Analysis of Greenhouse Gas Emissions

In this section, a method for calculating GHG emission from a landfill site without an active collection system is first described. Then, methods for calculating GHG emissions from landfill sites in which the collected LFG is utilized by flaring, conventional electricity generation technologies such as ICE and GT, and SOFC, are discussed. Finally, some parameters for comparing these technologies are introduced.

8.4.1 Landfill site without an active collection system

In a landfill site without an active collection system, not all of the methane generated is emitted into the atmosphere. A portion of the methane generated is oxidized while passing through soil and landfill covers. The fraction of methane that is oxidized is generally taken as 10% (Climate Leaders, 2004). The oxidation of methane reaction is given as follows:

If we neglect the NMOC portion of the LFG, total GHG emissions from this kind of a site may be expressed as shown in Eq. (8.6). It should be noted that the equations in this chapter are derived for a LFG composition of 50% CH4 and 50% CO2. However, similar equations may be derived with simple modifications to these equations for different ratios of CH4 and CO2.

GHG.uncoll if W



8.4.2 Landfill site with an active collection system

In a landfill site with an active collection system, LFG is recovered by vertical wells or horizontal collectors. The recovered gas can be flared or utilized to generate electricity by technologies such as gas turbines, ICEs, or fuel cells. In the following subsections, the methodologies for calculating the GHG emissions from landfill sites, which consist of flare and these electricity generating technologies, are discussed. Flaring

The combustion of methane may also be represented by Eq. (8.5). If we assume that all of the collected gas is flared, and a small portion of the collected gas is vented during the routine and unscheduled maintenance, total GHG emissions from the site can be found by using Eq. (8.7).

mGHG.coll = g ((1 — x(mQHG.uncollI + n«on x(mGHG.flareI ) (8'7)

where GHG generated due to flaring is mGHG.flare : Electricity generation technologies from LFG Internal combustion engine

GHG emissions per energy output of ICEs suitable for LFG operation are given in the literature (Lombardi et al., 2006). Using this emission data, the amount of collected LFG, electrical efficiency of the ICE, days of operation of the engine per year, and higher heating value of the fuel, one may calculate the total GHG emissions from such a landfill site using Eq. (8.9). In this equation, it is assumed that after year, tdown, engines stop operating and collected LFG is burned. There is also enough number of ICEs that can utilize LFG even at the year when its generation is at maximum level.

„ = (1 — ncoll)x \GHG.imco111 +ncoll x(mGHG.ICE )

"GHG.uncoll >y "/col^^GHG.flare11

y tdown where GHG generated from ICE can be calculated as mGHG.ICE = ( T /3651X ( mLFG.gen X hhv X ^ICE X fICE

Since there is insufficient data in the literature regarding GHG emissions from LFG fueled gas turbines, a simple model is developed by the authors. In this model, it is assumed that air gas composition is: 77.48% N2, 20.59 O2, 0.03% CO2, and 1.9% H2O. For the fuel-air ratio, A , the combustion equation may be written as shown in Eq. (8.11).

l(0.5CH4 + 0.5C02)+ (0.7748N2 + 0.205902 + 0.0003c02 + 0.019H2O ^ (8 U)

Exit gas composition of the combustor may be shown using Eqs. (8.12)-(8.15). 0.7748

Solving the energy balance around the control volume enclosing the combustor, as shown in Eq. (8.16), A hence exit gas composition may be calculated.

0 = -0.02 • nf • LHV+ nf • hf + nc i • hc i - nco • hc o (8.16)

The first term in the right-hand side of Eq. (8.16) denotes the assumed heat loss from the combustor.

The total GHG emissions from a landfill site, where a gas turbine is used for electricity production, may be calculated using Eq. (8.9), if the mGHG.ICE is replaced with mGHG.GT which is shown in Eq. (8.17).

(t/365)x| mtfrgen x-=-x-- I + (1 -t/365)xm„TT„ _ Solid oxide fuel cell

GHG emissions per LFG entering the system may be found using the SOFC model developed by the authors (Colpan et al., 2007). This SOFC model may be described as follows. The gas composition of the fuel channel exit can be found using chemical equilibrium equations and the relation between the electric current and the molar flow rate of hydrogen that is utilized. Then, the air utilization ratio, which measures the amount of excess air that should be sent to the air channel to carry away the unutilized heat in the fuel cell, is calculated. Hence, cell voltage, work output of the cell, and electrical efficiency of the cell are found. After finding the GHG emissions from the SOFC, the total GHG emissions from the landfill site may be calculated in a similar method as conducted with ICEs and gas turbines.

8.4.3 Comparison of LFG utilization technologies

The authors propose three parameters for comparing the usefulness of technologies in reducing the global warming in landfill sites. The first parameter is called 'GHG reduction ratio', as shown in Eq. (8.18). This ratio quantifies the GHG emission reduction when an active collection system is used. If there is no emission from the landfill site when an active system is used, this ratio is equal to 100%. If this ratio is equal to 1, it also means there is no contribution to global warming from this landfill site.

The second parameter is called 'specific lifetime GHG emission' which may be defined as the ratio of the total GHG emission from the landfill site in its lifetime to the total amount of useful energy produced from LFG. This ratio is shown in Eq. (8.19) and is useful to compare GHG emissions for the same amount of power produced from different technologies. From the point of view of global warming and energy, the lower the ratio is, the more effective the technology is.

n + m ixncoll xr/365xhhv/3.6xnel

CH4.gen CO2.gen) coU el

The third parameter is called environmental impact factor, which could be used to rank the electricity generation technologies according to their effectiveness in reducing the greenhouse gases. This ratio changes between 0 and 1. If it is equal to 1, it means it is the least effective way to reduce the greenhouse gases among the possibilities that have been studied. This parameter is shown in Eq. (8.20).

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