Introduction       

Biomass is stored energy. The plants majorly constitute it by harnessing solar energy and converting it into chemical energy by photosynthesis and is the stored living matter or energy that serves as primary or secondary fuel. It acts as an alternative energy source to produce biofuel by processing biomass. It provides fuel flexibility to meet the broad range of energy demands. All types of biomass produce 14% of global energy, and it is the primary energy source that contributes around 90% of the energy demand of some developing countries.

Biomass plays a significant role as a renewable energy source with a high potential for biofuel production that can be used in transportation, heating, and electricity. The biomass is produced by terrestrial and aquatic life. The terrestrial biomass includes agricultural residue, municipal waste, energy crops and plantations, woody biomass, and the aquatic biomass produced by aquatic plants and algae.

The biomass resources are categorized as energy crops, woody biomass or forestry, forestry wastes (wood, logs chips, leaves, bark, and sawdust). The agricultural residue is a byproduct of agriculture, as primary residue (rice straw, maize straw, wheat straw) and secondary residue( bagasse, rice husk).   Municipal waste collected from household waste mainly consists of organic waste used for energy recovery.

Biomass Energy Conversion Technologies and Applications

The energy conversion technologies used to convert biomass into usable energy that based on biomass type (chemical composition and physical nature), the biomass energy conversion occurs by the following means:

Thermo-chemical Technologies: This technique includes pyrolysis, combustion, biomass gasification, and liquefaction

Combustion: It is the process of oxidation of biomass. During this biomass, chemical energy changes into heat energy. Which is further produces electricity with the help of boiler and steam turbines. This method is suitable for biomass that has less moisture content 20 to 40%.

Gasification: Biomass is partially oxidized at high temperatures and produces combustible gases, this energy conversion process is known as gasification. The gaseous product is known as Syngas.

Pyrolysis: This method of energy production used to produce liquid fuel from biomass (bio-oil) in the absence of air is called pyrolysis. It can be efficient about 80%.

Biochemical Conversion: It is an anaerobic and fermentation process used to produce bio-energy from biomass.

Fermentation: Starch or sugar is converted into liquid fuel using microbes, called fermentation. The product obtained via this method is biobutanol, bioethanol, etc. the sugar and starch crops are used in the formation process.

Anaerobic Digestion: This method of energy conversion process organic matter (manure) converted into a mixture of gases with the help of microbes in the absence of oxygen. The main product obtained is biogas and bio-hydrogen. This method is suitable for higher moisture content biomass (80 to 90%).

Chemical Technology: This method is also referred to as a physiochemical biomass conversion process. This process produces high-density biofuels, in which vegetable oil and animal fat are used to produce bio-diesel through the esterification or transesterification process. The vegetable oil used to produce biofuel is first-generation biodiesel that includes sunflower oil and rapeseed soil that account for 10-15% and 80-85% of total biodiesel production.

Transesterification: In this method, fat or oil react with alcohol to produce biodiesel (fatty acid ester), referred to as transesterification.

Schematic diagram of different bioenergy products and routes. (Source: E4tech, 2009).

Biomass Gasification

It is the type of thermos chemical process in which solid carbon biomass is converted into gas which is referred to as Syngas or synthesis gas. That consists of CO, H2, and N2. The produce of Syngas is used to generate electricity. The most dominant method to produce Syngas is partial oxidation of solid biomass that gives CO and H2 is the different ratio that accounts for more than 85% of a total volume, whereas CO2 and CH4 constitute the rest volume. This treatment process is performed in the airtight chamber under lower pressure than atmospheric pressure.

The produced gas is subsequently cleaned to extract the impurities and trace elements and remove all pollutants in Syngas. Pure gas is used in electricity generation or chemical production. The Syngas consists of a combustible and non-combustible part that production depends on the type of biomass and type of process and operational condition used. Generally, the calorific value of produced gas is 4.5 to 6Mj/m3, concerning natural gas’s 10-50% calorific value.

Biomass consists of four different oxidation, Drying, Pyrolysis, and reduction processes.

Combustion of coal C + O2 → CO2 ΔH = -394 kJ/mol

Partial oxidation C + ½ O2 → CO ΔH = -111 kJ/mol

Hydrogen combustion H2 + ½ O2 → H2O ΔH = -242 kJ/mol

Drying Stage(Endothermic): In this phase, the biomass moisture is extracted at 100oC by changing into steam by raising the temperature to 150oC. The heat input is proportional to the moisture of biomass that derives via the previous phase.

Pyrolysis Phase(Endothermic): This phase is the thermal decomposition of carbonaceous material by anaerobic means, which produce solid component(char-high carbon content material) and liquid component( tar-complex and condensable organic matter at low temperature) and gas component (incondensable mixture of H2, Co2, CO  along with light hydrocarbons-pyrolyzed gas. In this phase, the molecule’s chemical bonds are broken and form lighter molecules. That happened between  250 to 700oC.

Biomass ↔ H2 + CO + CO2 + CH4 + H2O (g) + Tar + Char.

Reduction Stage(Endothermic): This is a gas-phase reaction. The coal and gas are extracted from the pyrolysis, transforming oxidation. The previous phase product is reacted with each other and given final Syngas. The following reaction takes place in this phase.

Boudouard reaction: C + CO2 ↔ 2CO ΔH = 172 kJ/mole

Character reform: C + H2O ↔ CO + H2 ΔH = 131 kJ/mole

Water gas displacement reaction: CO + H2O ↔ CO2 + H2 ΔH = -41 kJ / mole

Methanation: C + 2H2 ↔ CH4 ΔH = -7

The process of gasification occurs at a temperature ranging from 800-1100oC. In contrast, the temperature can range between 500 to 1600oC when oxygen is required in gasification.

Types And Application of Gasifier

The classification of gasifier is depend on the direction of air or gas flow and fuel in the reactor, that is as follow

Updraft Gasifier:

This type of gasifier is known for its counter flow of gases and fuel in the reactor. The fuel is loaded from the top and air from the bottom of the reactor, whereas producer gas is collected from the top gasifier. The reduction, pyrolysis, and drying zone are formed over the oxidation zone, and the oxidation zone is created at the bottom of the gasifier. The zone of reaction is the updraft gasifier. This reaction arrangement causes the producer gas to pass through the pyrolysis and drying zone before exiting the gasifier. The pyrolysis zone of the gasifier released the tar, and the water vapor from the drying zone moved with generated producer gas. The gas produced contains high moisture and tar ranging from 1-20g/Nm3. That application of that type of gas is best for thermally. The updraft is majorly used for thermal applications.

Reaction Zones In An Updraft Gasifier

Downdraft Gasifier:

This type of gasifier is known for the counter-current flow of fuel and gas. The fuel is loaded from the top, and the air is introduced on either side or top of the gasifier in the oxidation zone. The movement of fuel and air is in the same direction. The pyrolysis product passes from the oxidation (high temperature) and reduction zone before taking the exit from the gasifier. In this type, most of the tar is burned or cracked in the gasifier, and the generated producer gas is collected from the bottom of the gasifier with minimum tar and other condensable material. It is further divided into two categories an Imbert type downdraft gasifier and a constant diameter throat less gasifier.

Reaction Zones In An Downdraft Gasifier

Cross Draft Gasifier:

This type of gasifier can work with a wide range of fuels compared to the other two discussed gasifiers. This type is used for coal gasification—the high gas velocity with high gas temperature from the gasifier exit with poor carbon dioxide reduction.

Reaction Zones In An Crossdraft Gasifier

Fluidized Bed Gasifier:

It is a homogenous reactor bed with inert sand material. In this high alumina, refractory sand is used as bedding material, and fluidization occurs using gas and air. The uniform size of palletized biomass is used, and the fuel is loaded in the inert bed material, and then air is introduced at the bottom side of the bed in the gasifier. The reactions like oxidation-reduction pyrolysis take place simultaneously. In this type of gasifier, any fuel is used with a uniform and small size—the gas exit with high temperatures and high solid particulate matter with low efficiency. The generated producer gas needs extensive cleaning for thermal and mechanical application.

 Line Diagram of a Fluidized-bed Gasifier System

Biomass To Ethanol Production

The production of ethanol from biomass is bioethanol that is produced by the process of sugar fermentation but can also be produced by chemical means reacting the ethylene with steam. The sugar energy crop is used to produce bioethanol. Ethanol is a colorless clear, biodegradable fluid. That has low toxicity with little environmental impact. Different treatment is given to energy crops’ feedstock that includes pre-treatment, hydrolysis, fermentation, and ethanol recovery for producing bioethanol.

                  Strategies for cellulose hydrolysis and utilization by anaerobic and aerobic bacteria

Pre-treatment:

This is the costlier phase, which includes different types of pre-treatment methods like chemical, physical, physicochemical, and biological methods. The pre-treatment of the lignocellulosic biomass causes the release of cellulose from the complex lignocellulose material in plants. This method increases the sugar yield by more than 90%( theoretically).

The biological, chemical, and physicochemical pretreatment are categorized as traditional methods. In the physical pre-treatment method, the lignocellulose is disintegrated into a lignocellulosic mass with the help of extrusion, grinding, irradiation, and milling. They are passing through such a process to increase the surface area that makes them effectively available to the enzymatic hydrolysis action.

In chemical pre-treatment, organic acids, alkali, oxidative lignification method is conducted for the selective feedstock that helps to deconstruct or remove the lignin and hemicellulose part, in physicochemical which include both physical and chemical processes like stem explosion,  microwave irradiation, liquid hot water, and co2 explosion. In biological treatment, the microorganism breaks the complex lignocellulose materials and is further treated with hydrolysis. Microbes used in this process are white rot, brown rot, soft rot fungi, and bacteria.

Hydrolysis:

After the pre-treatment of biomass, hydrolysis action is required to produce sugar monomers. This phase is enzymatic and leads to the fermentation that can work on monomers. The process can be carried out by using either acids or enzymes. Hydrolysis by the acid is a more commonly used method that needs concentrated acid like H2SO4 HCl used at low temperature and gives 90% sugar recovery quickly. The cost is high for employing this method.

(C₆H₁₀O₅)n+H₃0+→H₂O+nC₆H₁₂O₆C₆H₁₀O₅n+H₃0+→H₂O+nC₆H₁₂O₆

Enzymatic hydrolysis use enzyme to degrade the complex carbohydrate and give monomeric sugar molecule und e controlled operating condition the temperature around  45–50oC and pH 4.8–5.0. This method is good and efficient in producing high sugar without causing corrosion. The efficiency can be influenced due to changes in pH, temperature, moisture, substrate concentration. The use of enzymes affects the cost of the overall process. The enzyme used is high in cost, for example, clostridium, cellulosomes, Erwinia, Thermonospora, Bacteriodes, Bacillus, Ruminococcus, Acetovibrio, and Streptomyces. Others include fungi such as Trichoderma, Penicillium, Fusarium, Phanerochaete, Humicola, and Schizophillum sp.

Fermentation:

In this biological process, ethanol is produced along with acid and gases from the monomeric sugar by using the microbes like bacteria yeast fungi. Saccharomyces cerevisiae is the most commonly used organism due to its high tolerance limit and yields production of ethanol. The organism produces ethanol by converting mannose, glucose, and fructose. Several fermentation technologies are used, like batch fermentation, continuous fermentation, separate hydrolysis, and fermentation.

Bioethanol recovery:

After the fermentation, the ethanol recovers from the broth. The broth’s water content decreases approx. 0.5% by volume, causing the formation of anhydrous ethanol with 99.5% volume. The process of ethanol recovery by the distillation process

Biogas Production From Waste Biomass

Biogas is the clean energy fuel used for lighting, cooking, water pumping, and some other applications, and is produced by microbial degradation of biological material in the absence of oxygen via the anaerobic process. It is a renewable fuel similar to natural gas or marsh gases due to gas composition that is CH4, CO2. Methane is also generated in landfill sites due to organic material decomposition via a natural process.

Organic waste is used to produce biogas irrespective of waste composition. The waste can be concentrated or liquid, solid or slurries of organic waste. Mainly manure is used as feedstock for biogas production.

The biogas production process involves 4 steps hydrolysis, acid-genesis, acetogenesis, methanogenesis.

Hydrolysis: It is the first step in which complex biomacromolecules like lipid, fat, carbohydrate, and protein are broken down by lipolytic, cellulolytic, and proteolytic bacteria respectively into its monomer units as glycerol, sugar, and amino acid. Microbes that are used for degradation or hydrolysis are Lactobacillus, Propionibacterium, Sphingomonas, Sporobacterium, Megasphaera, Bifidobacteriumare.

Acidogenesis: It is the further step that comes after hydrolysis in which acidogenic bacteria are used to convert or break down the product of hydrolysis product into organic acid, acetic acid, formic acid, propionic acid, lactic acid, butyric acid, succinic acid. Ketones and alcohol with some gases NH3, H2, and H2S. microbes that are acidogenic Clostridium spp., Bifidobacterium spp., Desulphovibrio spp., Corynebacterium spp., Lactobacillus spp

Acitogensis: Acitogensis is the third step after acidogenesis in which acetogenic bacteria work on the volatile fatty acid (propionic, lactic, and butyric acid) and change them into acetic acid or acetate, H2, and CO2 as byproduct release. Bacteria use di the processes are Syntrobacterwolinii and Syntrophomonaswolfeiare.

Methanogenesis: The final and last step, which is strictly anaerobic, produces methane by methanogenesis in two ways: Hydrogenotrophicmathanogensis and Aceticlasticmethanogenesis.

Biogas Production Process

Types of Biogas Plants

Fixed Dome-Type: This is one unit plant that consists of a digester, gas storage dome, and slurry displacement chamber. This type of plant is constructed with brick, stone, and cement sand mortar. The utmost precaution is required to construct the plant to avoid any leakage. The model is approved under this category is Janata and Desh Bandhu. Which is cylindrical shape with top is domed.

Fixed-dome type biogas plan

Floating Drum Type: The plant consists of two integral units: the gas holder and digester with inlet and outlet pipe. In this category, several plants were approved by the Ministry of Non-conventional Energy Sources (MNES), New Delhi.

Floating Drum Type Biogas Plant

Bag Type Biogas Plant: It has a portable unit that can be placed at any location also called Flexi type. It consists of 2 parts the upper is the gas storage part, and the lower is the digestor. This type of plant is made up of a neoprene rubber reinforced with nylon, and the gas is collected under low pressure. This type of plant is easy and quick installation, but the cost is high, short life, and low pressure, not so popular enough.

Factors Affecting Biogas Generation

Environmental parameters affect biogas production. It can affect pH, temperature, concentration, alkalinity, volatile acids, nutrient availability, toxic material. It can also affect operation factors like retention time, organic substrate and its concentration, rate of organic loading.

pH: It is an essential factor that influences bacterial action. Optimum pH is 6.8-7.2 for anaerobic digestion; methanogenic bacteria work above 5pH around 8Ph.

Temperature: The optimum temperature of 35oC is suitable for bacterial growth, temperature fluctuation can affect the overall process.

Nutrient availability: Methane production depends on biological growth to confirm satisfactory biodegradation. The primary nutrients are Sulphur, phosphorus, nitrogen, and potassium.

Toxic material: Certain toxic substances like ammonia, sulfide, heavy metal, volatile acid, and tannins hamper bacterial growth.

Substrate concentration: The production of methane affected by the manure and biomass composition that consist of cellulose, lignin, glucose, fructose protein, lipid, and pectin

C: N ratio: The anaerobic microbes require a 20:30:1 ratio of C to N with a more significant portion of the carbon that is easily degradable in anaerobic digestion. The altered ratio of C: N can hinder the overall reaction.

The Future Role of Biomass

Biomass energy is a competitive energy resource with other energy sources. It is sustainable, abundant, surplus biomass available, renewable. The contribution of biomass by India is 32% of the total energy consumption. The biomass power contribution is 8.63%, from biogas is 5.31%, and waste to energy is2.88%. The estimated potential from biomass residue is 1800MW. Bagasse cogeneration produces around 7000MW. Punjab is the top with the highest potential regarding bioenergy. The second is Maharashtra, Chhattisgarh, Uttar Pradesh, Andhra Pradesh, and Tamil Nadu are the leading states. There is various way to produce bioenergy like thermochemical and biochemical which have certain limitations. There is an urgent need for technological advancement to harness the energy from waste and biomass feedstocks efficiently.