Biomass Residues & Energy Conversion

Biomass Residues and Energy Conversion Pathways

• Technologies exist to convert biomass into usable fuels.
• Fermentation and anaerobic digestion technologies are well-understood and relatively simple.
• Gasification technologies are in pilot stages and are beginning to be commercialized.
• Varying treatments of biomass resources yield a variety of products.
• Domestic refuse can be burned for heat or converted via pyrolysis into low-calorific gas.
• Domestic refuse can also be digested to produce methane.
• Conversion methods are chosen based on resource type, location, economic factors, and market availability.
• Feedstock must be specified to match the conversion system.
• Feedstock preprocessing may be required to produce a high-value product.
• Pretreatment includes physical methods like size reduction, drying, and gasification and chemical and biological methods.

Pretreatment Importance

-Due to its low energy density, biomass transportation can be expensive, therefore densification is used to improve it.

• Biomass is compressed into pellets or briquettes for thermochemical conversion.
• Combustion easily releases energy stored in dry matter.
• Drying biomass is essential before it is burned.
• Drying is crucial for high efficiency in thermochemical conversion.
• Size reduction ensures a homogeneous mixture for efficient digestion.
• Pretreatment depends on the feedstock specifications of the conversion systems.
• Conversion products may require upgrading for usability.
• Upgrading techniques: separation, hydrotreating, distillation, and extraction.

Upgrading Techniques

• Distillation upgrades 10% crude ethanol to 95% commercial grade ethanol in alcoholic fermentation.
• Bio-oil from pyrolysis is upgraded via hydrotreating to improve its fuel value.
• The finished product can be converted into electrical energy.
• Effective utilization is essential due to biomass properties: dispersed, bulky, excess moisture, inconsistent quality, and variable composition.
• Compression into pellets or briquettes improves energy density for thermochemical conversion.
• Biomass with ≤10% moisture is preferred in thermochemical conversion for better quality products.
• Wet organic material can be treated using thermochemical and wet conversion (biochemical to aqueous ethanol) processes.

Biomass Energy Conversion

• Biomass energy conversion involves transforming energy from natural to useful forms.
• Biomass sources undergo a sequence of energy transformation stages.
• Conversion pathways include biomass selection, harvesting, preprocessing, biomass conversion, and product cleaning/treatment.
• Preprocessing is important to remove unwanted materials and excess moisture.
• Biomass may need to be screened, crushed, or milled before conversion.
• Auxiliary equipment is similar in combustion, pyrolysis, and gasification plants.
• Alcoholic fermentation or anaerobic digestion equipment varies due to different feedstocks.
• Combustion is a commercially used method to produce energy from biomass.
• Denser pellets improve energy density.
• Heat from combustion can be used as process heat or converted into electricity.
• Briquettes or pellets can be gasified to release fuel gas for heat or electricity production.
• Pyrolysis yields char, bio-oil, and fuel gas.
• Bio-oil can be used to produce chemicals or upgraded into high-quality fuel via hydrotreating.

Biomass Types

• Wet processes convert wet organic materials into premium fuels.
• Wet organic materials can be converted into biogas or aqueous ethanol.
• Stirring wet organic materials into slurry ensures a homogeneous mixture for microbial digestion into biogas.
• Pretreatment of wet organic materials can convert complex carbohydrates into simple sugars before ethanolic fermentation with yeast.
• Mechanical processing extracts fuel oils from plant parts.
• Certain plants yield light hydrocarbons as petroleum substitutes.
• Physical conversion methods transform raw materials into forms suitable for energy conversion.
• Briquetting and pelletization are simple physical conversion methods that densify biomass and reduce volume.
• Agrochemical routes extract fuel oil or light hydrocarbons from plants as petroleum substitutes.

Pelletization Process

• During pelletization, dry wood material is pulverized, after being screened and crushed.
• Wood pellets are widely used in thermochemical conversion.
• Sawdust, a timber industry byproduct, can be converted into pellets and briquettes.
• Rice husk can be converted into briquettes or pellets.
• Agriculture and forest residues can be converted into leaf briquettes.
• In charcoal briquette production, raw biomass is carbonized to produce charcoal, which is then converted into charcoal briquettes.
• Charcoal briquettes replace fossil fuels like coal.

Pelletization

• Pelletization includes: pulverizing, drying, and forcing waste wood through an extrusion device under pressure.
• Small particles, with a large surface area, increases material density, resulting in stronger pellets.
• Final pellet quality depends on raw materials and manufacturing processes.
• Wood pellets can contain up to 2% additives to ensure they form as a solid piece.
• Pelletization reduces moisture content to 7-10%, this increases heat value of the biomass.
• Pelletization facilitates the use of raw biomass in power plants and gasification systems.

Briquetting

• Briquettes are made from woody matter and replace fossil fuels like coal or charcoal.
• Briquettes can heat boilers in manufacturing plants.
• Briquettes have a moisture content as low as 4%, lower than green firewood's 65%.
• Briquetting compresses raw material, breaking down elasticity and squeezing out moisture.
• Lignin in biomass acts as a binder, helping solid particles of wood form a solid briquette.

Expelling Agro Products

• Vegetable oils, obtained through expelling processes, can be used as fuel in diesel engines.
• Plant oil has advantages and disadvantages, including higher viscosity and combustion deposits.
• Transesterification, a chemical conversion technique, upgrades vegetable oil to improve its properties.
• Non-edible oil seeds can be used to produce high-quality fuel through conversion processes.

Fuel Extraction

• Fuel extraction involves obtaining milky latex-like materials, called exudates, from plant cuts.
• Some plants require crushing to obtain the product if they cannot be tapped.
• The Euphorbia plant can be crushed to produce a petroleum substitute with a lower molecular weight than rubber.

Thermochemical Conversion Processes

• Thermochemical conversion processes involve biomass decomposition and oxidation to produce a product.
• Drying, pyrolysis, and gasification are the first steps in solid decomposition.
• The relative importance of these processes depends on conversion technology, raw material properties, and process conditions.
• Biomass combustion is more complex than pyrolysis and gasification, as biomass must be pyrolyzed and partially combusted before full combustion.
• During combustion, fuel gas must pass through pyrolysis and gasification to oxidize and achieve heat with stable compounds (CO2 and H2O).
• Pyrolysis, gasification, and combustion are often modeled in series but overlap without sharp boundaries.

Pyrolysis

• Pyrolysis is a thermal decomposition process that partially removes carbon from feedstock without adding hydrogen.
• Pyrolysis products include gas, non-condensable gases, condensable vapors (liquid), tar, and solid char.

Gasification

• Gasification uses a gasifying medium like oxygen, air, or steam.
• The products of pyrolysis react with the gasifying medium to produce a final gas product.
• Gasification adds hydrogen to the product, unlike pyrolysis.
• Gasification can be an efficient way to produce secondary fuels from biomass.

Gasification vs. Combustion

• Gasification and combustion are closely related, but gasification adds hydrogen and strips carbon from the feedstock.
• Combustion oxidizes hydrogen and carbon to produce H2O and CO2, releasing significant heat.
• When the amount of oxygen exceeds a certain value, the process transitions from gasification to combustion, producing flue gas.
• Gasification is partial combustion, while combustion is complete oxidation.

Thermal Process Advantages

• Thermal processes typically have higher throughput and can operate on any biomass form (wet or dry).

Key Reactions

• Pyrolysis: Biomass is heated without oxygen, releasing charcoal, oil, and gas.
• Gasification: Requires a gasifying medium (oxygen, air, or steam) with a partial oxygen supply, releasing fuel gas.
• Combustion: Complete oxidation of biomass requires a stoichiometric amount of oxygen, releasing hot combustion products.

Combustion Process

• Combustion is an exothermic reaction between oxygen and hydrocarbons in biomass.
• Biomass is oxidized into stable products, CO2 and H2O, reversing the photosynthesis process.

Combustion Stages

• Drying: Moisture evaporates from the biomass in the temperature range of 50-100°C, leaving ash, char, and volatile matter.
• Thermal Decomposition (Devolatilization): Volatile matter decomposes in the temperature range of 150-350°C, leaving ash and char.
• Partial Combustion (Gasification): Char is oxidized to CO2 and H2O, reacting with CO and other gases to release CO2 and H2.
• Complete Oxidation: All gas and char are converted into CO2 and water, releasing significant heat.

Combustibles of Solid Carbon

• Volatile matter is about 80% of woody biomass, with solid carbon around 20%.
• 80% of energy comes from the combustion of volatile matter, and 20% from the burning of solid carbon in the biomass.

Complete vs Incomplete Combustion

• Complete Combustion: Full oxidation of char and volatiles, produces CO2, H2O, significant heat, and trace species.
• Incomplete Combustion: Occurs with insufficient oxygen, producing CO2, H2O, heat, carbon monoxide, and soot particles with toxic byproducts like PAAH.
• Complete combustion gives off non-toxic byproducts like NOX, CO2, and water.

Applications of Combustion

• Heat and electricity can be derived from biomass combustion.
• Biomass, in pellet or briquette form, can be combusted with or without coal to produce electricity.