3. Wet biomass conversion processes

#### 3.1. Hydrothermal conversion process

2. Classification of thermochemical biomass conversion processes

Feedstock Technology Features

Hydrothermal conversion

22 Gasification for Low-grade Feedstock

\*

Biomass is the only renewable resource that can be directly converted in to concentrated energy products using thermochemical conversion [9]. Biomass consists of organic and inorganic matter and often significant amounts of moisture. Organic matter in biomass contributes to its calorific value. Organic matter can be further classified into cellulose, hemicellulose, and lignin. The inorganic matter is conventionally referred to as ash. Compared to conventional fuels, the oxygen content of biomass is typically very high, approximately ranging from 20 to 50% by weight. The moisture content plays a very important role in selecting the appropriate thermochemical conversion process. Heating value of biomass is heavily dependent on the moisture content. The LHV value is negative for biomass with a high moisture content (80%), since the heat released during the combustion process is not sufficient to evaporate all the water [10]. Therefore, biomass is usually dried under sunlight or through other methods, e.g., using recycled thermal energy as part of feed preparation. Solar and air drying in the production field is the preferred method of drying, primarily due to the lower cost. However, there are several conversion routes that use wet biomass feedstocks without the need for drying. Whether a conversion process uses wet or dry biomass as the feedstock is an important criteria for the classification of biomass conversion processes. A technology is categorized as a wet biomass process based on whether the moisture in the feedstock plays an important role in the process, either as a major reactant, or as physical media to maintain the reaction environment. In other words, wet biomass processes do not benefit from drying the feedstock, and often require the feedstock to contain certain quantity of moisture. Table 4 lists the wet and dry biomass feedstock processes, and these processes are discussed in detail in the rest of the chapter.

Wet biomass Biological\* Anaerobic digestion, or alcohol production from sugars by biomass hydrolysis

Dry biomass Oil extraction form seeds\* Trans-esterification or hydrogenation of vegetable oil from oil seeds to produce

Direct combustion Generate heat or power through the direct combustion of biomass

Slow pyrolysis Heating up the biomass in the absence of air (or oxygen) with slow heating rates to produce biochar and gaseous products Fast pyrolysis Extremely fast pyrolysis of biomass with very high heating rates resulting in crude oil like bio-oil and gaseous products Gasification Biomass is converted into the syngas or Substituted Natural Gas (Bio-SNG) using air or oxygen or hydrogen as the gasifying agent

conversion to methane, liquid fuels or chemicals

High pressure conversion to a hydrophobic oil. Often involves further catalytic

and fermentation

Supercritical gasification Conversion occurs under supercritical conditions Steam hydrogasification Uses hydrogen and steam as the gasifying agents

bio-diesel

These processes are outside the scope of this chapter and are not covered.

Table 4. Classification of biomass conversion processes.

Hydrothermal conversion has been studied for more than a 100 years. Friedrich Bergius, who would later receive the 1931 Nobel Prize in Chemistry along with Carl Bosch, developed the Bergius process that produces liquid fuel through hydrogenation of crude oil derived from hydrothermal treatment of coal. The technology was also applied to peat and plant material [11]. Hydrothermal conversion converts biomass into "bio-crude" through thermal depolymerisation under high pressures and moderate temperatures and has since then been studied by several research groups. A comprehensive review of the hydrothermal conversion process of the biomass is provided by Peterson et al. [12].

Hydrothermal processes can convert all types of biomass, including wet organic biomass, and typically involves the use of a catalyst to improve conversion efficiency. Hydrothermal liquefaction study conducted by Yang et al., shows that the process produces larger amount of oil product compared to other methods [12]. The product liquid fuel is easy to separate, for once it is cooled downed to the room temperature the gaseous product is emitted very quickly [13]. The product bio-crude, can be further processed into high-quality diesel or kerosene. The fast pyrolysis process, discussed later, is a dry conversion process that produces a bio-crude (or bio-oil) from dry biomass feedstock. Hydrothermal conversion process has lower efficiencies caused by the significant energy requirement of water evaporation.

Hydrothermal conversion processes can be further divided into supercritical hydrothermal and subcritical conversion processes. A supercritical hydrothermal conversion process developed by Aalborg University and commercialized by Steeper Energy under the name of "Hydrofraction" converts organic wastes into a raw bio-crude under supercritical conditions in the presence of K2CO3 catalyst [14]. Another process, referred to as the "Catliq" process uses Zirconia catalyst under supercritical conditions to produce a bio-crude with less than 6% oxygen content [15].

Shell research group has demonstrated a subcritical process named Hydro Thermal Upgrading (HTU) that converts the biomass into bio-crude with and without a catalyst [16]. Research has shown that in the presence of a catalyst with adequate activity, conversion could be accomplished at conditions that are less severe than supercritical. A number of catalysts including ones based on Ru, Carbon, and Ni have been proposed with the ultimate goal of developing an optimal hydrothermal conversion process under subcritical conditions [17].

#### 3.2. Supercritical gasification

The supercritical condition for water is the combination of T > 374C and P > 218 atm. Under these conditions, distinct liquid and vapor phases do not exist and the water exists as a single phase fluid [18]. The general reason to use supercritical conditions for wet feedstocks is to minimize the energy loss associated with water evaporation. Study performed by Demirbas [19] and Gadhe, & Gupta [20] shows that the supercritical water behaves as an organic solvent and exhibits extraordinary solubility toward organic compounds containing large nonpolar groups and most permanent gases. Transition of liquid water to the gas phase (steam) requires a large amount of heat, so-called "the heat of vaporization," which can be recovered in theory, but needs very efficient heat exchanger design. By operating the conversion process under supercritical conditions, uniform temperature profile along the reactor can be expected without the formation of multi-phase of water (liquid water, steam and/or superheated steam), which in turn results in efficient heat transfer between the product gas and feed inlet of the gasifier.

The VERENA group operates a somewhat larger pilot plant with 100 kg per hour throughput

Current Developments in Thermochemical Conversion of Biomass to Fuels and Chemicals

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

25

Steam hydrogasification uses steam and hydrogen as the gasifying agents and is especially suited for the conversion of wet feedstocks since it utilizes the water from the feedstock as a major reactant other than the physical media served as a thermal moderator [26]. Hydrogasification, using only hydrogen as the gasifying agent, is a well-known conversion technology but is not considered commercially viable due to several issues, including low conversion efficiencies and requirement of an external hydrogen source [27]. Research has shown that hydrogasification in the presence of steam significantly enhances the rate of methane formation under specific process conditions, thereby improving the overall process efficiency [28, 29]. This process, referred to as "steam hydrogasification," produces a product gas with a high methane content. The product gas also contains a considerable amount of unreacted steam along with CO, CO2, H2, and some higher molecular hydrocarbons. The product gas can then be converted into various fuels or chemical products. The impurities in the flue gas is significant during the steam hydrogasification. Cui et al., [30] developed Ni based catalyst and ZnO sorbent for contaminant and sulfur removal from the product gas and they were able to reduce

An example block flow diagram for Renewable Natural Gas (RNG) production is shown in Figure 1. The feedstock is turned into a slurry through a hydrothermal pre-treatment process (HTP) and is transported into the steam hydrogasification reactor (SHR) using a slurry pump. A portion of the necessary steam enters the reactor as water that is part of the slurry along with

The methane-rich gasifier product gas is then subjected to warm gas clean-up in order to remove contaminants such as sulfur and other species. Following this, the excess steam and CO is converted into hydrogen in a water gas shift reactor (WGS). This is an important aspect of the process: Even though the steam hydrogasification process needs hydrogen, it does not require an external source of hydrogen. The hydrogen is separated and fed back into the gasifier, making the process self-sustained in terms of the hydrogen supply. The process is

in Karlsruhe, Germany [25].

3.3. Steam hydrogasification

the components significantly.

additional superheated steam and recycled hydrogen.

Figure 1. Block diagram of RNG production by steam hydrogasification process.

currently undergoing demonstration [31].

Supercritical biomass gasifiers typically operate around 500 to 750C without a catalyst or at temperatures below 500C in the presence of a catalyst. The presence of supercritical water leads to rapid hydrolysis of biomass and high solubility of intermediate reaction products including gaseous species. These features make supercritical gasification as excellent tool for the conversion of very wet biomass feedstocks such as aquatic species and sewage sludge (a.k.a. biosolids), which normally require considerable drying before they can be gasified economically. Supercritical gasification also produces a high pressure product gas, thereby eliminating the need for the product gas compression required by most down-stream processes. A detailed discussion of process efficiency and other aspects of supercritical gasification is available in the article by [21]. For example, the gasification efficiency of a biomass feedstock with 80% water content using conventional steam reforming reaction is only 10%, while that of supercritical gasification can be as high as 70%. The main products are H2 and CO2 and the hydrogen is produced at very high pressure which is at the storage condition and reduces the storage cost. Also, the reactor operates at high reaction temperature, low residence time, and low biomass concentration.

However, there are several technological issues that must be overcome in order for supercritical processes to be commercially viable:


For these reasons, supercritical gasification processes are still in the development stage [23]. University of Twente operates a pilot plant and is involved in active research and development [24]. The VERENA group operates a somewhat larger pilot plant with 100 kg per hour throughput in Karlsruhe, Germany [25].

#### 3.3. Steam hydrogasification

3.2. Supercritical gasification

24 Gasification for Low-grade Feedstock

The supercritical condition for water is the combination of T > 374C and P > 218 atm. Under these conditions, distinct liquid and vapor phases do not exist and the water exists as a single phase fluid [18]. The general reason to use supercritical conditions for wet feedstocks is to minimize the energy loss associated with water evaporation. Study performed by Demirbas [19] and Gadhe, & Gupta [20] shows that the supercritical water behaves as an organic solvent and exhibits extraordinary solubility toward organic compounds containing large nonpolar groups and most permanent gases. Transition of liquid water to the gas phase (steam) requires a large amount of heat, so-called "the heat of vaporization," which can be recovered in theory, but needs very efficient heat exchanger design. By operating the conversion process under supercritical conditions, uniform temperature profile along the reactor can be expected without the formation of multi-phase of water (liquid water, steam and/or superheated steam), which in turn results in efficient heat transfer between the product gas and feed inlet of the gasifier.

Supercritical biomass gasifiers typically operate around 500 to 750C without a catalyst or at temperatures below 500C in the presence of a catalyst. The presence of supercritical water leads to rapid hydrolysis of biomass and high solubility of intermediate reaction products including gaseous species. These features make supercritical gasification as excellent tool for the conversion of very wet biomass feedstocks such as aquatic species and sewage sludge (a.k.a. biosolids), which normally require considerable drying before they can be gasified economically. Supercritical gasification also produces a high pressure product gas, thereby eliminating the need for the product gas compression required by most down-stream processes. A detailed discussion of process efficiency and other aspects of supercritical gasification is available in the article by [21]. For example, the gasification efficiency of a biomass feedstock with 80% water content using conventional steam reforming reaction is only 10%, while that of supercritical gasification can be as high as 70%. The main products are H2 and CO2 and the hydrogen is produced at very high pressure which is at the storage condition and reduces the storage cost. Also, the reactor operates

at high reaction temperature, low residence time, and low biomass concentration.

exchanger is critical to achieve desired energy conversion efficiency.

ical processes to be commercially viable:

by [22].

economic performance.

However, there are several technological issues that must be overcome in order for supercrit-

• Supercritical gasification processes need large heat input. Efficient design of heat

• The feeding of wet biomass is another barrier, although slurry pump has been used to feed into the high pressure vessel. However, achieving reliable feeding into supercritical gasification reactor under a very high operating pressure is still a significant challenge. • Other issues such as fouling, plugging of the feedstock, and corrosion are well reviewed

• Higher capital costs due the high operating pressure also have a negative impact on

For these reasons, supercritical gasification processes are still in the development stage [23]. University of Twente operates a pilot plant and is involved in active research and development [24]. Steam hydrogasification uses steam and hydrogen as the gasifying agents and is especially suited for the conversion of wet feedstocks since it utilizes the water from the feedstock as a major reactant other than the physical media served as a thermal moderator [26]. Hydrogasification, using only hydrogen as the gasifying agent, is a well-known conversion technology but is not considered commercially viable due to several issues, including low conversion efficiencies and requirement of an external hydrogen source [27]. Research has shown that hydrogasification in the presence of steam significantly enhances the rate of methane formation under specific process conditions, thereby improving the overall process efficiency [28, 29]. This process, referred to as "steam hydrogasification," produces a product gas with a high methane content. The product gas also contains a considerable amount of unreacted steam along with CO, CO2, H2, and some higher molecular hydrocarbons. The product gas can then be converted into various fuels or chemical products. The impurities in the flue gas is significant during the steam hydrogasification. Cui et al., [30] developed Ni based catalyst and ZnO sorbent for contaminant and sulfur removal from the product gas and they were able to reduce the components significantly.

An example block flow diagram for Renewable Natural Gas (RNG) production is shown in Figure 1. The feedstock is turned into a slurry through a hydrothermal pre-treatment process (HTP) and is transported into the steam hydrogasification reactor (SHR) using a slurry pump. A portion of the necessary steam enters the reactor as water that is part of the slurry along with additional superheated steam and recycled hydrogen.

The methane-rich gasifier product gas is then subjected to warm gas clean-up in order to remove contaminants such as sulfur and other species. Following this, the excess steam and CO is converted into hydrogen in a water gas shift reactor (WGS). This is an important aspect of the process: Even though the steam hydrogasification process needs hydrogen, it does not require an external source of hydrogen. The hydrogen is separated and fed back into the gasifier, making the process self-sustained in terms of the hydrogen supply. The process is currently undergoing demonstration [31].

Figure 1. Block diagram of RNG production by steam hydrogasification process.
