If you have been spending any amount of time reading about alternative energy sources, you are sure to have read about biomass fuels and their potential to replace conventional liquid fuels. However, what exactly are biomass fuels?
Biomass fuels are derived from recent biological origin feedstocks instead of fossil fuels. They have considerable potential to replace fossil fuels in settings where liquid or gaseous fuels are desirable and come in first, second, and third generations differentiated by feedstock.
With the proven role of fossil fuels in driving global climate change, biomass fuels offer a carbon-neutral alternative with the advantages of liquid fuels. However, they also have their drawbacks, particularly first-generation biofuels.
We will examine the following topics:
- What Biomass Fuels Are
- The Characteristics and Applications of The Different Types of Biomass Fuel
- What The Advantages and Disadvantages of Biomass Fuels Are
There are many interesting angles to this highly diverse group of fuels, so let us examine them in detail to gain a better understanding of how they can provide energy security for our collective future.
What Is Biomass And What Are Biomass Fuels?
Biomass fuels are fuels of recent biological origin, derived from living or recently-living organisms.
Biomass is organic material originating from living or recently-living organisms, such as plants and algae. These organisms trap solar energy during photosynthesis and convert water and carbon dioxide into energy.
The solar energy locked up in these organisms as biomass can be transformed into usable energy by humans through direct means such as burning it or converting it into electricity, or by indirect means such as processing it into liquid or gaseous biofuels.
Burning basic biomass fuels such as wood is something humans have done for as long as humans have used fire.
However, this sort of biomass fuel is highly inefficient, and modern biomass fuels are usually generated from waste or made to be used as liquid or gaseous biofuels.
The Advantages Of Biofuels For The Transportation Sector
Other renewable energy sources such as solar, wind, and water energy are useful for generating electricity or as energy sources for residential, commercial, or industrial applications. However, none of these sources are appropriate for the transportation sector.
Biofuels are liquid fuels designed primarily for use in transportation. They are ‘drop-in’ fuels, which can be used in existing fuel infrastructure without modifying it.
With growing concerns about the role of fossil fuels in causing global climate change and a desire to decrease reliance on imported fossil fuels, many governments are looking to displace fossil fuels with renewable biofuels produced from biomass.
The US federal government has established a target of displacing 30% of current domestic gasoline consumption in the transport sector with biofuels by 2030.
Current consumption in the transport sector is approximately 200 billion gallons per year; the United States must develop a commercial infrastructure to produce about 60 billion gallons of biofuel per year.
China, Australia, New Zealand, and the European Union have set similar targets for biofuel production.
The Promise And The Pitfalls Of Biomass Fuels
An additional incentive to displace fossil fuels with biomass fuels is the depletion of fossil fuel stocks. The world’s fossil fuels are rapidly depleting, and we will need something to take their place.
Biomass energy can create a lot of rural infrastructure and rural jobs, revitalizing impoverished communities.
It also has the potential to be used for rehabilitating degraded land. Plants can be planted on degraded land, and not only do those plants help the area regenerate but they can also be used as feedstocks for biomass fuels.
One problem with biomass fuels, at least the first generation or conventional biofuels, is the dilemma of using food crops for generating fuel. Doing so can divert food from hungry mouths or create a conundrum for farmers supplying biofuel refineries as food prices increase.
An additional problem hindering their implementation is that the price of fossil fuels discounts their environmental costs. However, biomass fuels are increasingly being adopted.
The raw biomass used to create biomass fuel is known as a feedstock.
Different biomass feedstocks include:
- Dedicated energy crops
- Wood and wood processing wastes
- Food crops and crop residues
- Municipal solid waste [MSW]
- Wet waste
When discussing biofuels, as a subset of biomass fuels, a key concept is generation. Different generations of biofuels are distinguished from each other based on feedstock.
The different generations are as follows:
First Generation Biofuels
First-generation biofuels use food crops as feedstocks, such as corn, soy, canola, sugar beets, and sugarcane.
First-generation biofuel technology is well-established and integrated into the market, with approximately 97% of gasoline sold in the United States being blended with corn ethanol.
Most gasoline sold in the US is E10 (10% ethanol). Some vehicles, known as flexible-fuel vehicles, can run on E85, which contains anything from 51 to 83% ethanol depending on season and geography.
Conventional or genetically modified yeasts such Saccharomyces cerevisiae are used to ferment C6 sugars in biomass high in carbohydrates, such as corn, wheat, or sugar cane, which generates ethanol. An acre of corn will yield about 400 gallons of ethanol.
In a process known as trans-esterification, ethanol (or other alcohols such as methanol) is combined with fats such as animal fat, vegetable oil, or recycled cooking oil resulting in the formation of biodiesel.
Biodiesel can be used on its own (B100) or blended with petroleum diesel (the most commonly used blend being 20% biodiesel or B20).
The two most widely used biofuels are ethanol and biodiesel.
Second Generation Biofuels
Second-generation biofuels use non-food biomass as feedstocks. These feedstocks include dedicated energy crops, agricultural residues, wood processing residues, municipal waste, and used cooking oil.
Second-generation biofuel technology is less well-established, as many of the feedstocks are high in lignocellulosic biomass. The processes to convert this to usable fuel are more complex than first-generation biofuels.
Lignocellulosic materials, the complex matrix of lignin, pectin, and polysaccharides forming plant cell walls, are more challenging to convert into fermentable sugars than grain-based feedstocks such as corn, being recalcitrant to being depolymerized.
Two significant approaches being researched are enzyme depolymerization and pyrolysis (a controlled burning process).
Enzymatic depolymerization isolates cellulose and then breaks it down into sugars, then fermented to ethanol.
Pyrolysis generates solid biomass fuel called biochar, pyrolytic oil, and syngas. Increasing temperatures lead to higher proportions of syngas. Syngas can be used to produce ethanol and diesel.
Extensive research has been carried out on their potential environmental and economic advantages, and the United States currently operates several commercial-scale cellulosic ethanol biorefineries.
Third Generation Biofuels
Third-generation biofuels are produced from algae, whether seaweed, microalgae, or cyanobacteria.
Microalgae such as Chlorella have very high productivity and lipid content of 60 to 70%. These lipids can be used to produce liquid biofuels by trans-esterification to produce biodiesel, or hydrogenolysis can generate kerosene-grade alkane that can be used for aviation.
Fourth Generation Biofuels
Fourth-generation biofuels use genetically modified algae for enhanced production of biofuel. They are still highly experimental, as there are considerable health risks and risks of environmental contamination associated with their use.
What Are The Different Biomass Feedstocks?
The different biomass feedstocks are dedicated energy crops, wood and wood processing wastes, food crops and crop residues, algae, municipal solid waste [MSW], and wet waste.
Let us look at these different biomass feedstocks, as they vary enormously in type.
Dedicated Energy Crops
Dedicated energy crops are non-food crops explicitly grown to provide biomass for fuel. These crops can be grown on land marginal for food crops.
There are two broad categories: herbaceous and woody.
Herbaceous energy crops are perennial grasses that can be harvested annually after an initial 2 to 3 year growth period to reach full productivity. These crops include:
- Tall fescue
- Sweet sorghum
Woody energy crops are fast-growing hardwood trees. These are harvested within 5 to 8 years of planting. These crops include:
- Hybrid willow
- Hybrid poplar
- Eastern cottonwood
- Black walnut
- Silver maple
- Green ash
Wood And Wood Processing Residues
Wood can be harvested from many different types of trees, which influences whether or not it is a good option for biomass fuel.
Wood collected from old-growth forests or slow-growing species of tree is environmentally unfriendly, whereas wood harvested from woodlots of fast-growing species planted explicitly for use as firewood is more sustainable.
Wood from commercial plantations of fast-growing softwoods, or wood harvested by foresters to improve forest health, also provides a sustainable option.
Wood need not be burned as firewood; it can also be processed into small particles and burned in fluidized bed reactors (where gas is passed through a bed of solid material at high enough velocities to fluidize it).
The processing of wood for pulp and lumber yields:
- leaves or needles
- unused branches
These waste streams are collectively called wood processing residues.
Such wood processing residues can be burned in incinerators or fluidized bed reactors.
Food Crops And Crop Residues
First-generation biofuels are made from food crops such as corn and soy, which are used as feedstocks for ethanol, and hence for biodiesel. However, this leads to the ‘food versus fuel’ conundrum.
There are ways to use agricultural crop residues for feedstock. Examples of crop residues are:
- Wheat straw
- Barley straw
- Oat straw
- Rice straw
- Sorghum stubble
- Corn stover (stalks, leaves, cobs, and husks)
- Bagasse (sugarcane waste)
The term ‘algae,’ when used of biomass feedstocks, refer to a range of different photosynthetic organisms, including:
- Macroalgae (seaweed)
- Cyanobacteria (previously called blue-green algae)
The products of their photosynthesis, namely carbohydrates, lipids, and proteins, can be converted into a range of different biofuels.
Municipal Solid Waste (MSW)
Municipal solid waste feedstocks include residential and commercial garbage, such as:
- Paper and cardboard
- Food wastes
- Yard trimmings
These feedstocks are burned in incineration plants to generate electricity.
Wet waste feedstocks include:
- Food wastes
- Organic wastes from industrial operations
- Treated sewage sludge from municipal wastewater
- Manure and manure slurries from concentrated livestock farming
Wet waste is put into biodigesters to break down, yielding biogas (composed mainly of methane), which can be packaged into gas cylinders for domestic cooking applications or burned to generate electricity.
What Are The Advantages And Disadvantages Of Biomass Fuels?
Biomass fuels have distinct advantages over fossil fuels, such as carbon neutrality (most of them) and sustainability (being renewable fuels). However, they also have disadvantages; for example, land used for growing fuel is unavailable for growing food.
The exact breakdown of advantages and disadvantages depends on the type of biomass fuel.
|Type Of Biomass Fuel||Advantages||Disadvantages|
|Dedicated Energy Crops||Reduced emissions Improve wildlife habitat Improve soil and water quality Improve farm productivity Diversify sources of income Increase soil carbon Require less fertilizer and water than food crops Less expensive to buy than food crops||Lignocellulosic material composing the cell walls of these feedstocks is resistant to depolymerization. It can be difficult to transport large quantities of biomass to processing facilities. More expensive to process than food crops.|
|Wood And Wood Processing Residues||Reduced emissions. The burning of wood processing residues means that otherwise wasted resources can be used. Harvesting of excess woody biomass reduces the risk of disease and fire.||Burning wood as firewood is a highly inefficient means of generating energy.|
|Food Crops And Crop Residues||Reduced emissions (but see disadvantage 1). The use of crop residues means that otherwise wasted parts of a crop can be used. The sale of crop residues provides another stream of income for farmers .||Using fossil fuels while growing food crops means that these biofuels still contribute substantially to climate change. Monocultures are environmentally unfriendly. Pesticide use creates an environmental burden. Food crops used for fuel reduce the amount of food available to feed people.|
|Algae||Reduced emissions. Highly-efficient fuels can be produced. Depending on the strain, it can be grown in any type of water from any source. Can be grown in wastewater from industry, agriculture, or aquaculture. Fast-growing.||Difficult and expensive to control the environment for optimal growth. Maintenance of strict environmental control requires expensive equipment and facilities. Large quantities of liquid water are required to grow algae. Water must be removed from the algae before it can be processed.|
|Municipal Solid Waste [MSW]||Reduced emissions. Reduced volume of waste going to landfills.||It cannot be used for liquid fuels.|
|Wet Waste||Reduced emissions. Waste disposal problems can be solved. Additional revenue streams for rural economies can be created.||It cannot be used for liquid fuels.|