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Biomass - Anaerobic Digestion

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Anaerobic digestion (AD) is a biological process in which biodegradable organic matters are broken-down by bacteria into biogas, which consists of methane (CH4), carbon dioxide (CO2), and other trace amount of gases. The biogas can be used to generate heat and electricity. Oxygen-free is the primary requirement of AD to occur. Other important factors, such as temperature, moisture and nutrient contents, and pH are also critical for the success of AD. AD can be best occurred at two range of temperatures, mesophilic (30-40°C) and thermophilic (50-60°C). In general, AD at mesophilic temperature is more common even though digestion at thermophilic temperature has the advantages of reducing reaction time, which corresponding to the reduction of digester volume. Moisture contents in greater than 85% or higher are suitable for AD.

The types of anaerobic digesters include Covered Lagoon, Batch Digester, Plug-Flow Digester, Completely Stirred Tank Reactor (CSTR), Upflow Anaerobic Sludge Blanket (UASB), and Anaerobic Sequencing Batch Reactor (ASBR), and others. The complete-mix, plug-flow, and the covered anaerobic lagoon are three types of the digesters that are recognized by the USDA's Natural Resource Conservation Service (NRCS) in the form of "National Guidance provided to States."

The complete-mix digester is a large, vertical poured concrete or steel circular container. Today's complete-mix digester can handle organic wastes with total solid concentration of 3% to 10%. Complete-mix digesters can be operated at either the mesophilic or thermophilic temperature range with a hydraulic retention time (HRT) as brief as 10-20 days. 

The basic plug-flow digester design is a long linear through, often built below ground level, with an air-tight expandable cover. Organic wastes is collected daily and added to one end of the trough. Each day a new "plug" of organic wastes is added, slowly pushing the other manure down the trough. Plug-flow digesters are usually operated with a total solid concentration of 11%-13% at the mesophilic temperature range, with a HRT from 20-30 days. 

A cover lagoon is an earthen lagoon fitted with a floating, impermeable cover that collects biogas as it is produced from the organic wastes. The cover is constructed of an industrial fabric that rests on solid floats laid on the surface of the lagoon. The cover can be placed over the entire lagoon or over the part that produces the most methane. An anaerobic lagoon is best suited for organic wastes with a total solid concentration of 0.5%-3%. Cover lagoons are not heated. 

Covered lagoon digester O&M is simple and straightforward compared to complete-mix and plug-flow digesters. The capital cost for covered lagoon can be less than those required for the complete-mix and plug-flow types of conventional digesters. However, a key issue for covered lagoon is that digestion is dependent on temperature, therefore biogas production varies seasonally if the lagoon is not externally heated. This means that methane production is greater in summer than in winter. In general, a daily biogas production in summer could be averaged 35% higher than in winter. This may make end-use applications more problematic than plug flow and completed mix digesters. Another concern is that it can take an anaerobic lagoon as long as 1-2 years to achieve its "steady state" biogas production potential. 

Production of renewable energy, improvement on environmental pollution in air and water, reduction of agricultural wastes, and utilization of byproducts as fertilizers from anaerobic digestion (AD), has increased the attractiveness of the application of AD. AD technology is well developed worldwide. Of the estimated 5300-6300 MW worldwide anaerobic digestion capacity, Asia accounts for over 95% or 5000-6000 MW. Traditional, small, farm-based digesters have been used in China, India and elsewhere for centuries. The number of digesters of this type and scale is estimated to exceed 6 million. European (EU) companies are world leaders in development of the AD technology. Currently, EU has a total generating capacity of 307 MW from AD technology. The countries in EU with the largest development figures are Germany (150 MW), Denmark (40 MW), Italy (30 MW), Austria and Sweden (both 20 MW). Germany led the small on-farm digesters for odor control. Italy developed a series of farm AD systems. Larger, centralized anaerobic digestion plants, which utilize animal manure and industry waste in a single facility, are a newer development and most prevalent in Denmark where there are 18 plants (worldwide there are 50 or so, all within Europe). Municipal solid waste digestion is the newest area for anaerobic digestion. The most recent is for source-separated feedstocks, for which there are estimated to be over 150 commercial-scale plants. These plants have a combined capacity in excess of 6 million tons per year and the number of plants planned is increasing rapidly. 

Biogas to Energy Technologies

Basic technologies for the utilization of digester gas:

  • Medium-Btu Gas Use
    Medium-Btu biogas can be used in a number of ways. Typically after condensate and particulate removal, the biogas is compressed, cooled, dehydrated and then be transported by pipeline to a nearby location for use as fuel for boiler or burners. Minor modifications are required to natural-gas-fired-burners when biogas is used because of its lower heating value. Another alternative for biogas applications is to generate steam using a boiler onsite. The biogas, after condensate and particulate removal and compression, is burned in a boiler. The customer for this steam would need to be close to the site since high pressure steel insulated pipeline is expensive and heat is lost during transport.
  • Generation of Electric Power using reciprocating engines, gas turbines, steam turbines, Microturbine, and Fuel Cell:
    Electricity generated on-site using a reciprocating engine, steam turbine, or gas turbine, is being actively used. When a reciprocating engine is used, the biogas must have condensate and particulates removed. In order to move fuel gas into a gas turbine combustion chamber, the biogas must have most of the visible moisture and any particulates removed and then compressed. Using a steam turbine requires generating the steam first. Microturbine can be used to generate electricity at a capacity as small as 30 kW. However, issues exist in the high cost for biogas clean up and limited engine running time when a Microturbine is applied. The microturbine technology has not been commercialized. High cost associated with biogas clean up is also an important issue for potential application of the fuel cell technology.
  • Injection into an existing natural gas pipeline
    Biogas can be upgraded into high-Btu gas and injected into a natural gas pipeline. As compared with other power generation alternatives, the capital cost for sale of upgraded pipeline quality gas is high because treatment systems that are used to remove CO2 and impurities are required. Also, upgraded gas needs a significant amount of compression to conform to the pipelines pressure at the interconnect point. However, the advantage of pipeline quality gas technology is that all the biogas produced can be utilized.
  • Conversion to other chemical forms
    It is possible to convert the biogas to another form such as methanol, ammonia, or urea. Of these three options, conversion to methanol is the most economically feasible. In order to convert high methane content gas to methanol, water vapor and carbon dioxide must be removed. In addition, the gas must be compressed under high pressure, reformed, and catalytically converted. This tends to be an expensive process, which results in about 67 percent loss of available energy.

Opportunities for Biogas to Energy Development in California

  • Biogas from Animal Manure in California
    The tax incentives of the late 1970's and early 1980's encouraged the construction of approximately 18 commercial farm scaled digesters for energy production in California. Only 5 of those systems are running today and 3 of these are on pig farms and 2 of these are on dairy farms. Only 0.37 MW of power is generated from existing 5 digesters in CA although the total potential for animal waste to energy in California dairies is over 105 MW. Energy can be produced from different types of livestock including dairy, swine, poultry, turkeys and sheep and lambs wastes in California. California dairies have 1.4 million milk cows and is the second leading state in total number of milk cows. There are 2,308 dairy farms in California with an average size of 602 cows. Currently, only less than 1 percent of the livestock manure generated in CA is utilized.
    Livestock Population VS Production per animal (lb./day) Potential energy production (Btu/year) Electrical Potential (MW) Power Potential (kWh/animal/day)
    Dairy Cows 1,420,000 6.2 9.64E+12 73.37 1.24
    Swine 210,000 1.64 3.77E+11 2.87 0.328
    Poultry layers 25,632,000 0.048 1.35E+12 10.25 0.0096
    Poultry broilers 230,300 0.034 8.57E+09 0.07 0.0068
    Turkeys 21,000,000 0.091 2.09E+12 15.93 0.0182
    Sheep and lambs 420,000 0.92 4.23E+11 3.22 0.184
    Total 1.39E+13 105.71
  • Biogas Gas from Sewage Wastewater Treatment Plants in California
    There are 242 sewage wastewater treatment plants existing in California. Anaerobic digesters exist in a number of sewage treatment plants. About 38 MW of electrical power is generated from existing 10 sewage wastewater treatment plants. There are 12 sewage treatment plants that utilize the biogas to produce hot water or heat the digester. The rest of 220 sewage wastewater treatment plants either don't recover biogas produced from anaerobic digester or do not have anaerobic digesters on sites. About 36 MW of electrical potential can be recovered from the 220 sewage wastewater plants. Biogas to electricity potential is estimated from existing 220 sewage treatment plants. As shown from the chart below, except two medium sites (1000 kw < biogas to electricity potential < 5000 kW), most of the sewage treatment sites have a small electrical potential (<1000 kW). There are 168 sewage treatment plants that have a biogas to electricity potential of less than 200 kW.

Research Plan on Potential Development of Anaerobic Digestion Technology

Nearer term opportunities

  • Conduct information outreach to educate California communities, policy makers and California AD industry on opportunities and benefits associated with AD development in California.
  • Conduct solicitation on AD development
  • Establish a forum to coordinate, plan and evaluate AD development
  • Help assist in technology development, environmental responsiveness and community oriented financing of AD projects
  • Encourage research activities on improving biogas yield and electricity conversion efficiency, and reducing cost of AD.
  • Encourage research activities on small-scale engine generator to fit the need of a typical size using AD technology.

Longer term opportunities 

  • Encourage research activities on improving biogas yield and electricity conversion efficiency, and reducing cost of AD.
  • Encourage research activities on small-scale engine generator to fit the need of a typical size using AD technology.
  • Development of AD using advantaged technologies (i.e., high rate at high solid concentration, thermophilic temperature, advantaged digester design)
  • Encourage research activities on improving biogas yield and electricity conversion efficiency, and reducing cost of AD.