**Current Developments in Thermochemical Conversion of Biomass to Fuels and Chemicals** Current Developments in Thermochemical Conversion of Biomass to Fuels and Chemicals

DOI: 10.5772/intechopen.71464

Chan Seung Park, Partho Sarothi Roy and Su Hyun Kim Chan Seung Park, Partho Sarothi Roy and

Additional information is available at the end of the chapter Su Hyun Kim

http://dx.doi.org/10.5772/intechopen.71464 Additional information is available at the end of the chapter

#### Abstract

Biomass is one of the largest concentrated carbon source available for producing renewable energy. Thermochemical conversion of biomass has been used for centuries in various settings. Biomass typically has a higher oxygen and volatile matter content than other solid carbon feedstocks, resulting in increased reactivity during conversion by thermochemical pathways. Moisture content of the biomass feedstock exerts significant influence on the conversion process and is an important criteria used to classify various thermochemical conversion technologies. This chapter discusses the current status and future outlook of thermochemical biomass conversion processes.

Keywords: biomass, gasification, pyrolysis, hydrothermal treatment, steam hydrogasification, combustion

### 1. Introduction

Biomass has always considered as one of the major energy source for the world. Biomass can be defined as plant materials and animal waste, although broader definitions that include other forms of carbonaceous waste are used in the renewable energy context. Earth's primary source of biomass is the plant matter that grows through photosynthesis. The carbon stored in the biomass is from the carbon dioxide consumed during photosynthesis and is ultimately converted back to carbon dioxide during any energy generation processes. As is well known, biomass based processes are often carbon neutral, i.e., do not add additional carbon dioxide to the atmosphere, or have a very low carbon footprint. For these reasons, biomass is the largest and most widespread carbon source for producing renewable energy and is relatively free of fluctuation problems inherent to wind and solar energy. A comprehensive inventory of biomass

© 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

resources in the United States potentially available for energy production is available as so-called "billion-ton study" by the U.S. Department of Energy [1].

From the thermodynamic point of view, at the typical biomass conversion temperature of 800C, the product gas typically has higher concentrations of more valuable C2+ species. Table 3 shows product gas composition of a biomass gasifier operated by the Milena project [6], a well-known biomass gasification demonstration project in Europe, along with the gas composition of a typical coal gasifier operated at higher temperatures (1400C), which more

Current Developments in Thermochemical Conversion of Biomass to Fuels and Chemicals

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

21

It also shows the product gas has lower syngas ratio (syngas or synthesis gas is a mixture of hydrogen and carbon monoxide and syngas ratio is the molar ratio of Hydrogen to CO), which typically provides more flexibility during use in downstream processes that require specific syngas ratios. Syngas ratio can be increased using the well-known, commercially matured, water-gas shift process. Syngas ratio reduction is achieved through techniques such as membrane separation, and presents a number of technical challenges [7]. Higher CH4 content is also beneficial since the product gas is often used as a fuel in combustion engines or boilers. C2+, i.e., gaseous carbon species with a higher carbon number, can potentially be used as a feedstock in chemical production. Such high value co-products often provide an additional revenue stream, improving the overall economics of the biomass

In conclusion, biomass is the only concentrated renewable carbon source that can be converted into fuels and chemicals with a zero or very low carbon footprint. Unlike biological processes that only convert part of the biomass, thermochemical processes can generally convert all the carbon in the feedstock. Biomass is a reactive, desirable feedstock for thermochemical processes due to the higher oxygen content compared to coal. Thermochemical conversion of biomass offers significant versatility since the product gas can be converted into fungible liquid fuels, thereby offering a pathway to reduce the carbon intensity of the transportation sector. The product gas from most thermochemical processes can also be converted into high

Some thermochemical process such as flash pyrolysis and hydrothermal liquefaction directly produce a liquid product. Thermochemical processes can also handle intermediate products and waste biomass from biological conversion processes [8]. High value chemicals production from biomass with energy as a major co-product may be the path to economic viability in the

This chapter presents an overview of the different thermochemical processes that convert biomass into a high energy content gaseous or liquid product and/or additional thermal energy. A discussion of the unique aspects of different technologies from various perspectives,

Mole fraction, % H2 CO CO2 CH4 C2+ Equilibrium value 51 45 3 1 0 Measured (Milena FB gasifier) [6] 25 33 18 15 6

Table 3. Comparison of product gas composition under equilibrium conditions with those from a biomass gasifier.

value chemicals such as ethylene and BTX (Benzene, Toluene, and Xylene).

including energy storage and transportation is also provided.

closely represents the thermodynamic equilibrium values.

gasification plant.

near future.

The oldest energy conversion process used by humans is biomass combustion in open air to produce heat. Biomass burning is still a dominant process in many parts of the world and thermochemical conversion of biomass to energy has a long scientific history. Since then, various thermochemical processes for biomass conversion have been developed to overcome the primary limitation of combustion; it only produces thermal energy along with the flue gases. Thermochemical biomass conversion to gaseous and liquid fuels has been studied and practiced for centuries. Production of a number of chemical compounds from biomass is also important application of the thermochemical process. The first such example is charcoal production from wood around 4000 B.C.

Compared to coal, which is the most widely used conventional solid fuel for energy production, biomass typically contains a higher oxygen and volatile matter content and lower ash [2]. This high oxygen content makes biomass a good fuel although oxygen itself does not contribute toward the energy value of the fuel. The higher oxygen content results in reduced air (oxygen) requirement during the combustion reaction. Table 1 summarizes the Lower Heating Value (LHV) of different fuels. Coal has a higher LHV than biomass per unit mass of the fuel. However, once the volume of air required for complete combustion is taken into account (LHV per mass per air mix), biomass's value is higher than that of coal, and is even comparable to methane.

Nearly 80% of the carbon in the biomass is typically considered "organic," i.e., bounded to hydrogen or oxygen. Organic carbon is highly volatile compared to elementary carbon, resulting in improved reactivity and thus makes biomass an attractive feedstock for thermochemical production of fuels and chemicals, especially from the conversion perspective. Table 2 summarizes the typical oxygen and volatile content of coal and biomass. Conversion of the volatile portion of the biomass feedstock into gaseous species starts around 225 to 300C and is mostly complete around 500 to 600C [4]. While elemental, non-volatile carbon decompose at temperatures above 800C, much higher temperatures (>1200C) are desirable to avoid potential problems associated with ash softening [5]. Thus, thermochemical conversion of biomass can be performed at the much lower temperatures than is needed for coal, with higher conversion efficiency.


Table 1. Comparison of LHV values of methane, coal, biomass and hydrogen.


Table 2. Oxygen and volatiles content of coal and biomass.

From the thermodynamic point of view, at the typical biomass conversion temperature of 800C, the product gas typically has higher concentrations of more valuable C2+ species. Table 3 shows product gas composition of a biomass gasifier operated by the Milena project [6], a well-known biomass gasification demonstration project in Europe, along with the gas composition of a typical coal gasifier operated at higher temperatures (1400C), which more closely represents the thermodynamic equilibrium values.

resources in the United States potentially available for energy production is available as so-called

The oldest energy conversion process used by humans is biomass combustion in open air to produce heat. Biomass burning is still a dominant process in many parts of the world and thermochemical conversion of biomass to energy has a long scientific history. Since then, various thermochemical processes for biomass conversion have been developed to overcome the primary limitation of combustion; it only produces thermal energy along with the flue gases. Thermochemical biomass conversion to gaseous and liquid fuels has been studied and practiced for centuries. Production of a number of chemical compounds from biomass is also important application of the thermochemical process. The first such example is charcoal

Compared to coal, which is the most widely used conventional solid fuel for energy production, biomass typically contains a higher oxygen and volatile matter content and lower ash [2]. This high oxygen content makes biomass a good fuel although oxygen itself does not contribute toward the energy value of the fuel. The higher oxygen content results in reduced air (oxygen) requirement during the combustion reaction. Table 1 summarizes the Lower Heating Value (LHV) of different fuels. Coal has a higher LHV than biomass per unit mass of the fuel. However, once the volume of air required for complete combustion is taken into account (LHV per mass per air mix), biomass's value is higher than that of coal, and is even comparable to methane.

Nearly 80% of the carbon in the biomass is typically considered "organic," i.e., bounded to hydrogen or oxygen. Organic carbon is highly volatile compared to elementary carbon, resulting in improved reactivity and thus makes biomass an attractive feedstock for thermochemical production of fuels and chemicals, especially from the conversion perspective. Table 2 summarizes the typical oxygen and volatile content of coal and biomass. Conversion of the volatile portion of the biomass feedstock into gaseous species starts around 225 to 300C and is mostly complete around 500 to 600C [4]. While elemental, non-volatile carbon decompose at temperatures above 800C, much higher temperatures (>1200C) are desirable to avoid potential problems associated with ash softening [5]. Thus, thermochemical conversion of biomass can be performed at the much lower temperatures than is needed for coal, with higher

LHV Methane Bituminous coal Biomass Hydrogen

Components Bituminous coal Biomass Oxygen (wt.%, dry ash free, daf) 14 43 Volatiles (wt.%, daf) 42 82

MJ/kg fuel [3] 47 27 18 120 MJ/kg fuel/air mix 2.62 2.44 2.60 3.36

Table 1. Comparison of LHV values of methane, coal, biomass and hydrogen.

Table 2. Oxygen and volatiles content of coal and biomass.

"billion-ton study" by the U.S. Department of Energy [1].

production from wood around 4000 B.C.

20 Gasification for Low-grade Feedstock

conversion efficiency.

It also shows the product gas has lower syngas ratio (syngas or synthesis gas is a mixture of hydrogen and carbon monoxide and syngas ratio is the molar ratio of Hydrogen to CO), which typically provides more flexibility during use in downstream processes that require specific syngas ratios. Syngas ratio can be increased using the well-known, commercially matured, water-gas shift process. Syngas ratio reduction is achieved through techniques such as membrane separation, and presents a number of technical challenges [7]. Higher CH4 content is also beneficial since the product gas is often used as a fuel in combustion engines or boilers. C2+, i.e., gaseous carbon species with a higher carbon number, can potentially be used as a feedstock in chemical production. Such high value co-products often provide an additional revenue stream, improving the overall economics of the biomass gasification plant.

In conclusion, biomass is the only concentrated renewable carbon source that can be converted into fuels and chemicals with a zero or very low carbon footprint. Unlike biological processes that only convert part of the biomass, thermochemical processes can generally convert all the carbon in the feedstock. Biomass is a reactive, desirable feedstock for thermochemical processes due to the higher oxygen content compared to coal. Thermochemical conversion of biomass offers significant versatility since the product gas can be converted into fungible liquid fuels, thereby offering a pathway to reduce the carbon intensity of the transportation sector. The product gas from most thermochemical processes can also be converted into high value chemicals such as ethylene and BTX (Benzene, Toluene, and Xylene).

Some thermochemical process such as flash pyrolysis and hydrothermal liquefaction directly produce a liquid product. Thermochemical processes can also handle intermediate products and waste biomass from biological conversion processes [8]. High value chemicals production from biomass with energy as a major co-product may be the path to economic viability in the near future.

This chapter presents an overview of the different thermochemical processes that convert biomass into a high energy content gaseous or liquid product and/or additional thermal energy. A discussion of the unique aspects of different technologies from various perspectives, including energy storage and transportation is also provided.


Table 3. Comparison of product gas composition under equilibrium conditions with those from a biomass gasifier.
