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March 5, 2019March 26, 2019

Potential Air Quality Impacts of Anaerobic Digestion Of Dairy Manure

Waste to Worth home | More proceedings….

Abstract

Anaerobic digestion (AD) of livestock manure is better known for the economic return derived from biogas for energy rather than for its, inherent, environmental benefits. The effect of AD of dairy manure on the emissions of odor, ammonia (NH3), and greenhouse gases (GHG) including: carbon dioxide (CO2), nitrous oxide (N2O), and methane (CH4), during manure storage and also in subsequent land applications will be presented. Air samples were collected in 10-L Tedlar bags, at pertinent locations within the AD system, and shipped immediately to the lab for odor analyses by a trained odor panel using the “Dynamic Dilution Forced-choice Olfactometer.” Measurements of GHG emissions from both AD and non-AD manure storages were made using a floating chamber and a photoacoustic gas analyzer (INNOVA model 1412). Emissions of GHG were determined using the standard closed chamber method from field plots applied with AD and non-AD manure. Although odor analyses of collected air samples indicated increased detection threshold (D/T), odor strength (intensity) and unpleasantness (hedonic tone) decreased after AD of manure. Data indicated significantly higher fluxes of GHG from land applied with non-AD manure than from land applied with AD manure. Injection of non-AD manure further increased CH4 flux from applied manure. More than 50% emissions of CO2 and CH4 were observed during the first 3 days after manure was land applied. Emissions of GHG from the anaerobic lagoon holding AD manure, during all four seasons, were significantly lower than from the anaerobic lagoon with non-AD manure. In contrast, the reverse was observed with NH3 emissions suggesting potential increased emissions of NH3 during storage of post AD manure.

Authors

Pius Ndegwa, Washington State University ndegwa@wsu.edu

H.S. Joo, Biological Systems Engineering, Washington State University, PO Box 646120, Pullman, WA 99164; J.H. Harrison, E. Whitefield, Animal Sciences, Washington State University, 2606 West Pioneer, Puyallup, WA 98371; A.J. Heber, J.Q. Ni, Agricultural & Biological Engineering, Purdue University, 225 South University Street, West Lafayette, IN 47907

The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2013. Title of presentation. Waste to Worth: Spreading Science and Solutions. Denver, CO. April 1-5, 2013. URL of this page. Accessed on: today’s date.

March 5, 2019July 15, 2020

Biogas Utilization and Cleanup

Introduction

Biogas generated from anaerobic digestion processes is a clean and environmentally friendly renewable fuel. But it is important to clean, or upgrade, biogas before using it to increase its heating value and to make it useable in some gas appliances such as engines and boilers.

Biogas Utilization

While most large farms use their biogas for heat and power, it is worthwhile to consider all the options before deciding which path to take, including direct sale of biogas to an off-farm buyer.

Raw animal manure biogas contains 55 to 65% methane (CH₄), 30 to 45% carbon dioxide (CO₂), traces of hydrogen sulfide (H₂S) and hydrogen (H₂), and fractions of water vapor. For the anaerobic digestion of sludge or landfill processes, traces of siloxanes may also be found in biogas. These siloxanes mainly originate from silicon-containing compounds widely used in various industrial material or frequently added to consumer products such as detergents and personal care products. This article will not address the cleanup of biogas of siloxanes.

Biogas is about 20% lighter than air and has an ignition temperature in the range of 650 to 750 degrees C. (1,200-1,380 degrees F.). It is an odorless and colorless gas that burns with a clear blue flame similar to that of natural gas. However, biogas has a calorific value of 20-26 MJ/m3 (537-700 Btu/ft3) compared to commercial quality natural gas’ caloric value of 39 MJ/m3 (1,028 Btu/ft3).

Biogas can potentially be used in many types of equipment, including:

Internal Combustion (Piston) Engine – Electrical Power Generation, Shaft Power
Gas Turbine Engine (Large) – Electrical Power Generation, Shaft Power
Microturbine Engine (Small) – Electrical Power Generation
Stirling Heat Engine – Electrical Power Generation
Boiler (Steam) Systems
Hot Water Systems
Process Heaters (Furnaces)
Space or Air Heaters
Gas Fired Chiller – Refrigeration
Absorption Chiller – Refrigeration
Combined Heat and Power (CHP) – Large and Small Scale – Electrical Power and Heat
Fuel Cells – Electrical Power, Some Heat

There are a variety of end uses for biogas. Except for the simplest thermal uses such as odor flaring or some types of heating, biogas needs to be cleaned or processed prior to use. With appropriate cleaning or upgrade, biogas can be used in all applications that were developed for natural gas.

The three basic end uses for biogas are:

production of heat and steam
electricity generation
vehicle fuel

Production of heat or steam

The most straightforward use of biogas is for thermal (heat) energy. In areas where fuels are scarce, small biogas systems can provide the heat energy for basic cooking and water heating. Gas lighting systems can also use biogas for illumination.

Conventional gas burners are easily adjusted for biogas by simply changing the air-to-gas ratio. The demand for biogas quality in gas burners is low, only requiring a gas pressure of 8 to 25 mbar and maintaining H₂S levels to below 100 ppm to achieve a dew point of 150 degrees C.

Electricity Generation or Combined Heat and Power (CHP)

Combined heat and power systems use both the power producing ability of a fuel and the inevitable waste heat. Some CHP systems produce primarily heat, and electrical power is secondary (bottoming cycle). Other CHP systems produce primarily electrical power and the waste heat is used to heat process water (topping cycle). In either case, the overall (combined) efficiency of the power and heat produced and used gives a much higher efficiency than using the fuel (biogas) to produce only power or heat.

Other than high initial investments, gas turbines (micro-turbines, 25-100 kW; large turbines, >100 kW) with comparable efficiencies to spark-ignition engines and low maintenance can be used for production of both heat and power. However, internal combustion engines are most cmmonly used in CHP applications. The use of biogas in these systems requires removal of both H₂S (to below 100 ppm) and water vapor.

Fuel cells are considered the small-scale power plants of the future for production of power and heat with efficiencies exceeding 60% and low emissions. One of the largest digester/fuel cell units is located in Washington State. The fuel cell, located at the South Treatment Plant in Renton, WA, can consume about 154,000 ft³ of biogas a day to produce up to 1 megawatt (1,000,000 watts) of electricity. That’s enough to power 1,000 households, but it’s being used instead for the operation of the plant.

Vehicle fuel

Gasoline vehicles can use biogas as a fuel provided the biogas is upgraded to natural gas quality in vehicles that have been adjusted to using natural gas. Most vehicles in this category have been retro-fitted with a gas tank and a gas supply system in addition to the normal petrol fuel system. However, dedicated vehicles (using only biogas) are more efficient than these retro-fits.

Biogas Cleanup Or Upgrading

Biogas cleaning is important for two reasons: (1) to increase the heating value of biogas, and (2) to meet requirements for some gas appliances (engines, boilers, fuel cells, vehicles, etc). Desired biogas cleaning or upgrading purposes are summarized in Figure 1. “Full treatment” implies that biogas is cleaned of CO₂, water vapor, and other trace gases, while “reforming” is conversion of methane to hydrogen.

Figure 1. Alternative biogas utilization and required cleanup

CO₂ Removal

For many of the simpler biogas applications such as heaters or internal combustion engines or generator systems, carbon dioxide (CO₂) removal from biogas is not necessary and CO₂ simply passes through the burner or engine. For more demanding biogas/engine applications, such as vehicles that require higher energy density fuels, CO₂ is routinely removed. Removing CO₂ increases the heating value and leads to a consistent gas quality similar to the natural gas. Carbon dioxide can be removed from biogas economically through absorption or adsorption. Membrane and cryogenic separations are other possible processes.

Pressurized counter-current scrubbing of CO₂ and H₂S from biogas can be accomplished in water. For removal of CO₂ in particular; pH, pressure, and temperatures are critical. High pressures, low temperature, and high pH increases CO₂ scrubbing from biogas. Use of Ca(OH)₂ solutions can completely remove both CO₂ and H₂S. Both CO₂ and H₂S are more soluble in some organic solvents such as polyethyleneglycol and alkanol amines that do not dissolve methane. These organic solvents can thus be used to scrub these gases from biogas even at low pressures. Systems using these kinds of organic solvents can remove CO₂ down to 0.5% from the biogas.

However, use of organic solvents is much more expensive than water systems. Adsorption of CO₂ on solids such as activated carbon or molecular sieves is possible although it requires high temperatures and pressures. These processes may not be cost-effective because of associated high temperature and pressure drops. Cryogenic separation is possible because at 1 atm, methane has a boiling point of -106^oC, whereas CO₂ has a boiling point of -78^oC. Fractional condensation and distillation at low temperatures can thus separate pure methane in liquid form, which is convenient for transportation. Up to 97% pure methane can be obtained, but the process requires high initial and operational investments. Membrane or molecular sieves depend on the differences in permeability of individual gas components through a thin membrane. Membrane separations are quickly gaining in popularity. Other chemical conversions are technically viable, but their economics are poor for practical biogas-cleaning.

Water Vapor Removal

Straight from the digester, biogas will generally be saturated with vapor. Besides reducing the energy value of biogas, water can react with H₂S to create ionic hydrogen and/or sulfuric acid, which is corrosive to metals. Refrigeration or sensible pipe-work design can condense and remove the water. The biogas is normally compressed before cooling to achieve high dew points. Alternative water vapor removal mechanisms include adsorption on: (1) silica gel and Al₂O₃ at low dew points, (2) glycol and hygroscopic salts at elevated temperatures, and 3) molecular sieves.

Removal of Hydrogen Sulfide

Hydrogen sulfide in biogas needs to be removed for all but the most simple burner applications. Hydrogen sulfide in combination with the water vapor in raw biogas can form sulfuric acid (H₂SO₄), which is very corrosive to engines and components. At concentrations above 100 parts per million by volume (ppmv), H₂S is also very toxic. Activated carbon can be used to remove both H₂S and CO₂. Activated carbon catalytically converts H₂S to elemental sulfur. Hydrogen sulfide can also be scrubbed out from biogas in either: NaOH, water, or iron salt solutions. A simple and inexpensive process is dosing a stream of biogas with O₂, which oxidizes H₂S to elemental sulfur. Oxygen dosing can reduce H₂S to below 50ppm levels from biogas [Warning: IMPROPERLY DOSING A BIOGAS STREAM WITH O₂ CAN CREATE AN EXPLOSION HAZARD]. Iron oxide also removes H₂S as iron sulfide. This method can be sensitive to high water vapor content of the biogas. In addition to clean up of biogas of H₂S after it has been produced, available methods of reducing H₂S content from produced biogas include: co-digestions, multiphase digestion, reactor pH buffering, and removal of sulfur from feed substrates.

Conclusions

Biogas produced from animal waste can be a valuable energy resource. By combusting waste methane (biogas), a powerful greenhouse gas is eliminated that would otherwise be released. If used in simple burners for cooking or lighting the gas may not need to be treated prior to use. However, for uses that require the gas to be used in internal combustion engines, boilers or fuel cells, the biogas will probably need to be pretreated in order to remove corrosive or dangerous contaminants. The primary contaminant of biogas is hydrogen sulfide. This chemical will also react with water to form corrosive acids that can attack metals and plastics. Hydrogen sulfide is also toxic and sufficient quantities also present a possible health hazard if not treated.

Additional Resources

Intro | Feedstocks | Processing | Utilization

References

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Contributors

Authors

R. Scott Frazier, Renewable Energy Extension Engineer, Oklahoma State University, robert.frazier@okstate.edu
Pius Ndegwa, Washington State University

Peer Reviewers

Jerry Martin II, Environmental Engineer, USDA – ARS
Daniel Ciolkosz, Extension Associate, Penn State

March 5, 2019April 2, 2021

Introduction to Biogas and Anaerobic Digestion

Harnessing energy from livestock waste.

Intro | Feedstocks | Processing | Utilization

On-farm biogas production has long been a topic of interest for farmers, with historical records of biogas production going back several hundreds of years. In modern livestock production systems, for example, the benefits of producing biogas are significant and include:

provision of supplemental renewable energy
odor reduction
reduction of emissions of greenhouse gases
pathogen control
waste biostabilization
nutrients are preserved and transformed into plant-available forms

The economics of biogas production, however, are sometimes difficult to justify unless the accompanying environmental benefits and other by-products are considered.

What Is a Biogas?

Biogas is a by-product of the anaerobic (without oxygen) breakdown of organic matter. The organic matter could be any of a number of materials, but on the farm, it most often comprises animal manure or other agricultural waste.

The most important component in biogas is methane, a flammable gas that can be used in furnaces, for cooking, or even as an engine fuel. However, biogas also contains carbon dioxide and small amounts of hydrogen, hydrogen sulfide, nitrogen, and water vapor.

What Is a Digester?

A digester is a sealed vessel or container in which anaerobic digestion of organic matter occurs. The bacteria “feed” off the manure and, in the process, release biogas as a by-product. This process is referred to as anaerobic digestion, and the sealed vessel or container is thus usually referred to as an anaerobic digester. Anaerobic digestion also occurs in the anaerobic zones of open or unsealed swamps, bogs, and wastewater lagoons.

Today, farmers in developed countries are using digesters primarily to improve the quality of their manure and to reduce manure odors, the energy content of the methane being simply a by-product. However, as the price of energy increases, more farmers are looking at using anaerobic digestion as a way to generate supplemental heat and electricity for their farms. Digesters are a popular technology in rural areas of the developing world, where electricity and petroleum fuels are often unavailable or unaffordable.

What Does a Digester Look Like?

This is a 600,000-gallon plug-flow digester that creates biogas using the manure from 1,000 dairy cows.

Physically, digesters can come in many different shapes and sizes, varying from simple earthen lagoons to complex steel and concrete structures. In North America, the most common commercial farm digesters are usually buried concrete tanks with heavy plastic covers.

Take a virtual tour of one regional digester (hydraulic mix type) located in Cayuga County, NY. More…

How Does a Digester Work?

Fresh biomass entering a digester is supplied with anaerobic bacteria by the existing digested biomass, which is tremendously rich in these microbes. The digester tank provides a conducive environment for anaerobic microbes to “digest” the biomass, resulting in digested solids, liquids, and biogas. In general, the anaerobic digestion is a living process, requiring favorable conditions (temperature, moisture content, oxygen exclusion,and pH) and a steady food supply in order to flourish.

Manure from this dairy barn is automatically collected and delivered to a nearby anaerobic digester.

What Goes into a Digester?

Livestock manure is the most popular material, or feedstock, for anaerobic digestion on the farm, but almost any type of organic matter can be digested, including food waste, forestry residue, animal processing waste, and field crops.

What Can Go Wrong?

Probably the biggest problem in a digester occurs when the digester’s pH drops too low. In general, acid-forming bacteria grow much faster than methane-forming bacteria. This can reduce the pH to an unfavorable level for methane-forming bacteria, thus inhibiting the activity of methanogens. This is referred to souring and may result in failure or crashing of the anaerobic digester. In most cases, however, the pH is self-regulating, but bicarbonates are sometimes used to maintain consistent pH. The optimal pH range is between 6.8 to 8.5. Restarting a digester that has “soured” is not an easy task. Typically, the approach is to open the digester, excavate the soured material, then refill and restart the digester. This is a costly and unpleasant task and should be avoided whenever possible.

There are safety risks in dealing with biogas, including explosion, asphyxiation, disease, or hydrogen sulfide poisoning. Operators must be aware of the potential hazards and take preventative measures.

How Is Biogas Used?

Biogas generated from anaerobic digestion processes is a clean and environmentally friendly renewable fuel. There are many uses for this fuel, including use in engines, generation of electricity, heat and hot water systems, and even refrigeration.

This generator makes electricity using biogas from a digester on a 1,000 cow dairy farm.

Contributors to This Document