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Click link to expand for a text description of Figure 8.5
Diagram examines bioenergy conversion technologies of various different energies. F stands for Feedstock. PP stands for Preprocessing, C stands for conversion and PEP stands for primary energy product.
Thermal Conversion
F: Lignocellulose (all sources)
PP: Densification
C: Combustion
PEP: Power, Heat, Steam
Chemical Conversion
F: N/A
PP: N/A
C: Transesterification
PEP: Bio-diesel
Thermochemical Conversion
F: Lignocellulose (all sources)
PP: N/A
C: Torrefaction
PEP: Bio-coal
C: Gasification
PEP: Syngas
C: Pyrolysis
PEP: Charcoal, methanol, syngas, bio-oil
Biochemical conversion
F: Lignocellulose (all sources)
PP: Cellulose to Sugars
C: Fermentation
PEP: Ethanol
F: Sugars & Starches (Agricultural Crops)
PP: N/A
C: Fermentation
PEP: Ethanol
F: Land Fill gas & Biogas
PP: N/A
C: Anaerobic Digestion
PEP: Pipeline quality gas, CNG, LNG
A feedstock is "any renewable, biological material that can be used directly as a fuel, or converted to another form of fuel or energy product." According to the Office of Energy Efficiency and Renewable Energy, "biomass feedstocks are the plant and algal materials used to derive fuels like ethanol, butanol, biodiesel, and other hydrocarbon fuels".
There are two basic categories of biomass material: woody & .... non-woody! "Lignocellulose" is woody biomass. (For an excellent description and discussion, see Sources of biomass from the Wisconsin Grasslands Bioenergy Network. This is not required reading.)
From the Environmental and Energy Study Institute, here is a list of some of the most "common (and/or most promising)" biomass feedstocks--
- Grains and starch crops – Sugar cane, corn, wheat, sugar beets, industrial sweet potatoes, etc.
- Agricultural residues – Corn stover, wheat straw, rice straw, orchard prunings, etc.
- Food waste – Waste produce, food processing waste, etc.
- Forestry materials – Logging residues, forest thinnings, etc.
- Animal byproducts – Tallow, fish oil, manure, etc.
- Energy crops – Switchgrass, miscanthus, hybrid poplar, willow, algae, etc.
- Urban and suburban wastes – Municipal solid wastes (MSW), lawn wastes, wastewater treatment sludge, urban wood wastes, disaster debris, trap grease, yellow grease, waste cooking oil, etc.
Logistics
Feedstock logistics encompass all of the operations necessary to harvest the biomass and move it to the conversion reactor at the biorefinery (or the heat and/or electricity generation facility), including the processing steps necessary to ensure that the delivered feedstock meets the specifications of the biorefinery conversion process. A biorefinery is where "biomass is upgraded to one or more valuable products such as transport fuels, materials, chemicals, electricity and, as a byproduct, heat" (Source: "What is a Biorefinery?" by Bernstsson, Snadén, Olsson, and Åsblad. This article provides an excellent explanation of various biorefining processes if you are so inclined!). Conventionally, facilities that generate electricity and/or heat through direct thermal conversion (combustion) are not considered biorefineries unless they first convert the biomass into a "novel" material like biogas. In the chart at the top of this page, biorefineries are used in all technologies except for thermal conversion.
Logistics includes harvest and collection, preprocessing, storage and queuing, handling and transportation, and is used in all four of the technologies in the chart above.
In its natural form, most biomass is bulky, relatively wet, and due to its low bulk-density, costly to transport. Preprocessing includes production steps, like chipping, grinding, compacting and drying, that turn biomass into what is properly called feedstock.
Biomass densification is the compression or compaction of biomass to reduce its volume per unit area. Densification is used for solid fuel applications (e.g., pellets, briquettes, logs). Drying biomass improves the grinding process, and results in smaller more uniform particles of biomass.
For cellulosic biomass, mechanical (e.g., crushing) and thermochemical (e.g., hydrolysis) pretreatments are necessary.
Many herbaceous feedstocks, for example, corn stover, are only harvested over a few weeks during the year in the U.S. Corn Belt. To maintain a continuous supply of this feedstock to biorefineries, storage is required. Biological degradation can reduce the amount of biomass available for bioenergy production and also impact the conversion yield, by altering biomass chemical composition.
Unprocessed biomass leaving the field or forest is often bulky, aerobically unstable, and has poor flowability and handling characteristics. These traits can make raw biomass handling and transportation inefficient. Transport can be expensive, especially as distance increases.
The video below from the U.S. Department of Energy may help you visualize some of the processes involved with harvesting and using various feedstocks. Please watch the following (3:39) video:
PRESENTER: Nearly a billion dollars a day-- that's how much we spend on oil imports in the US. Oil that powers our nation's transportation systems and industries.
But here's something to think about-- a strong biofuels industry could meet much of our demand. Biofuels are made from organic materials, or biomass, grown in our own fields and forests. A booming biofuels industry would also keep a lot of the money we spend on imported oil in the country-- plus it would reduce our dependence on foreign oil, and create jobs in rural America.
In fact, we can use homegrown biomass to replace or supplement almost every product that comes from a typical barrel of crude oil. These are things like gasoline, diesel, jet fuel, and other consumer products, like plastics. Much of our imported oil could be replaced with sustainable, renewable biofuels and products made in the USA.
Check this out. This is the Billion-Ton Update Study by the US Department of Energy. This study found that potential biomass resources could produce about 85 billion gallons of biofuels a year-- that's about a third of the oil we use.
OK, so what kinds of plant materials or feedstocks can be converted to fuels? And, where will they come from? America is already using biomass that comes from agriculture and forest operations across the country. These are non-food plants grown specifically for energy. American farms all across the US can produce a wide variety of energy crops. These are plants that are grown because of their high energy content-- crops like switchgrass or fast-growing hybrid poplar trees.
And energy crops can also be grown on marginal, degraded, or underused agricultural land, helping farms expand and become more productive. Agricultural waste can even be converted into biofuels.
Look at this-- farmers can gather and sell corn stalks and wheat straw to be converted to biofuels, making their lands even more profitable. This is non-edible plant material left over from crop harvests that's been collected from farm land instead of going to waste.
So, how do you take plants and make them into fuels and other products? No matter what kind of plant you start with, the first steps are to break them down. The US Department of Energy, partnering with private industry, is making these steps a lot more efficient and affordable. Together, they're developing new machinery and processes specific to the various biomass crops.
This equipment is harvesting, baling, grinding, and condensing these raw plants into energy-ready materials-- materials like these energy dense pellets, ready for the biorefinery. From there, energy-ready biomass feedstocks are transported to one of many biorefineries sprouting up in communities across the country. Here, they can be further broken down, converted into biofuels, and made ready for use.
Homegrown biomass feedstocks-- creating jobs in rural America, generating clean renewable fuels, and reducing our dependence on foreign oil.
Conversion
The figure at the top of this page shows four categories of biomass processing: thermal, thermochemical, biochemical, and chemical. This is a very helpful starting place for understanding the different inputs and outputs associated with biomass.
To Read Now
Visit the the Wisconsin Grasslands Bioenergy Network and read closely, Bioenergy Conversion Technologies.