Results and Discussion

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Generated and collected LFG and GHG emissions for each scenario were calculated using the methodology described in Sections 8.4 and 8.5. Then, to find the most effective technology, a comparison of the different scenarios was carried out.

Annual gas generation rates for all components of the LFG, i.e., methane, carbon dioxide, and NMOC, were calculated by LandGEM software. The results are shown in Fig. 8.2. As can be seen from this figure, LFG generation increases until the final year it accepts the waste. Then it decreases exponentially. For this landfill site, which has a 20-year lifetime, the site continues releasing GHGs for 120 years more after it stops accepting waste as can be seen from this figure.

Taking an average collection efficiency of 75%, collected and uncollected LFG and its components were calculated for each year and shown in Fig. 8.3.

For a landfill site without an active collection system, some amount of methane will be oxidized and converted into carbon dioxide. Remaining gases will be released into the atmosphere. Given that high amounts of methane, which is 25 times more contributing to global warming than carbon dioxide, are released in this case, this gas should be collected and utilized since it has a considerable amount of heating value and high global warming potential. In this study, different technologies for utilizing the collected gas were considered. These include flaring, electricity generation technologies such as ICE, gas turbine, and SOFC. Annual GHG emission from from the landfill site for each technology is shown in Fig. 8.4. For example, in the final year that the site will accept waste, i.e. 2028; 366,831 tonnes CO2 eq. could be released to the atmosphere from a site without an active collection system. Using the most economical solution, which is flaring, GHG emissions would be much lower, 153,456 tonnes CO2 eq. However, there is no electricity production when flaring is used. In the case where a gas turbine is used to utilize the LFG, GHG emissions would be slightly lower than the case of flaring, which is found to be 151,404 tonnes CO2 eq. The most effective technologies for reducing GHG emissions are ICE and SOFC. For the peak year, when ICE and SOFC are utilized, the site produces GHG emissions of 127,430 and 134,208 tonnes CO2 eq, respectively. It should be noted that, for all technologies, it is considered in the calculations that many power generators of the same type operate together, and they may be replaced with new ones if necessary. Additionally, after the year 2088, due to the low methane generation, it is assumed that collected gas will be sent to gas flare instead of the power generator.

Fig. 8.2 Annual gas generation of LFG and its components.

As previously mentioned, the results obtained by using the methodology discussed in Section 8.4 were used in constructing Fig. 8.4. When modeling an ICE, the specific GHG emission ratio of the ICE, which has unit of tonnes eq.CO2/MWh of an existing engine, was taken from literature (Lombardi et al., 2006) and used in Eq. (8.10). In the case of the gas turbine, a simple model was developed by the authors as discussed in Section Using input data given in Table 8.1, the fuel/air ratio on a molar basis was calculated to be 0.070935. According to this ratio, exit gas composition of the gas turbine was found to be 72.3% N2, 12.6% O2, 6.7% CO2, and 8.4% H2O. Finally, in the case of the SOFC, the model is discussed in Section For the type of fuel used in this study, it is reasonable to assume a 0.65 V cell voltage. At this voltage, the corresponding current density and electrical efficiency are 0.28 A/cm2 and 40.3 %, respectively. It is also found that for

1 ton of LFG entering a SOFC system, 0.98595 ton CO2.eq GHG is emitted to the atmosphere.

50000 I 45000 | 40000 S 35000 'g 30000 | 25000 § 20000 ¡» 15000 if 10000 S 5000 * 0

2008 2028 2048 2068 2088 2108 2128 2148


Fig. 8.3 Collected and uncollected amount of LFG and its components.

O o" 200000

tn u

2008 2028 2048 2068 2088 2108 2128 2148


Figure 8.4 Total GHG emissions for various LFG utilization methods.

Figure 8.5 shows the comparison between different technologies operating at controlled landfill sites in terms of their effect on production of GHGs. As shown in Fig. 8.5, the simplest solution, which is flaring, will reduce the GHGs by 58%. Hence, this result reveals the fact that an active collection system together with a

Collected methane Uncollected methane Collected CO2 Uncollected CO2

Collected NMOC Uncollected NMOC

— Collected LFG Uncollected LFG

2008 2028 2048 2068 2088 2108 2128 2148


Fig. 8.3 Collected and uncollected amount of LFG and its components.

2008 2028 2048 2068 2088 2108 2128 2148

Year gas flare would be very effective in reducing the GHG emission if an economical solution is desired and there is no consideration of getting benefit from this gas to convert it into electricity. This figure also shows that using an ICE results in the highest GHG reduction ratio, which is slightly higher than the ratio when SOFC is used. The gas turbine has the least global warming reduction potential of the electricity production technologies studied in this chapter.

Fig. 8.5 GHG reduction ratio for different scenarios.

Since each technology has different electrical efficiency and global warming potential, a more meaningful comparison between the controlled landfill sites studied may be conducted calculating the total GHG emissions in the lifetime per total amount of energy produced for each technology. The results of this comparison are shown in Fig. 8.6. It may be seen from this figure that the SOFC has the lowest specific lifetime GHG emission among the technologies studied, which is 2.3836 tonnes CO2.eq/MWh, when the SOFC is only used for electricity generation. Since the SOFC has a high exhaust temperature, useful heat may be produced which would increase the fuel utilization efficiency of the system. Producing work and heat at the same time, which is called cogeneration, the specific lifetime GHG emission may be further reduced to 1.1217 tonnes CO2.eq/MWh, as shown in Fig. 8.6. In Fig. 8.7, environmental impact factor of the electricity generation technologies are compared. As it can be seen from this figure, gas turbine has an environmental impact factor of 1, since it has the highest specific lifetime GHG emission, and SOFC with cogeneration case has the lowest environmental impact factor, which is equal to 29%.

Fig. 8.6 Specific lifetime GHG emission for different scenarios.
Environment Impact factor

Fig. 8.7 Environmental impact factor for different scenarios.

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