11.3 Renewable energy and biofuels

Renewable energy and biofuels are crucial for reducing our reliance on fossil fuels. This section covers various biofuel types, including alcohol-based, biodiesel, and gaseous fuels, as well as advanced options like algal biofuels and drop-in alternatives.

Biofuel production involves different biomass feedstocks and biorefinery processes. From first-generation crops to algae, these feedstocks are converted into biofuels through biochemical or thermochemical methods, often in integrated biorefineries that maximize efficiency and product diversity.

Biofuel Types

Alcohol-based Biofuels

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• Bioethanol produced from fermentation of sugars or starches by microorganisms (yeast, bacteria)
• Cellulosic ethanol derived from cellulose in plant cell walls, requires pretreatment to break down cellulose into fermentable sugars
• Biobutanol has higher energy density than ethanol, can be used in existing gasoline engines without modification

Biodiesel and Renewable Diesel

• Biodiesel produced by transesterification of vegetable oils or animal fats with an alcohol (methanol), creating fatty acid methyl esters (FAME)
• Renewable diesel produced by hydrotreating vegetable oils or animal fats, resulting in a hydrocarbon fuel chemically similar to petroleum diesel
• Both can be used in diesel engines with little to no modifications, reduce emissions compared to petroleum diesel

Gaseous Biofuels

• Biogas produced by anaerobic digestion of organic matter (manure, sewage, food waste) by bacteria, consists primarily of methane and carbon dioxide
  • Can be used directly for heating and electricity generation or upgraded to biomethane by removing CO2 and impurities
• Biohydrogen produced by fermentation of sugars by bacteria or algae, or by gasification of biomass
  • Challenges include low yields, storage, and distribution infrastructure

Advanced Biofuels

• Algal biofuels produced from microalgae, which can be grown in ponds or photobioreactors
  • High oil content, do not compete with food crops for land, can utilize wastewater or industrial CO2 emissions
• Drop-in biofuels are functionally equivalent to petroleum fuels, can be used in existing infrastructure without blending limits
  • Examples include bio-based gasoline, jet fuel, and diesel produced from biomass via advanced conversion technologies (pyrolysis, gasification, syngas fermentation)

Biofuel Production

Biomass Feedstocks

• Biomass is organic matter used as a feedstock for biofuel production, includes crops, residues, and wastes
• First-generation feedstocks are sugars and starches from food crops (corn, sugarcane, soybeans)
  • Controversial due to competition with food production and land use changes
• Second-generation feedstocks are lignocellulosic biomass (agricultural residues, wood, grasses), do not compete with food
• Third-generation feedstocks are algae, which can be grown on non-arable land and utilize waste streams

Biorefinery Processes

• Biorefineries convert biomass into biofuels and other products (chemicals, materials, power) using various conversion technologies
• Biochemical conversion uses enzymes and microorganisms to break down biomass into sugars for fermentation
  • Pretreatment is required to break down lignocellulose into accessible sugars, methods include dilute acid, steam explosion, and enzymatic hydrolysis
• Thermochemical conversion uses heat and catalysts to convert biomass into intermediate products (syngas, bio-oil) which can be upgraded into biofuels
  • Processes include pyrolysis (heating in the absence of oxygen), gasification (partial oxidation to produce syngas), and hydrothermal liquefaction (converting wet biomass into bio-oil)
• Biorefineries can be integrated with existing industrial facilities (sugar mills, pulp and paper mills) to improve efficiency and economics