Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Earth science
Reexamination Certificate
2001-12-14
2003-08-26
McElheny, Jr., Donald E. (Department: 2862)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Earth science
Reexamination Certificate
active
06611760
ABSTRACT:
TECHNICAL FIELD
The present invention provides a method and system for estimating the rate of gas production by a landfill or other subsurface body of material in which a gas flow is generated. It also provides a method for estimating the effective gas permeability of a gas generating landfill.
BACKGROUND
Estimation of the rate of landfill gas (LFG) generation is required for calculation of non-methane organic compound (NMOC) emissions under current US law, for successful design of LFG collection systems for LFG-to-energy projects, and for other LFG control systems. With respect to NMOC emissions, United States regulations allow a landfill owner to calculate emissions using a tiered approach based on estimates and/or measurements of LFG generation and NMOC concentrations within the landfill. Tiers 1 and 2 utilize a formula for LFG generation that is based in part on the size and age of the landfill and that does not involve direct measurement of LFG. Because this formula is designed to be conservative, estimates of LFG generation by this method are likely to be higher than the actual rate, especially for landfills in arid environments where low refuse moisture content may limit LFG generation. Tier 3 involves measurements from which the LFG generation rate is calculated. Similar measurement and estimation methods are typically employed to estimate LFG generation rates when designing LFG-to-energy projects and LFG control systems.
The Tier 3 methodology is generally not employed for calculation of NMOC emissions unless calculations by Tier 1 and 2 methods indicate that the NMOC emissions exceed 50 megagrams per year (MG/yr). United States regulations require the landfill owner to install an LFG control system unless the NMOC emissions calculated by Tiers 1, 2, or 3 are less than 50 MG/yr. Operation of the control system is then required until NMOC emissions drop below 50 MG/yr, which will occur eventually for a closed landfill as it ages. Periodic recalculation of NMOC emissions is required, however, to demonstrate that emissions are below this threshold, resulting in additional expense. The Tier 3 methodology is time consuming and expensive, and, as described below, does not provide a reliable estimate of the LFG generation rate or NMOC emission rate. Overestimation of LFG generation by any of these methods is costly to the landfill operator if it results in estimated NMOC emissions greater than 50 MG/yr and requires the installation of an LFG control system. Over- or under-estimation of the LFG generation rate is also costly if it results in an over- or under-designed LFG collection or control system.
The Tier 3 method involves extracting gas from a well or cluster of wells completed in landfilled materials and measuring pressure drawdown in monitoring probes completed at various depths and distances from the extraction well(s) to determine the extraction wells' “radius of influence” (ROI). The Tier 3 ROI is typically taken to be the distance at which no measurable pressure drawdown occurs. Pressure drawdown is defined as the difference between “average static pressure” in the landfill measured prior to gas extraction and the average pressure measured during extraction. Average pressures are used in an attempt to remove the influence of atmospheric pressure fluctuations on the measurements. The assumption is made that the “average static pressure” is determinable as a reference pressure to calculate pressure drawdown after extraction begins.
FIG. 1
, a generalized plot of pressure versus distance from an extraction well, illustrates some of the measurements associated with the Tier 3 methodology. The pressure drop, or “influence” at a given distance from the extraction well is defined as:
I={overscore (P)}
0
−{overscore (P)}
e
(1)
where
{overscore (P)}
0
is the average static absolute pressure
101
(see
FIG. 1
) and
{overscore (P)}
e
is the average extraction absolute pressure
102
(see also
FIG. 1
)
As further seen in
FIG. 1
, the ROI
103
may be determined directly as the distance from the extraction well at which the measured I≦0 (within measurement error
104
) or by extrapolating the measured I values using a semi-logarithmic regression. The accuracy of the pressure measurements is specified to be ±0.02 mm of mercury or 4×10
−4
pounds per square inch (psi).
Gas samples are also collected during extraction from the extraction well and monitoring probes and analyzed for nitrogen to determine whether leakage of atmospheric air into the landfill from the surface is contributing significantly to the flow to the extraction well(s). Nitrogen concentrations in excess of 20% are taken to indicate excess surface leakage. If surface leakage is not indicated by gas analysis or by negative gauge pressures in shallow monitoring probes, then the rate of gas extraction by the well(s) is assumed to be equal to the rate of LFG generation within the volume of landfill materials encompassed by the ROI. Landfill materials outside the ROI are not considered to contribute to gas flow to the extraction well.
The Tier 3 methodology rests entirely on the assumption that the gas extraction rate equals the LFG generation rate within the volume of the refuse between the extraction well and the ROI. This assumption is inconsistent with fundamental principles of gas flow to wells. To illustrate this point, assume that the LFG generation rate is uniform throughout the landfill and that the effective gas permeability of the refuse is much larger than the gas permeability of the cover so that the vertical pressure gradient in the refuse is negligible. In this case, the average difference in pressure between refuse and the atmosphere due to flow through the cover is given simply by Darcy's Law (Al'Hussainy and others, 1966):
q
LFG
=
k
c
μ
Δ
P
0
b
c
or
(
2
)
Δ
P
0
=
q
LFG
μ
c
b
c
k
c
(
3
)
where
q
LFG
is the gas generation rate unit area of landfill
k
c
is the effective gas permeability of the cover
&mgr; is the dynamic viscosity of the LFG
b
c
is the cover thickness
&Dgr;P
0
is the pressure differential P
0
−P
a
P
a
is the atmospheric pressure
P
0
is the pressure in the refuse.
Given the assumption of a uniform LFG generation rate and an areally extensive landfill, the static pressure in the refuse is
{overscore (P)}
0
={overscore (P)}
a
+&Dgr;P
0
(4)
where {overscore (P)}
a
is the average atmospheric pressure. Given the assumptions above, {overscore (P)}
0
is uniform throughout the landfill.
For small pressure differentials, the pressure drop created by an extraction well (assuming an ideal gas and steady-flow conditions and ignoring compressibility effects) is given by:
Δ
P
e
=
-
Q
e
μ
2
π
k
r
b
r
P
D
(
5
)
where
k
r
is the effective horizontal gas permeability of the refuse,
Q
e
is the well extraction rate,
P
D
is an appropriate dimensionless pressure solution for flow to the well,
&Dgr;P
e
is the difference between static and flowing pressure, and
b
r
is the thickness of the refuse.
For the case of a well fully penetrating a highly permeable refuse in a lined landfill with a relatively low permeability cover, the appropriate P
D
function is that given by Hantush (1964) for a leaky, confined formation without fluid storage in the confining bed:
P
D
=
K
0
(
r
/
B
)
;
B
=
(
k
r
b
r
b
c
k
c
)
1
/
2
(
6
)
where K
0
is the modified Bessel function of zero order.
Thus, equation (5) becomes
Δ
P
e
=
-
Q
e
μ
2
π
k
r
b
r
K
0
(
r
/
B
)
(
7
)
The average absolute pressure within the refuse during extraction is then
{overscore (P)}
e
={overscore (P)}
0
+&Dgr;P
e
(8)
The generalized absolute pressure in the refuse
102
based on (7) is illustrated in
FIG. 1
along with its relationship to the static pressure
101
. In the Tier 3 methodolog
Bentley Harold W.
Smith Stewart J.
Tang Jinshan
Walter Gary R.
Williamson Christian T.
Hydro Geo Chem, Inc.
Lawrence R. Oremland, P.C.
McElheny Jr. Donald E.
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