LandGEM: EPA’s Landfill Gas Emissions Model – Part 3

Using LandGEM
LandGEM is flexible enough to allow the user to input either site-specific data (if available) or default parameters provided by the model. LandGEM contains two sets of default parameters. Clean Air Act (CAA) is based parameters or inventory defaults. The CAA defaults are derived from for MSW landfill emission requirements defined by the Clean Air Act. Appropriately, this default set results in conservative (high) emission projections. Inventory Defaults (except those associated with wet, bioreactor landfills) derive from emission factors utilized by the EPA’s “Compilation of Air Pollutant Emission Factors” (AP-42). This is a less conservative set of assumptions and can be used to project average emissions rates. Both are useful for estimated emission results when site-specific test data is absent.

And this will be the position of most LFG system design engineers and analysts. Even established landfills may be lacking consistent or complete historical data for LFG system planning and design. Furthermore, it is often more economical (and politically acceptable) to expand an existing landfill rather than site, permit and construct a brand new landfill. When this occurs, previous LFG system design assumption may need to be rethought or even thrown out.

LandGEM is an Excel based software package that is actually a series of interrelated spreadsheets. These spreadsheets, to quote from the User’s Manual, are described as follows:

Intro—“Contains an overview of the model and important notes about using LandGEM.” This would include information on what must be entered in the Users Input spreadsheet, a short description of the model’s methodology and default data, and what types of adjustments should be made when comparing model results with actual field data.

User Inputs—“Allows users to provide landfill characteristics, determine model parameters, select up to four gases or pollutants (total landfill gas, methane, carbon dioxide, NMOCs, and 46 air pollutants), and enter waste acceptance rates.” The landfill characteristics to be entered on this spreadsheet include the landfill’s name, opening and closing year (unless the model is used to calculate the closing year), and the landfill’s capacity. The user also decides which model parameters to use (AP-42, CAA, site data) and the types of gases to measure (total LFG, methane, carbon dioxide, and NMOCs). Lastly, the user inputs the amount of waste the landfill is expected to receive each year of its operational lifetime (up to a maximum of 80 years, the model’s waste acceptance limit).

Pollutants—“Allows users to edit air pollutant concentrations and molecular weights for existing pollutants and add up to 10 new pollutants Input Review Allows users to review and print model inputs.” These pollutants include hazardous air pollutants (HAP) listed in Title III of the 1990 Clean Air Act amendment and volatile organic compounds (VOCs) listing in 40 CFR 51.100(s). The user can adjust anticipated pollutant concentrations and add additional pollutants to the list based on field data. A mini-report is provided on an additional spreadsheet that provides a quick review of the user’s inputs and data modifications.

Methane—“Calculates methane emission estimates using the first-order decomposition rate equation.” This is where the first-order decomposition computation is performed. It shows the user inputted waste acceptance rates for each operational year, the subsequent amount of waste in place after each year of additional waste receipts less the amount of anticipated decomposition, and methane emissions in cubic meters for each year. Note that the methane production rate projections extend beyond the maximum 80 years of waste disposal operations (up to year 140, a difference of 60 years—twice as long as the typical post closure care period for a landfill). This reflects the fact that waste decomposition and methane production continues long past the last day of waste disposal and closure of the landfill.

Results—“Shows tabular emission estimates for up to four gases/pollutants (selected in the User Inputs worksheet) in megagrams per year, cubic meters per year, and user’s choice of a third unit of measure (average cubic feet per minute, cubic feet per year, or short tons per year).” This sheet provides a summary table where all four major pollutant categories (total LFG, methane, carbon dioxide, and NMOCs) are tabulated. The user can convert this data to either in English or Metric units.

Graphs—“Shows graphical emission estimates for up to four gases/pollutants (selected in the User Inputs worksheet) in megagrams per year, cubic meters per year, and user’s choice of a third unit of measure (selected in the Results worksheet).” As the previous spreadsheet provides a tabular summary, this spreadsheet provides a graphical summary in the form of four line graphs that trace the production of pollutants over the operational lifetime of the landfill and beyond. All the graphs follow the same pattern with a steep curved increase in production until a peak is reach (coinciding with the last year of waste disposal operations), followed by a shallow decline curve out to the model’s last calculation in year 140.

Inventory—“Displays tabular emission estimates for all gases/pollutants for a single year specified by users.” The user enters a particular year (from year 1 to 140) and this spreadsheet provides an estimate for the emission rates of all of the pollutants listed in the previous “Pollutants” spreadsheet described above.

Report—“Allows users to review and print model inputs and outputs in a summary report.” This spreadsheet takes all of the results described above and summarizes them in a narrative format for easy and convenient print out of hard copy reports for manual review.

LandGEM Results and Applications
As a simple example, we can run the model for a hypothetical landfill with a 30-year operating lifetime (from 2012 to 2042) that receives an average of 500 metric tons (megagrams) of waste each year. This example would utilize standard CAA default values for K (1/0.05 year), Lo (170 M³/megagram), NMOC concentrations (4,000 ppm as hexane), and methane percentage of total LFG (50%). The model will report on the standard pollutant gasses (total LFG, methane, carbon dioxide and NMOCs) without any additions or revisions to the pollutants list.

Accumulated waste received peaks at 15,000 metric tons in year 2042, the year of landfill closure. The resultant gas production rates are illustrated in the following graphs.

The first graph takes into account the different molecular weights of each pollutant to provide estimates in terms of megagrams per year. The second is by volume, as measured in cubic meters per year. Note that with methane assumed to be 50% of total landfill gas, the methane curve coincides with the carbon dioxide curve. Methane production peaks in year 2043 at 6.713E+04 cubic meters per year.

The Bottom Line
So why is modeling LFG production in general and LandGEM in particular so important? LFG management systems are expensive, usually more expensive than the landfill’s other primarily mechanical landfill system for managing and removing leachate. An LFG management system consists of extensive piping, fittings, extraction wells, condensate drip legs, and blower/flare apparatus. Gas probes can cost as much as $8,000 with gas wells costing more and $10,000 each. Header and lateral pipelines can cast around $100 per linear foot to install. The blower/flare apparatus can have a price tag as high as $50,000. On a per-acre basis, the entire system can cost between $30,000 to $40,000 per acre, with large (100 acres or more) landfills spending in the millions of dollars for a complete system.

In addition to the capital costs of installing the system, operations and maintenance for the landfill gas system maintenance can also be a major line item in the landfill operator’s budget. Annual maintenance averages $50 to $70 per well with the maintenance of the header pipelines and other appurtenances averages $2 to $2.50 per linear foot. Total annual gas system costs can be $500 per acre. Over a 30-year post-closure care period, LFG system management costs can exceed $15,000 per acre, again costing in the millions of dollars for large landfills.