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Monitoring of waste degradation processes for sustainable MSW landfills - Gas Monitoring

Gas Monitoring Systems

The production of gas increases in bioreactor landfills as compared to conventional landfills along with the increased rate of waste degradation. A gas collection and control system (GCCS) is used to collect the primarily methane and CO2 gases that are produced as organic matter in the landfill is oxidized. The gas is either flared off, run through a passive biofilter system before entering the atmosphere, or used for energy generation. Because of the detrimental environmental effects of greenhouse gases, energy recovery is especially important in landfills with leachate recirculation and increased gas generation. To harness energy, the gas is first pressurized into pipes, water vapor removed in a condensing tank, compressed, chilled, reheated, and sent to a generator to generate electricity which is then distributed to the grid.

1

Figure 6: Typical landfill gas-to-energy process schematic. (EPA, 2013)

Important parameters to measure in the resulting gas include:

  • Volume of total gas (ft3/min)
  • Methane, CH4 - % by volume
  • Carbon dioxide, CO2 - % by volume
  • Oxygen, O2 - % by volume
  • Other trace gases
  • Pressure
  • Temperature

 

The total gas production can be estimated using a mass flow meter, orifice plate, or pitot tube. (MPCA, 2009) A mass flow meter can only measure mass of gas per time, whereas orifice plates and pitot tubes can measure mass or volume of gas per time. The flow meters require the pressure and the temperature of the gas in order to calculate the flow rate. The pressure drop can be determined using a differential pressure transducer, and the temperature is measured using an in-line temperature gauge (Townsend et al. 2015). Mass flow meters also require the composition data of the gas before a flow rate can be determined. The composition of the gas at the exit site is measured using a landfill gas analyzer, see Figure 7 (Giannis et al. 2008).

The measurement of gas properties is important in determining the progression of waste decomposition; a graph similar to Figure 1 can be constructed for a particular landfill using carbon dioxide, methane, oxygen, and nitrogen concentrations over time. Gas composition data can also be used to predict the future gas production for a new batch of waste. The following formula is used to estimate gas generation:

 

G(t) = 2LokMoe-kt

where G(t) is the amount generated, Lo and k are the methane generation potential and rate constant, respectively (determined experimentally), and Mo is the mass of solid waste in the batch (Townsend et al. 2015). The coefficient of 2 assumes an approximately 50/50 CO2/CHcomposition; this value is adjusted based on the composition measured by the landfill gas analyzer.

 

gas analyzer diagram

Figure 7: Landfill gas analyzer diagram of a Nova system. The collected landfill gas passes into the analyzing machine and gas composition is reported. (Nova Gas, 2015)

Several times a year, in addition to monitoring the collected gas, the surface of all bioreactor landfills must be monitored for methane leaks as per the New Source Performance Standards (NSPS) 40 CFR 60. The “serpentine method” is carried out during dry meteorological conditions:

  1. A forward-looking infrared (FLIR) camera is used to detect potential leaks in landfill surface, especially the areas surrounding gas wellheads.
  2. A trained inspector carries a portable monitoring device. One option is a flame ionization detector (FID) portable gas analyzer.
  3. The inspector traverses the landfill by foot with the analyzer several inches above the ground surface.
  4. The upwind and downwind methane concentrations are measured.
  5. The inspector walks a serpentine path in order to cover the entire landfill surface, with the sampler continuously monitoring methane emissions.
  6. The path should not cross itself over the entire landfill surface. A separate range of concentrations is recorded for leachate recirculation areas and non-circulation areas (if applicable).
  7. Any methane concentrations greater than 500 ppm should be studied, repaired, and reevaluated according the NSPS standards.

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