## 7 What if the Pits Enter the Methane-Forming Phase? C. Knop of Carbon Sequestration, Inc. has reported the results of monitoring for evidence of buried wood slash decomposition via sensors inserted in a proprietary design of gas wells constructed of perforated pipe (Knop [49]). The sensors measure oxygen, temperature, humidity, CO\({}_{2}\), and methane. To date, CO\({}_{2}\) and methane levels are at or near the limit of detection. Thus, the method of surrounding the buried biomass with low-permeable clay appears to be preventing decomposition. Figure 5 shows the evolution of the composition of biogas generated in municipal landfills. It does not show volume versus time. If buried wood follows the EPA LandGem model for the decomposition of municipal waste (EPA [44]), then the volume of biogas reaches a maximum at the beginning of Phase IV and begins an exponential decline. Thus, after the onset of methane formation  
\[\text{Q}_{\text{CH4}}=\text{A}_{0}\times\text{e}^{-\text{kt}} \tag{15}\]  
where  
\(\text{Q}_{\text{CH4}}=\) the volumetric methane generation at time t;  
\(\text{A}_{0}=\) the pre-exponential factor;  
\(\text{k}=\) the time constant;  
\(\text{t}=\) time. \(\text{A}_{0}\) depends on the parameter \(\text{DOC}_{\text{f}}\), the integrated fraction of biomass that can decompose and that will decompose at time infinity. The latest Puro.Earth Standard Terrestrial Storage of Biomass Methodology for CO\({}_{2}\) Removal (2023) indicates that, with proper burial pit design, the A\({}_{0}\) should be adjusted to match an asymptotic integrated decomposition of 8.8% (Puro.Earth [42]). Since the methane formation phase generates a biogas molar ratio of about 1:1 CH\({}_{4}\)/CO\({}_{2}\), this means that asymptotic integrated methane formation from the biomass carbon will be less than 4.4% (we believe that, with a proper pit design, it may be much less). What is not known is the time constant, which determines how long it takes to reach the asymptote. A LandGem model for the decomposition of municipal waste in a landfill in Bahrain indicates that it will take about 100 years to reach 78% of the asymptote ([PERSON] et al. [51]; [PERSON] [6]). Woody biomass degradation should be exceedingly slow compared to the degradation of municipal waste due to the cross-linking of cellulose and, thus, its dense packing [6]. Thus, it may take several hundreds or even thousands of years to reach the asymptote. This has several implications. First, if and when significant methane formation is detected by the sensors in the simple low-cost perforated plastic pipe gas wells used by Carbon Sequestration, Inc., there will be plenty of time to install a network of gas well pipes that can be connected to a header and directed to a flare, where the biomethane could be burned and converted to bio-CO\({}_{2}\), if needed. But, will gathering and burning really be necessary? The instantaneous rate of methane formation will be exceeding low, and the total methane formation will be spread over the time to reach the asymptote. Methane has a lifetime in the atmosphere of only about 12 years (IEA (2022) [52]). Thus, the cumulative amount of methane in the atmosphere at long times since wood burial will also be exceeding low. Thus, gathering the biogas and burning may not be necessary. To illustrate this point, the exponential decline of methane formation coupled with the rate of the methane remaining in the atmosphere as oxidation takes place was modeled in Excel assuming an A\({}_{0}\) of 0.00172 (adjusted to match the total asymptotic decomposition of 8.8%) and a conservative (fast decomposition) time constant k of 0.0388. This leads to an asymptotic conversion of buried biomass carbon to methane of 4.4% (after 500 years), with 97.7% of the conversion occurring within 100 years. Time slices of 0.5 years were used. This was coupled with an assumed linear decline of methane formed in each time slice to zero over 12 years.