Biomass power plants: operation, advantages and disadvantages
A biomass energy plant is a facility that generates electricity or heat by
using organic materials as fuel. These materials, known as biomass, can include
wood, agricultural waste, or even algae. Instead of letting these resources go
to waste, they are burned or processed to produce energy. This form of energy
is renewable, as the materials used can be replenished naturally. Biomass
energy plants offer a more sustainable alternative to fossil fuels because they
help reduce carbon emissions and utilize locally available resources. This
makes them an environmentally friendly option for power generation.
Biomass explained—renewable energy from plants and animals
Biomass is renewable organic
material that comes from plants and animals. Biomass can be burned directly for
heat or converted to liquid and gaseous fuels through various processes.
Biomass was the largest source of
total annual U.S. energy consumption until the mid-1800s. In 2023, biomass
accounted for about 5% of U.S. total primary energy consumption. Biomass is
used for heating and electricity generation and as a transportation fuel.
Biomass is an important fuel in many countries, especially in developing
countries for cooking and heating.
Biomass sources for energy
include:
Wood and wood processing
waste—firewood, wood pellets, and wood chips, lumber and furniture mill sawdust
and waste, and black liquor from pulp and paper mills
Agricultural crops and waste
materials—corn, soybeans, sugar cane, switchgrass, woody plants, algae, and
crop and food processing residues, mostly to produce biofuels
Biogenic materials in municipal
solid waste—paper products; cotton and wool products; and food, yard, and wood
wastes
Animal manure and human sewage
for producing biogas (renewable natural gas)
Biomass can be converted to energy in different ways
Biomass is converted to energy
through various processes, including:
· Direct
combustion (burning) to produce heat
· Thermochemical
conversion to produce solid, gaseous, and liquid fuels
· Chemical
conversion to produce liquid fuels
· Biological
conversion to produce liquid and gaseous fuels
Direct combustion is the most common method for converting biomass to useful energy. All biomass can be burned directly for heating buildings and water, for providing industrial process heat, and for generating electricity in steam turbines.
Thermal Conversion
Biomass can be burned by thermal conversion and used for energy. Thermal conversion involves heating the biomass feedstock in order to burn, dehydrate, or stabilize it. The most familiar biomass feedstocks for thermal conversion are raw materials such as municipal solid waste (MSW) and scraps from paper or lumber mills.Different types of energy are created through direct firing, co-firing, pyrolysis, gasification, and anaerobic decomposition.
Before biomass can be burned, however, it must be dried. This chemical process is called torrefaction. During torrefaction, biomass is heated to about 200° to 320° Celsius (390° to 610° Fahrenheit). The biomass dries out so completely that it loses the ability to absorb moisture, or rot. It loses about 20 percent of its original mass, but retains 90 percent of its energy. The lost energy and mass can be used to fuel the torrefaction process.
Direct Firing and Co-Firing
Most briquettes are burned
directly. The steam produced during the firing process powers a turbine,
which turns a generator and produces electricity. This electricity
can be used for manufacturing or to heat buildings.
Biomass can also be co-fired, or burned with a fossil fuel. Biomass is
most often co-fired in coal plants. Co-firing eliminates the need for
new factories for processing biomass. Co-firing also eases the demand for coal.
This reduces the amount of carbon dioxide and other greenhouse gases released
by burning fossil fuels.
Pyrolysis
Pyrolysis is a related method of
heating biomass. During pyrolysis, biomass is heated to 200° to 300° C (390° to
570° F) without the presence of oxygen. This keeps it from combusting and
causes the biomass to be chemically altered.
Pyrolysis produces a dark liquid
called pyrolysis oil a synthetic gas called syngas,
and a solid residue called biochar. All of these components can be used
for energy.
Pyrolysis oil, sometimes called
bio-oil or biocrude, is a type of tar. It can be combusted to generate
electricity and is also used as a component in other fuels and plastics.
Scientists and engineers are studying pyrolysis oil as a possible alternative
to petroleum.
Syngas can be converted into fuel
(such as synthetic natural gas). It can also be converted into methane and
used as a replacement for natural gas.
Biochar is a type of charcoal. Biochar is a carbon-rich solid that is
particularly useful in agriculture. Biochar enriches soil and
prevents it from leaching pesticides and other nutrients
into runoff. Biochar is also an excellent carbon sink. Carbon sinks
are reservoirs for carbon-containing chemicals, including greenhouse gases.
Gasification
Biomass can also be directly
converted to energy through gasification. During the gasification process, a
biomass feedstock (usually MSW) is heated to more than 700° C (1,300° F) with a
controlled amount of oxygen. The molecules break down, and produce syngas
and slag.
Syngas is a combination of hydrogen and carbon monoxide. During gasification,
syngas is cleaned of sulfur, particulates, mercury, and other pollutants.
The clean syngas can be combusted for heat or electricity, or processed into
transportation biofuels, chemicals, and fertilizers.
Slag forms as a glassy, molten liquid. It can be used to make
shingles, cement, or asphalt.
Industrial gasification plants
are being built all over the world. Asia and Australia are constructing and
operating the most plants, although one of the largest gasification plants in
the world is currently under construction in Stockton-on-Tees, England. This
plant will eventually be able to convert more than 350,000 tons of MSW into
enough energy to power 50,000 homes.
Anaerobic Decomposition
Anaerobic decomposition is the
process where microorganisms, usually bacteria, break down material
in the absence of oxygen. Anaerobic decomposition is an important process
in landfills, where biomass is crushed and compressed, creating an
anaerobic (or oxygen-poor) environment.
In an anaerobic environment,
biomass decays and produces methane, which is a valuable energy
source. This methane can replace fossil fuels.
In addition to landfills, anaerobic decomposition can also be implemented
on ranches and livestock farms. Manure and other
animal waste can be converted to sustainably meet the energy needs of the farm.
Biofuel
Biomass is the only renewable energy source that can be converted into liquid biofuels such as ethanol and biodiesel. Biofuel is used to power vehicles, and is being produced by gasification in countries such as Sweden, Austria, and the United States.Biofuels do not operate as efficiently as gasoline. However, they can be blended with gasoline to efficiently power vehicles and machinery, and do not release the emissions associated with fossil fuels.
Biochar
Biochar, produced during pyrolysis, is valuable in agricultural and environmental use.Biochar is used in Brazil’s
Amazon rainforest in a process called slash-and-char. Slash-and-char
agriculture replaces slash-and-burn, which temporarily increases the soil
nutrients but causes it to lose 97 percent of its carbon content. During
slash-and-char, the charred plants (biochar) are returned to the soil, and the
soil retains 50 percent of its carbon. This enhances the soil and leads to significantly
higher plant growth.
Black Liquor
When wood is processed into paper, it produces a high-energy, toxic substance called black liquor. Until the 1930s, black liquor from paper mills was considered a waste product and dumped into nearby water sources.Hydrogen Fuel Cells
Biomass is rich in hydrogen, which can be chemically extracted and used to generate power and to fuel vehicles. Stationary fuel cells are used to generate electricity in remote locations, such as spacecraft and wilderness areas. Yosemite National Park in the U.S. state of California, for example, uses hydrogen fuel cells to provide electricity and hot water to its administration building.However, there is a debate as to whether this technology will become sustainable or economically possible. The energy that it takes to isolate, compress, package, and transport the hydrogen does not leave a high quantity of energy for practical use.
Biomass and the Environment
Biomass is an integral part of Earth’s carbon cycle. The carbon cycle is the process by which carbon is exchanged between all layers of Earth: atmosphere, hydrosphere, biosphere, and lithosphere.The carbon cycle takes many forms. Carbon helps regulate the amount of sunlight that enters Earth’s atmosphere. It is exchanged through photosynthesis, decomposition, respiration, and human activity. Carbon that is absorbed by soil as an organism decomposes, for example, may be recycled as a plant releases carbon-based nutrients into the biosphere through photosynthesis. Under the right conditions, the decomposing organism may become peat, coal, or petroleum before being extracted through natural or human activity.
Between periods of exchange, carbon is sequestered, or stored. The carbon in fossil fuels has been sequestered for millions of years. When fossil fuels are extracted and burned for energy, their sequestered carbon is released into the atmosphere. Fossil fuels do not reabsorb carbon.
In contrast to fossil fuels, biomass comes from recently living organisms. The carbon in biomass can continue to be exchanged in the carbon cycle.
In order to effectively allow Earth to continue the carbon cycle process, however, biomass materials such as plants and forests have to be sustainably farmed. It takes decades for trees and plants such as switchgrass to reabsorb and sequester carbon. Uprooting or disturbing the soil can be extremely disruptive to the process. A steady and varied supply of trees, crops, and other plants is vital for maintaining a healthy environment.
Algal Fuel | Algae Biomass Energy
Algae is a unique organism that has enormous potential as a source of biomass energy. Algae, whose most familiar form is seaweed, produces energy through photosynthesis at a much quicker rate than any other biofuel feedstock—up to 30 times faster than food crops!Algae can be grown in ocean water, so it does not deplete freshwater resources. It also does not require soil, and therefore does not reduce arable land that could potentially grow food crops. Although algae releases carbon dioxide when it is burned, it can be farmed and replenished as a living organism. As it is replenished, it releases oxygen, and absorbs pollutants and carbon emissions.
Algae takes up much less space than other biofuel crops. The U.S. Department of Energy estimates that it would only take approximately 38,850 square kilometers (15,000 square miles, an area less than half the size of the U.S. state of Maine) to grow enough algae to replace all petroleum-fueled energy needs in the United States.
Algae contains oils that can be converted to a biofuel. At the Aquaflow
Bionomic Corporation in New Zealand, for example, algae is processed with heat
and pressure. This creates a “green crude,” which has similar properties to
crude oil, and can be used as a biofuel.
Algae’s growth, photosynthesis, and energy production increases when carbon
dioxide is bubbled through it. Algae is an excellent filter that absorbs carbon
emissions. Bioenergy Ventures, a Scottish firm, has developed a system in which
carbon emissions from a whiskey distillery are funneled to an algae pool. The
algae flourishes with the additional carbon dioxide. When the algae die (after
about a week) they are collected, and their lipids (oils) are converted
into biofuel or fish food.
Algae has enormous potential as an alternative energy source. However, processing it into usable forms is expensive. Although it is estimated to yield 10 to 100 times more fuel than other biofuel crops, in 2010 it cost $5,000 a ton. The cost will likely come down, but it is currently out of reach for most developing economies.
Biomass of Algae
The third-generation biorefinery focuses on using algal biomass as a feedstock. Algal biomass contains proteins, lipids, and carbohydrates, making it versatile for producing various products. These include pigments, vitamins, and biofuels like biodiesel and biomethane.
The process involves cultivating, harvesting, and converting biomass into biofuels and chemicals. Techniques like membrane filtration help concentrate the biomass, though sustainable methods are still under study. While biofuel production is cost-competitive, coproducts offer additional potential.
People and Biomass
Advantages
Biomass is a clean, renewable
energy source. Its initial energy comes from the sun, and plants or algae
biomass can regrow in a relatively short amount of time. Trees, crops, and
municipal solid waste are consistently available and can be managed
sustainably.
If trees and crops are sustainably farmed, they can offset carbon emissions
when they absorb carbon dioxide through respiration. In some bioenergy
processes, the amount of carbon that is reabsorbed even exceeds the carbon
emissions that are released during fuel processing or usage.
Many biomass feedstocks, such as switchgrass, can be harvested on marginal
lands or pastures, where they do not compete with food crops.
Unlike other renewable energy sources, such as wind or solar, biomass energy is
stored within the organism, and can be harvested when it is needed.
Disadvantages
If biomass feedstocks are not
replenished as quickly as they are used, they can become nonrenewable. A
forest, for instance, can take hundreds of years to re-establish itself. This
is still a much, much shorter time period than a fossil fuel such as peat. It
can take 900 years for just a meter (three feet) of peat to replenish itself.
Most biomass requires arable land to develop. This means that land used for
biofuel crops such as corn and soybeans are unavailable to grow food or provide
natural habitats.
Forested areas that have matured for decades (so-called “old-growth forests”)
are able to sequester more carbon than newly planted areas. Therefore, if
forested areas are not sustainably cut, re-planted, and given time to grow and
sequester carbon, the advantages of using the wood for fuel are not offset by
the trees’ regrowth.
Most biomass plants require fossil fuels to be economically efficient. An
enormous plant under construction near Port Talbot, Wales, for instance, will
require fossil fuels imported from North America, offsetting some of the
sustainability of the enterprise.
Biomass has a lower “energy density” than fossil fuels. As much as 50 percent
of biomass is water, which is lost in the energy conversion process. Scientists
and engineers estimate that it is not economically efficient to transport
biomass more than 160 kilometers (100 miles) from where it is processed.
However, converting biomass into pellets (as opposed to wood chips or larger
briquettes) can increase the fuel’s energy density and make it more
advantageous to ship.
Burning biomass releases carbon monoxide, carbon dioxide, nitrogen oxides, and
other pollutants and particulates. If these pollutants are not captured and
recycled, burning biomass can create smog and even exceed the number
of pollutants released by fossil fuels.
FAST FACT
Balancing Biomass
The Union of Concerned Scientists
helped develop A Balanced Definition of Renewable Biomass, which are practical
and effective sustainability provisions that can provide a measure of assurance
that woody biomass harvests will be sustainable.
FAST FACT
FAST FACT
FAST FACT
World's Top Biofuel Crops (HowStuffWorks)
1. switchgrass2. wheat
3. sunflower
4. cottonseed oil
5. soy
6. jatropha
7. palm oil
8. sugarcane
9. canola
10. corn
Biomass provided about 5% of U.S. energy in 2023
In 2023, biomass accounted for
about 5% of U.S. energy consumption, or about 4,978 trillion British thermal
units (TBtu). The types, amounts, and the percentage shares of total biomass
energy consumption in 2023 were:
- Biofuels—2,662 TBtu—53%
- Wood and wood waste—1,918 TBtu—39%
- Municipal solid waste, animal manure, and sewage—398 TBtu—8%
The industrial sector is the largest consumer of biomass for energy in the United States
The amounts—in TBtu—and percentage shares of total U.S. biomass energy use by consuming sector in 2023 were:
- Industrial—2,225 TBtu—45%
- Transportation—1,788 TBtu—36%
- Residential—450 TBtu—9%
- Electric power—329 TBtu—7%
- Commercial—185 TBtu—4%
The industrial sector accounted
for the highest total annual U.S. biomass consumption in 2023 in terms of
energy content and percentage share. The wood products and paper industries use
biomass in combined heat and power plants for process heat and to generate
electricity for their own use.
The transportation sector
accounted for the second-highest amount and percentage share of biomass (as
biofuels) consumption in 2023.
The residential and commercial
sectors use firewood and wood pellets for heating. Commercial sector biomass
consumption includes biogas produced and consumed by municipal sewage treatment
facilities and waste landfills.
The electric power sector uses
wood and biomass-derived wastes to generate electricity for sale to the other
sectors.
The operation of a biomass plant
A biomass power plant produces
electricity from the steam that is released during the combustion of plant or
animal matter in a combustion chamber. This process is done in several steps:
1. Combustion: The biomass is
burned in a combustion chamber.
2. Steam production: The biomass
releases heat that heats water in a boiler. The water is transformed into
steam, which is sent under pressure to turbines.
3. Electricity production: The
steam turns a turbine which in turn drives an alternator. Thanks to the energy
supplied by the turbine, the alternator produces an alternating electric
current. A transformer raises the voltage of the electric current produced by
the alternator so that it can be more easily transported in medium and high
voltage lines.
4. Recycling: At the exit of the
turbine, part of the steam is recovered to be used for heating. This is called
cogeneration.
The rest of the steam is again
transformed into water thanks to a condenser in which cold water from the sea or
a river circulates. The water thus obtained is recovered and recirculated in
the boiler to start another cycle.
The advantages of biomass
The main advantage of biomass
power plants is simple: they allow to create energy without using fossil fuels,
thanks to ecological resources. With biomass, it is also possible to recover
waste and reuse it to create energy.
This mode of electricity
production has, in principle, a neutral carbon balance, because it rejects a
relatively low quantity of CO2, similar to the quantity consumed by the plants
during their growth phase.
In a world where ecology and
respect for the environment are increasingly important, the use of natural
fuels is necessarily a welcome solution. However, this solution is contested by
many associations...
Different mechanisms allow to create energy with biomass:
1. Combustion of raw material:
This is the most frequently adopted method. It relies on the combustion of raw
material, such as wood, in order to create steam, which is necessary for the
turbine to function.
One detail is particularly
pointed out by environmental associations: deforestation.
Wood being the main material
consumed by biomass power plants, the increasing presence of this mode of
energy production will potentially cause a need in trees more and more
important. It will be necessary to be careful that the exploitation of the
resources is not too intensive.
Example of use: from residue of sugar cane production: bagasse. This form of biomass is used instead of coal to fuel thermal power plants. This practice allows to use the natural resources of the islands in the DROM-COM (Reunion, Guadeloupe, ...).
2. Gasification:
This is an innovative alternative that consists, through a thermochemical
process, of transforming solid biomass into combustible gas that can be used in
multiple ways. With this process, the raw material is transformed into a gas,
which will then be used as fuel.
Example of use: Gasification can
create a biogas, for example thanks to hemp. Gasification is a high-temperature
thermo-chemical process (between 800°C and 1400°C) that can be used for various
inputs such as dry biomass, solid recovered fuels or wood waste. Hemp can therefore
be used in this process.
Similarly, there is an
association in Japan called HySTRA, which aims to develop a reliable and
economical hydrogen supply network. Thanks to this association, a terminal for
unloading this fuel is being built in Japan, and a lignite gasification plant
is also being built in Australia.
3. Methanization:
The methanization process is sometimes used by biomass power plants. In this
case, the organic materials used, such as household waste, paper, cardboard or
manure, are not burned, but fermented.
Example of use: Methanization
will create a biogas used for the operation of cars and trucks. It is a
technique in full expansion! The number of methanization sites injecting biogas
into the gas networks has largely increased in 2020. As of December 31, 2020,
France had 214 biogas plants.
A national information portal for
the general public dedicated to methanization has recently been created. Called
MéthaFrance, its objective is to provide educational answers to questions that
French people may have about the development of methanization in our
territories.
Characteristic of Biomass Electric Power Generation
Through photosynthesis,
chloroplasts of plants absorb carbon dioxide (CO2) from the air and utilize it
to grow the plant’s body. Consequently, power generation fueled by biomass is
regarded as carbon neutral as it only releases carbon dioxide that was absorbed
during the plant’s growth. Using biomass fuels as an alternative to fossil
fuels enables power generation to reduce CO2 emissions globally and
consequently to contribute to the prevention of global warming. Utilizing
domestic biomass resources also contributes to revitalizing the domestic
economy through the creation of new business fields and the provision of
employment opportunities to the local community. These opportunities include
not only power plant operation jobs but also harvesting and collecting biomass
from forests and transporting biomass to power stations.
Read Related: Biomass Combined Heat and Power (CHP)
How a biomass combustion plant works
Historically, firewood was the
first form of energy used by humanity, even before the adoption of coal, gas
and oil, and it is still widely used for domestic heating through fireplaces or
pellet stoves. The current large-scale exploitation, on the other hand, is much
more intense in countries with favorable legislation (e.g. Germany, Austria,
Denmark and Spain), while in others, like Italy, it is mainly hampered by
public opinion and by the opposition of local committees.
But how is energy produced from
biomass? There are several processes that make it possible to transform
organic materials from plants and animals into energy, but all share a basic
operation. They take place inside a thermal plant, where the combustion of
organic materials generates heat, which transforms the water of the
thermodynamic circuit into steam. The steam rotates a turbine which puts into
action the rotor of an alternator, producing alternating electric current.
What are biomass cogeneration plants used for
A biomass cogeneration unit
is useful in all cases where simultaneous production of electricity and heat is
necessary, such as district heating or certain industrial productions. Inside,
the biomass can be used raw or treated.
In the first case, the raw
biomass is conveyed to a burner which generates heat. The heat in turn heats a
working fluid (usually water) that transforms into steam, acquiring mechanical
energy through a pressure increase. The mechanical energy powers a turbine
connected to a generator, which produces electricity. Part of the heat produced
is conveyed to a hot water circuit which powers the thermal utilities.
In the case of biomass treatment,
for example, through anaerobic digestion of organic materials in the
absence of oxygen, the plant is powered by biogas, which has characteristics
similar to methane gas. This biofuel powers an internal combustion engine
connected to an electric generator and allows the simultaneous generation of
electrical energy, hot water and steam.
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