Clean Energy from Landfills and Anaerobic Digesters
Every day in the United States, on average, every man, woman and child throw away 4.5 pounds of waste. With a country population of over 300 million people almost 675,000 tons of waste per day must be disposed. This waste is generically made up of food scraps, packaging of all types, paper waste, broken toys and a host of other begotten items that, literally, clutter our lives. In the last decade of the 20th century many communities, concerned about the growing solid waste volumes, implemented recycling programs to reduce the waste stream. At the beginning of the 21st century many of these recycling programs are still going, but markets for recycled materials are not growing, and the panacea described in most recycling programs of recycling 50%-100% of our waste stream never materialized and today most of the solid waste created by our society ultimately ends up at a landfill.
Why use a landfill for waste disposal? Primarily, it is a matter of economics. Landfilling is a simple, environmentally sound, yet (relatively speaking) economic and effective way to dispose of solid waste and has controllable side effects. Side effects? The two most significant side effects or problems for landfills are 1) land use and 2) gas generation (source of odors). A modern day landfill near a metropolitan area can take in thousands of tons per day; cover hundreds of acres for operations. When municipal solid waste, with upwards of 70%-90% of organic material, is disposed in a landfill and covered up, millions of cubic feet per day of gas can be generated. Landfill gas (LFG) has its own distinct odor, and is arguably, the biggest complaint about landfills.
The good news is that landfill gas contains, generally, 50%-55% methane and can be a stable, reliable, and valuable source for energy. It is this potential energy source and the substantial opportunities to harvest gas from more than 700 landfills identified by the USEPA, and use that gas to generate electricity or clean the gas to provide Renewable Natural Gas (Clean Energy) that has allowed the gas to fit as a viable fuel source with our national green energy program.
Landfill gas is a highly reliable and long-term sustainable fuel source, but it is not the same as natural gas and must be treated accordingly. Where the gas is, how it is created, what it contains, and how it is extracted is important in being able to select the right equipment and how to understand and trouble shoot the system when problems arise.
Where Clean Energy Gas Comes From
A general understanding of the processes of anaerobic decomposition of organic material is necessary as a foundation to understand the impact of the gas on equipment selection. Landfill gas is produced by the process of anaerobic decomposition of organic waste. Basically, anaerobic bacterial cause the breakdown of complex organic material (vegetable and plant matter, wood and paper products, etc.) into simpler forms such as organic acids and ultimately to methane and carbon dioxide. The latter part of this process is known as methanogenesis. This process also occurs in the intestines of humans and animals. The constituents of the gas are created by the process and the organic material included in the process. Gas created by a landfill is called Landfill Gas and gas created by a digester is called Digester Gas. Both gases are what we call today Clean Energy. Although the various gases have many similarities there are also significant characteristics of each. A brief discussion of the more common methanogenetically produced gases and their characteristics are provided below:
Landfill – General Range of Landfill Gas (LFG) Constituents
|45 – 58
|32 – 45
|Less than 1, more than 1 may indicate air leak in collection system
|Trace to 5
|Trace – may indicated subsurface fire in landfill
|Hydrogen sulfide and other sulfur compounds
|Varies greatly by landfill – generally 10-200 PPMV
|Volatile organic compounds (VOC’s)
|Less than 2
|Always present in landfill gas from household products – Higher in new gas collection systems, declines over time
Characteristics of LFG are as follows:
- Specific gravity is close to specific gravity of air
- Typical temperature range is 60 to 125 degrees F within the landfill.
- Component gases (methane, carbon dioxide, water vapor and others) tend to stay together but may separate through soil and liquid contact.
- Secondary constituents (trace gases) may cause nuisance odors, environmental pollution, equipment problems and may create a health risk.
Digester – Characteristics of Digester Gas are as follows:
- Similar constituents of LFG. Primarily produced and used in municipal waste water treatment facilities. Organic material, typically, in slurry form (high percent of liquids) in controlled temperature range to maximize methanogenesis process.
- Methane range generally higher than landfill gas and may range up to 65%.
- The primary goal of a digester is to reduce solids content through organic oxidation and purify liquid discharge for safe disposal in public waters.
- is a generic term used to describe a broad range of gases produced by the anaerobic digestion or fermentation of organic material including plant matter, manure, municipal solid waste, or any other biogas feedstock. The primary constitutes are methane and carbon dioxide. Recent interest has been developed in cow, pig and chicken manure.
- other forms of similar gases that may be generated in large enough volumes to make it financially feasible to capture the gas include coal bed and coal seam methane streams.
Common Uses for Clean Energy
Since the mid 1980’s, Clean Energy gas has been identified as a reliable and stable fuel source for use in engines or turbines to create electricity or used in boiler systems as a replacement for natural gas. Clean Energy gas projects are typically small when compared to the overall US energy market and have been typically shunned by the large utilities, not because of the fuel but because national energy growth has been steady increasing and the utilities, typically, are concerned with adding substantial new energy capacity that is much more than the typical three megawatt Clean Energy gas project. This has allowed small developers or landfill-owning municipalities to be the champions for Clean Energy gas and it is not unexpected that certain uses for Clean Energy gas have been more successful than others. These common uses have also provided the best opportunity for financial gain from the project.
Every year or so a new research and development program may merge to use Clean Energy gas in a fuel cell, sterling engine or other application but the common uses for Clean Energy gas provided in the list below still offer the most opportunity for success.
High BTU Projects
High BTU projects have always seemed the “Holy Grail” of the Clean Energy gas industry. The goal of just “stripping” the gases you do not want and leaving the gas you do sounds very simple. Unfortunately, with Clean Energy gas it is never as simple as it may seem. High BTU projects describe projects that “strip” the carbon dioxide from the methane to create, in effect, natural gas (NG). Natural gas is approximately, free of moisture (dry), 96% methane and no more than 4% nitrogen, carbon dioxide, and oxygen. Any gas that goes into a natural gas pipeline is usually required to meet these very stringent requirements.
Clean Energy gas is 100% saturated, 50% methane, 43% carbon dioxide, 5% nitrogen, 1% or less oxygen, and less than 1% non-methane organic compounds. It is fairly easy to strip the carbon dioxide out of Clean Energy gas. However, this has the effect of doubling the nitrogen content (in our example from 5% to 10%) which causes our gas to exceed pipeline quality specifications.
Currently, there are approximately twelve (12) high-BTU projects nationwide that are in commercial operations, and a fair number of projects (10-12) reportedly under development. The more successful projects have gained access to the pipeline without needing to meet the more stringent pipeline requirements discussed above. To be able to do by-pass the current pipeline requirements would be on a case by case basis. Most of the current and proposed projects have selected pressure-swing absorption as the technology but there are also a number of membrane projects as well as mechanical strippers.
Regardless of the specific technology chosen for a high BTU project the science of how they work is generally the same. The stripping of gases uses high pressure to force the gas through a molecular sieve or absorption material that separates the gases by molecular size. The product gas or methane (the larger molecule of carbon dioxide or nitrogen) is trapped because of its size, allowing the CO2 and nitrogen to continue to move. The trapped methane is directed through the process one way and the waste gases (carbon dioxide and nitrogen) are directed in a different path. As much as 15% of available methane may be lost through methane still commingled with the waste gas.
Most successful Clean Energy gas projects use the gas for electrical generation. The success appears to be from both a process and a financial standpoint. Although there have been no specific studies conducted, a general review of failed or unsuccessful electrical generation projects appear to point the blame at vastly overestimating the availability of the gas. Further, the overestimating of available Clean Energy gas appears to be from an over-reliance on desktop gas generation models and installing more generation capacity than available gas. The problems can be further compounded by using the excess generating equipment to pull harder on the landfill to get more gas and produce more revenue that the landfill becomes overstressed and even less Clean Energy gas is available.