Technology Descriptions
The following are descriptions of different bioenergy technologies:
Direct Combustion
Pile burners
Sometimes called “wet cells.” Burns solid materials in a
pile. Can accept a wide range of particle sizes. Simple in
construction and therefore relatively inexpensive. Can handle
stringy materials but has poor turndown. Also includes big bale
and “whole tree” burners. Can range in size from residential
stoves to several million Btuh.
Grate burners
Includes two kinds of grates: stationary and traveling. Requires particles
sufficient in size to not fall through the grate. Provides a method of cofiring.
Suspension burners
One method is to suspend the particles by centrifugal force in a cyclonic
burner. A second method is with an “air stoker” which blows the material into
the combustion chamber above a grate with combustion air. Requires small dry
particles so they will burn quickly while in suspension; therefore, sometimes
used with biomass pulverizers.
Fluidized bed burners
Fluidized bed burners consist of a bed of hot, inert small particles (such as
sand) that can be fluidized by gas injected from beneath the bed. Particles
injected into the bed or above the bed burn while suspended in the bed. Can
accept a wide range of particle sizes including stringy materials and has
infinite turndown, but is the most expensive burner system.
Cofiring
Usually means burning wood together with coal; however, it can be other biomass
materials and other fossil fuels. Typically cofiring is down either with a
commercial-scale grate burner or in a pulverized coal utility boiler. If in a
grate, particles must be big enough to not fall through the grate. If passed
through a coal pulverizer, wood particles must be 0.25-inch maximum.
Thermal gasification
Fixed bed
Fixed bed gasifiers include cross-flow, updraft, and downdraft, with the
latter the most common and producing the highest quality gas. Producer gas has
an energy content of roughly 150 Btu/cf; about 15% that of natural gas.
Fluidized bed
Fluidized bed gasifiers operate either without oxygen or with limited oxygen and
consist of a bed of hot, inert small particles (such as sand) that can be
fluidized by gas injected from beneath the bed. Can handle a range of biomass
particle sizes and shapes including mixtures.
Pyrolysis
Slow
Slow pyrolysis systems are basically thermal gasification systems that are
“tuned” to maximize charcoal yields, which are on the order of 25%. The
processes may be in the form of continuous furnaces, or closed vessels
(retorts), or kilns. Charcoal plants range from 50 to several hundred tons of
input per day. All charcoal in the US is made from wood and its primary markets
are for recreational cooking and activated carbon.
Fast
Fast pyrolysis systems are basically thermal gasification systems that are
“tuned” to maximize distillate (pyrolysis oil) yields, which are on the order of
60-70% with wood. Distillates have roughly 75,000 to 80,000 Btu/gal. The largest
commercial fast pyrolysis plants are on the order of 60-70 tons per day input.
Other
Thermal Depolymerization
A process that mixes biomass with water and reacts it at high pressures to
breakdown the biomass and create distillate products. One commercial plant has a
capacity of 200 tons per day of input.
Hydrolysis / Fermentation
Fermentation processes use micro-organisms to digest sugars in the biomass
to ethanol or other liquid fuel products. If the biomass is in the form starch
or cellulose, it must be broken down (hydrolyzed) into simple sugars compatible
with the micro-organisms. Most ethanol plants being constructed today are
between 50 and 100 million annual gallons of capacity.
Bio-gasification
A type of fermentation process, anaerobic digestion is a bio-gasification
process used to breakdown wastewaters or animal manures in slurry form. Produces
a biogas that is half carbon dioxide and half methane, and has roughly 500-700
Btu/cf (versus natural gas at 1000 Btu/cf). Can substitute for natural gas.
Chemical / Esterification
In this context, usually refers to reacting animal fats, waste greases, and
vegetable oils with ethanol or methanol catalysts to produce a liquid fuel
product (an ester or more commonly, biodiesel) than can be easily substituted
for diesel fuels. Biodiesel plants typically range from 3 to 10 million gallons
per year annual capacity.
Mechanical
Pellets
Biomass materials that are ground to a dimension less than the pellet
diameter can be compressed into pellets with ring-die machines. Typically sold
in bags and used for residential heating and typically has dimensions of
0.25-inch to 0.5-inch. Not cost effective if the material is not already at
least already partially ground up. Pellet plants typically produce 2-5 tons per
hour. Virtually all pellets are made from wood.
Briquettes / Pressed Logs
Briquettes and pressed logs are formed by compressing the ground up biomass
material through a die. Typically used for fireplaces or campfires although
briquetting sometimes done to increase biomass density for transportation.
Briquette and pressed log plants typically produce 2 to 5 tons per hour.
Emerging Conversion Technologies
Ethanol from Cellulose: Hydrolysis
Uses either acid or enzymes to break down cellulose based materials (e.g.,
trees, grasses, MSW) into simple sugars which can be fermented into ethanol or
liquid fuel products. Usually requires a mild acid pretreatment process to
expose the cellulose. The Iogen Company’s enzyme-based process is closest to
commercialization.
Ethanol from Cellulose: thermal gasification
One method thermally gasifies the biomass materials, and then passes the
gases and vapors across a catalyst which converts them into ethanol. A second
method thermally gasifies the biomass materials, and then passes the gases and
vapors through a fermentation tank where specialized micro-organisms convert the
biomass into ethanol.
Liquefaction
Dimethyl ether (DME)
Although commercial in other parts of the world, no DME is produced in the
US.
Fisher-Tropisch
Fisher Tropisch process first gasifies the biomass and then passes the gas
across a catalyst where it is converted into liquids. These liquids can be used
for chemical feedstocks or used either by themselves or in blends with fossil
diesel to fuel diesel engines.
Whole Tree Burners
Part of an overall system for harvesting whole trees, transporting these
whole trees to a storage pile under an air inflated dome at the power plant for
drying, then burning the whole segments of the trees with lengths up to 20 feet
at time in a giant pile burner. Prototypes of the burners have been built and
tested in Minnesota. The trees are typically grown as short rotation crops, so
that they have relatively long stems with small branches, which allows them to
be easily managed in stacks.
Stirling engines
Thermal-gasification to recip engines |
