4. Dry biomass conversion processes

#### 4.1. Gasification

Gasification, which implies incomplete combustion (also commonly referred to as partial oxidation) of the carbonaceous feedstock, is one of the most attractive options to convert biomass into various high value products such as liquid and gaseous fuels, chemicals and electricity. Gasification is the most popular among the thermochemical conversion processes with the exception of direct combustion. Gasification processes have several advantages and disadvantages over other conversion technologies. The main advantages are that the gasification feedstock can be any type of biomass including agricultural residues, forestry residues, by-products from chemical processes, and even organic municipal wastes. Moreover, gasification typically converts the entire carbon content of the feedstock, making it more attractive than enzymatic ethanol production or anaerobic digestion where only portions of the biomass material are converted to fuel. The second advantage is that the product gas can be converted into a variety of fuels (H2, Bio-SNG, synthetic diesel and gasoline, etc.) and chemicals (methanol, urea). The other benefit of the biomass gasification process is lowered CO2 emissions, compact equipment setup with higher thermal efficiency [32]. Thus gasification is most suitable to produce chemicals that can be alternatives to petroleum based products.

The dual fluidized bed reactor configuration is a well-known option for the gasification of biomass feedstock. This configuration uses two separate reactors, one for the combustion and

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• Provides improved process efficiencies and avoids the challenges related to ash melting by operating at lower gasification temperatures (normally greater than 800C but below

• Other fuel sources can be used for the combustion step to overcome the low heating value of the biomass feedstock. These fuels include char by-product from the reduction reactor

• Air is only used in the combustion reactor and does not enter the reduction reactor, thereby preventing nitrogen dilution of syngas, a major problem in air blown gasifiers [34].

The heat required for the reduction reaction is supplied through the bed material (typically sand) from the combustion reactor. The bed material is continuously circulated between the two reactors while the ash is removed from the bed material using cyclones and the gases from the two reactors are not allowed to mix. The Milena project gasifier uses the two reactor configuration [35].

Tar formation in a gasifier during gasification process is significantly affected by reactor/gasifier configuration. Cao et al. [36] introduced an innovative fluidized bed gasifier which can produce tar free product flue gas. The factors that affects the biomass gasification process most are: the reaction temperature, residence time and oxygen to biomass ratio [37].According to this study,

A major challenge of biomass gasification is to overcome the higher specific capital and operating costs. This is due to the much smaller plant sizes (normally less than 500 tons per day) compared to coal gasification plants (tens of thousands of tons per day). The plant size is determined by biomass availability and related logistic issues and transportation costs inherent to any distributed resource. Other challenges include the presence of undesirable species such as alkali compounds in biomass ash. Alkali materials such as sodium and potassium cause slagging and fouling problems [38]. Most biomass gasifiers operate below the ash softening temperature to avoid ash melting. The lower temperatures also lead to lower capital cost requirement, resulting in favorable process economics. However, lower temperatures often result in the formation of undesired tar, which leads to severe operational problems. A number of catalysts and process configurations have been developed to address this issue, but tar problems still persist [39]. Addition of a catalytic tar cracker to the outlet of the gasifier to decompose the tars into smaller molecules has been considered [40]. Washing out the tars while the product gas is cooling down has also been proposed, but this approach requires rigorous treatment of the washing water. Tar formation is still a major challenge and is regarded as the "Achilles heel" of biomass gasification processes. These issues are not to be underestimated and careful attention is

the optimum residence time is 1.6 s and the optimum oxygen to biomass ratio is 0.4.

Benefits of the dual bed configuration for biomass gasification are [33]:

the other for the reduction reaction.

the ash softening point).

or other designated fuels such as methane.

required in the design and operation of biomass gasifiers.

Gasification technology for biomass conversion is commercially applied in China: in 1990, China built more than 70 biomass gasification projects for household cooking and each of them can supply energy for 800–1600 families [32] whereas in India, a perspective way of electricity generation is gasification.

Gasification processes are primarily designed to produce synthesis gas (syngas, a mixture of hydrogen and carbon monoxide) by converting the feedstock under reducing (oxygen deficient) conditions in the presence of a limited amount of gasifying agent such as air or oxygen [5]. Gasification consists of three major steps. The first step is devolatilization of the dried feedstock to produce the fuel gas for the second step, which is combustion. The combustion step produces the necessary heat and reducing environment required for the final step. The final step (so-called reduction step, char gasification step or syngas production step), is the slowest reaction phase in gasification, and often governs the overall gasification reaction rate. These 3 steps can be shown as:

Devolatilization: Feedstock Ò Fuel gas +Char.

Combustion: Fuel gas + Air Ò Flue gas + Heat (25% of carbon)

Reduction: Fuel gas, Char + Heat Ò Syngas (75% of carbon)

Gasification: Feedstock + Air Ò Syngas + Flue gas + Ash.

Approximately 25% of carbon in the feedstock is consumed in the combustion step to provide the heat and reducing environment for the reduction step. A detailed discussion of gasification, including minor steps and considerations is available elsewhere [5].

The dual fluidized bed reactor configuration is a well-known option for the gasification of biomass feedstock. This configuration uses two separate reactors, one for the combustion and the other for the reduction reaction.

Benefits of the dual bed configuration for biomass gasification are [33]:

4. Dry biomass conversion processes

electricity generation is gasification.

These 3 steps can be shown as:

Devolatilization: Feedstock Ò Fuel gas +Char.

Combustion: Fuel gas + Air Ò Flue gas + Heat (25% of carbon)

tion, including minor steps and considerations is available elsewhere [5].

Reduction: Fuel gas, Char + Heat Ò Syngas (75% of carbon)

Gasification: Feedstock + Air Ò Syngas + Flue gas + Ash.

Gasification, which implies incomplete combustion (also commonly referred to as partial oxidation) of the carbonaceous feedstock, is one of the most attractive options to convert biomass into various high value products such as liquid and gaseous fuels, chemicals and electricity. Gasification is the most popular among the thermochemical conversion processes with the exception of direct combustion. Gasification processes have several advantages and disadvantages over other conversion technologies. The main advantages are that the gasification feedstock can be any type of biomass including agricultural residues, forestry residues, by-products from chemical processes, and even organic municipal wastes. Moreover, gasification typically converts the entire carbon content of the feedstock, making it more attractive than enzymatic ethanol production or anaerobic digestion where only portions of the biomass material are converted to fuel. The second advantage is that the product gas can be converted into a variety of fuels (H2, Bio-SNG, synthetic diesel and gasoline, etc.) and chemicals (methanol, urea). The other benefit of the biomass gasification process is lowered CO2 emissions, compact equipment setup with higher thermal efficiency [32]. Thus gasification is most suit-

able to produce chemicals that can be alternatives to petroleum based products.

Gasification technology for biomass conversion is commercially applied in China: in 1990, China built more than 70 biomass gasification projects for household cooking and each of them can supply energy for 800–1600 families [32] whereas in India, a perspective way of

Gasification processes are primarily designed to produce synthesis gas (syngas, a mixture of hydrogen and carbon monoxide) by converting the feedstock under reducing (oxygen deficient) conditions in the presence of a limited amount of gasifying agent such as air or oxygen [5]. Gasification consists of three major steps. The first step is devolatilization of the dried feedstock to produce the fuel gas for the second step, which is combustion. The combustion step produces the necessary heat and reducing environment required for the final step. The final step (so-called reduction step, char gasification step or syngas production step), is the slowest reaction phase in gasification, and often governs the overall gasification reaction rate.

Approximately 25% of carbon in the feedstock is consumed in the combustion step to provide the heat and reducing environment for the reduction step. A detailed discussion of gasifica-

4.1. Gasification

26 Gasification for Low-grade Feedstock


The heat required for the reduction reaction is supplied through the bed material (typically sand) from the combustion reactor. The bed material is continuously circulated between the two reactors while the ash is removed from the bed material using cyclones and the gases from the two reactors are not allowed to mix. The Milena project gasifier uses the two reactor configuration [35].

Tar formation in a gasifier during gasification process is significantly affected by reactor/gasifier configuration. Cao et al. [36] introduced an innovative fluidized bed gasifier which can produce tar free product flue gas. The factors that affects the biomass gasification process most are: the reaction temperature, residence time and oxygen to biomass ratio [37].According to this study, the optimum residence time is 1.6 s and the optimum oxygen to biomass ratio is 0.4.

A major challenge of biomass gasification is to overcome the higher specific capital and operating costs. This is due to the much smaller plant sizes (normally less than 500 tons per day) compared to coal gasification plants (tens of thousands of tons per day). The plant size is determined by biomass availability and related logistic issues and transportation costs inherent to any distributed resource. Other challenges include the presence of undesirable species such as alkali compounds in biomass ash. Alkali materials such as sodium and potassium cause slagging and fouling problems [38]. Most biomass gasifiers operate below the ash softening temperature to avoid ash melting. The lower temperatures also lead to lower capital cost requirement, resulting in favorable process economics. However, lower temperatures often result in the formation of undesired tar, which leads to severe operational problems. A number of catalysts and process configurations have been developed to address this issue, but tar problems still persist [39]. Addition of a catalytic tar cracker to the outlet of the gasifier to decompose the tars into smaller molecules has been considered [40]. Washing out the tars while the product gas is cooling down has also been proposed, but this approach requires rigorous treatment of the washing water. Tar formation is still a major challenge and is regarded as the "Achilles heel" of biomass gasification processes. These issues are not to be underestimated and careful attention is required in the design and operation of biomass gasifiers.

The study performed by van de Kaa, Kamp and Rezaei [41], investigated the technology dominance of the three different dry thermochemical conversion of biomass. They found that the gasification technology has the highest potential of becoming the commercial technology for biomass conversion in the Netherlands.

#### 4.2. Bio-SNG production by gasification

#### 4.2.1. Current status of bio-SNG R&D projects

Bio-SNG is a fuel made from syngas produced by biomass gasification with major constituent of natural gas for potential use in household or transportation. Various R&D projects on biomass gasification are underway in many EU nations with specific objectives of enhanced generation and distribution of renewable energy and consequent reduction in greenhouse gas evolution: Current status of 3 Bio-SNG projects in Austria, the Netherlands, and Sweden is introduced.

#### 4.2.2. SNG demonstration plant (Austria, Güssing)

• Fluidized bed gasification demonstration plant in operation at Güssing, Austria burning wood chips. Details are summarized in Figures 2–4 and Tables 5–7.

#### 4.2.3. ECN SNG project (Netherlands)

• At present, EU is setting legally-binding objectives of 20% CO2 reduction by 2020 and further 60–80% by 2050 and thus the Netherlands is planning to increase the bio-SNG portion to 20% of primary energy generation source in compliance. Dutch ECN [44] has already performed feasibility study on production of SNG from biomass gasification since 2002 with fluidized bed gasifier consisting of gas purification system, and subsequent methanation and SNG upgrading processes. Details of ECN-initiated bio-SNG plant R&D stages and unit processes are shown in Figures 5 and 6.

The ECN bio-SNG plant gasifier is named as "MILENA," with circulating fluidized bed gasifier and bubbling fluidized combustor as main elements proper. The gasifier operates at 800C, with various feedstocks. Overall gasification performance and processes of the ECN

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• Sweden has currently set 50% proportion of renewable energy sources in the national energy generation and concomitant reduction of CO2 evolution by 40%. Besides, they are planning to exclude reliance on fossil fuel sources in national transport system by 2030. Göteborg Energi initiated relevant R&D by performing feasibility studies on various

bio-SNG plant are shown in Figure 7 and Table 8.

Figure 3. Process flow diagram of the gasification plant [43].

4.2.4. GoBiGas project (Sweden, Göthenburg)

Figure 4. 1 MW capacity SNG demonstration plant [43].

Current Developments in Thermochemical Conversion of Biomass to Fuels and Chemicals http://dx.doi.org/10.5772/intechopen.71464 29

Figure 3. Process flow diagram of the gasification plant [43].

The study performed by van de Kaa, Kamp and Rezaei [41], investigated the technology dominance of the three different dry thermochemical conversion of biomass. They found that the gasification technology has the highest potential of becoming the commercial technology

Bio-SNG is a fuel made from syngas produced by biomass gasification with major constituent of natural gas for potential use in household or transportation. Various R&D projects on biomass gasification are underway in many EU nations with specific objectives of enhanced generation and distribution of renewable energy and consequent reduction in greenhouse gas evolution: Current status of 3 Bio-SNG projects in Austria, the Netherlands, and Sweden is

• Fluidized bed gasification demonstration plant in operation at Güssing, Austria burning

• At present, EU is setting legally-binding objectives of 20% CO2 reduction by 2020 and further 60–80% by 2050 and thus the Netherlands is planning to increase the bio-SNG portion to 20% of primary energy generation source in compliance. Dutch ECN [44] has already performed feasibility study on production of SNG from biomass gasification since 2002 with fluidized bed gasifier consisting of gas purification system, and subsequent methanation and SNG upgrading processes. Details of ECN-initiated bio-SNG plant

wood chips. Details are summarized in Figures 2–4 and Tables 5–7.

R&D stages and unit processes are shown in Figures 5 and 6.

for biomass conversion in the Netherlands.

28 Gasification for Low-grade Feedstock

4.2. Bio-SNG production by gasification

introduced.

4.2.1. Current status of bio-SNG R&D projects

4.2.2. SNG demonstration plant (Austria, Güssing)

4.2.3. ECN SNG project (Netherlands)

Figure 2. Gasifier type [42].

Figure 4. 1 MW capacity SNG demonstration plant [43].

The ECN bio-SNG plant gasifier is named as "MILENA," with circulating fluidized bed gasifier and bubbling fluidized combustor as main elements proper. The gasifier operates at 800C, with various feedstocks. Overall gasification performance and processes of the ECN bio-SNG plant are shown in Figure 7 and Table 8.

#### 4.2.4. GoBiGas project (Sweden, Göthenburg)

• Sweden has currently set 50% proportion of renewable energy sources in the national energy generation and concomitant reduction of CO2 evolution by 40%. Besides, they are planning to exclude reliance on fossil fuel sources in national transport system by 2030. Göteborg Energi initiated relevant R&D by performing feasibility studies on various


Table 5. Summary of the project.


SNG demonstration plant already supplied hydrogen fuel via natural gas distribution network with accumulated operation record exceeding 10,000 h. The methane concentration and calorific value of the SNG thus produced were 96.5–97.5% and 10.8 kWh/Nm<sup>3</sup> (HHV basis) which will be used as a basis for construction of a commercial scale gasifier with 80–100 MW capacity after successful operation of the demonstration plant for reliability substantiation. Overall unit processes and their flow diagram are illustrated in

LHV MJ/Nm3 34.20 CH4 % 94.81 CO2 % 0.47 H2 % 1.55 H2O % 0.16 N2 % 2.67

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Pyrolysis is the thermal decomposition of the feedstock in the absence of oxygen. The products of biomass pyrolysis are char, bio-oil (also referred to as bio-crude) and gases including methane, hydrogen, carbon monoxide, and carbon dioxide. Pyrolysis can be further classified into slow and fast pyrolysis based on the residence time of the solid biomass in the reactor. Fast pyrolysis is normally conducted under medium to high temperatures (usually 450–550C) at very high heating rates and short residence time (e.g., milliseconds to

The objective of the process is to maximize the liquid yield and minimize the production of char and gases. This requires fast heating of the biomass and produces bio-oil (60% by weight) and other products including gas and char [49]. On the other hand, slow

Figure 8.

Main components

Table 7. Bio-SNG composition [43].

a few seconds).

4.3. Slow/fast pyrolysis

Figure 5. Overall timeline of the Dutch bio-SNG project [44].

Table 6. Syngas composition [43].

gasification technologies since 2006 and has completed fundamental engineering design as a result of primary stage progress during 2008–2013, thus enabling construction of a 20 MW bio-SNG demonstration plant. The production capacity of the 20 MW bio-SNG demonstration plant was equivalent to fuel 15,000–20,000 hydrogen fuel cell vehicles, based on the same type of gasifier as that of aforementioned Güssing, Austria. The bio-


Table 7. Bio-SNG composition [43].

Figure 5. Overall timeline of the Dutch bio-SNG project [44].

SNG demonstration plant already supplied hydrogen fuel via natural gas distribution network with accumulated operation record exceeding 10,000 h. The methane concentration and calorific value of the SNG thus produced were 96.5–97.5% and 10.8 kWh/Nm<sup>3</sup> (HHV basis) which will be used as a basis for construction of a commercial scale gasifier with 80–100 MW capacity after successful operation of the demonstration plant for reliability substantiation. Overall unit processes and their flow diagram are illustrated in Figure 8.

#### 4.3. Slow/fast pyrolysis

gasification technologies since 2006 and has completed fundamental engineering design as a result of primary stage progress during 2008–2013, thus enabling construction of a 20 MW bio-SNG demonstration plant. The production capacity of the 20 MW bio-SNG demonstration plant was equivalent to fuel 15,000–20,000 hydrogen fuel cell vehicles, based on the same type of gasifier as that of aforementioned Güssing, Austria. The bio-

Items Contents

30 Gasification for Low-grade Feedstock

Capacity • 8 MWth(2MWe)

History • R&D since 2002

Table 5. Summary of the project.

Table 6. Syngas composition [43].

Main components (vol %)

Minor components

Gasifier type

Final Product

Feedstock • Wood chip(Moisture 20–30%) 2300 kg/h

• Dual fluidized bed steam gasifier, • two-stage gas cleaning system • gas engine with an electricity generator

• Commercial bio-SNG plants: announced as 20–200 MW

• 1 MW demonstration SNG plant construction completed • 8-year of ongoing R&D in gas conditioning and synthesis of SNG

• Currently used as power and heat by operation of gas engine • SNG synthesis is separately studied at Paul-Scherrer Institute, Sweden (1000 h demonstration experiment linked with slip stream

• 10 M EURO invested (government subsidy of 6 M EURO inclusive) • Plant operation cost per annum at 15% of total investment • Recorded cumulative operation time of more than 60,000 h • R & D Project under name of "GAYA" as of 2010

• heat utilization system

H2 % 35–45 CO % 22–25 CH4 % 10 CO2 % 20–25

C2H4 % 2–3 C2H6 % 0.5 C2H2 % 0.4 O2 % <0.1 N2 % 1–3 C6H6 g/m<sup>3</sup> <sup>8</sup> C7H8 g/m<sup>3</sup> 0.5 C10H8 g/m<sup>3</sup> <sup>2</sup> TARS mg/m3 20–30

Pyrolysis is the thermal decomposition of the feedstock in the absence of oxygen. The products of biomass pyrolysis are char, bio-oil (also referred to as bio-crude) and gases including methane, hydrogen, carbon monoxide, and carbon dioxide. Pyrolysis can be further classified into slow and fast pyrolysis based on the residence time of the solid biomass in the reactor. Fast pyrolysis is normally conducted under medium to high temperatures (usually 450–550C) at very high heating rates and short residence time (e.g., milliseconds to a few seconds).

The objective of the process is to maximize the liquid yield and minimize the production of char and gases. This requires fast heating of the biomass and produces bio-oil (60% by weight) and other products including gas and char [49]. On the other hand, slow

Figure 6. Process step of pilot and demo plants [45].

pyrolysis takes several hours to complete with bio-char being the main product. Pan et al. [50] performed the slow pyrolysis of Nannochloropsis sp. (a kind of green microalga) and showed that the catalytic pyrolysis can produce the fuel with low oxygen content and higher heating value than the pyrolysis product without catalyst. Fast pyrolysis has attracted considerable attention in recent years. Fast pyrolysis efficiency, in addition to the residence time and operating temperature, is strongly dependent on the particle size of the feedstock as rapid and efficient heat transfer through the particle is critical. Most fast pyrolysis processes use a maximum particle size of 2 mm. Pyrolysis processes can be built in relatively small scales and are well suited for lignocellulosic feedstocks. Efficient thermal energy input to the reactor is critical since the pyrolysis process is endothermic

and heat transfer rates play a major role in the conversion process. High moisture content biomass must be dried prior to the conversion process. Besides oil and gas, bio-char is an important pyrolysis product. Bio-char is well-known as a soil amendment as it is highly absorbent and increases the soil's ability to retain water and nutrients. This fast process product yields more than 70 wt.% biomass when operates at atmospheric pressure and

/kg biomass) 0.82

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SNG-process characteristics Milena Carbon conversion (%) 80 Biomass-to-SNG efficiency (%) 66.3 Overall process efficiency (%) 82

Figure 7. Process flow diagram and unit process of the ECN bio-SNG plant [47].

Table 8. Performance of the ECN bio-SNG plant [46].

Gas yield(Nm<sup>3</sup>

Current Developments in Thermochemical Conversion of Biomass to Fuels and Chemicals http://dx.doi.org/10.5772/intechopen.71464 33

pyrolysis takes several hours to complete with bio-char being the main product. Pan et al. [50] performed the slow pyrolysis of Nannochloropsis sp. (a kind of green microalga) and showed that the catalytic pyrolysis can produce the fuel with low oxygen content and higher heating value than the pyrolysis product without catalyst. Fast pyrolysis has attracted considerable attention in recent years. Fast pyrolysis efficiency, in addition to the residence time and operating temperature, is strongly dependent on the particle size of the feedstock as rapid and efficient heat transfer through the particle is critical. Most fast pyrolysis processes use a maximum particle size of 2 mm. Pyrolysis processes can be built in relatively small scales and are well suited for lignocellulosic feedstocks. Efficient thermal energy input to the reactor is critical since the pyrolysis process is endothermic

Figure 6. Process step of pilot and demo plants [45].

32 Gasification for Low-grade Feedstock

Figure 7. Process flow diagram and unit process of the ECN bio-SNG plant [47].


Table 8. Performance of the ECN bio-SNG plant [46].

and heat transfer rates play a major role in the conversion process. High moisture content biomass must be dried prior to the conversion process. Besides oil and gas, bio-char is an important pyrolysis product. Bio-char is well-known as a soil amendment as it is highly absorbent and increases the soil's ability to retain water and nutrients. This fast process product yields more than 70 wt.% biomass when operates at atmospheric pressure and

toward the acidity of the bio-oil. Typical pH of the bio-oil is in the range of 2.0–3.0 [16]. The viscosity of bio-oils increases during storage and the physical properties undergo considerable changes [21]. The changes in the physical properties are attributed to the self-reaction of various compounds in the bio-oil including polymerization reactions [22, 25]. These reactions, occurring during storage, increase the average molecular weight of

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The resulting corrosive nature presents serious obstacles to any efforts aimed at the transportation and centralized refining or upgrading of the bio-oils. Also, the unstable nature of bio-oils often necessitates minimizing storage times and local upgrading, instead of transportation to a centralized facility. Such local upgrading is done by means of hydro-deoxygenation using hydrogen, often in the presence of catalysts. This normally adds capital and operating cost to the bio-oil production process. Gasification and co-gasification of bio-crude to syngas have

Most of the fast pyrolysis projects are still in laboratory scale with an exception of a few, including KIOR project [53] and BTG-BV in the Netherlands, which was originally developed by University of Twente [54]. These processes are regarded as pre-commercial, or demonstra-

Direct combustion of biomass is the oldest energy production process in human history. It is still by far the most widely used biomass conversion process. It is the most common biomass to power generation method commercially available [41]. The scale can be very small to relatively large, ranging from 1 MW up to 100 MW. Co-firing of biomass with coal is the effective way for lowering the greenhouse gas emissions. A wide range of technology options ranging from the simple fire stove to the advanced boiler system with fluidized furnace using pulverized fuel are available. Precise control of mixing between the biomass fuel and oxygen source (generally, air) is a critical aspect of advanced combustion systems in order to achieve improved thermal efficiency and minimize of criteria pollutant emissions including particulate matter (PM), nitrogen oxide (NOx), carbon mon-

For industrial and centralized domestic heat and power generation, several designs including stoker burners, grate boilers and/or dense fluid-bed combustor are used ranging from a few kilowatts to 10 MW. Combustion efficiency has improved remarkably in recent decades and has reached over 90% from around 55% in 1980 (FBC). Recently, development of combustion systems with pressurized fluidized beds have enabled direct electricity production without requiring steam generation, since process utilizes the fluidized bed as combustion chamber of

For a very large-scale direct combustion (larger than 300 MW), co-firing biomass with pulverized coal has been recommended. Pulverized coal combustion technology is well established and co-firing is an attractive option that can reduce the carbon dioxide emissions

the bio-oil and also lead to other storage related issues such as phase separation.

been tried, with reasonable success [5].

tion stage technologies.

4.4. Direct combustion

oxide (CO) and hydrocarbons.

the gas turbine [55].

Figure 8. Process diagram of the Swedish GoBiGas plant [48].

moderate temperatures (450C). Oxygen and water are major by-products during fast pyrolysis and these components degrades the fuel quality to the low-grade fuel compared to conventional hydrocarbon fuel [51].

Duman et al. [52] performed a comparison study of slow and fast pyrolysis: medium level of bio-oil is produced by slow pyrolysis of cherry seeds with only 21 wt.% yields whereas the bio-oil of 44% yield is produced by fast pyrolysis of the same feedstock.

Flash pyrolysis is an emerging technology and there are several key issues that need to be addressed. The most critical problems are associated with the quality of the "bio-oil," dictated by the physical and the chemical properties. Some of these problems are discussed below. Ideally, bio-oil should be interchangeable with petroleum crude oil so that the transportation and refining infrastructure can be used in existing form or with minor modifications. Based on this reasoning, the properties of bio-oil are often compared to that of petroleum crude oil. However, bio-oil has serious physical and chemical property issues and it is difficult to use it in existing petroleum refineries [15–18, 21–23].

Bio-oil is known to be extremely corrosive and this nature causes serious problems related to handling and transportation. The Total Acid Number (TAN) required for crude oil refineries is normally less than 2. Typical bio-oil TAN values range from 50 to as high as 200 [24]. Bio-oil typically contains 15–30% water. Besides water, components present in high concentrations are hydroxyl-acetaldehyde and acetic and formic acids. These oxygenated compounds along with various other species such as phenolic compounds contribute toward the acidity of the bio-oil. Typical pH of the bio-oil is in the range of 2.0–3.0 [16]. The viscosity of bio-oils increases during storage and the physical properties undergo considerable changes [21]. The changes in the physical properties are attributed to the self-reaction of various compounds in the bio-oil including polymerization reactions [22, 25]. These reactions, occurring during storage, increase the average molecular weight of the bio-oil and also lead to other storage related issues such as phase separation.

The resulting corrosive nature presents serious obstacles to any efforts aimed at the transportation and centralized refining or upgrading of the bio-oils. Also, the unstable nature of bio-oils often necessitates minimizing storage times and local upgrading, instead of transportation to a centralized facility. Such local upgrading is done by means of hydro-deoxygenation using hydrogen, often in the presence of catalysts. This normally adds capital and operating cost to the bio-oil production process. Gasification and co-gasification of bio-crude to syngas have been tried, with reasonable success [5].

Most of the fast pyrolysis projects are still in laboratory scale with an exception of a few, including KIOR project [53] and BTG-BV in the Netherlands, which was originally developed by University of Twente [54]. These processes are regarded as pre-commercial, or demonstration stage technologies.

#### 4.4. Direct combustion

moderate temperatures (450C). Oxygen and water are major by-products during fast pyrolysis and these components degrades the fuel quality to the low-grade fuel compared

Duman et al. [52] performed a comparison study of slow and fast pyrolysis: medium level of bio-oil is produced by slow pyrolysis of cherry seeds with only 21 wt.% yields whereas

Flash pyrolysis is an emerging technology and there are several key issues that need to be addressed. The most critical problems are associated with the quality of the "bio-oil," dictated by the physical and the chemical properties. Some of these problems are discussed below. Ideally, bio-oil should be interchangeable with petroleum crude oil so that the transportation and refining infrastructure can be used in existing form or with minor modifications. Based on this reasoning, the properties of bio-oil are often compared to that of petroleum crude oil. However, bio-oil has serious physical and chemical property issues and it is difficult to use it in

Bio-oil is known to be extremely corrosive and this nature causes serious problems related to handling and transportation. The Total Acid Number (TAN) required for crude oil refineries is normally less than 2. Typical bio-oil TAN values range from 50 to as high as 200 [24]. Bio-oil typically contains 15–30% water. Besides water, components present in high concentrations are hydroxyl-acetaldehyde and acetic and formic acids. These oxygenated compounds along with various other species such as phenolic compounds contribute

the bio-oil of 44% yield is produced by fast pyrolysis of the same feedstock.

to conventional hydrocarbon fuel [51].

34 Gasification for Low-grade Feedstock

Figure 8. Process diagram of the Swedish GoBiGas plant [48].

existing petroleum refineries [15–18, 21–23].

Direct combustion of biomass is the oldest energy production process in human history. It is still by far the most widely used biomass conversion process. It is the most common biomass to power generation method commercially available [41]. The scale can be very small to relatively large, ranging from 1 MW up to 100 MW. Co-firing of biomass with coal is the effective way for lowering the greenhouse gas emissions. A wide range of technology options ranging from the simple fire stove to the advanced boiler system with fluidized furnace using pulverized fuel are available. Precise control of mixing between the biomass fuel and oxygen source (generally, air) is a critical aspect of advanced combustion systems in order to achieve improved thermal efficiency and minimize of criteria pollutant emissions including particulate matter (PM), nitrogen oxide (NOx), carbon monoxide (CO) and hydrocarbons.

For industrial and centralized domestic heat and power generation, several designs including stoker burners, grate boilers and/or dense fluid-bed combustor are used ranging from a few kilowatts to 10 MW. Combustion efficiency has improved remarkably in recent decades and has reached over 90% from around 55% in 1980 (FBC). Recently, development of combustion systems with pressurized fluidized beds have enabled direct electricity production without requiring steam generation, since process utilizes the fluidized bed as combustion chamber of the gas turbine [55].

For a very large-scale direct combustion (larger than 300 MW), co-firing biomass with pulverized coal has been recommended. Pulverized coal combustion technology is well established and co-firing is an attractive option that can reduce the carbon dioxide emissions from coal. However challenges associated with co-firing with biomass such as changes in ash properties, fouling of heat exchanger, etc. still need to be addressed [56]. Biomass torrefaction is promising process that improves the usefulness of biomass as a fuel by heating the biomass in the absence of air under mild temperatures (230–300C). The resulting biomass fuel is a desirable feedstock for entrained-flow reactors or in pulverized coal fired boilers with cofiring of biomass [57].

Author details

Chan Seung Park1

References

California, Riverside, CA, USA

GO-102995-2135

\*, Partho Sarothi Roy1 and Su Hyun Kim2

2 Institute for Advanced Engineering, Yongin-si, Gyeonggi-do, Korea

1 College of Engineering - Center for Environmental Research and Technology, University of

Current Developments in Thermochemical Conversion of Biomass to Fuels and Chemicals

http://dx.doi.org/10.5772/intechopen.71464

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[1] Perlack RD, Wright LL, Turhollow AF, Graham RL, Stokes BJ, Erbach DC. Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply. Oak Ridge, Tennessee: Oak Ridge National Laboratory; 2005. DOE/

[2] Drift A. Role of gasification in a bio-based future. In: 24th European Biomass Conference

[3] GREET by Argonne National Laboratory. The Greenhouse Gases, Regulated Emissions, and Energy Use in Transportation Model. Argonne, IL: GREET 1.8d.1 ed; 2010

[4] Gaur S, Reed TB. Thermal Data for Natural and Synthetic Fuels. CRC Press: Marcel Dekker;

[5] Higman C, van der Burgt M. Gasification. Gulf Professional Publishing: Elsevier Science;

[6] Milena Indirect Gasifier. MILENA Biomass Gasification Process [Internet]. 2017. Avail-

[7] Peer M, Mahdeyarfar M, Mohammadi T. Investigation of syngas ratio adjustment using a polyimide membrane. Chemical Engineering and Processing Process Intensification.

[8] Öhrman OGW, Weiland F, Pettersson E, Johansson A-C, Hedman H, Pedersen M. Pressurized oxygen blown entrained flow gasification of a biorefinery lignin residue. Fuel

[9] Patel M, Zhang X, Kumar A. Techno-economic and life cycle assessment on lignocellulosic biomass thermochemical conversion technologies: A review. Renewable and Sus-

[10] Swaaij W, Kersten S. Biomass Power for the World. CRC Press: Pan Stanford Publishing;

Processing Technology. 2013;115:130-138. DOI: 10.1016/j.fuproc.2013.04.009

tainable Energy Reviews. 2016;53:1486-1499. DOI: 10.1016/j.rser.2015.09.070

and Exhibition; 1–4 June; Vienna, Austria; 2015. ISBN: ECN-L-15-063 EN

able from: http://www.milenatechnology.com [Accessed: 2017-08-01]

\*Address all correspondence to: cspark@cert.ucr.edu

1998. pp. 102-108. ISBN 0-8247-0070-8

2015. p. 33. ISBN 978-981-4669-24-5

2011. 33, 167-173 p. ISBN 978-0-7560-8528-3

2009;48(3):755-761. DOI: 10.1016/j.cep.2008.09.006

Arce et al., [58] studied the performance of the different types of biomass fuel combustion process in a counter-current fixed bed reactor in the temperature range of 740–1300C to check the effects of different factors and find the optimum condition. According to the study, the ignition front propagation speed and the highest temperature that is reached at the fixed bed combustor affects the combustion process most.

Oxy-combustion is an emerging technology that uses pure oxygen in the combustor. The advantage is that after the cooling of flue gas, nearly pure carbon dioxide is produced without any nitrogen or nitrogen oxides. However, the use of pure oxygen (or oxygen enriched air) results in higher capital and operating costs. This needs to be balanced against the cost/energy savings related to carbon dioxide capture. This technology is still in the research and demonstration stage. As more cost effective processes for oxygen production such as membrane separation are developed, oxy-combustion will presumably become a more attractive option for both biomass and fossil feedstocks.
