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Landfill Gas Quality and Quantity

Significance of Landfill Gas
Potential energy recovery of methane Methane is a potent greenhouse gas Explosive danger Health hazards associated with trace gases Odor nuisance

Legislative Issues
Public Utility Regulatory Policy Act (PURPA-1978) governs the sale of electric power generated by LFGto-energy plants (and other renewable energy sources)Federal tax credits and state regulations which provide financial assistance and incentives to recover and reuse LFG PURPA only calls for renewable energy if it is cost competitive with conventional polluting resources. Many of the benefits of renewables are not included in the price, such as clean air

RCRA Subtitle D
RCRA, Subtitle D and Chapter 17-701, FAC, with respect to LFG monitoring,control, and recovery for reuse Concentration of methane cannot exceed 25% of the lower explosive limit in on-site structures

NSPS and Emission Guidelines
Promulgated under the Clean Air Act New and existing landfills Capacities equal to or greater than 2.75 million tons Regulates methane, carbon dioxide and NMOCs Require
– Well designed/operated collection system – Control device capable ofreducing NMOCs by 98%

National Emission Standards for Hazardous Air Pollutants: Municipal Solid Waste Landfills Additional requirements for landfills constructed since Nov. 2000 Additional controls for HAPs identified in the CAA

AP-42 Emission Factors (EF)
An EF is related to the quantity of pollutants emitted from a unit source Important for developing control strategies,applicability of permitting programs, evaluating effects of sources and mitigation When site specific data are not available, EFs are used to estimate area-wide emissions
– For a specific facility – Relative to ambient air quality

EFs for LFG
EFs provided for controlled and noncontrolled and secondary emissions from landfills EFs developed for NOx, CO, PM, SO2 NMOCs HAPs, others (HCl, H2S, CH4) Methanogenesis Reactions
CH3COO- + H2O ---> CH4 + HCO3acetate bicarbonate + water ---> methane +

hydrogen + carbon ---> methane + water dioxide

4H2 + CO2 ---> CH4 + 2H2O

Favorable Conditions for Methanogenesis
Sufficient moisture content Sufficient nutrients Absence of oxygen and toxics Relatively neutral pH, 6.7 - 7.2 Alkalinity greater than 2000 mg/l as calcium carbonate VolatileAcids less than 3000 mg/L as Acetic Acid Internal temperature between 86o F and 131oF

Properties of Methane
Molecular Formula: Heating value: Solubility in water: Ratio of O2:CH4 req. for combustion: CH4 2350 Jg-1 17 mg/L 2

Gas Composition - Major Gases
Methane (45 - 60 % by volume) Carbon Dioxide (40 - 60 % by volume) Nitrogen (2 - 5 % by volume) Oxygen (0.1 - 1.0 % by volume) Ammonia(0.1 - 1.0 % by volume) Hydrogen (0 - 0.2% by volume)

Gas Composition - Trace Gases (less than 0.6 % by volume)
Odor causing compounds Aromatic hydrocarbons Chlorinated solvents Aliphatic hydrocarbons Alcohols Polyaromatic hydrocarbons

Estimating Gas Quantities
Gas Yield Duration of Gas Production Shape of Batch Production Curve Lag Time Estimate

Gas Yields
3 - 90 L/kg dry Stoichiometric Estimate of Gas Potential
1 CH a Ob N c + (4 − a − 2b + 3c )H 2O → 4 1 1 (4 − a + 2b + 3c )CO2 + (4 + a − 2b − 3c )CH 4 + cNH 3 8 8

Problems with Stoichiometric Estimates
Some fractions are not biodegradable (lignin, plastics) Moisture limitations Toxins Some fractions are not accessible (plastic bags)

Biochemical Methane Potential
Sample Mixed MSW Mixed Yard Waste Office PaperNewsprint Magazine Food Board Milk Carton Wax Paper

Methane Yield, m 3 /kg VS 0.186 - 0.222 0.143 0.369 0.084 0.203 0.343 0.318 0.341

From Owens, J.M. and D.P. Chynoweth

Duration of Gas Production
Waste composition (degradability) Moisture conditions For first order kinetic models, controlled by first order reaction rate constant (k)

Estimates of Gas Production Rates
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