4 Biomass Conversion Methods and Principles Explained
Biomass conversion methods include thermal, chemical, biochemical and electrochemical.
One of the most important facts about biomass is that it can be used to generate energy, known as Biomass Energy or Bioenergy.
This article discusses biomass and bioenergy, as outlined below;
-Biofuel as A Source of Biomass Energy
-Indirect Production of Energy: Biomass Conversion
-Utilization of Biomass Energy
What is Biomass Energy?
Biomass energy is simply a form of energy produced or generated (directly/indirectly) by living organisms, or organic matter. Common examples of biomass from which energy is derived include plants like soy, corn, and sugarcane .
Generally, biomass energy is released in the form of heat, when biomass is broken down by combustion (burning). This heat is either used directly, or converted in order to generate electricity.
The use of biomass energy dates far back in the history of man, beginning from the use of firewood to cook and keep warm.
Primarily, biomass energy comes from the Sun. It is stored in green plants, algae, and other chlorophyll-pigmented organisms through the process of photosynthesis . This means that biomass energy is a converted and stored form of solar energy.
Biofuel as A Source of Biomass Energy
Biofuel is the term used to refer to biomass or any of its derivatives which is directly used to generate energy.
As a fuel, biomass has the advantage of being versatile, in the sense that it can be converted to any of various forms of biofuel.
Ethanol is one significant example of a liquid biofuel, which is derived from biomass, through the process of fermentation. Sugarcane, maize and wheat are all examples of plant biomass that can ferment go produce ethanol.
Biodiesel is yet another example of a liquid biofuel, and it is produced when ethanol is reacted with fatty acids.
The primary residue from the Kraft Process (which involves the manufacture of paper from pulpwood) is black liquor; a toxic, carbon-rich substance formed when hemicellulose and lignin are both removed from woody biomass .
Before the 1930s, black liquor was discarded as waste into water bodies, causing pollution in many cases. However, the recovery boiler, invented in the early 1930s, provided a device with which black liquor could be used as a fuel.
Firewood is the most basic example of biomass as a source of energy. It produces heat energy when burnt, which is applied directly for various purposes.
Indirect Production of Energy: Biomass Conversion
When biomass is used to generate energy in its natural form; it can be said to have directly produced energy. Indirect production of energy occurs when biomass is converted from its original, natural form before it is used to generate energy.
There are three main ways in which biomass is converted, so as to produce energy. Each of them is discussed as follows;
1). Thermal Conversion
As the name implies, thermal conversion is carried out basically by the application of heat. This heat is applied in order to alter the properties of the biomass and convert it into a more suitable and efficient fuel.
Common example of biomass which us used as feedstock for thermal conversion include Municipal Solid Waste (MSW), forest residue and industrial organic waste.
The main methods of thermal conversion of biomass include pyrolysis, firing/co-firing, torrefaction, and gasification.
Pyrolysis and Thermal Conversion of Biomass
Pyrolysis is a method of biomass conversion involving the application of thermal energy under anoxic conditions.
It is carried out simply by heating biomass to temperatures ranging from 200-1000°C in the absence of oxygen. In many cases, the temperature may reach high levels up to 800°C. This however depends on the nature of biomass feedstock and the required outcomes of the process.
Another concise way to describe pyrolysis, is as a process of thermal decomposition of biomass, occurring without the presence of oxygen.
Oxygen is excluded from pyrolysis solely in order to prevent combustion from occurring. This allows the biomass to undergo physiochemical changes without completely breaking down.
Products of pyrolysis may vary based on the conditions of the process and the nature of feedstock used. However, the main products include;
-Pyrolysis Oil, also called Bio-oil; which a vicious, dark, liquid. Pyrolysis oil is fairly similar in its physical characteristics to tar, and it can be burnt to generate heat which may be converted to electricity using a steam turbine. It can also be used as an ingredient in the manufacture of plastics and other renewable fuels.
-Biochar, which is the solid residue from the pyrolysis of biomass. It is rich in carbon and very useful as an additive to the soil . Because biochar is carbon-rich, its addition to soil serves as a means of carbon sequestration, which is helpful in addressing global warming.
-Syngas; which is a flammable, mixture of gases including carbon monoxide (CO), hydrogen (H2), carbon dioxide, and methane alongside trace amounts of other gases. It can be used as a fuel or converted to biogas.
Firing/Co-firing and Thermal Conversion of Biomass
Firing and co-firing do not involve the thermal conversion of biomass itself; however, they involve the conversion of biomass energy to electricity.
In firing (or ‘direct’ firing), biomass is made to undergo combustion. This produces heat energy which is used to operate a steam turbine and generate electricity. Firing implies that biomass is the sole source of energy, i.e.; the fuel which is burned to produce energy.
On the other hand, co-firing involves the combustion of biomass, together with another fuel (usually a fossil fuel). Co-firing is usually carried out in coal power plants, where biomass such as wood, is added to the coal.
Through the practice of co-firing, the demand on fossil fuels is reduced, as well as the associated greenhouse gas emissions.
Torrefaction and Thermal Conversion of Biomass
There is much similarity between torrefaction and pyrolysis.
During torrefaction, biomass is heated to temperatures typically between 200-320°C, in the absence of oxygen .
This heating changes the physicochemical characteristics of biomass, causing it to lose its moisture and much of its mass. However, in the same process, the biomass develops more suitable properties to be used as a fuel.
The product of torrefaction is known as a briquette. These materials are usually dry, hydrophobic (i.e.; they repel moisture) and dark in color. Briquettes can be seen as a derivative of biomass, and generally have a high energy density.
Gasification and Thermal Conversion of Biomass
Biomass gasification is a method of thermal conversion which transforms biomass into a group of mostly gaseous products, along with the release of energy.
In the process of gasification, biomass feedstock is heated under controlled conditions, to temperatures above 700°C; in the presence of predetermined amounts of oxygen.
Main products of biomass gasification include syngas (hydrogen, carbon dioxide, carbon monoxide, hydrogen and methane) as well as slag; which as dark, carbonaceous liquid that can be used to manufacture cement and asphalt.
2). Biochemical Conversion
Because biomass is biologically created and composed of atoms and molecules of chemical elements, it can be broken down by biochemical agents and processes.
Biochemical conversion of biomass, specifically involves the use of microorganisms like bacteria; as well as naturally-occurring chemicals like enzymes, to breakdown biomass.
The products of biochemical biomass conversion include biofuels like ethanol and methane. Common methods of biochemical conversion include anaerobic digestion, hydrolysis, composting and fermentation. These methods are discussed briefly as follows;
Anaerobic Digestion and Biochemical Conversion of Biomass
In the process of anaerobic digestion (AD), biomass is broken down by bacteria in the absence of oxygen .
The main product of anaerobic digestion is biogas, which is dominantly composed of methane. Biogas is useful as a fuel, and can reduce the need for fossil fuels in some applications.
Anaerobic digestion is often applied to organic waste in landfills, as well as livestock waste and other forms of agricultural biomass.
Hydrolysis and Biochemical Conversion of Biomass
Also referred to as enzymatic hydrolysis, this method of biomass conversion can be viewed as an intermediate process rather than a major one.
In hydrolysis, enzymes are used to break down (or ‘depolymerise’) lignocellulose polymers in (woody) plant biomass . It is seen as an intermediate process because it converts biomass into products that are more suitable for anaerobic digestion or fermentation.
The products of enzymatic hydrolysis of biomass are less-complex sugars like monosaccharides and oligosaccharides. On the other hand, some of the enzymes which are used to break down the lignocellulose polymers in woody biomass, during hydrolysis, include arabinofuranosidases, acetylxylan esterases, 4‐O‐glucuronoyl methylesterases, and alpha‐galactosidases .
Composting and Biochemical Conversion of Biomass
Composting is one of the most simple and basic methods of biomass conversion.
It is carried out by heaping biomass together in such a manner (and for a period of time) that allows it to undergo microbial decomposition under mildly controlled conditions. ‘Controlled conditions’ in this case merely refers to the act of limiting the exposure of the biomass to extreme heat, excessive moisture, and any other condition(s) that could directly alter the decomposition process.
Biomass that is used as feedstock for composting includes agricultural and forest residue, paper, mill residue, as well as food waste.
The breakdown of biomass in composting, differs from the approach in other conversion methods like pyrolysis and anaerobic digestion, because it does not exclude oxygen. Also, composting may be seen as a thermal process because of the increase in temperature of the heaped biomass as it is broken down by fungi and bacteria.
The products of composting may vary in their physicochemical composition based on the nature and composition of the original biomass (feedstock) that was subjected to the process. These products are however often used as manure to boost soil fertility. Composting is also a means of addressing the challenges of organic waste disposal .
Fermentation and Biochemical Conversion of Biomass
Fermentation can be described as a biochemical process by which organic matter (biomass) is broken down by microorganisms under anaerobic (no-oxygen) conditions.
More specifically, during fermentation, the carbohydrate components (starch, cellulose, etc.) in biomass, are broken down to produce simpler compounds like organic acids and alcohols. The most common (and perhaps useful) product of fermentation is ethanol.
Under well-controlled conditions, bacteria or yeast may be added to the biomass to facilitate the breakdown. The ethanol which is produced, can also be dehydrated and distilled to improve its concentration, purity and suitability for use as a fuel. A solid residue is usually produced as well, and this can be used as livestock feed or as a fuel.
3). Electrochemical Conversion
As the name implies, electrochemical conversion of biomass involves the breakdown or alteration of biomass through electrochemical processes (electron transfer, redox reactions).
Some terms used to refer to electrochemical biomass conversion are electro-synthesis, electro-fermentation, and electrochemical hydrogenation (ECH).
Electrochemical biomass conversion may be carried out in a fuel cell , and usually leads to the oxidation or reduction of biomass. Products of electrochemical conversion include ethanol and bio-oil.
4). Chemical Conversion
The Fischer-Tropsch synthesis is a classic example of a chemical conversion process for biomass.
It involves the conversion of synthesis gas (a derivative of biomass) through a range of chemical, catalytic reactions, to produce useful chemicals and fuels.
The chemical reactions typically occur at temperatures between 150-300°C. Also, the main products of these reactions are liquid hydrocarbons, including alkanes.
Utilization of Biomass Energy
By far the most common ways in which biomass energy is utilized, are domestic heating purposes, and electricity generation.
In order to access the energy (from biomass) for these purposes, various approaches are applied. One of these is the direct burning of biomass (such as wood) or any of its combustible derivatives (such as biogas, briquette, black liquor, or biodiesel) to release bioenergy in the form of heat. Another is the use of heat from burning biomass, to produce steam that drives a turbine and generates electricity .
In the United States in 2016, biomass energy was used to generate roughly 71.4 billion kilowatt hours of electricity, which amounts to 2 percent of the total electricity which was generated in the country that year .
In 2018, biomass accounted for 45 percent of renewable energy, and 5.1 percent of total energy in the United States.
Up to 33 percent of all biomass energy (produced globally) is derived from woody biomass like firewood, wood pellets, mill residue, paper industrial waste, and forest residue.
Black liquor (which is a biofuel derived from biomass during the Kraft process for manufacturing paper), provided about 27 percent of biomass-derived electricity in the United States in 2016. Organic municipal solid waste (which is a type of biomass) accounted for 20 percent of biomass-derived electricity in the same year.
The most common feedstock used in electricity generation from organic municipal solid waste include food waste, wood matter, yard waste and rubber.
Other forms (and derivatives) of biomass such as landfill gas (biogas) account for the remaining percentage of biomass-derived electricity.
Biomass energy is simply energy derived from organic matter.
The primary source of bioenergy is the Sun; meaning that biomass energy is itself a stored form of solar energy which is accumulated in plants through photosynthesis.
When used solely to produce energy, biomass can alternatively be referred to as biofuel. Also, biomass can be used either directly or indirectly to produce energy. The direct use of biomass can be illustrated by the burning of firewood to produce heat energy for domestic purposes.
Indirect use of biomass to produce energy, generally involves a form of conversion. Four main types of biomass conversion can be carried out, namely; thermal, chemical, biochemical, and electrochemical conversion.
Each of these types of conversion are aimed at altering the physicochemical properties of biomass, in order to make it more efficient as a fuel. Some of the processes involved to achieve such alterations are fermentation, anaerobic decomposition, electro-synthesis and pyrolysis.
Another important conversion, is the generation of electricity from biomass energy. Such electricity, when derived, can be referred to as ‘biomass-derived electricity’. The basic approach involves using bioenergy (that is; biomass energy) in the form of heat, to produce steam which can be used to drive a turbine generator.
In the United States and beyond, biomass-derived electricity is gaining importance as one of the renewable forms of power. Some examples of biomass which is used as feedstock (fuel/biofuel) in biomass power plants, include wood, organic municipal solid waste, paper, mill residue, black liquor, biodiesel and biogas.
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