Second-generation biofuels are biofuels produced from non-food-based feedstocks. These feedstocks include non-edible oils (or fats), lignocellulosic biomass and waste, and sugar and starch crop residues. The second-generation advanced liquid biofuels include bioethanol, biojet, biobutanol, bio-oil, dimethyl-ether, biomethanol, and biodiesel. Some of these advanced biofuels are characterized either by their ability to be used in existing internal combustion engines or blend with existing petroleum fuels.

Different pathways are available for second-generation liquid biofuel production. The pathways include biochemical fermentation, transesterification/esterification, and pyrolysis.

Biochemical fermentation

Biochemical fermentation is applied to convert starch and sugar-type agricultural residues into bioethanol or biobutanol. The yield of bioethanol is dependent on the structure of the biomass, method of pretreatment, and sugar type (five or six-carbon sugars). It also depends on the sugar concentration in the hydrolysates, conditions for the fermentation process, and the type of microorganisms.

Integrating fermentation and hydrolysis is a vital step in achieving high ethanol yield and process sustainability. The primary steps for producing bioethanol from lignocellulosic biomass include pretreatment and/or detoxification, hydrolysis, fermentation, and product recovery. To generate a slurry prior to pretreatment, crushing the feedstock and mixing it with water is needed.

The type of feedstock has a significant impact on the selection of suitable pretreatment technology. Carbohydrate biochemical fermentation resulting from biomass hydrolysis occurs in the presence of specific microorganisms. These microorganisms can be yeast or bacteria and the reaction occur at temperatures of around 38 °C with either a continuous, fed-batch, or batch mode bioreactor.

Microorganism selection for biochemical fermentation depends on the reactor configuration, fermentation conditions, and pretreatment conditions. The major advantage of biochemical fermentation is that the route is well-developed. Also, the acid/enzymatic step allows for the diversification of feedstock. The main disadvantage is the unavailability of microorganisms that can convert both types of sugars.

DuPont Danisco, a joint venture between DuPont and Danisco converts non-food biomass into cellulosic ethanol.

Gevo produces advanced biofuels and renewable chemicals via fermentation of biomass. The main material produced by the company is isobutanol.

Green Biologics, a UK-based energy company produces biobutanol through fermentation of sugars obtained from the cellulosic pre-treatment process.

Transesterification/esterification

Transesterification/esterification reaction is employed to produce biodiesel. In the transesterification reaction, the oil (free fat acids (FFA) and triglycerides) reacts with an alcohol (usually methanol) in the presence of a catalyst. The products are glycerol or water and alkyl esters (fatty acid methyl ester (FAME)).

Alkaline catalysts (NaOH and KOH) are the most commonly used catalysts. However, these catalysts need less FFA and water in the oil. Thus, feedstock properties limit transesterification. Transesterification is unsuitable for feedstock with a large amount of free fatty acids such as waste cooking oil.

Therefore, to convert waste cooking oils to biodiesel, more catalytic routes have been developed. These routes include enzymes, acid catalysts, and heterogeneous catalysts. Several researchers have investigated the use of biological waste-derived heterogeneous catalysts due to their environmental benefits, non-toxicity, water tolerance, and high activity.

Other researchers have focused on completely eliminating catalysts by using supercritical fluids. This includes direct supercritical transesterification and subcritical hydrolysis for lipid extraction integrated into supercritical esterification.

The advantage of supercritical transesterification and subcritical hydrolysis is that they have a high tolerance for FFA and water. This makes these routes more desirable for wet biomass (algae) and waste cooking oils.

Gushan Environmental Energy, a renewable energy company in China, produces biodiesel using a variety of feedstocks like waste cooking oil, animal fat, and vegetable oil.

Pyrolysis

Pyrolysis is the thermal decomposition of organic matter in the complete absence of oxygen or presence of inert nitrogen. The temperature range for pyrolysis is between 280 ℃ and 850 °C. However, the temperature requirement of pyrolysis depends on the type of pyrolysis process involved, the desired products, and the nature of the feedstock.

Pyrolysis can be classified into various categories depending on the residence time of the vapor phase and heating rate.

Slow pyrolysis refers to a batch process that is characterized by a long vapor residence time of around 5 to 30 mins. Slow pyrolysis is also defined by low reactor temperatures ranging from 300 to 500 °C and slow heat transfer rates up to 2 °C/s.

Intermediate pyrolysis takes place around heat transfer rates up to 1000 °C/s and a temperature range between 300 and 500 °C.

Intermediate pyrolysis may sometimes be considered semi-continuous.

Fast pyrolysis refers to a continuous process and needs larger heat transfer rates greater than 1000 °C/s. It also requires a shorter vapor residence time of around 1 s, and a relatively high-temperature range between 450 ℃ to 600 °C. Additionally, fast pyrolysis requires feedstock drying. On the other hand, slow and intermediate pyrolysis processes can work with higher moisture feedstocks.

Although fast pyrolysis can produce high bio-oil yields of around 75 wt.% expressed on a biomass basis, intermediate pyrolysis can produce a better-quality bio-oil (low viscosity and tar yield). Instead of inert nitrogen, hydrogen can be used for pyrolysis.

Hydro-pyrolysis occurs under a pressurized hydrogen atmosphere. The result is the formation of fewer compounds in the bio-oil with higher selectivity and reduction of unsaturated hydrocarbons. Pyrolysis in the presence of nitrogen may lead to random cracking. However, the presence of hydrogen prevents it and facilitates the systematic bond cleavage in the biomass structure. This increases the quality of the produced bio-oil.

Pressurized water may be utilized to achieve biomass thermochemical conversion into bio-oil through hydrothermal liquefaction. The usual operating temperatures of hydrothermal liquefaction are between 250 ℃ and 374 °C and the operating pressures range from 4 to 22 MPa.
Microwave pyrolysis is another pyrolysis mode with a high potential for advanced biofuel production. The basic difference is that microwaves are used to heat the biomass particles from within the reactor system. No external high-temperature heat source is needed for heat transfer.

Several companies are already employing pyrolysis and gasification to produce biofuels. Chemrec, a Swedish energy company, uses gasification of waste from paper mills to produce syngas. The syngas is then converted to biofuels.

Future research directions

The availability of feedstock determines the process feasibility for each of the pathways described above. Pathways that allow the use of diversified feedstocks has a vital advantage over other pathways. Pyrolysis allows the use of diversified feedstocks from the household, food, and municipal wastes and sewage sludge to animal manures, including biomass residues from forest and agricultural activities.

Future research on the process integration of biochemical fermentation and hydrolysis is needed to achieve product recovery process and process feasibility. The research should be focused on improving the efficiency and economic feasibility of supercritical transesterification and subcritical hydrolysis to make them viable for commercial applications.