Anaerobic Digestion: From Waste to Revenue
Anaerobic digestion is defined as a series or collection of processes by which microorganisms break down biodegradable material in the absence of oxygen. This is the process used by GCES to convert waste into biogases for uses as heat and power.
Agricultural, Farming & Husbandry Masses Utilized for Biogas Production:
GCES partners with dozens of food industries to develop solutions for the creation of biogases. These biogases are created through the breakdown of organic materials. The origin of the substrates include:
- Pig, cattle, fish & foul slurry
- Energy crops such as grain & grasses
- Produce waste from vegetables, fruits & nuts
The main components required to create biogases are:
- Cellulose & Hemicelluloses
These masses are broken down into the following three scenarios and associated digestion processes.
- High Solids (Dry-Stackable Substrate)
- High Solids (Wet-Pumpable Substrate)
- Low Solids (Wed-Pumpable Substrate)
High dry solids are designed to process materials that have a solid content of between 25 to 40% and process slurries without the addition of other liquids such as water. Traditionally these dry digestion systems are continuous vertical plug flow digesters or batch tunnel horizontal digesters. Vertical plug flow digesters are cylindrical upright tanks that are continuously fed feedstock. The feedstock comes in through the top of the digester and utilizes gravity to flow downward during the digestion process. With batch tunnel digesters, the feedstock is deposited into a chamber with a gas-tight door. With either of these digesters is mixing done inside the digester.
High wet solids are designed to process thick slurry. The thick slurry requires a higher energy input to move the input during processing. This thick material is often associated with abrasion problems, but their advantage is a lower land requirement due to lower volumes associated with moisture. High solid digesters require regular correction of performance calculations as larger fractions of the feedstock mass are potential converted into biogas. These calculations include gas production, retention time, kinetics, dilution, etc.
Low wet solids transport material through the digestion system using standard pumps that require a low energy input but a large area of land due to increased volumes required with their digestion models larger liquid-to-feedstock ratio. The benefits beyond low energy consumption that come from digestion in a liquid environment includes a more thoroughly circulated material and improved contact between the bacteria and its feedstock. This permits the bacteria to more readily access feedstock and increase the rate of gas production.
The Digestion Process:
The digestions process to produce biogas has four phases:
Phase 1: Hydrolysis
During Hydrolysis long chain organic compounds such as proteins, fats, carbohydrates, etc. are split into more simple organic compounds such as amino acids, fatty acids, and sugars through a bacterial action. The most common of these long chain organic compounds include animal proteins and waste from cattle, pigs, chicken and fish husbandry, and production food waste including that from canneries and bakeries, byproducts of beer, and alcohol manufacturing, human and animal sewage waste, and any other process that releases methane gas.
Phase 2: Acidogenesis
During phase 2 (Acidogenesis), the products of Hydrolysis (phase 1), are metabolized by acidogenic bacteria and broken down into short chain fatty acids such as acetic acid, propionic acid, butyric acid, valeric acid, and alcohol. Acetic acid, hydrogen and carbon dioxide are created and serve as the initial products for the formation of methane. A key component during this phase is the concentration of hydrogen also known as the hydrogen partial pressure.
Phase 3: Acetogenesis
Through the Acetogenesis phase, the organic acids and alcohols break down from acetogenic bacteria into carbon dioxide, hydrogen, and acetic acid which are the main components for the production of biogas.
Phase 4: Methanogenesis
During this final phase biogas is created. Biogas is a combustible gas that occurs when methane and carbon dioxide is converted into methanogenic microorganisms or archaea.
Digestion systems can be configured in either a single stage digestion system or a two-stage digestion system formation. There are advantages and disadvantages to both systems. For example, using a single stage system has a smaller initial construction and equipment cost but a allows for less control of the reactions occurring in the system which can greatly reduce output. The biological reaction of the various species in a single stage reactor can be in direct competition with one another. The two bacteria reaction types are known as acidogenic bacteria, which through the production of acids reduces the pH of the tank, and methanogenic bacteria, which operates strictly in a defined pH range. The anaerobic reactions of these two bacteria occurs in a pond and are contained within a anaerobic lagoon or pool with natural anaerobic sludge.
In a two-stage digestion system, also known as a multistage digestion system, multiple digestion containers are optimized to bring maximum control over the bacterial communities developing within the digester. As with single stage digestion, there are multiple types of bacterial processes. In the first reaction vessel, hydrolysis, acetogenesis and acidogenesis occur simultaneously. During this reaction feedstock is added and controlled. That organic material is then heated to the ideal operating temperature (either mesophilic or thermophilic) and then pumped into a methanogenic reactor. During this process, a sterilization process may also occur heating the matter to a temperature that kills the harmful bacteria found in waste.
Once these bacteria reactions have taken place, a residence time occurs which allows the feed bacterial to begin degrading and developing gases. This time ranges in most cases from 14 to 40 days depending on the type of feedstock and the digestion system utilized. In some cases, in an up flow anaerobic sludge blanket digestion system, a hydraulic residence time can be as short as even an hour with solid retention times can be up to 90 days. Continuous digesters utilize mechanical and hydraulic devices which continually mix the contents enabling the bacteria and the feed to be in contact with one another. This also allows for a more consistent volume within the digestion tanks as the excess material is continuously extracted.
Biogas yield depends on the composition of the substrates utilized in the process. Energy crops typically increase the yield substantially with solid feed systems. Solid feed systems are those where chopped energy crops are continuously fed into the process.
A Complete Waste to Energy Solution:
A biogas plant consists of the following components:
- Liquid Manure Storage with Pump
- Feeding System for Solid Biomass
- Disinfection Unit
- Bio-Digester with Stirrers and Foil Covering
- Storage Tank System
- Combined Heat & Power Generators
Slurry and biomass are stored in the liquid manure storage area where feeding liquid material and additional biomass quantities are feed into the system. This liquid material is pumped from the liquid storage area into the heated digester which is the main area of any biomass plant. The mix in this digester is always specific to the input materials and must be well mixed and homogenous so that the bacteria and substrates are in close contact. This ensures a high gas yield. Included in a digester’s design is a pressure protection system which adjusts to both over-pressured and under-pressured scenarios as well as a gas holder or foil covering.
When this substrate is digested it is then pumped into a larger storage tank, repository, or lagoon. In this condition, it can be used for fertilizer, bedding, or pelletized into other uses.
A combined heat and power generator (CHP), also known as a cogeneration unit, converts the biogas generated into electricity and heat. Options available from GCES includes world-leading high and medium speed gas engines, complete drive systems, distributed energy systems, and fuel injection systems as stand-alone systems or as part of a larger solution.
Power is generated by a CHP unit through combustion which converts the biogas into electricity. This electricity can be used to power facilities, equipment, as a heat source, or as revenue through either private or public power sale.
Environmental conditions greatly affect the digestion of biomass with one of the most important factors being temperature. Temperature influences both the speed and stability of the anaerobic digestion process which in turn effects biogas production. There are two optimum temperature ranges that can be utilized in creation of bacteria to aid the digestion process:
- Mesophilic which is between 32 and 45°C
- Thermophilic which is between 50 and 65°C
During Mesophilic temperature ranges, methane bacteria experiences optimum growth which is why many biogas development facilities operate within this temperature range. The output is high gas yields and strong process stability.
Thermophilic digestion at 55°C is advantageous in processes that utilize animal by-products or organic waste. With this process a disinfection is required and thermophilic temperature ranges are considered for the operation of the plant. The disinfection process is simple and requires the mass to be heated at one hour at 70°C. This often results in a higher gas yield, but considerations must be made to assure stability as there is increased sensitivity to disturbances and irregularities during thermophilic digestion.
Waste heat from the cogeneration plant is often used to produce the heat necessary to heat the bacteria to the optimal digestion temperature.
To learn more about biofuels and the profit potential read this.