IRENA Statistics Measuring Small-Scale Biogas 2016 PDF

Measuring small-scale biogas capacity and production Copyright © IRENA 2016 Unless otherwise stated, this publication and material featured herein are the property of the International Renewable Energy Agency (IRENA) and are subject to copyright by IRENA. Material in this publication may be freely...

Measuring small-scale biogas capacity and production Copyright © IRENA 2016 Unless otherwise stated, this publication and material featured herein are the property of the International Renewable Energy Agency (IRENA) and are subject to copyright by IRENA. Material in this publication may be freely used, shared, copied, reproduced, printed and/or stored, provided that all such material is clearly attributed to IRENA and bears a notation that it is subject to copyright (© IRENA), with the year of the copyright. Material contained in this publication attributed to third parties may be subject to third party copyright and separate terms of use and restrictions, including restrictions in relation to any commercial use. ISBN 978-92-95111-12-7 (PDF) This report should be cited: IRENA (2016), Measuring small-scale biogas capacity and production, International Renewable Energy Agency (IRENA), Abu Dhabi About IRENA The International Renewable Energy Agency (IRENA) is an intergovernmental organisation that supports countries in their transition to a sustainable energy future, and serves as the principal platform for international co-operation, a centre of excellence, and a repository of policy, technology, resource and financial knowledge on renewable energy. IRENA promotes the widespread adoption and sustainable use of all forms of renewable energy, including bioenergy, geothermal, hydropower, ocean, solar and wind energy, in the pursuit of sustainable development, energy access, energy security and low-carbon economic growth and prosperity. www.irena.org Prepared by Samah Elsayed, IRENA Programme Officer, Capacity Building in Statistics For further information or to provide feedback, please contact the IRENA statistics team ([email protected]). This report is available for download: www.irena.org/publications. Disclaimer This publication and the material featured herein are provided “as is”, for informational purposes. All reasonable precautions have been taken by IRENA to verify the reliability of the material featured in this publication. Neither IRENA nor any of its officials, agents, data or other third-party content providers or licensors provides any warranty, including as to the accuracy, completeness, or fitness for a particular purpose or use of such material, or regarding the non-infringement of third-party rights, and they accept no responsibility or liability with regard to the use of this publication and the material featured therein. The information contained herein does not necessarily represent the views of the Members of IRENA, nor is it an endorsement of any project, product or service provider. The designations employed and the presentation of material herein do not imply the expression of any opinion on the part of IRENA concerning the legal status of any region, country, territory, city or area or of its authorities, or concerning the delimitation of frontiers or boundaries. CONTENTS 1. INTRODUCTION............................................................................................................... 1 What is a biogas digester?.................................................................................................................................. 1 Why are biogas statistics collected?................................................................................................................ 1 Guidance structure............................................................................................................................................... 2 2. THE MAIN PARAMETERS AFFECTING BIOGAS PRODUCTION......................................... 3 Digester technologies.......................................................................................................................................... 3 Plant capacity......................................................................................................................................................... 4 Total feedstock volume....................................................................................................................................... 4 Feedstock properties........................................................................................................................................... 5 Feedstock retention time.................................................................................................................................... 5 Temperature........................................................................................................................................................... 5 3. PLANT CAPACITY............................................................................................................ 6 Administrative data.............................................................................................................................................. 6 Survey of standardised biogas plant types................................................................................................... 7 Survey of biogas plant dimensions.................................................................................................................. 7 4. BIOGAS PRODUCTION AND CONSUMPTION............................................................... 12 Plant capacity........................................................................................................................................................ 13 Appliance use........................................................................................................................................................ 13 Feedstock use....................................................................................................................................................... 15 Fuel substitution................................................................................................................................................... 18 Direct measurement............................................................................................................................................19 5. OTHER INFORMATION ABOUT BIOGAS PRODUCTION................................................. 20 Financial and technical performance........................................................................................................... 20 Emissions reductions........................................................................................................................................... 21 Energy access and socioeconomic impacts................................................................................................. 21 REFERENCES........................................................................................................................... 23 APPENDIX 1: BIOGAS PLANT CALCULATION TABLES............................................................ 25 APPENDIX 2: BIOGAS DATA COLLECTION IN CHINA............................................................ 29 Rural renewable energy statistics system................................................................................................... 29 Green county monitoring system.................................................................................................................. 30 1. INTRODUCTION Interest in small-scale biogas technologies has increased in recent years across Africa, Asia and Latin America. This has been driven by the socio- KEY REQUIREMENTS FOR ` economic and environmental benefits of using DIGESTER OPERATION biogas, such as reduced firewood and kerosene consumption, lower emissions of greenhouse gases An airtight environment with no and indoor air pollutants and the possibilities to use oxygen biogas to treat human and animal wastes (Bunny & Besselink, 2006). Given this trend, biogas use needs Moderate temperatures to be accurately monitored and measured. No light Small feedstock particles with This guide aims to help energy statisticians to broken down fibres and minimal measure and estimate the capacity and production lignin (wood) content of biogas plants, as well as other aspects of biogas For some feedstocks a starter production. The main focus is on data collection bacteria is also necessary from small-scale household, communal or farm biogas plants that produce biogas in a continuous monitoring of renewable energy targets may only process (i.e. feedstocks are added and biogas need very basic data that can be obtained from a removed every day). few biogas questions in a census or household survey. The following are some examples of how The methods described here may also be used for biogas statistics may be used and the sorts of data large-scale biogas plants or plants producing that might be needed. electricity, but such facilities should have monitoring devices to measure production more easily and National energy statistics accurately. In some countries, biogas contributes significantly to the national energy balance. It may be used both WHAT IS A BIOGAS DIGESTER? as a fuel and to generate electricity. To produce these statistics, biogas data collection should focus The main part of a biogas plant is the digester, which on plant capacity and production of biogas and is an airtight container in which bacteria break down electricity. organic waste through a process of anaerobic fermentation.1 This generates a gas (biogas) that is Project and policy monitoring mostly methane and carbon dioxide (CO2). This gas More detailed biogas statistics may be required to can be used for cooking, heating and lighting, or it monitor implementation of biogas projects, can be used to generate electricity. As more material programmes and policies. Important variables to is added to the digester, a liquid waste (slurry) is monitor can include: the number and size of biogas also produced, which can be used as a fertiliser. plants installed; their condition; the amount of biogas produced; and a range of socio-economic Biogas digesters can vary greatly in capacity, impacts. ranging from small-scale units used by households to larger communal and industrial digesters. Tracking progress towards targets Feedstocks added to the digester can include many Household biogas plants are often used to produce types of biomass such as animal, food and biogas for cooking, as biogas is a clean and modern agricultural waste, but materials that are difficult for alternative to using solid biofuels. Statistics about the bacteria to digest (e.g. wood) should be the numbers of households using biogas are needed avoided. The amount of biogas produced depends to track progress towards clean energy goals, such on a range of factors including the type and amount as the energy access target under the Sustainable of biomass used, the digester size and temperature. Development Goal for energy (SDG 7) “to ensure access to affordable, reliable, sustainable and WHY ARE BIOGAS STATISTICS COLLECTED? modern energy for all.” The amount and type of biogas data collected will Measuring environmental impacts depend on how this information will be used. For Biogas can reduce the environmental impact of example, project and policy monitoring may require energy use in many ways. Switching to biogas can the collection of specific and detailed data, while reduce CO2 emissions from energy use, as well as methane emissions (if biogas is produced from 1 Anaerobic fermentation means that the fermentation or digestion occurs in the absence of oxygen. 1 waste). It can also have positive benefits for indoor GUIDANCE STRUCTURE air pollution and land degradation when it replaces the use of solid biofuels. To measure these impacts, The remainder of this document is divided into four information may need to be collected about several sections. social and environmental indicators as part of the collection of biogas statistics. The first section describes the main variables influencing digester operation such as the digester International energy statistics technology, the type and amount of feedstock used, This guide, produced as part of the statistics plant capacity and digester temperature. capacity building programme at the International Renewable Energy Agency (IRENA), is intended to The second section looks at two ways to collect data help countries collect and report their biogas data in about the capacity of a biogas plant. This includes an internationally comparable format. guidance about how to make volume calculations based on typical biogas plant designs. In energy balances, biogas data is usually reported in megajoules (MJ), so conversion factors to convert A third section describes five different ways to from cubic metres of gas to MJ are given in section estimate or measure biogas production and 4 of the guidance. Other advice is also given in the consumption, including: estimates based on text about where different elements of biogas digester size; estimates based on appliance use; production and consumption should be recorded in estimates based on feedstock use; estimates based an energy balance on fuel substitution; and direct measurement of gas production. With the methodologies and techniques described here, it is hoped that the availability of biogas data The final section describes how the environmental will improve, so that IRENA’s statistics will capture and socio-economic impacts of biogas use may be more information about this rapidly growing part of monitored, including: emissions reductions; effects the renewable energy sector. on women and children; and health benefits. 2 2. THE MAIN PARAMETERS AFFECTING BIOGAS PRODUCTION This section presents an overview of some of the the dome at the top of the digester. Biogas is main variables that affect biogas production removed from the digester using a pipe attached to including: the digester type; digester size (measured the top of the dome. either as volume or expected production level); the type and amount of feedstock used in the digester; As biogas is produced, slurry is pushed out from the feedstock retention time; and temperature. digester through the outlet pipe into a displacement tank. When the biogas is used, this slurry flows back into the digester. Some designs may also include an KEY DIGESTER TYPES additional gas storage tank connected to the gas outlet pipe. Fixed dome plant The typical Chinese fixed dome plant often consists (Hemisphere, Deenbandhu and of a cylindrical digester with a round top and flat or Chinese designs) curved bottom. Other variations of the fixed dome Floating drum plant plant include the Deenbandhu and Deenbandhu 2000 model biogas plants developed in India, which Balloon/bag digester have a dome at the top and a curved base (Kudaravelli, 2013). Another type is the CAMARTEC model designed by GIZ for use in Tanzania, which is DIGESTER TECHNOLOGIES built as a series of brick rings in the shape of a dome on top of a flat base (Sasse et al, 1991). There are a number of different designs of small- scale biogas digesters and recording the type of Floating drum plant digester is important for estimating its capacity, particularly in cases where the digester is partly underground. If the owner of the digester has an operator’s manual or watched it being installed, then they should know what type of digester they have. If they do not know, then the data collector will need to look at the digester to determine its type. Fixed dome plant Figure 2: Floating drum plant The floating drum plant was designed and developed in India. It comprises a brick lined pit that is often partly underground (the digester) and a drum above ground is used as the gas collector. A popular design is the Khadi and Village Industries Commission (KVIC) digester. The drum is typically Figure 1: Fixed dome plant made of steel although some newer designs use fiberglass reinforced plastic. The fixed dome plant was originally developed in China, where there are now several million of these Water and feedstocks are combined in a mixing pit types of biogas plant. which then flows into the underground digester through the inlet pipe. As gas is produced, it is The digester in a fixed dome plant consists of an collected in the drum, which moves up and down a underground pit lined with concrete or brick, with an central guide pipe depending on the amount of gas inlet pipe that is used to add feed to the digester. being stored. The gas is held under pressure from Gas is produced under pressure and is stored under 3 the weight of the drum, which can be increased with capacity in this way. For example, in India, the the addition of weights. expected amount of gas produced each day may be called the plant capacity or plant size. Biogas As more feedstocks are added, slurry flows out production can be reported in this way because the through the outlet pipe. A gas outlet pipe is also feedstocks used in India usually do not change very attached to the drum to remove gas from the plant. much, leading to a predictable daily production level. In other countries (e.g. China), the feedstocks A variation of this design is the small-scale above used for biogas production vary greatly from place ground floating drum plant developed in India by to place and at different times, so plant capacity is the Appropriate Rural Technology Institute (ARTI). recorded as the volume of the plant rather than the The ARTI biogas plant is made for the digestion of volume of production per day (INFORSE South Asia, household food waste and is made from two plastic 2007). water tanks with their tops removed and the smaller tank placed upside down inside the larger one. Pipes To avoid confusion, this guidance will use “rated are added to the outer tank to add feedstocks and daily gas production” in calculations when the remove the slurry and a gas outlet pipe is added to calculations use an expected daily production level the top of the inner tank (AIDG, 2009). and will use “total plant volume” if the calculations are referring to the volume of the plant. Balloon/bag digester Rated daily gas production The rated daily gas production is the volume of gas that a biogas plant is designed to produce each day if operated under optimal conditions. Measured in m3/day, this shows the amount of biogas that is produced (not the amount of methane). The energy content of biogas will mostly depend on the methane content of the biogas, which should be about 65%. This needs to be taken into account when converting figures from gas production to energy production. Biogas will also contain other Figure 3: Balloon digester gases such as carbon dioxide, nitrogen and small amounts of hydrogen, but these have little impact Balloon digesters are often used in Latin American on the energy content of the biogas. countries. This type of digester is usually made from a large, strong plastic bag connected to a piece of Total plant volume drainpipe at either end, with these pipes being used The total plant volume is the sum of two to add feedstocks and remove slurry. components: the digester volume and the gas storage volume. It is measured in m3. To avoid damage to the bag, the digester is usually placed in a trench and the trench is slightly deeper The digester volume is the maximum amount of at the slurry outlet so that the slurry will settle there. slurry that the plant can hold, while the gas storage As gas is produced the top of the bag inflates and volume is the amount of gas it can hold when full of the gas can be removed through an outlet pipe in slurry. The latter is usually a proportion of the the top of the bag. Gas pressure can be increased by former. The digester volume may also include a placing weights on top of the bag (Vögeli et al, safety margin or “dead zone volume”, which is used 2014). to prevent waste overflow on days when biogas production is higher than normal or biogas use is In China, a common variation of this design is the unusually low. bag digester, which is a circular concrete, brick or plastic lined container covered with a plastic bag or TOTAL FEEDSTOCK VOLUME tent. As with a fixed dome plant, inlet and outlet pipes are used to add feedstocks and remove slurry The total feedstock volume is the average amount and the gas is collected and removed from the bag. of material added to the biogas plant each day. This includes the amount of waste and other feedstocks PLANT CAPACITY added each day (waste volume) and the amount of water added to these inputs (added water volume). The capacity of a biogas plant is the maximum total The total feedstock volume should be recorded in volume of gas and slurry that it can contain. m3/day, but this can be measured in litres and However, not all countries measure biogas plant converted to cubic metres (with three decimal 4 places) by dividing the measured amount by 1,000 matter divided by the total (wet) weight of the (i.e. 1 m3 = 1,000 litres). feedstock and is recorded as a percentage of the total (wet) weight. Waste volume The waste volume is the volume of organic material So, for example, if 1 kilogram (kg) of household added to the digester. This can include animal, kitchen waste contains 600 g of water, then its human, food and agricultural waste, as well as other moisture content is 60% and its total solid content is feedstocks collected and used in the digester. 40%.2 Added water volume Volatile solid content For optimal biogas production, water must usually The volatile solid content is the proportion of the be added to the waste, and other feedstocks put solid material that can be digested by the bacteria into the digester. The owner or operator of the and turned into biogas in the digester. This will be biogas plant should know how much water they different for each type of waste or feedstock. must add when they are putting material into the digester. However, if this is not known or cannot be The volatile solid content is measured as the weight measured directly, the added water volume can be of volatile solids in the material divided by the total assumed to equal the waste volume (most biogas weight of solids in the material and is recorded as a plant manuals recommend adding water to the percentage of the total solid content. Estimates of waste in a ratio of 1:1). the volatile sold content of different feedstocks are not measured in the field, but are usually taken from the biogas literature. MAIN PARAMETERS FEEDSTOCK RETENTION TIME Plant capacity The retention time is the average amount of time Total feedstock volume that feedstocks will stay in the digester before they Feedstock properties are pushed out through the outlet pipe. The Feedstock retention time retention time is measured in days and is simply Temperature calculated as the digester volume divided by the total feedstock volume (i.e. daily waste and added water volume). Because the digester volume is usually measured in m3, this is why the feedstock FEEDSTOCK PROPERTIES volume is also usually recorded in m3. For each type of material added to the biogas TEMPERATURE digester, the amount of biogas produced will depend on how much of the material can be The temperature within the digester is an important digested and converted into biogas by the bacteria variable that affects the speed of gas production in the digester. (production per day) and the total amount of biogas that can be produced from any feedstock. The amount of material (feedstock) that can be digested will depend on two variables: the total In small-scale biogas plants, the bacteria that solid content and the volatile solid content of the produce biogas work most effectively when the material added to the digester. If these two figures temperature of the slurry is 20-45°C. Within this are known, then the amount of biogas that can be range, biogas production will vary, so the size of a produced can be calculated using figures found in biogas plant is usually designed to ensure that daily various studies (see Section 4 for some examples). gas production is maximised. Total solid content In colder climates, the slurry may need to be heated Most organic material contains a significant amount to keep it within this temperature range, in which of water. This is known as its moisture content and, case some of the biogas may be used for this as it is only water, does not contribute to biogas purpose. If this is the situation in a country, then this production. share of the biogas production should be recorded as consumption in the energy sector in that country. The total solid content of a feedstock is the opposite of this and is the amount of dry matter present in the material. This is measured as the weight of the dry 2 Note that this calculation does not include any additional water added to the waste before it is put into the digester. 5 3. PLANT CAPACITY The section starts by describing how administrative If there is a wide variety of different biogas plant data and local knowledge should be used to develop designs used in a country or if biogas plants are built the overall approach to collecting biogas capacity in many different sizes, then a more complicated statistics. It then presents two options for collecting questionnaire design may be needed to collect the this data: measurements required to calculate capacity. 1. Survey of standardised plant types Sampling and interpretation of results 2. Survey of biogas plant dimensions To reduce the cost of data collection, survey locations are often organised into clusters and To simplify calculations, tables at the end of this information about the number of biogas plants in report present estimates of total plant volume for a different parts of a country can be used as part of range of different digester types and dimensions the process to select survey locations. (diameter, height, length, etc.). For example, at a very simple level, administrative ADMINISTRATIVE DATA data can show where most biogas plants have been built, so that the selection of survey sites can focus In many countries, most small-scale biogas plants on those areas. For a more complex survey design, have been built under biogas projects or this information can also be combined with other programmes supported by the government, non- relevant variables (e.g. local climatic data) to stratify governmental organisations (NGOs) or other similar locations before selecting clusters. agencies. These organisations may have databases that record useful information such as the number of Administrative data is also be useful for interpreting biogas plants built in different areas, the type and the survey results, so that the total capacity size of biogas plants built and possibly even the recorded in the survey can be multiplied by an detailed location of biogas plants (for an example appropriate factor to produce an estimate for the from China, see Appendix 2).3 whole country. If administrative data such as this is available, it If the administrative data includes details about should be collected as part of the development of a variables such as the size and types of biogas plants biogas survey, because it can be used to improve in different parts of a country, it may also be used to survey design. Even if such information is not adjust the survey results to make them more available, obtaining local knowledge and expert representative (post-stratification). opinion may still be useful to assist with some aspects of the survey design. Annual updating between surveys The third important use of administrative data is to Biogas plant designs and capacities update national biogas statistics between surveys. Information about the different types of biogas plants in a country can be used in the questionnaire Most types of biogas digester should work for many design. years, so collecting detailed biogas data every year will not be cost-effective. Instead, data can be For example, if all of the biogas plants in a country collected using surveys every five years, and are built to one design, then the dimensions that administrative data can be used to update these should be measured and recorded in the biogas statistics (with estimates) in the years between survey can be standardised on the questionnaire, surveys. the conversion of these measurements into volumes can be simplified, and the training of data collectors For example, the number of new biogas plants built can focus on these measurements. Furthermore, if in a country each year can be compared to the biogas plants are built in a limited number of number at the start of the year, so that capacity and standard sizes that are well known to plant owners production figures can be increased by a similar or can be easily recognised in the field, then it may proportion. If administrative data is available split be possible to collect capacity data using a few into plant types and/or size categories, then the simple categories (see Module M1 below). 3 Other useful data sources for survey development include household or business energy surveys, population and housing censuses, living standards surveys or agricultural censuses. If information about energy use is collected in these surveys (e.g. the types of energy used for cooking in households), then this can also be used to develop a sampling strategy for a biogas survey. Indeed, where biogas is important in a country or growing rapidly, energy statisticians should make sure that it is included as a possible answer to any energy questions in national surveys (see Section 5). 6 national statistics can be updated using this more M1: Plant capacity (rated daily gas production) detailed information. 1. Have you used your biogas plant in the last The main problem with annual revisions such as this year? (tick one) is that they do not take into account the capacity of biogas plants that stop working in a year. This Yes should be subtracted from the capacity of new No – I stopped using it last year plants built in a year to get the net additions to No – I haven’t used it for over a year capacity. However, considering that most biogas plants have probably been built in the last 5-10 years 2. What is the main type of waste that this in many countries, such an adjustment may not be plant is designed to use? (tick one) necessary in most places. Cattle SURVEY OF STANDARDISED BIOGAS PLANT TYPES Poultry If administrative data suggests that most biogas Don’t know plants in a country have been built to a standard specification and in a limited number of sizes, then 3. How much gas is the plant designed to it may be possible to collect data by asking owners produce each day? (tick one or write in) or operators about the size of their biogas plant. Don’t know (go to ….) For example, biogas plants built under the National 1.2 m3/day Domestic Biogas and Manure Program (NDBMP) in 1.6 m3/day Bangladesh have been built in twelve different sizes, 2.0 m3/day with six different levels of expected biogas 2.4 m3/day production for owners that will feed the digester 3.2 m3/day with cattle or poultry waste. Deenbandhu biogas 4.8 m3/day plants in India are also often built to take cattle Other (write in) waste and produce one of four possible levels of daily gas production: 2 m3, 3 m3, 4 m3 and 6 m3 (Kudaravelli, 2013). Daily gas production statistics can be converted to total plant volumes during data processing, using If biogas plant owners or operators know the the design specifications for biogas plants built in a expected daily gas production of their plant or have country (for example, see Table 1). an operating manual or guidebook, then this information can be collected using questions such as It is also recommended that plant owners should be those shown in questionnaire Module M1. asked whether they have used their plant in the last year. This information can be used to estimate how It is recommended that this approach should only be many biogas plants stop working each year. Routing used where capacity is measured as daily gas in the questionnaire (“go to” instructions) can also production, as biogas plant owners will probably not direct the data collectors to collect the know the total volume of their plant and will not be measurements of a biogas plant if its owner does not able to answer questions about this. know the rated daily gas production. Table 1: Standard sizes (models) of fixed dome SURVEY OF BIOGAS PLANT DIMENSIONS biogas plants used in Bangladesh If there are many different types and sizes of biogas Rated daily gas Effective digester volume plants in a country or if plant owners do not know production (m3) the capacity of their plants, then it will be necessary (m3/day) Cow Poultry to measure or estimate the dimensions of each plant and calculate its total volume. 1.2 3.0 2.3 1.6 3.8 3.0 Most biogas plants are built using one of the designs 2.0 4.8 3.9 described in Section 2 and, for each of these, the total plant volume can be calculated from one or 2.4 5.8 4.5 two simple dimensions (usually diameter and length 3.2 7.8 6.0 or height). For situations where part or all of a biogas plant is buried under ground, these 4.8 11.8 9.3 dimensions may be difficult to measure directly. Source: derived from NDBMP (2013). Therefore, the following text also presents some 7 ideas about how these dimensions may be M2: Plant capacity (type and dimensions) estimated. 1. Indicate the type of biogas plant that is To calculate gas production, it may also be being measured. (tick one) necessary to know how the total plant volume is divided into the digester volume and gas storage Fixed dome plant (hemisphere) volume. In most cases, these two volumes cannot be Fixed dome plant (Deenbandhu) measured directly, so Table 2 below presents Fixed dome plant (Chinese design) average values for the digester and gas storage Floating drum plant volumes (as a proportion of total plant volume). These figures should be adjusted to reflect local Balloon/bag digester biogas plant designs if expert knowledge or other Non-standard design (go to Q3) information is available. 2. Write in the dimensions of the biogas plant Table 2 also presents multiplication factors that can as indicated below. (to the nearest cm) be used to estimate the total plant volume (and its two components) from the rated daily gas m cm production of different types of plants. The main Diameter assumption in the table is that the gas storage Digester height (floating drum) volume in a biogas plant is usually designed to hold Gas holder height (floating drum) 60% of the rated daily gas production. Again, these Length (balloon/bag digester) figures can be adjusted with local information if available. 3. If non-standard design, sketch the plant below and show the main dimensions Questionnaire Module M2 presents some questions that could be used to record biogas plant dimensions. This should be adjusted to reflect the types of digesters used in a country (i.e. some of these questions may not be needed). The remainder of this section describes how data collectors can measure these dimensions in the field and how they can be used to calculate plant volume for each of the main types of biogas digester. Table 2: Proportions and multiplication factors to convert total plant volume and rated daily gas production into digester volume and gas storage volume Digester and gas storage volumes Multiplication factors to convert as a share of total plant volume gas production to plant volume Biogas plant type Gas Gas Digester Total Digester Total storage storage volume volume volume volume volume volume Fixed dome plant 80% 20% 100% 2.4 0.6 3.0 Floating drum plant 70% 30% 100% 1.4 0.6 2.0 Balloon/bag digester 75% 25% 100% 1.8 0.6 2.4 Note: If a fixed dome plant has a rated daily gas production of 1.2 m3/day, the multiplier above suggests that total plant volume would be 1.2 x 3 = 3.6 m3. However, the above figures are averages and should be replaced by figures based on local biogas plant designs where available (e.g. see Table 1). 8 Fixed dome plant (hemisphere design) Fixed dome plant (Deenbandhu design) This plant design consists of a hemisphere with a flat The Deenbandhu design combines a hemisphere bottom. Countries that use this design of biogas with a curved base. Based on the specifications of a plant include: Viet Nam (KT Model); Tanzania number of different sizes of Deenbandhu digesters (CAMARTEC Model); and Cambodia (modified given in Kudaravelli (2013), the depth of the curved Deenbandhu design). base of this design (k) is usually about 40% of the radius (r) of the digester. r D D k Figure 4: Fixed dome plant (hemisphere design) Measurement: For this type of biogas plant, it is only Figure 5: Fixed dome plant (Deenbandhu) necessary to measure the diameter of the base of the digester (D). This can be done as follows: Measurement: To calculate the volume of a Deenbandhu digester, it is also only necessary to If the dome is located above ground, the measure the maximum diameter (D) of the digester diameter can be calculated by measuring the and this can be done in the same way as described circumference of the dome at its base and for the hemisphere design. dividing this measurement by 3.14 (π). An alternative is to measure across the top of the Volume calculation: For data processing, it is dome and divide the measurement by 1.57. necessary to calculate the radius (r) of the digester (r = D/2) and the depth of the curved base (k If the dome is buried underground, the diameter = 0.4r). The total plant volume (Vp) can then be can be estimated by measuring the distance calculated as the volume of a hemisphere and from the gas outlet pipe to the closest side of sphere segment: the displacement tank and multiplying the answer by 2. (This assumes that the 2 3 1 2 2 = + (3 + ) displacement tank has not been built partly on 3 6 top of the dome). The digester volume (Vd) and gas storage volume Volume calculation: For data processing, the total (Vg) can be calculated as before (by multiplying the plant volume (Vp) can be calculated as the volume total plant volume by 0.65 and 0.35 respectively) of a hemisphere: and a lookup table for these volumes across a range of diameters is given in Table 11 in Appendix 1. 3 2 = 3 2 Following Table 2, the digester volume (Vd) can be calculated by multiplying the total plant volume by 0.65 and the gas storage volume (Vg) can be found by multiplying the total plant volume by 0.35 (for variations to these calculations, see Heegde, 2010). A lookup table showing total plant volume, digester volume and gas storage volume for a range of diameters is given in Table 10 in Appendix 1. 9 Fixed dome plant (Chinese design) Floating drum plant The Chinese design of a fixed dome plant consists of Floating drum plants are usually circular, so their a hemisphere on top of a cylinder with a curved or volume can be calculated as the volume of two flat bottom. This type of digester is common in cylinders. The larger cylinder is the digester, which China and may also be found in other countries that may be partly or totally underground and the have received assistance and training from the smaller cylinder is the gas storage tank, which is Biogas Institute of the Ministry of Agriculture in placed on top and inside this. China (BIOMA). D D H Figure 6: Fixed dome plant (Chinese design) TAKING MEASUREMENTS Measurement: A standard plant design has been Measurement: For this type of biogas plant, it is developed by the Biogas Training Centre in Sichuan, necessary to measure both the diameter of the China (LGED, 2002) and the volume of plants built digester (D) and the height of the digester (H). This following that specification can be calculated from can be done as follows: the diameter of the plant (which can be measured as described previously). The diameter of the digester can be measured directly if a tape can be placed across the top of Volume calculation: the total plant volume (Vp) the gas storage tank. Alternatively, it can also following this design can be calculated from the be calculated by measuring the circumference measured diameter (D) as follows: of the digester or gas storage tank and dividing this measurement by 3.14 (π). If the 3 circumference of the digester is measured, the = thickness of the walls of the digester should be 2.2368 subtracted from the answer to get the internal The digester volume (Vd) and gas storage volume diameter of the digester. (Vg) can be calculated as before and a lookup table for these volumes across a range of diameters is If the digester is completely above ground, then given in Table 12 in Appendix 1. the height of the digester should be measured from its base to the top of the digester wall. If it Above ground plants: Plants built above ground is partly underground, then the maximum are likely to have a flat bottom. In such cases, the height of the gas storage tank should be height of the cylindrical part of the digester should measured (or estimated if it is not full) and the be measured and the volume can be calculated digester height should be estimated as a using a combination of the formulae for a multiple of this. For example, the gas storage hemisphere design and the balloon digester design volume in the KVIC floating drum design is 30% (see below). of the total plant volume, so the digester height can be calculated by multiplying the height of Other combinations of a cylinder with a flat base and the gas storage tank by 2.3 (i.e. 70/30). dome or curved top may also exist. In such cases, height and diameter should be measured and local Volume calculation: The total plant volume (Vp) can formulae should be developed to calculate total be calculated as the sum of the digester volume (Vd) plant volume from these measurements. 10 and the gas storage volume (Vg), where these are The diameter of the digester should be calculated as follows: measured from top to bottom across one of the ends of the digester when the bag is fully expanded. If the bag is not fully expanded, this = may be estimated measuring the distance and 2 asking the biogas plant owner how much higher = 30/70 × it expands. Alternatively, if the bag is made of two pieces of plastic (with visible seams), it may = + be possible to measure across the top of the bag from seam to seam and divide that measurement by 1.57 to calculate the diameter. The calculations above assume that floating drum plants are cylindrical and are built so that the gas The length of the bag should be simply storage and digester volumes are in the ratio of measured from one end to the other. This 30:70. If the ratio of gas and digester volumes are measurement should be taken across the top of different in a country (or plants are not circular), the bag in a straight line (by holding the tape then the above calculations should be adjusted to tight) and should not include any part of the reflect this. Similarly, if floating drum digesters are bag that would form the ends of the cylinder built to several different designs in a country, then it when fully expanded. Again, it may be possible may be necessary to use a more complicated to measure from seam to seam if the circular questionnaire and take more measurements so that ends of the bag have been joined to the main volume can be calculated. part of the cylinder with visible seams. The above calculations have been based on the Volume calculation: The total plant volume (Vp) can standard specifications for a KVIC plant and a be calculated as the volume of a cylinder as follows: lookup table (based on this) showing total plant volume, digester volume and gas storage volume for different diameters and heights is given in Table 13 = in Appendix 1. 2 The digester volume (Vd) and the gas storage Balloon digester volume (Vg) can be calculated by multiplying the When fully expanded the shape of a balloon digester total plant volume by 0.75 and 0.25 respectively. will be similar to that of a cylinder, so the total plant volume can be calculated as the volume of a A lookup table showing total plant volume, digester cylinder. A review of the literature on balloon volume and gas storage volume for various digesters suggests that the digester volume is diameters and lengths of balloon digesters is given around 75% of the total plant volume and the gas in Table 14 in Appendix 1. storage volume is around 25%, so these two figures can be used to divide the total plant volume into Chinese bag digester: The volume of a bag digester these two components. of the Chinese design (circular digester with a bag on top) can be measured and calculated in a similar way to the floating drum plant.4 However, if the digester is partly or totally underground and cannot be measured directly, it will be necessary to ask the D biogas plant owner to estimate the depth (height) of the digester. The gas storage volume (in the bag) should be L about one-quarter of the total plant volume, so it Figure 7: Balloon digester can be calculated as the measured digester volume (Vd) divided by three (i.e. 25/75 = 1/3), but the ratio Measurement: To calculate the volume of a balloon of gas storage and digester volumes should be digester, it is necessary to measure the diameter of checked with local biogas plant designs and the digester (D) and the length of the digester (L). adjusted in the calculation as necessary. This can be done as follows: 4 By measuring and calculating the digester volume, estimating the gas storage volume relative to that and adding the two volumes together to get total plant volume. 11 4. BIOGAS PRODUCTION AND CONSUMPTION Five different methodologies are presented here for calculating or estimating biogas production and CONVERSION FACTORS consumption. Estimates can be based on plant capacity, appliance use or feedstock use or by comparing fuel use in households with and without 1 m3 of biogas = 0.65 m3 of methane a biogas plant (the fuel substitution method). Biogas 1 m3 of methane = 34 MJ of energy production may also be measured directly. 1 m3 of biogas = 22 MJ of energy 1 m3/day of biogas = 8,060 MJ/year Biogas production is usually measured or estimated in cubic metres over a period of time, but it should A summary of the advantages and disadvantages of be converted and reported in energy units. The main the different estimation methodologies is given in calculations are to convert biogas production into Table 3. The more complicated methodologies will methane production and then convert that into give more detailed and accurate estimates of biogas energy production (in MJ). The methane content of production, but will be more difficult and expensive biogas should be about 65% and 1 m3 of methane to implement. Therefore, the choice of methodology contains 34 MJ of energy, so 1 m3 of biogas should should be based on the purpose of the survey and contain about 22 MJ of energy. availability of resources. Table 3: Summary of the different methods to estimate biogas production Estimation methodology Advantages Disadvantages Plant capacity: Useful to make estimates from The accuracy of estimates will Estimate production from administrative data or simple surveys of depend on how much is known capacity data using a plant capacity. Can be used to about capacity utilisation. This capacity utilisation figure. produce annual estimates of method may be difficult to use production in between biogas surveys. if many different types and sizes of biogas plants exist. Appliance use: This requires a small number of Biogas users may not know how Collect data on appliance questions that can be integrated into much they use biogas use (hours per day and existing household surveys. It can be appliances. There may be a gas per hour) and use this used to produce detailed estimates of difference between biogas to estimate consumption. consumption as part of larger surveys or consumption and production. as a relatively small energy survey. Feedstock use: This method calculates production Data collection requires a Collect data on digester based on feedstock use rather than detailed set of questions and size, technology and assumptions about capacity utilisation. more complex calculations to feedstock use and It is most suitable for including in produce results. Estimates of calculate the expected detailed household energy surveys. feedstock inputs may not be level of gas production. Results can also be used to calculate accurate, but errors may be capacity utilisation figures. lower than in other methods. Fuel substitution: This requires a small number of More complex analysis is Collect data on fuel use in questions that can be integrated into required to adjust for households with/without or existing household surveys. This household characteristics in before/after connection to approach is particularly useful for the comparison of fuel use. biogas and estimate project monitoring and for examining Total energy use may also differ consumption from the use socio-economic and environmental between households using or of other fuels. aspects of fuel switching. not using biogas. Direct measurement: This method will give the most accurate Measuring devices are Biogas production is and detailed measurements of biogas expensive and may require recorded over a period of production. It is only likely to be cost- specialised training to use. It time by mechanical effective for large-scale production or may also still be necessary to devices (flow meters). for detailed research into the design estimate annual production and operation of biogas plants. from measurements taken over a short time period. 12 PLANT CAPACITY APPLIANCE USE The simplest way to estimate annual biogas A second method to estimate biogas production is production is to calculate the theoretical level of to collect information about the use of biogas- production from plant capacity data and adjust this powered appliances (e.g. lamps and stoves). This for capacity utilisation (where possible). can then be used to calculate the annual biogas consumption of households. Measurement: For this method, plant capacity can be taken from administrative data or the results of Measurement: Survey questions should collect capacity surveys. information about the types of household appliances using biogas, the amount of biogas used Calculation: If biogas capacity is recorded as the in each appliance (its power rating) and the average rated daily gas production of biogas plants (see number of hours each appliance is used every day. Section 3) then the maximum potential biogas production in a country is calculated as total plant An example of the questions that could be used is capacity multiplied by 8,060 (i.e. to convert from shown in questionnaire Module M3. Questions 1-3 daily biogas production in m3/day to biogas energy ask about the power rating and use of biogas lamps, production in MJ/year). questions 4-5 ask about biogas used for cooking and questions 6-7 ask if the household has to burn If capacity is recorded as total plant volume (in m3), excess biogas (biogas flaring).5 then it should be converted to estimated daily gas production using figures such as those shown in Data should be collected about the average power Table 2. For example, Table 2 shows that the total rating of each biogas lamp and each burner on a plant volume of fixed dome plants should be about biogas stove. So for example, if a household has one twice the daily volume of biogas production, so total lamp that uses 60 litres of biogas an hour and plant volume (in m3) divided by two will give an another that uses 100 litres/hour, the average power estimate of total daily gas production (in m3/day). rating of the biogas lamps would be 80 litres/hour. This can then be multiplied by 8,060 to get a figure Similarly, if a biogas stove has two burners and a for biogas energy in MJ/year. total power rating of 5,000 watts, the power rating of each burner would be 2,500 watts. The result of these calculations is the maximum theoretical or potential level of gas production in a Data collectors should ask about these power country, but this assumes that all biogas plants are ratings, but should also check the answers to these operated under optimal conditions (e.g. with the questions by looking at technical manuals or on any correct preparation of feedstocks, ideal operating labels on appliances (if available). They should also temperatures, etc.). Real operating conditions are be very careful to record the power rating of each unlikely to be ideal, so this figure should be lamp and burner and not the total power rating of multiplied by a capacity utilisation factor (%) to take all lamps and burners. this into account. In countries where the power ratings of biogas The main advantage of this methodology is that lamps and stoves are standardised, it may also be plant capacity data can be converted into estimates possible to collect this data using a limited choice of of biogas production with simple calculations. The answers (e.g. lamps of 100 W, 200 W and 300 W) main problem is that the results of these calculations rather than asking about the exact power rating of should be adjusted for capacity utilisation, which will appliances. often be unknown and could be as low as 40-60% (NDBMP, 2013). Calculation: Biogas energy consumption can be calculated in four steps. To solve this problem, capacity utilisation may be estimated from local expert knowledge or from a Step 1: All of the measurements of the power rating more detailed survey of sample of biogas plant of appliances should be converted into a common owners (comparing rated and actual production unit of m3/hour. For data recorded in litres per hour, levels). This information may also be obtained the measurements should be divided by 1,000. For gradually over time as more biogas data is collected data recorded in watts, the measurements should be in a country. divided by 6,100 (i.e. 1 m3 of biogas contains 22 MJ, which is the same as 6,100 watt-hours). 5 If a household has a biogas plant that produces more biogas than they can use, then they may have to burn the excess biogas for safety reasons. This can occur if an over-sized biogas plant has been built by mistake or if it has been designed to manage a waste problem. 13 M3: Appliance use Step 2: The second stage is to complete the set of power rating measurements by using estimates in 1. Do you use any biogas lamps? (tick one) cases where the household did not know this information or it could not be obtained from Yes manuals or labels on appliances. No (go to Q4) There are many ways of replacing missing data with substituted values (imputation). The simplest is to 2. What is the average power rating of each calculate the average of all recorded measurements lamp? (tick don’t know or write in number in and use that value for all missing data. Alternatively, either litres/hour or watts) national minimum standards for appliances can be used or average values may be taken from other Don’t know studies and used. Some examples of minimum Gas use in litres/hour standards for biogas burners and estimates of Power in watts biogas use in lamps are shown in Table 4. 3. On average, how many hours per day do Table 4: Examples of appliance power ratings you use each lamp? (write in no. of hours) Biogas lamps (averages) hrs/day Cambodia 0.049 m3/hr Gas lamp 1 Ethiopia 0.048 m3/hr Gas lamp 2 India 0.093 m3/hr Gas lamp 3 Biogas stove burners (minimum standards) 4. What is the power rating of each burner on China 0.380 m3/hr your biogas stove? (tick don’t know or write India 0.450 m3/hr in number in either litres/hour or watts) Kenya 0.500 m3/hr Don’t know Sources: Khandelwal and Gupta (2009); KBS (2013). Gas use in litres/hour Power in watts Step 3: The third stage is to calculate the average daily biogas consumption of each household by 5. On average, how many hours per day do multiplying the average power rating of biogas you use each burner for cooking and boiling lamps (m3/hr) by the total number of hours they are water? (write in no. of hours) used and the power rating of burners by the total number of hours they are used for cooking and hrs/day flaring. This should give the daily biogas Burner 1 consumption (in m3) for lighting, cooking and flaring Burner 2 in each household. Burner 3 Step 4: The last stage in the calculation is to multiply these results by 8,060 to convert the figures into 6. Do you also burn excess biogas? (tick one) MJ/year per household. These figures can then be used to produce regional and national estimates, Yes using the appropriate multiplication factors to No (go to……) convert the sample results to population estimates. 7. On average, how many hours per day do When this is done, the biogas consumption for you use each burner to burn excess biogas? lighting and cooking should be recorded as final (write in no. of hours) consumption in the appropriate end-use sectors in the energy balance, any biogas used for flaring hrs/day should be recorded as losses and the total of all Burner 1 three uses can be recorded as primary energy Burner 2 supply. Burner 3 14 FEEDSTOCK USE M4: Feedstock use Biogas production can also be estimated from 1. Do you feed the digester with waste from information about the amount and type of any of the following animals? (write in no. of feedstocks used in digesters. The amount of data animals providing waste for digester) required is similar to other methodologies, but the calculation of biogas production is slightly more No. complicated. However, these calculations can also Buffalo provide useful information about the operation of Cows biogas digesters in a country. Calves Measurement: Digester volume is used to calculate Sheep/goats biogas production, so this must be measured or Pigs estimated (see previous section). In addition, data Hens also needs to be collected about the weight and Horses volume of different feedstocks added to the Humans digester each day. Questionnaire Module M4 shows some questions that could be used. 2. How much of the following types of waste do you usually add to the digester each day? The first question asks about animal waste. Rather (write in amount in kg) than asking about the amount of waste added to the digester, this asks about the number of animals that Kg/day provide waste for the digester. Various studies have Cereals/grains reported the weight of waste produced by different Rice straw animals and these figures can be used in the Wheat straw calculations (see Table 5). Grass Corn stalk The second question asks more directly about the Fruit waste weight of other materials added to the digester (in kg/day). This should not include any water Vegetable waste added to the waste (this is a separate question). Fats Mixed food waste Cereals/grains are high value inputs with a lot of Mixed organic waste energy content that may only be added if respondents are deliberately trying to increase gas 3. When you feed the digester, how much production. The other inputs are all wastes. If water do you add compared to the amount respondents cannot divide their waste inputs by of waste? (tick one, check that this includes type, then these inputs can be recorded as mixed the water added to animal wastes) food waste or mixed organic waste. Half as much water If biogas plant owners are unable to answer this An equal amount of water question, then it may be possible to estimate the Twice as much water inputs by asking about the container used to collect Three times as much water the waste (probably a large bucket) and then by Over three times as much water weighing one bucket full of waste and asking them Don’t know how many bucket loads they add a day. Another alternative is to ask about (or calculate) the volume of the bucket and assume that 1 litre = 1 kg, then Calculation: For any one feedstock, daily biogas calculate the weight added from the number of production can be estimated using the following bucket loads per day (as described above). equation (Fulford, 2015): The last question asks about how much water is added to the digester. Wastes are usually mixed = × × × 1+ with water when they are added to a digester, so the Where: easiest way to ask about this is to ask about how much water is added compared to the amount of - G is the biogas production (in m3/day) waste added. (Note, this should include the water - C is the biogas potential, which is the maximum added to both animal and other wastes). amount of gas that can be produced from 1 kg of volatile solids in a feedstock (in m3/kg) - Vd is the digester volume (in m3) 15 - S is the initial concentration of volatile solids in Table 6.7 These figures can then be added together the slurry (in kg/m3) to get the total weight of volatile solids added each - R is the feedstock retention time (in days) day, which can be divided by the total feedstock - k is a constant indicating the rate of gas volume. production at a given temperature So, for example, if the weight of volatile To simplify this equation, IRENA has calculated gas solids added each day is 5.6 kg/day and the production across a wide range of temperatures and daily feedstock volume is 0.08 m3/day, the retention times, so that biogas production can be initial concentration of volatile solids (S) is calculated as follows: 5.6/0.08 = 70 kg/m3. × × Table 5: Animal waste feedstock properties = 1000 Animal Total Volatile production solids Where G, Vd and S are the same as before and Y is a (kg/day) (kg/day) yield factor based on temperature and the feedstock retention time (see Table 7).6 Buffalo 14 1.94 Cow 10 1.42 Assuming that the digester volume (Vd) has already Calf 5 0.50 been calculated, it is only necessary to calculate the feedstock retention time (R) and initial Sheep/goat 2 0.44 concentration of volatile solids (S) in order to Pig 5 1.00 calculate daily biogas production and this can all be done in the following four steps. 100 hens 7.5 2.77 Horse 10 2.24 Step 1: First, the total feedstock volume should be Human 0.2 0.03 calculated. This starts by multiplying the number of animals recorded in the survey by the total waste Sources: Kudaravalli (2014); Fulford (2015); Lorimor et al production per day (from Table 5) and adding to this (2004); SEAI (2012). the total weight of all other feedstocks used. This gives the daily waste input in kg. This figure can be Table 6: Other feedstocks volatile solid content assumed to be about the same as the volumetric input in litres. The answer to question 3 is then used Animal Volatile to multiply that value to take into account the added solids (in %) water (e.g. if they add twice as much water then the Cereals/grains 0.81 waste input should be multiplied by three). The final Rice straw 0.36 value is divided by 1,000 to convert from litres to m3. Wheat straw 0.39 Step 2: The feedstock retention time is calculated by Grass 0.51 dividing the digester volume by the total feedstock Corn stalk 0.43 volume. So, for example, if the digester volume is 5.0 m3 and the total daily feedstock volume Fruit waste 0.14 (including water) is 0.08 m3/day, the feedstock Vegetable waste 0.16 retention time (R) is 5.0/0.08= 62.5 days. Fat 0.83 Step 3: The initial concentration of volatile solids (S) Mixed food waste 0.08 is calculated by dividing the weight of volatile solids Mixed organic waste 0.26 added each day by the daily waste inputs. The weight of volatile solids from animal waste can be Sources: Rajendran et al (2012); Fulford (2015); SEAI (2012). calculated from the figures shown in Table 5 and the numbers of animals providing waste for the Step 4: The final step in the calculation is to use the digester. The weight of volatile solids from other feedstock retention time and temperature in the wastes can be calculated using the figures shown in 6 These yield factors have been derived by calculating gas production for different combinations of temperature and feedstock retention time, using values of C and k that are typical for the feedstocks used in small-scale digesters. This is a simplification of the first equation, which would require calculating daily gas production for each and every feedstock used in the digester. 7 This proportion is calculated from the total solid content in a feedstock (in percent) multiplied by its volatile solid content (measured as a percentage of solid content). For feedstocks not shown in Tables 6 and 5, estimates of these values can be found in many different biogas studies. 16 country to find the yield factor and use this to With a yield factor (Y) of 3.33, digester volume (Vd) calculate daily gas production and annual energy of 5.0 m3 and initial concentration of solids (S) of production in MJ. 70 kg/m3, daily gas production would be calculated as follows: So, for example, if the feedstock retention time of a biogas plant is 62.5 days and the average 3.33 × 5 × 70 = = 1.17 / temperature in a country is 23 °C, the yield factor (Y) 1000 would be 3.33. The figure for daily gas production can then be A table of average annual temperatures in different converted into annual energy production in countries and areas is given in Table 15 in megajoules by multiplying by 8,060, as before Appendix 1. If a biogas digester is buried (i.e. 1.17 m3/day x 8,060 = 9,430 MJ/year). underground, an additional 2 °C should be added to the average temperature to account for this. Table 7: Yield factors for biogas production, by temperature and feedstock retention time Feedstock retention Temperature (°C) time (in days) 16-18 19-21 22-24 25-27 28-30 31-33 6-10 5.41 7.98 10.83 13.59 15.91 18.33 11-15 4.73 6.79 8.99 11.09 12.88 14.74 16-20 4.21 5.90 7.68 9.37 10.82 12.32 21-25 3.79 5.22 6.70 8.11 9.33 10.59 26-30 3.44 4.69 5.95 7.15 8.20 9.28 31-35 3.16 4.25 5.35 6.39 7.32 8.26 36-40 2.91 3.88 4.86 5.78 6.60 7.44 41-45 2.71 3.58 4.45 5.27 6.02 6.77 46-50 2.53 3.32 4.10 4.85 5.53 6.21 51-55 2.37 3.09 3.81 4.49 5.11 5.74 56-60 2.23 2.89 3.55 4.18 4.75 5.33 61-65 2.10 2.72 3.33 3.91 4.44 4.98 66-70 1.99 2.57 3.13 3.67 4.17 4.67 71-75 1.89 2.43 2.95 3.46 3.93 4.40 76-80 1.80 2.30 2.80 3.27 3.71 4.15 81-85 1.72 2.19 2.66 3.10 3.52 3.94 86-90 1.65 2.09 2.53 2.95 3.34 3.74 91-95 1.58 2.00 2.41 2.81 3.19 3.56 96-100 1.52 1.92 2.31 2.69 3.04 3.40 17 FUEL SUBSTITUTION M5: Fuel use Given that biogas replaces other fuels, biogas 1. How much of the following types of fuel do consumption can be estimated by looking at the you use for cooking each day? (write in quantity of other fuels consumed by households amount in kg or litres) with and without biogas plants. This approach is often used as part of biogas projects, where fuel Fuelwood Kg/day consumption is compared before and after a Charcoal Kg/day household is connected to a biogas plant. Kerosene Litre/day Alternatively, fuel consumption can be compared Bottled gas (LPG) Litre/day across a sample of households with and without biogas plants, after correcting for other factors such as household size and location. Table 8: Biogas energy content comparisons For example, a biogas survey in Bangladesh found Amount and Volume of biogas with that households were saving 249 kg/month of type of fuel same energy content (m3) fuelwood on average after switching to biogas, Unadjusted Adjusted ranging from 186 kg/month for households with a 1.6m3 plant to 394 kg/month for those with a 1 kg fuelwood 0.70 0.25 4.8m3 plant (iDE, 2011). Another analysis in Nepal 1 kg charcoal 1.40 0.65 estimated that fuelwood use in households with 1 litre kerosene 1.60 1.60 biogas plants was 1,100 kg/year less than the 2,400 kg/year used in households without biogas 1 litre LPG 1.05 1.05 plants (Somanathan and Bluffstone, 2015). 1 kg LPG 2.10 2.10 The two main disadvantages of this approach are Note: Energy content of liquid propane gas (LPG) is given in the difficulties of measuring accurately the use of kilograms and litres, in case it is recorded in kg. other fuels by households and the possibility that total energy use in a household may change if they The latter adjustment is important because some have a biogas plant. For example, houses switching fuels are used in much less efficient cooking stoves. from fuelwood to biogas may use more total energy For example, biogas stoves are usually about three if their biogas plant produces a lot of biogas. times more efficient than fuelwood stoves, so if the use of biogas in a household reduces the Measurement: Information about the amounts of consumption of fuelwood for cooking by 6 kg/day, different fuels used in a household can be collected the biogas energy required to replace this is only using a very simple question, such as that shown in equivalent to the energy content of 2 kg/day of Questionnaire Module M5. Households with a biogas fuelwood. plant could be asked in one survey how much they were using before it was installed and now, but their A more complex questionnaire format could ask answers about fuel use before the plant was about the different devices used for cooking and installed could be unreliable. Thus, this question make more detailed adjustments for stove efficiency should be asked in two surveys, before and after a (for further information about stove efficiencies, see: biogas plant is installed (e.g. as part of project Berkley Air Monitoring Group, 2012). monitoring); otherwise, a sample of households with and without biogas plants can be asked this Table 8 shows the equivalent amount of biogas that question in one survey. would be required to replace one unit of fuel consumption used for cooking, with the adjusted These questions can be asked using a variety of column showing the figure after adjusting for stove different units. However, it is advisable to record efficiency. The adjusted figures can then be used responses in kg or litres per day. So, for example, if with the survey data to estimate the energy a household buys kerosene in 1 litre bottles and they provided by biogas. last about 4-5 days on average, then the daily consumption would be 1/4.5 = 0.22 litres/day. For example, if connection to a biogas plant reduces the average fuelwood consumption of households Calculation: To calculate the energy provided by by 6 kg/day, this suggests that the fuelwood energy biogas, it is necessary to compare the consumption used for cooking has been replaced by 1.5 m3/day of of fuels in households with and witho