Energy Yields from a Farm-Based Anaerobic Digestion System
Table of Contents
- What Is a Farm-Based Anaerobic Digestion System?
- Organic Materials Added to a Farm-Based AD System
- Factors Contributing to Energy Yield from an Anaerobic Digester
- Introducing New Materials into a Digester
- Estimating Energy Yields from Feedstocks
- Energy Balance
This Infosheet gives the basic information required to calculate the type, amount and economic value of energy that could be expected to be produced from a farm-based anaerobic digestion system. It also provides an example of an energy balance for a digester.
What Is a Farm-Based Anaerobic Digestion System?
A farm-based anaerobic digestion (AD) system is a sealed, heated container located on a farm that breaks down organic materials to produce biogas. This biogas, containing approximately 60 per cent methane, is used to generate energy.
One feature of a farm-based AD system is the liquid component of the effluent from the digester is spread on a local land base as a crop nutrient source. The solid component (if available) may be spread as nutrient, used as a livestock bedding material, or sold as a compost or bio-material.
More information on farm-based AD systems can be obtained from the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) Infosheet titled Anaerobic Digestion Basics.
Figure 1. A farm-based anaerobic digestion system operating in Ontario.
Organic Materials Added to a Farm-Based AD System
The organic materials which are suitable for digestion can be placed into three general categories:
- Byproducts produced by a farming operation, including materials such as manure, bedding, feed waste, and runoff from silos.
Energy crops, including any crop grown for the express purpose of producing energy. The most common crop currently used in digesters is corn silage. There are many other crops that could be used such as forages or whole plant sugar beets.
- Off-farm source materials, includes a wide range of products and byproducts that are not part of a farming operation such as materials from food processing plants, source-separated organics, grease trap residuals, and other non-agricultural source materials.
The regulatory requirements for adding organic materials depends on the types and combinations of materials used. In some circumstances, a Certificate of Approval (C of A) under the Environmental Protection Act (EPA) may have to be obtained for both the AD facility and the application of the mixed materials on land. Recent changes to the EPA and Nutrient Management Act (NMA) will allow a limited percentage of certain off-farm source materials to be added without the requirement for a C of A (however requirements in the NMA will have to be met). Also this legislation change allows the anaerobic digestion output (which could be solid or liquid) from farm-based mixed AD facilities to fit within the definition of an agriculture source material if over 50 per cent of the inputs are generated at an agricultural operation (even if a C of A is required under Part V of the Environmental Protection Act for the AD system).
Contact the Agriculture Information Contact Centre for more information on this topic at 1-877-424-1300 or e-mail: firstname.lastname@example.org.
Factors Contributing to Energy Yield from an Anaerobic Digester
The energy yield from an anaerobic digester depends on the material used to "feed" the digester. There are five main characteristics of any given feedstock that affect the energy yield.
Dry Matter Content
Typically, the amount of available energy from a material rises with increased dry matter (DM) content. Materials with very low dry matter content (such as washwater or highly diluted manure) will have a very low (or negative) energy yield - that is, the energy required to heat the material for digestion may come close to or exceed the amount of energy produced by the digester. These materials may be used to dilute other materials or used if there is a financial incentive for treatment (e.g. a tipping fee).
Materials with a high dry matter content (>20 per cent DM) require different operational practices for the digester. These materials will have to be mixed with other, more dilute incoming materials, or be mixed with recycled effluent from the digester. Alternative ways of harvesting energy could also be considered instead of digestion (e.g. combustion of the material to produce heat in a biomass combustion system when the material is >70 per cent DM).
Most agricultural labs can perform dry matter determinations. Also, information sources such as OMAFRA's NMAN software program may provide useful estimates for the dry matter content of farm-based materials.
Volatile Solids Content
In addition to the basic dry matter content of a matter, the ability of that material to be broken down effectively in the digester must be considered (e.g. sand has a high dry matter content, but does not digest). Volatile Solids (VS) are organic compounds of animal or plant origin. They are sometimes called organic total solids (OTS). A higher VS value typically results in a higher energy yield. This value is specified as a percentage of dry matter content. For most materials used in farm-based AD systems, this value ranges from 63 to 98 per cent of the dry matter in the material. Several labs in Ontario can complete this measurement.
For most digesters, there is a recommended upper limit of volatile solids loading per day. If this limit is exceeded, the digester will become more difficult to operate on a stable basis and problems such as poor biogas production or foaming could occur. This limit is typically expressed as kilograms of volatile solids per day per cubic meter of digester capacity (kg/day/m3). Values of over 4.5 kg/day/m3 approach the operational limits of a digester, especially during start-up.
Biogas Output per Tonne of Volatile Solids
Biogas output is a measurement of biogas production of in the time period the material is expected to be in the digester. A higher value of biogas output per tonne of volatile solids gives a higher energy yield. This number varies greatly depending on the type and condition of the material. Information from Germany indicates a range of 200 m3 to 4,500 m3 of biogas per tonne of volatile solids for different materials typically used in a digester. There are charts available giving a range of yields expected. However, knowledge from a similar digester operating with the same materials or from a specialized lab that digests the material for a specified period (Figure 2) is recommended to ensure expected biogas production is achieved from the material.
Figure 2. Biogas production sampling technique.
Methane Content in Biogas
Biogas consists of methane, carbon dioxide, hydrogen sulphide, water vapour and other constituents. The methane content in biogas from agricultural sources typically ranges from 50 to 65 per cent. Many Ontario-based labs have the capability to measure methane content.
Inhibiting Components in the Feedstock
High nitrogen content of feedstocks could inhibit the digestion process especially at higher operational temperatures. Swine and poultry manure may have high enough nitrogen levels to cause this inhibiting effect. Materials such as copper sulphide or antibiotics in the feedstock may also inhibit digestion. Lab studies or knowledge from similar digesters running with the same materials is often necessary to ensure proper operation.
Introducing New Materials into a Digester
Digestion is a biological process. It takes time for this process to adjust to a different material being introduced into a digester.
Before a new product is introduced, there should be a plan developed for speed of introduction and steps taken to monitor and react to changes. For example, a plan could be to add new materials starting with 10 per cent of the full loading and gradually increasing to full loading in a four week period. This part could also include a concept to slow introduction of new materials and/or change agitation times if excessive foaming is detected in the digester.
Estimating Energy Yields from Feedstocks
Basic information is available to estimate energy yields for many feedstocks and combinations of feedstocks without full testing. However, to obtain an accurate estimate of expected yield, information will often need to be obtained from a consultant familiar with biogas technology. The consultant will typically use lab tests, results from similar facilities and experience to predict the yield. Figure 3 gives a summary of estimated biogas and energy yields calculated for three common feedstocks based on experience in Europe.
Source: Böhni Energie & Umwelt, Systemoptimierungen Wirtschaftlichkeitsuntersuchungen Umsetzung
m3/t = cubic metre per tonne
kWh/t = kilowatt hour per tonne
* Assumes 35 per cent conversion of biogas energy to electricity, 45 per cent conversion of biogas energy to heat. Some of this heat will be required to heat the digester. Electrical efficiency can vary from 25 to 42 per cent.
Energy Yield from Livestock
Using Figure 3, a basic estimate of biogas and energy production can be completed. The following steps show the calculations needed for a dairy farm that has 140 milking cows (plus replacements). Note: the calculations below are for information purposes only. Individual assessment by qualified personnel is required for a final design of a facility.
140 cows plus replacements produce approximately 5,600 tonnes per year of manure (OMAFRA MSTOR software program)
Electricity yield: 5,600 tonnes/year x 48 kWh/tonne = 269,000 kWh/yr (730 kWh per day)
Heat yield: 5,600 tonnes/year x 62 kWh = 350,000 kWh per year (950 kWh per day)
On cold winter days, 50 per cent of the heat might be required to maintain digester temperature. Thus, on the coldest winter day, 475 kWh/day of surplus heat is available. (At 3,413 BTU (British Thermal Unit) per kWh, this would provide heat equivalent to a standard 100,000 BTU furnace for 16 hours.)
For economic calculations below, 25 per cent of the total heat was assumed to be used by the farmstead as a heat source, giving a usable heat yield of 87,500 kWh/yr.
Energy Yield from Adding Off-Farm-Source Material
A farm-based digester can blend up to 10 - 25 per cent off-farm source material and work effectively. The following calculations are for the dairy farm described above with the addition of 10 per cent bakery waste.
560 tonnes/yr of off-farm source material will be added (10 per cent of 5,600 tonnes of manure)
Additional electricity yield: 560 tonnes/yr x 490 kWh/tonne = 274,000 kWh per year
Additional heat yield: 560 tonnes/yr x 630 kWh/tonnes = 360,000 kWh per year
- Total electricity generated from this farm:
269,000 (manure) + 274,000 (bakery waste) = 543,000 kWh/yr (1450 kWh/day)
Total heat yield generated from this farm:
350,000 (manure) + 360,000 (bakery waste) = 710,000 kWh/year
In this case, significantly more heat is produced than is produced from manure only. In many circumstances at livestock farms, there will not be sufficient use for all of this heat. For the following economic calculations, it is assumed that 25 per cent of the heat is utilized giving a usable heat yield of 175,000 kWh per year
Gross Value of Electricity and Heat
If the electricity from the above examples is sold at a value of 12¢/kWh (the approximate value of power from biogas systems if sold through the Renewable Energy Standard Offer Program, including peak power production), the current annual potential gross value of the electricity is $32,000 for the manure alone, or $65,000 for the manure mixed with off-farm-source materials. See OMAFRA Factsheet Anaerobic Digestion and the Renewable Energy Standard Offer Program and Infosheet Considerations and Opportunities for Building a Farm-Based Anaerobic Digester System in Ontario, August 2007 for more details on pricing..
Figure 4. Hopper holding corn silage to be fed into digester.
Using the above assumptions that 25 per cent of the total available heat is utilized, then the value of heat as a natural gas replacement ($0.05 per kWh) is $4,300 for the manure alone and $8,600 for the manure mixed with off-farm-source materials.
Energy Yield from Energy Crops
Many farms in Europe are using energy crops to produce biogas. The primary crop used is corn silage. Based on Figure 3, the following calculations are made using corn silage from one hectare.
An average crop on good soils produces 45 tonnes of corn silage
Electricity yield: 45 tonnes x 335 kWh per tonne = 15,000 kWh
- Total heat yield: 45 tonnes x 425 kWh per tonne = 19,000 kWh
- Usable heat yield =19,000 x 25% = 4,750 kWh
Note: this calculation uses the assumption that the farmstead uses 25 per cent of the heat. Many cash crop farms will not have any opportunity to utilize this available heat.
Total usable energy yield per hectare is 19,750 kWh (with heat recovery) or 15,000 kWh (without heat recovery).
This is the amount of energy available from one year's worth of crops. To capture the electrical power, roughly two kW of electrical generation capacity are required to operate on a continuous basis to capture the 15,000 kWh of available power (assuming 8,000+ hours of operation per year).
Gross Value of Electricity and Heat from Energy Crops
If the electricity from the above examples is sold at a value of 12¢/kWh (the approximate value of power from biogas systems if sold through the Renewable Energy Standard Offer Program, including peak power production), the current annual potential gross value of the electricity is $1,800 per ha. Using the previous assumption that 25 per cent of the total available heat is utilized, then the value of heat as a natural gas replacement ($0.05 per kWh) is $240 per hectare (ha).
Energy Input versus Output Comparison
There will be energy expended to produce renewable energy. Using corn silage as an example, the following inputs are required to grow, harvest, transport and digest 1 hectare of corn silage having a yield of 45 tonnes/ha (from OMAFRA data).
Energy Used to Produce and Digest Crop
940 kWh of energy in the 87.5 litres of fuel used to grow, harvest and transport one hectare of corn silage
423 kWh of energy in the 42.3 m3 of natural gas is required for the nitrogen to fertilize the crop
650 kWh of electricity is required to operate the digester
Total Energy Used = 2,014 kWh/ha
Calculations above have indicated that the Total Usable Energy Produced from one hectare of corn silage using an AD system is 19,750 kWh if some heat is utilized and 15,000 kWh if only the electricity value is captured.
10.2 per cent of the energy generated is required to produce and digest the crop if some heat is utilized
13.4 per cent of the energy generated is required to produce and digest the crop if no heat is utilized
Note: these calculations do not take into account hidden energy uses, such as driving the farm truck, energy required to produce concrete, etc.
Farm-based digesters have the capability to efficiently produce electrical and heat energy from organic materials produced on and off the farm. However, careful assessment should be completed to ensure that materials used will produce energy in an economically viable manner.
Figure 5. It takes about 10 - 14 per cent of the energy produced is required to plant, harvest and digest an energy crop.
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