Energy yields from a farm-based anaerobic digestion system

Introduction

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.

Organic Materials Added to a Farm-Based AD System

The organic materials which are suitable for digestion can be placed into three general categories:

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: ag.info.omafra@ontario.ca.

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.

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.

Estimate:

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.

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..

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.

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.

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

Energy Balance

Note: these calculations do not take into account hidden energy uses, such as driving the farm truck, energy required to produce concrete, etc.

Summary

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.