Choice of method

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The IPCC methodology for estimating CH4 emissions from SWDS is based on the First Order Decay (FOD) method. This method assumes that the degradable organic component (degradable organic carbon, DOC) in waste decays slowly throughout a few decades, during which CH4 and CO2 are formed. If conditions are constant, the rate of CH4 production depends solely on the amount of carbon remaining in the waste. As a result emissions of CH4 from waste deposited in a disposal site are highest in the first few years after deposition, then gradually decline as the degradable carbon in the waste is consumed by the bacteria responsible for the decay.

Transformation of degradable material in the SWDS to CH4 and CO2 is by a chain of reactions and parallel reactions. A full model is likely to be very complex and vary with the conditions in the SWDS. However, laboratory and field observations on CH4 generation data suggest that the overall decomposition process can be approximated by first order kinetics (e.g., Hoeks, 1983), and this has been widely accepted. IPCC has therefore adopted the relatively simple FOD model as basis for the estimation of CH4 emissions from SWDS.

Half-lives for different types of waste vary from a few years to several decades or longer. The FOD method requires data to be collected or estimated for historical disposals of waste over a time period of 3 to 5 half-lives in order to achieve an acceptably accurate result. It is therefore good practice to use disposal data for at least 50 years as this time frame provides an acceptably accurate result for most typical disposal practices and conditions. If a shorter time frame is chosen, the inventory compiler should demonstrate that there will be no significant underestimation of the emissions. These Guidelines provide guidance on how to estimate historical waste disposal data (Section 3.2.2, Choice of Activity Data), default values for all the parameters of the FOD model

(Section 3.2.3, Choice of Emission Factors and Parameters), and a simple spreadsheet model to assist countries in using the FOD method.

Three tiers to estimate the CH4 emissions from SWDS are described:

Tier 1: The estimations of the Tier 1 methods are based on the IPCC FOD method using mainly default activity data and default parameters.

Tier 2: Tier 2 methods use the IPCC FOD method and some default parameters, but require good quality country-specific activity data on current and historical waste disposal at SWDS. Historical waste disposal data for 10 years or more should be based on country-specific statistics, surveys or other similar sources. Data are needed on amounts disposed at the SWDS.

Tier 3: Tier 3 methods are based on the use of good quality country-specific activity data (see Tier 2) and the use of either the FOD method with (1) nationally developed key parameters, or (2) measurement derived country-specific parameters. The inventory compiler may use country-specific methods that are of equal or higher quality to the above defined FOD-based Tier 3 method. Key parameters should include the half-life, and either methane generation potential (Lo) or DOC content in waste and the fraction of DOC which decomposes (DOCf ). These parameters can be based on measurements as described in Box 3.1.

A decision tree for choosing the most appropriate method appears in Figure 3.1. It is good practice for all countries to use the FOD method or a validated country-specific method, in order to account for time dependence of the emissions.

The FOD method is briefly described in Section 3.2.1.1 and in more detail in Annex 3A.1. A spreadsheet model has been developed by the IPCC to assist countries in implementing the FOD: IPCC Spreadsheet for Estimating Methane Emissions from Solid Waste Disposal Sites (IPCC Waste Model) ^The IPCC Waste Model is described in more detail below and can be modified and used for all tiers.

Figure 3.1 Decision Tree for CH4 emissions from Solid Waste Disposal Sites

Figure 3.1 Decision Tree for CH4 emissions from Solid Waste Disposal Sites

Urinary Catheter Removal Algorithm

Note:

1. Good quality country-specific activity data mean country-specific data on waste disposed in SWDS for 10 years or more.

2. Key parameters mean DOC/Lo, DOCf and half-life time.

3. See Volume 1 Chapter 4, "Methodological Choice and Identification of Key Categories" (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

Note:

1. Good quality country-specific activity data mean country-specific data on waste disposed in SWDS for 10 years or more.

2. Key parameters mean DOC/Lo, DOCf and half-life time.

3. See Volume 1 Chapter 4, "Methodological Choice and Identification of Key Categories" (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

1 See the attached spreadsheets in Excel format. <IPCC Waste Model.xls>.

3.2.1.1 First Order Decay (FOD) METHANE EMISSIONS

The CH4 emissions from solid waste disposal for a single year can be estimated using Equations 3.1. CH4 is generated as a result of degradation of organic material under anaerobic conditions. Part of the CH4 generated is oxidised in the cover of the SWDS, or can be recovered for energy or flaring. The CH4 actually emitted from the SWDS will hence be smaller than the amount generated.

Equation 3.1

CH4 emission from SWDS

CH4 Emissions =

2 CH4 generatedx T - RT

• (1 - OXT )

_ x _

CH4 Emissions = CH4 emitted in year T, Gg

T = inventory year x = waste category or type/material

Rt = recovered CH4 in year T, Gg

OXT = oxidation factor in year T, (fraction)

The CH4 recovered must be subtracted from the amount CH4 generated. Only the fraction of CH4 that is not recovered will be subject to oxidation in the SWDS cover layer.

METHANE GENERATION

The CH4 generation potential of the waste that is disposed in a certain year will decrease gradually throughout the following decades. In this process, the release of CH4 from this specific amount of waste decreases gradually. The FOD model is built on an exponential factor that describes the fraction of degradable material which each year is degraded into CH4 and CO2.

One key input in the model is the amount of degradable organic matter (DOCm) in waste disposed into SWDS. This is estimated based on information on disposal of different waste categories (municipal solid waste (MSW), sludge, industrial and other waste) and the different waste types/material (food, paper, wood, textiles, etc.) included in these categories, or alternatively as mean DOC in bulk waste disposed. Information is also needed on the types of SWDS in the country and the parameters described in Section 3.2.3. For Tier 1, default regional activity data and default IPCC parameters can be used and these are included in the spreadsheet model. Tiers 2 and 3 require country-specific activity data and/or country-specific parameters.

The equations for estimating the CH4 generation are given below. As the mathematics are the same for estimating the CH4 emissions from all waste categories/waste types/materials, no indexing referring to the different categories/waste materials/types is used in the equations below.

The CH4 potential that is generated throughout the years can be estimated on the basis of the amounts and composition of the waste disposed into SWDS and the waste management practices at the disposal sites. The basis for the calculation is the amount of Decomposable Degradable Organic Carbon (DDOCm) as defined in Equation 3.2. DDOCm is the part of the organic carbon that will degrade under the anaerobic conditions in SWDS. It is used in the equations and spreadsheet models as DDOCm. The index m is used for mass. DDOCm equals the product of the waste amount (W), the fraction of degradable organic carbon in the waste (DOC), the fraction of the degradable organic carbon that decomposes under anaerobic conditions (DOCf), and the part of the waste that will decompose under aerobic conditions (prior to the conditions becoming anaerobic) in the SWDS, which is interpreted with the methane correction factor (MCF).

Where:

DDOCm = mass of decomposable DOC deposited, Gg

W = mass of waste deposited, Gg

DOC = degradable organic carbon in the year of deposition, fraction, Gg C/Gg waste

DOCf = fraction of DOC that can decompose (fraction)

MCF = CH4 correction factor for aerobic decomposition in the year of deposition (fraction)

Although CH4 generation potential (Lo)2 is not used explicitly in these Guidelines, it equals the product of DDOCm, the CH4 concentration in the gas (F) and the molecular weight ratio of CH4 and C (16/12).

Equation 3.3 Transformation from DDOCm to Lo

Where:

CH4 generation potential, Gg CH4 mass of decomposable DOC, Gg fraction of CH4 in generated landfill gas (volume fraction) molecular weight ratio CH4/C (ratio)

Using DDOCma (DDOCm accumulated in the SWDS) from the spreadsheets, the above equation can be used to calculate the total CH4 generation potential of the waste remaining in the SWDS.

first order decay basics

With a first order reaction, the amount of product is always proportional to the amount of reactive material. This means that the year in which the waste material was deposited in the SWDS is irrelevant to the amount of CH4 generated each year. It is only the total mass of decomposing material currently in the site that matters.

This also means that when we know the amount of decomposing material in the SWDS at the start of the year, every year can be regarded as year number 1 in the estimation method, and the basic first order calculations can be done by these two simple equations, with the decay reaction beginning on the 1st of January the year after deposition.

2 In the 2006 Guidelines Lo (Gg CH4 generated) is estimated from the amount of decomposable DOC in the SWDS. The equation in GPG2000 is different as Lo is estimated as Gg CH4 per Gg waste disposed, and the emissions are obtained by multiplying with the mass disposed.

Where:

T = inventory year

DDOCmax = DDOCm accumulated in the SWDS at the end of year T, Gg

DDOCmai-i = DDOCm accumulated in the SWDS at the end of year (T-1), Gg

DDOCmdT = DDOCm deposited into the SWDS in year T, Gg

DDOCm decompT = DDOCm decomposed in the SWDS in year T, Gg k = reaction constant, k = ln(2)/ti/2 (y-1)

The method can be adjusted for reaction start dates earlier than 1st of January in the year after deposition. Equations and explanations can be found in Annex 3A.1.

CH4 generated from decomposable DDOCm

The amount of CH4 formed from decomposable material is found by multiplying the CH4 fraction in generated landfill gas and the CH4 /C molecular weight ratio.

Equation 3.6 CH4 generated from decayed DDOCm

Where:

CH4 generatedT = amount of CH4 generated from decomposable material

DDOCm decompT = DDOCm decomposed in year T, Gg

F = fraction of CH4, by volume, in generated landfill gas (fraction)

16/12 = molecular weight ratio CH4/C (ratio)

Further background details on the FOD, and an explanation of differences with the approaches in previous versions of the guidance (IPCC, 1997; IPCC, 2000), are given in Annex 3A.1.

SIMPLE FOD SPREADSHEET MODEL

The simple FOD spreadsheet model (IPCC Waste Model) has been developed on the basis of Equations 3.4 and 3.5 shown above. The spreadsheet keeps a running total of the amount of decomposable DOC in the disposal site, taking account of the amount deposited each year and the amount remaining from previous years. This is used to calculate the amount of DOC decomposing to CH4 and CO2 each year.

The spreadsheet also allows users to define a time delay between deposition of the waste and the start of CH4 generation. This represents the time taken for substantial CH4 to be generated from the disposed waste (see Section 3.2.3 and Annex 3A.1).

The model then calculates the amount of CH4 generated from the DDOCm, and subtracts the CH4 recovered and CH4 oxidised in the cover material (see Annex 3A.1 for equations) to give the amount of CH4 emitted.

The IPCC Waste Model provides two options for the estimation of the emissions from MSW, that can be chosen depending on the available activity data. The first option is a multi-phase model based on waste composition data. The amounts of each type of degradable waste material (food, garden and park waste3, paper and cardboard, wood, textiles, etc.) in MSW are entered separately. The second option is single-phase model based on bulk waste (MSW). Emissions from industrial waste and sludge are estimated in a similar way as for bulk MSW. Countries that choose to use the spreadsheet model may use either the waste composition or the bulk waste option, depending on the level of data available. When waste composition is relatively stable, both options give similar results. However when rapid changes in waste composition occur, options might give different

3 'garden waste' may also be called 'yard waste' in US English.

outputs. For example, changes in waste management, such as bans to dispose food waste or degradable organic materials, can result in rapid changes in the composition of waste disposed in SWDS.

Both options can be used for estimating the carbon in harvested wood products (HWP) that is long-term stored in SWDS (see Volume 4, Chapter 12, Harvested Wood Products). If no national data are available on bulk waste, it is good practice to use the waste composition option in the spreadsheets, using the provided IPCC default data for waste composition.

In the spreadsheet model, separate values for DOC and the decay half-life may be entered for each waste category and in the waste composition option also for each waste type/material. The decay half-life can also be assumed to be the same for all waste categories and/or waste types. The first approach assumes that decomposition of different waste types/materials in a SWDS is completely independent of each other; the second approach assumes that decomposition of all types of waste is completely dependent on each other. At the time of writing these Guidelines, no evidence exists that one approach is better than the other (see Section 3.2.3, Halflife).

The spreadsheet calculates the amount of CH4 generated from each waste component on a different worksheet. The methane correction factor (MCF - see Section 3.2.3) is entered as a weighted average for all disposal sites in the country. MCF may vary by time to take account of changes in waste management practices (such as a move towards more managed SWDS or deeper sites). Finally, the amount of CH4 generated from each waste category and type/material is summed, and the amounts of CH4 recovered and oxidised in the cover material are subtracted (if applicable), to give an estimate of total CH4 emissions. For the bulk waste option, DOC can be a weighted average for MSW.

The spreadsheet model is most useful to Tier 1 methods, but can be adapted for use with all tiers. For Tier 1 the spreadsheets can estimate the activity data from population data and disposal data per capita (for MSW) and GDP (industrial waste), see Section 3.2.2 for additional guidance. When Tier 2 and 3 approaches are used, countries can extend the spreadsheet model to meet their own demands, or create their own models. The spreadsheet model can be extended with more sheets to calculate the CH4 emissions if needed. MCF, OX and DOC for bulk waste can be made to vary over time. The same can easily be done to other parameters like DOCf. New half-lives will require new CH4 calculating sheets. Countries with good data on industrial waste can add new CH4 calculating sheets and calculate the CH4 emissions separately for different types of industrial waste. When the spreadsheet model is modified or countries-specific models are used, key assumptions and parameters should be transparently documented. Details on how to use the spreadsheet model can be found in the Instructions spreadsheet.

The model can be copied from the 2006 Guidelines CDROM or downloaded from the IPCC NGGIP website < http://www.ipcc-nggip.iges.or.jp/ >.

Modelling different geographical or climate regions

It is possible to estimate CH4 generation in different geographical regions of the country. For example, if the country contains a hot and wet region and a hot and dry region, the decay rates will be different in each region.

Dealing with different waste categories

Some users may find that their national waste statistics do not match the categories used in the model (food, garden and park waste, paper and cardboard, textiles and others as well as industrial waste). Where this is the case, the spreadsheet model will need to be modified to correspond to categorisation used by the country, or country-specific waste types will need to be re-classified into the IPCC categories. For example, clothes, curtain, and rugs are included in textiles, kitchen waste is similar to food waste, and straw and bamboo are similar to wood. The national statistics may contain a category called street sweepings. The user should estimate the composition of this waste. For example, it may be 50 percent inert material, 10 percent food, 30 percent paper and 10 percent garden and park waste. The street sweepings category can then be divided into these IPCC categories and added on to the waste already in these categories. In a similar manner, furniture can be divided into wood, plastic or metal waste, and electronics to metal, plastic and glass waste. This can all be done in a separate worksheet set up by the inventory compiler.

Adjusting waste composition at generation to waste composition at SWDS

The user should establish whether national waste composition statistics refer to the composition of waste generated or waste received at SWDS. The default waste composition statistics presented here are the composition of waste generated, not waste sent to SWDS. The composition should therefore be adjusted if necessary to take account of the impact of recycling or composting activities on the composition of the waste sent to SWDS. This could be best done in a separate spreadsheet set up by the inventory compiler, to estimate the amounts of each waste material generated, then subtract estimates of the amount of each waste material recycled, incinerated or composted, and work out the new composition of the residual waste sent to SWDS.

Open burning of waste at SWDS

Open burning at SWDS is common in many developing countries. The amount of waste (and DDOCm) available for decay at SWDS should be adjusted to the amount burned. Chapter 5 provides methods how to estimate the amount of waste burned. The estimation of emissions from SWDS should be consistent with estimates for open burning of waste at the disposal sites.

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