upgraded biofuels [2, 3]. Approximately 70–77% of the global wood harvest is either used or is potentially available as a renewable energy source.

Table 1.2 Production and utilization of wood fuel, industrial round wood, sawn wood, wood-based panels, pulp for paper, and paper and paper board, year 2010.

The most efficient utilization of these resources comes when they are converted to liquid and gaseous products by appropriate technologies. Non-commercial biomass (biofuels) is the main source of energy available in the rural areas. An estimation by the Food and Agriculture Organization (FAO) shows that the global production of wood fuel and round wood reached 3410 million m³ during 2010 [2–4]. Just over half of this was wood fuel, where 90% of that is being produced and consumed in developing countries. On the other hand, industrial round wood production, totaling around 1542 million m³ in 2010, is produced and consumed both by North and Central America and Europe.

1.2 Features of the Different Generations of Biomass

Broadly, biomass can be categorized as first, second, third, and fourth generation. First generation biomass refers to traditional plant biomass like sugar and starch crops. Second generation biofuels include bioethanol and biodiesel produced from the residual, non-food parts of crops, and from other forms of lignocellulosic biomass, such as wood, grasses, and municipal solid wastes [5]. Third and fourth generation biofuels include algae-derived fuels, such as biodiesel from microalgae oil, bioethanol from micro algae and seaweeds, the fine chemicals and H2 from green microalgae, and microbes by sub- and supercritical extraction processes. Further these extracted microalgae can be utilized as biomass in thermochemical or biochemical routes of conversion [6]. Drop in fuels like green gasoline, green diesel, and green aviation fuel produced from biomass are also considered as fourth generation biofuels [7]. Efforts are also underway to genetically engineer organisms to open the secrete of these fourth generation hydrocarbon fuels. In Figure 1.1, both food and non-food biomass have been integrated in the sequential downward stream for establishment of the biorefinery concept towards energy surplus. Generation-wise details of the biomass diversifications are presented in Table 1.3 [7, 8].

Table 1.3 Generation-wise biomass distribution with its features.

Figure 1.1 Biomass feedstock distribution in term of food and non-food basis for bio-refinery.

At present, biomass represents approximately 14–18% of the world's total energy consumption [3, 4]. In order to utilize these resources properly, biomass should be converted to energy that can meet a sizeable percentage of demands for fuel and chemicals. Efficient utilization of biomass as a potential feedstock depends on general information about the composition of plant species, heating value, production yields and bulk density. Organic component analysis reports on the kinds and amounts of plant chemicals, including proteins, oils, sugars, starches, and lignocelluloses (fibers) required much attention about their behavior [1, 7].

1.3 Analysis of Biomass

The main components of biomass are cellulose, hemicelluloses, and lignin:

Cellulose or carbohydrate is the principal constituent of wood and other biomass and forms the structural framework of wood cells. It is a polymer of glucose with a repeating unit of C6H10O5 strung together by β-glycosidic linkages. The β-linkages in cellulose form linear chains that are highly stable and resistant to chemical attack because of the high degree of hydrogen bonding that can occur between chains of cellulose. Hydrogen bonding between cellulose chains makes the polymers more rigid, inhibiting the flexing of the molecules that must occur in the hydrolytic breaking of the glycosidic linkages. Hydrolysis can reduce cellulose to a cellobiose repeating unit, C12H22O11, and ultimately to glucose, C6H12O6. Heating values for cellulose may be slightly different based upon the feedstock [8, 9].

Hemicellulose consists of short, highly branched chains of sugars. In contrast to cellulose, which is a polymer of only glucose, a hemicellulose is a polymer of five different sugars. It contains five-carbon sugars (usually D-xylose and L-arabinose), six-carbon sugars (D-galactose, D-glucose, and D-mannose) and uronic acid. The sugars are highly substituted with acetic acid. The branched nature of hemicellulose renders amorphous properties which is relatively easy to hydrolyze to its constituent sugars compared to cellulose. When it hydrolyzed, the hemicellulose from hardwoods releases products which high in xylose (a five-carbon sugar). The hemicellulose that contained in softwoods, by contrast, yields six more carbon sugars [7, 8].

Lignin is the major non-carbohydrate, polypenolic structural constituent of wood and other native plant materials that encrusts the cell walls and helps in cementing the cells all together. It is a highly polymeric substance, with a complex, crosslinked, highly aromatic structure and having the molecular weight of about 10 000 derived principally from coniferyl alcohol (C10H12O3) by extensive condensation and polymerization [1, 8, 9].

For the efficient utilization of biomass, feedstock engineers are particularly evaluating the hemicellulosic component and the distribution among cellulose, hemicelluloses, and lignin. Table 1.4 gives an idea of the organic components of some of the dedicated energy crops, common sugar, and starch crops, respectively.

Table 1.4 Organic components and composition of lignocelluloses biomass (dry basis).

1.3.1 Proximate and Ultimate Analysis of Biomass

Analysis of biomass and its characteristics is generally accomplished by both proximate and ultimate analysis. Proximate analysis separates the products into four groups: (i) moisture, (ii) volatile matter, consisting of gases and vapors driven off during torrefaction or pyrolysis, (iii) fixed carbon, the non-volatile fraction of biomass, and (iv) ash, the inorganic residue that remains after combustion. The remaining fraction is a mixture of solid carbon (fixed carbon) and mineral matter (ash), which can be distinguished by further heating the sample in the presence of oxygen; the carbon is converted to CO2 and only leaving the ash [9]. Table 1.5 provides both the proximate and ultimate analysis (dry basis) for a wide range of biomass materials. Ultimate analysis deals with the determination of the carbon and hydrogen in the material, are found in the gaseous products after combustion. Using these analysis, the molecular weight analysis becomes simpler. For example, cellulose and starch having the generic molecular formula C1H1.7O0.83, hemicelluloses can be represented by C1H1.6O0.8 and wood by C1H1.7O0.83. Typical thermochemical properties of some selected biomass materials based on proximate and ultimate analysis are given below (Table 1.5) [9–15].

Table 1.5 Thermochemical properties of the selected biomass (proximate and ultimate analysis).

The calorific value of the char and the conversion efficiency based on calorific value are given in Table 1.5. The higher heating value (HHV) of the biomass is calculated by implementing the HHVs of lignocellulosic fuels, as the equation given below [16]:

(1.1)

Chaniwala and Parikh [17] have developed an empirical correlation based on elemental and proximate analysis to predict the HHV of raw biomass as stated below:

(1.2)

Here C, H, S, O, N, and A refer to the weight percent of carbon, hydrogen, sulfur, oxygen, nitrogen, and ash in biomass respectively.

1.3.2 Inorganic Minerals' Ash Content and Properties

Fuel contains various impurities in the form of incombustible components mainly known as ash. Ash itself is undesirable, since it requires purification of the flue gas for particles with subsequent ash and slag disposal as a result. The ash from wood comes primarily from soil and sand absorbed into the bark. Wood also contains salts thus having the importance to the combustion process. They are primarily potassium (K), and partly sodium (Na), based salts resulting in sticky ash, which may cause deposits in the boiler unit. The Na and K contents in wood are normally so low that they will not cause problems for traditional heating technologies. Typical mineral fractions in wood chips expressed as percentage of the dry matter (DM) of the wood are shown in Table 1.6. Apart from all these individual analysis processes, NREL researchers have developed a very interesting and rapid analysis method for biomass composition using near-infrared (NIR) spectroscopy. By applying this technique, the light reflected off a biomass sample is analyzed to determine the sample's composition [8, 18].

Table 1.6 Total inorganic components of plant biomass (dry basis).

1.4 Biomass Conversion Routes

By a number of processes, biomass can be converted into solid, liquid, and gaseous fuels. The technologies include thermal, thermochemical, and biochemical conversions. Reactions involved during conversion are hydrolysis, dehydration, isomerization, oxidation, de-hydrogenation, and hydrogenation. The actual processes included these technologies are combustion, pyrolysis, gasification, alcoholic fermentation, liquefaction, and so on [8]. A schematic flow diagram for biomass conversion is shown in Figure 1.2. The main products of conversion technologies are energy (thermal, steam, electricity), solid fuels (charcoal, combustibles), and synthetic fuels (methanol, methane, hydrogen gas, etc.). These can be used for different purposes such as cooking, lighting, heating, water pumping, electricity generation, and as industrial and transport fuels. Biomass fuels and residues can be converted to energy via thermal, biological, chemical, and physical processes.

Figure 1.2 Different conversion routes to get end products (liquid and gases). (Adopted from Mohanty et al., 2014 [3])

In a commercial process, biodiesel is produced by the reaction of vegetable oil or animal fat with methanol in the presence of base or acid catalysts. Concerns over the downstream processing of the homogeneous transesterification processes have motivated intense research on the heterogeneously catalyzed transesterification process [18, 19]. In general, heterogeneous biodiesel production processes have few numbers of unit operations, with simpler separation and purification steps for products as no neutralization process is required. There are three types of solid catalysts: acid, base, and enzyme. Solid base catalysts, such as alkaline–earth metal hydroxide, oxides, and alkoxides such as Ca(OH)2, CaO, and Ca(CH3O)2 function as effective catalysts for the transesterification of triglycerides [18, 20]. The main advantage of solid acid catalysts is their ability to carry out the esterification of free fatty acids and transesterification of triglycerides simultaneously [20–23]. Moreover, these are reactive on esterification and transesterification reactions at relatively low temperatures (i.e., 80 °C), as shown in Figure 1.3 [8].

Figure 1.3 Reaction scheme of transesterification reaction.

Lipase has been shown to have a high catalytic reactivity to produce high quality biodiesel [18, 20–23]. As lipases break down natural lipids and oils into free fatty acids and glycerol, therefore this group of enzymes is widely used in the leather and detergent industries. Recent findings show that an alternative acyl acceptor, such as methyl acetate is used to replace methanol, and it can obtain methyl ester yield up to 92%. In addition, the byproduct (glycerol) has a more expansive market, which can further be used for H2 production, acrolein, or several other chemicals [20].

In thermal conversion, combustion is already practiced widely, where as; gasification attracts high level of interest as it offers higher efficiencies compared to combustion. Pyrolysis is interesting as it results into liquid product that offers advantages in storage, easy transport and versatility in applications, although it is still at a stage of early development [8, 23].

1.4.1 Pyrolysis

There are different types of pyrolysis carried out under various operating conditions, among which fast, intermediate, flash, and slow having the substantial importance in the conversion of biomass to different liquid and gaseous products

(1.3)

(1.4)

1.4.1.1 Fast Pyrolysis

Currently, targeting the liquids production through fast pyrolysis is capturing the interest. The main features of fast pyrolysis are high heating rates and short vapor residence time. It generally requires a feedstock prepared with smaller particle sizes and a design that removes the vapors quickly from the presence of the hot solids. There are a number of different reactor configurations that can achieve this, including ablative systems, fluidized beds, stirred or moving beds, and vacuum pyrolysis systems.

Fast pyrolysis occurs in few seconds or less. Therefore, not only chemical reaction kinetics but also heat and mass transfer processes, as well as phase transition phenomena, play important roles. The critical issue is to bring the reacting biomass particle to an optimum process temperature and to minimize its exposure to the intermediate (lower) temperatures that favor formation of charcoal. This can be achieved by using smaller particles in fast pyrolysis as biomass decomposes to generate vapors, aerosols, and charcoal. After cooling and condensation, a dark brown liquid bio-oil is formed having the heating value of about half that of conventional fuel oil. Fast pyrolysis is an advanced process, with carefully controlled parameters to give higher yields of liquid. The essential features of the fast pyrolysis process for producing liquids are: (i) very high heating and heat transfer rates at the reaction interface, (ii) which usually requires a finely ground biomass feed, a carefully controlled pyrolysis reaction temperature of around 450–600 °C and a vapor phase temperature of 400–450 °C, short vapor residence times of typically less than 2 s, and rapid cooling of the pyrolysis vapor to produce the bio-oil product. The main product (bio-oil) is obtained in yields of up to 75% wt on a dry feed basis (in case of wood), together with byproduct char and gases which are used within the process so there are no waste streams other than flue gas and ash. During pyrolysis, how different variants within the main operating parameters affect the yield and product distribution is tabulated in Table 1.7 [24]. Some researchers have defined this process as thermolysis, in which a material, like biomass, is rapidly heated to high temperatures in the absence of air (specifically oxygen).

Table 1.7 Range of the main variants with main operating parameters and characterization for pyrolysis methods. (Adopted from Mohanty et al., 2014 [3])

1.4.1.2 Intermediate Pyrolysis

During intermediate pyrolysis the reactor is operated at temperatures ranging between 400 and 550 °C and the reactor consists of two coaxial conveyor screws, an inner screw and a covering screw widely known as a pyrolyzer. When the outer screw transports the biochar from one end to the other end of pyrolyzer, the chars act as a heat carrier with bed formation. The intermediate pyrolysis of biomass is carried out in a very reasonable way, resulting in bio-oil with low tar yields and viscosity, which is distinctive in intermediate pyrolysis in comparison to fast pyrolysis. Typically, this reactor has the flexibility to provide a moderate residence time [25]. This is only the case for woody biomass, when it leads towards a herbaceous stream it fluctuates to larger extent leading to a liquid phase high in water, acids, and tars. In terms of other feedstock like straws, grasses, or industrial residues from agricultural products like husks the picture is very different. The intermediate pyrolysis prevents the formation of high molecular tars with dry and brittle chars suitable for other applications like biofertilization and gasification. The advantage of such pyrolysis is that the non-milling character endures with the pellet charged to the pyroformer. The ease of access for larger sized feedstock offers the opportunity to separate it easily as a char; and to enrich a tied gasifier with the low ash content of biochar from the pyroformer [25]. The haloclean process was primarily developed for the thermal treatment of halogenated polymeric wastes. Any contaminated biomass can be handled inside a kiln heated from outside, with single- or double-screw rotation either clockwise or anticlockwise or both as per the equipment design and flexibility. This pyrolysis facilitates operating conditions for preventing the formation of high molecular tar and enhancing quality, that is, the dryness and brittleness of the char which can be further utilized for the purpose of fertilization and carbon sequestration. In this case, mechanical briquetting is not required for the processing of feedstock.

1.4.1.3 Slow Pyrolysis

Slow pyrolysis is also termed as carbonization due to similarities in its process conditions, like low temperature and more residence time. It can be divided into traditional charcoal making and more modern processes that are characterized by slower heating rates, relatively long solid and vapor residence times, and usually a lower temperature than fast pyrolysis, typically 400 ± 10 °C. The target product is often the char, but this is accompanied by liquid and gas products, although these are not always recovered. Traditional processes, using pits, mounds, or kilns, generally involve some direct combustion of the biomass, usually wood, as a heat source in the kiln. Liquid and gas products are often not collected but escape as smoke, with consequent environmental issues [1, 25]. It can be characterized by slow biomass heating rates, low temperatures, and lengthy gas and solid residence times. Depending on the system, heating rates are about 0.1 to 2 °C per second and prevailing temperatures are around 500 °C. Gas residence time may be greater than 5 s. During conventional pyrolysis, the biomass is slowly devolatillized; hence tar and char are the main products. This process yields a different range of products whose form and characteristics are dependent on the temperature, oxygen level, and process time used.

1.4.1.4 Torrefaction

This is a thermochemical treatment of biomass in the temperature range of about 200 to 320 °C, a kind of mild pyrolysis process that improves the fuel properties of biomass. It is carried out under atmospheric conditions and in the absence of oxygen. During this process, the water contained in the biomass, as well as superfluous volatiles, are removed, while the biopolymers (cellulose, hemicelluloses, and lignin) partly decompose by giving off various types of volatiles. The final product is the remaining solid, dry, blackened material which is referred to as torrefied biomass or biocoal [26, 27]. Torrefied products and volatiles are formed, resulting in a hardened, dried, and more volatile-free solid product. The product is at much higher energy density than the raw biomass, increasing the distance over which the biomass can be transported to plants for use or further processing, because of its relative lower weight and volume. Torrefied biomass is also hydrophobic, meaning it can be stored in the open space for long periods without taking up water, similar to the infrastructures used for coal. Torrefied biomass requires less energy to crush, grind, or pulverize and the same tools as for crushing coal can be used. Therefore, a well-developed biomass refinement method must interact and be integrated to obtain a biomass to liquid (BTL) process with high well-to-wheel efficiency.

Other developments have led to slow/intermediate pyrolysis technologies to create that are of much attention for 3-different pyrolysis product distribution in wide ranges. These are generally based on a horizontal tubular kiln where the biomass is moved at a controlled rate through the kiln; these include agitated drum kilns, rotary kilns, and the screw pyrolyzer [28]. In several cases these have been adapted for biomass pyrolysis from their original uses, such as the coking of coal with production of towns-gas or the extraction of hydrocarbons from oil shale (e.g., the Lurgi twin-screw pyrolyzer). Although some of these technologies have well-established for commercial applications, yet and yet considerable numbers of commercial applications are still under development to acquire potential market value with biomass to biochar production. The liquid fraction of the pyrolysis products consists of two phases: an aqueous phase containing a wide range of organo-oxygenate compounds with low molecular weight, and a non-aqueous phase containing insoluble organics (mainly aromatics), phenolic compounds of higher molecular weight. This non-aqueous phase is called bio-oil, which is a product of current interest. The ratios of acetic acid, methanol, and acetone of the aqueous phase were higher than those of the non-aqueous phase. For char production, one has to focus on low temperature and low heating rate; however, for maximum flue gas production a high temperature, low heating rate, and long residence time process would be preferable [29]. Distinct involvement of components' during the pyrolysis process is summarized in Figure 1.4.

Figure 1.4 Biomass component pyrolytic conversion for biorefineries. (Adopted from Mohanty et al., 2014 [3])

1.4.1.5 Gasification

This is an alternative thermochemical conversion technology suitable for the treatment of biomass or other organic matter, including municipal solid wastes or hydrocarbons such as coal. It involves partial combustion of biomass under a gas flow containing a controlled level of oxygen at relatively high temperatures (500–800 °C) yielding a main product of combustible producer gas/syngas with some char with low carbon percent. The main reaction involved during the gasification process is given below.

Partial oxidation can be represented by these reaction schemes:

(1.5)

(1.6)

(1.7)

Although designed for produce gas, under some conditions gasifiers can produce reasonable yields of producer gas, syngas, and char for an effective energy decentralization process [28]. The syngas production from biomass gasification can be reformed into a variety of chemicals like methanol, olefins, green diesel, gasoline, and wax through Fischer Tropsch routes [30].

1.4.1.6 Hydrothermal Carbonization

This is a completely different process involving the conversion of carbohydrate components of biomass (from cellulose) into carbon-rich solids in water at elevated temperatures and pressures [31]. Under acidic conditions with catalysis by iron salts the reaction temperature may be as low as 200 °C. The process may be suitable to concentrate the carbon (%) and to handle the high moisture content in the waste streams that would otherwise require drying before pyrolysis, making it complementary to pyrolysis and a potential alternative to anaerobic digestion.

1.4.1.7 Combustion

Combustion is the rapid oxidation of fuel to obtain energy in the form of heat. For combustion, biomass is used as the main feedstock and is primarily composed of carbon, hydrogen, and oxygen. Further it can produce H2, CO, CO2, and water by partial combustion [9]. Combustion takes place in the presence of excess air; therefore carbon dioxide and water are the pivotal components of gasification. At lower temperatures, formation of hydrocarbons takes place during gasification in the fluidized bed reactors. The flame temperature can go beyond 2000 °C, depending on various factors like the heating value and the moisture content of the fuel, the amount of air used to burn the fuel, and construction of the furnace. For combustion, mainly a combustor is used as the device to convert the chemical energy of fuels into high temperature exhaust gases [15, 16].

1.5 Bio-Oil Characteristics and Biochar

Bio-oil is typically a dark brown liquid with a smoky acrid smell. It tends to have relatively high water content – typically in the range of 20 to 25% [9]. The water comes from the pyrolysis conversion process, as well as from the initial water in the biomass feedstock. When the water content of the bio-oil is in the 20 to 25% range, it is entirely miscible in bio-oil (i.e., it does not separate). At higher moisture levels, the water can tend to separate from the bio-oil. To prevent this from happening, it is desirable to have the incoming biomass feedstock dried to 10% moisture content, or less, before it is fed into the pyrolysis conversion process [1, 8, 23]. Bio-oil characteristics vary somewhat, depending on the production technology and the type of biomass feedstock from which the bio-oil is produced. This means that bio-oil fuel specifications are likely to be fairly important. Bio-oil's energy content is in the range of 18–23 MJ/kg. (At the higher end of this range, there will typically be greater amounts of suspended char in the bio-oil.) Conventional heating oil has an energy content of about 42 ± 1 MJ/kg (lower heating value), thus bio-oil has about 52 to 58% as much energy as heating oil per gallon. However, it is interesting to note out that bio-oil weighs about 40% more per gallon than heating oil [9]. Bio-oil is a free flowing liquid. Its viscosity tends to be slightly higher than conventional no. 2 fuel oil. As the water content in bio-oil increases, its viscosity decreases (as does its energy content). Bio-oil is moderately acidic, having a pH in the range of 2.5 to 3.5 (similar to the acidity of vinegar). This means that bio-oil fuel storage tanks will need to be made of a material that will not corrode due to acidic character of the fuel (i.e., they will need to be made of materials such as stainless steel, plastic, fiberglass, etc.). Bio-oils are multicomponent mixtures comprised of different size molecules derived primarily from the depolymerization and fragmentation reactions of three key biomass building blocks: cellulose, hemicellulose, and lignin. Therefore, the elemental composition of bio-oil resembles that of biomass rather than that of petroleum oils [32, 33]. This raises a significant issue regarding the use of bio-oil in existing residential or commercial installations, since most of the existing fuel storage tanks used for heating oil are likely to be made of plain mild steel or stainless steel that is vulnerable to corrosion from bio-oil. As a result, it will generally be necessary to install a new fuel storage tank if bio-oil is to be used for an existing heating oil installation [9, 32]. Bio-oil is a complex mixture of oxygenated compounds, which carries potential drawbacks as well as potential benefits: from a fuel storage perspective, bio-oil is not as stable as petroleum fuel. However, bio-oil developers (such as Dyna-Motive, part of Dynamotive Energy Systems Corporation) have found that bio-oil samples stored for over a year have remained stable [34]. Producing bio-oil with a lower ash (char) content and/or a lower water content helps in prolong stability of bio-oil during storage [29].

Growing concerns about climate change have brought biochar into the limelight. Combustion and decomposition of woody biomass and agricultural residues results in the emission of a large amount of carbon dioxide. Biochar can store this CO2 in the soil, leading to a reduction in GHGs emission and enhancement of soil fertility. In addition to its potential for carbon sequestration, biochar has many other advantages [23]. It can increase the available nutrients for plant growth, increase water retention, and reduce the amount of fertilizer used by preventing the leaching of nutrients out of the soil. It can reduce methane and nitrous oxide emissions from soil, thus further reducing GHGs emissions, and can be utilized in many applications as