**2.5 Other challenges**

*Biomass for Bioenergy - Recent Trends and Future Challenges*

enzymatic—are summarized in **Table 2**.

any additional catalyst [27, 28].

of hydrogen ion (H<sup>+</sup>

**Breakdown methods**

Subcritical water

meet the needs.

an added necessary step of acid neutralization, the formation of byproducts that create an inhibitory effect in the fermentation [22, 23] and other negative environmental impacts. Thus, the current methods have undesirable processes and do not

Subcritical water (99.97°C < T < 374.15°C; 217.76 atm < P) is an alternative way to hydrolyze lignocellulosic biomass in an environmentally friendly manner by only operating temperature and pressure conditions. Significant advantages of subcritical water over commonly used biomass breakdown methods—alkali, acidic, and

The chemical properties of water are greatly changed at high temperatures and pressures due to the reduction of hydrogen bonding, which causes changes in dissociation, solubility, diffusivity, and reactivity [24]. Subcritical water has a lower relative dielectric constant and a higher ionic product than ambient water. When the temperature of water increases from ambient temperature to 250°C, its relative dielectric constant decreases from around 80 to nearly 27, which is similar to that of acetone at ambient temperature [25, 26]. Furthermore, the ion product of subcritical water substantially increases with temperature; therefore, subcritical water can catalyze chemical reactions such as hydrolysis and degradation without the use of

Ionic product numbers of water (Kw) at various temperatures and pressures showed that when pressure is around 35 MPa and temperature is in sub- and supercritical regions under 400°C, Kw values are always higher than 1 × 10<sup>−</sup>14. The Kw increases to its maxima (~10<sup>−</sup>11) between 200 and 300°C and does not respond to changes in pressure when in this temperature range. The molar concentrations

higher than those under room temperature. Therefore, the hydrolysis yield in these regions is expected to be high, and biomass polymers could be broken down into

The presence of a weak acid in subcritical water media can also improve hydrolysis of biomass materials. The use of carbon dioxide is as a pressurizing gas caused the formation of carbonic acid that plays a catalytic role in effective solubilization of biomass [32]. Some studies indicated that the addition of small amounts of hydrogen peroxide can enhance lignin removal and modify cellulose structure

Complexity and diversity of the biomass materials considerable affect the solubilization efficiency of these materials. The differences in the content and composition of resulted hydrolysates can change the yield of the biofuel or target compound produced from these biomass hydrolysates. The more degraded organics containing hydrolysates can positively affect the yield of certain various value-added products; for instance, production of gaseous products by hydrothermal gasification

Alkali/acidic Requires harsh conditions; uses corrosive, hazardous chemicals; high costs of chemicals;

*Comparison of alkali, acidic and enzymatic biomass breakdown methods with subcritical water treatment.*

neutralization step; decomposes released sugars. Enzymatic Ineffective unless coupled with an acid treatment; high cost; time consuming.

formation of inhibitory byproducts; recovery problem of chemicals; requires a

Takes place in a safe solvent (water); no corrosive, hazardous, or toxic chemicals are needed; hydrolysis efficiency can be enhanced by operating temperature and pressure; use of recyclable heterogeneous catalysts can make the process more effective.

their smaller molecular weight components efficiently [29–31].

**Advantages and disadvantages**

toward favoring enzymatic hydrolysis [33, 34].

) and hydroxide ion (OH<sup>−</sup>) in these regions are almost 30 times

**8**

**Table 2.**

Although energy demands are continuous, biomass materials are seasonal. Some biomass feedstocks have advantages in terms of production, harvesting, storage, and transportation compared to others. Non-food biomass such as energy crops (switchgrass, miscanthus, kenaf, etc.) have advantages over food crops (corn, sugarcane, sugar beet, sweet sorghum, etc.). Perrenial energy crops such as switchgrass and miscanthus do not need to be replanted each year and they do not require special care and high maintenance to grow. On the other hand, agricultural biomass residues (corn stover, wheat straw, rice husk, crop peels, pulps, etc.) as promising low-cost feedstocks since they do not need additional land for biomass growth and the land used for agriculture belongs to these types of biomass materials. Forest biomass are also large source of materials for biofuels and other value-added products production. However, high costs of their harvesting and transportation limit their use. In addition to the advantage and disadvantage listed above, different sources of biomass feedstocks do not have same composition, uniform size and shape, etc., that considerable affect efficiency of conversion processes for a specific product. Therefore, biomass feedstocks for a bio-refinery needs to be standardized.
