Guidelines on remediation of contaminated sites
Appendix
5.1
Gas production - an empirical model for approximate estimates
This Appendix describes a model for assessing the rate of gas production as a function of time. Many different models have been developed, none of which are capable of taking into account all the factors which influence gas generation. Since knowledge of the specific parameters is also often inadequate, using a simple 1st order degradation model is suggested. For this reason, the main use of the model is to provide estimates for consideration.
Calculation formulae
The rate of gas production is given by /1/:
 |
(1) |
where: dP/dt = annual gas production (Nm3/t ž year)
Ptot = total gas production (Nm3/t)
k = the degradation constant (year-1)
t = time (years)
The degradation constant is given by:
 |
(2) |
where: t½ = half-life period (years)
Data basis
The calculation is based on data concerning:
 | Total gas production |
| Half-life period |
Literature on the subject states that, given optimum conditions, total gas production lies between 320-430 Nm3 per ton of waste. However, in actual fact total gas production will be considerable smaller. Values of 180-210 Nm3/t for total gas production have been measured in Danish landfills. These values are supported by Swedish and German calculations.
Empirical figures for half-life periods are listed in Table 1.
Table 1
Half-life periods for types of waste.
Half-life period, t½ |
Years |
Domestic waste /2/ Sludge /2/ Industrial and commercial waste /2/ Bulky waste /2/ Building and construction waste /2/ |
1 2 5 10 15 |
Easily decomposable waste /3/ Moderately degradable waste /3/ |
1-1.5 15-25 |
If the investigation in question has included measurements of simultaneous values of water content and waste temperature, half-life periods can be determined as a function of this /1/.
Example
On the basis of the calculation formulae (equation 1 and 2), curves of annual gas generation and remaining gas potential can be traced. Figure 1 shows gas generation and remaining gas potential as functions of time after disposal for a waste cell with a total gas generation (Ptot) of 200 Nm3/t and a half-life period t½ of 10 years.
The annual gas generation dP/dt after 10 years is calculated as follows:
| dP/dt | = | Ptot · k ·e-k·t |
| = | 200 ·0.07 ·e-0.07 ·10 | |
| = | 6.95 Nm3/t |
Figure 1.
Yealy gasproduction and remaining gas as a function of time after deposition
in a landfill with halftime = 10 years
References
| /1/ | Lossepladsgas – Transport og Produktion
(‘Landfill Gas – Transport and Generation’) Erling Vincentz Fisher
Examination project, spring 1992, the Technical University of Denmark. [Tilbage] |
| /2/ | Gas i lossepladser (‘Gas in Landfills’) ATV
meeting, March 1993. [Tilbage] |
| /3/ | Noter om: Kontrollerede lossepladser (‘Notes on
Controlled Landfills’) Thomas H. Christensen and others. Teknisk forlag, 1982. [Tilbage] |