Sustainability | Renewable Energy

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Low Carbon Energy - Biogas from AD Plants; Waste-to-Energy

diagram of anaerobic digester
diagram of anaerobic digestion process

Anaerobic digestion (AD)- Low Carbon Climate  Solution 

Instead of waste simply ending up in landfills, or being incinerated, waste can be turned into renewable energy by use of anaerobic digestion (AD). Anaerobic digestion is the process of producing energy from agricultural waste (such as livestock manure, and/ or other organic waste from agricultural processes), wastewater, municipal waste streams, energy crops, and/ or other crops typically used in biomass/ biofuel production. Through the use of anaerobic digesters, farms can be entirely powered by waste from their livestock, food waste, crop/ agricultural plant waste (waste normally simply disposed of on most farms), energy crops/ other organic biodegradable material, and wastewater.

AD can be used for municipal, commercial, and industrial waste, in addition to agriculture. There are a number of applications, besides farms, for AD plants; such as businesses and municipalities. AD can be used in many situations for converting organic matter.

Examples of feedstocks for AD are food processing waste, biodegradable organic material, sewage, and other waste streams. The use of AD can make farm waste, sewage, and wastewater treatment facilities energy neutral or even energy positive; translating to huge cost savings for municipalities.

Where is the energy produced through AD used?

The renewable energy generated through AD can become energy for - the municipal power and gas grid, just the farm(s) with the AD units (as in on-site anaerobic digesters on farms), energy to heat and cool homes, or even fuel for the transportation sector. (AD can produce fuel only after being converted into a usable form for transportation fuel, as discussed below). Organic waste finds a constructive purpose in an AD biogas plant, as the waste is transformed into renewable energy.

Other than energy, what else is produced through anaerobic digestion? 

AD also produces numerous beneficial by-products (digestate by-products) for farms. Examples are animal bedding, and low-cost fertilizer or enhanced organic fertilizer (from digestate that has a large quantity of nutrients and can enhance soil).

The molecules of the organic waste used in an anaerobic digester are broken down by the micro-organisms in the AD plant into a useful form, like glucose. The "digested" raw material is used to create biogas. The remaining material can then be harvested as nitrogen and phosphorus-rich digestate, which can be used as fertilizer. The remaining material can also be used in animal bedding.

Biogas can be purified (the gas can also be upgraded with hydrogen) for use as pipeline-quality synthetic natural biogas for the grid. Biogas can also be converted into biomethane. Biomethane can be converted into liquefied natural gas (LNG), or compressed natural gas (CNG). These types of gas fuels, technically bio-natural gas, are used for vehicles, tractors, or other farm equipment. The use of these fuels requires vehicles to be modified to run on LNG or CNG (see below).

Anaerobic digestion process and applications

anaerobic digesters

Anaerobic digestion uses micro-organisms (enzymes, bacteria etc...) in an anaerobic digester. AD can be used in the form of a municipal anaerobic digestion power plant. However, AD is more commonly an on-site anaerobic power plant, as in an anaerobic digester on a large farm. Anaerobic digesters break down organic material and create biogas (in addition to biomethane, LNG, and CNG). Biogas can be converted into syngas (SNG), after being upgraded with hydrogen. 

The anaerobic process also occurs naturally in landfills. The process of using methane produced naturally in landfills (for municipal energy grids) is one example of the use of biogas as renewable energy. Clean, renewable energy, in the form of biogas, is created through the process of methanation in a wide variety of AD plants, biogas plants, and methanation plants.

AD plants are found on farms, on landfills, and in municipalities throughout the world. Because an AD plant generates biogas (and/ or biomethane) that is often burned on-site to generate heat, energy, or both - the process of anaerobic digestion easily can be integrated into combined heat and power (CHP) plants as well.

Waste-to-energy in conjunction with AD

In addition to AD, waste-to-energy is produced in landfills, using landfill refuse and landfill gases (methane and CO2). The use of AD in a biomass plant and waste-to-energy are cost-effective ways to produce renewable energy. Waste-to-energy also leads to less landfill waste. It is a constructive way for farms, businesses, and municipalities, to dispose of waste. Waste-to-energy creates renewable energy for municipal grids, energy for farms, heat for homes, and even transportation fuel.

Biogas can be upgraded with hydrogen, creating biomethane. Like conventional natural gas, biomethane can be used as a transportation fuel. This requires an additional process, to LNG or CNG.

When biogas is used for transportation, as LNG or CNG - both fuels can be used in place of diesel, given modifications to the vehicles in question. With the use of these fuel types, there are significant greenhouse gas emissions (GHGs) reductions.

A selection of buses in Oslo, Norway are run on CNG produced from sewage treatment and organic waste. These buses see a dramatic (around 70%) reduction in GHGs compared to fossil fuel-burning vehicles. Food waste and other waste processed through AD also bring the benefit of reducing GHGs substantially by reducing landfill waste. Also, when AD is used for on-site electrical generation, and energy generation for a municipality, farm, or wastewater facility, GHGs overall is greatly reduced. Energy produced by AD has a very low overall carbon footprint.

anaerobic digestion power plant in cannock
Poplars AD plant, Cannock, England

The AD plant at Cannock, Staffordshire, England (called the Poplars AD plant), is an example of a successful, large-scale AD plant. The £24 million project treats commercial and industrial waste streams to create, through methanation, around 6MW of renewable energy for the national grid. The Poplars plant also produces SNG and other products through the AD process.

The Poplars plant shows that a large-scale anaerobic digestion project is viable. AD has been successful in many commercial operations as well; from Indiana, to China and India, and in dozens of countries throughout the world.

Another large-scale anaerobic digestion facility with success in a commercial application is in Orlando, Florida, from food (and other) waste sourced primarily from the Walt Disney World Resort. The AD plant near Disney World is fed with waste from the amusement park's operations to produce renewable energy. The California company  Clean World developed a biodigester for Sacramento. This Sacramento biogas plant transforms 100 tons of food waste per day into natural gas, electricity, and fertilizer.

Please see:

Renewable energy: biomass and biofuel

Please also see:

Gasification - syngas from fossil fuels and environmentally friendly versions

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