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Combustible-gas production from domestic, municipal and industrial refuse deposited in landfill sites
4pplied Energy 34 (1989) 21-34
Combustible-Gas Production from Domestic, Municipal and Industrial Refuse Deposited in Landfill Sites N. G a r d n e r & S. D. P r o b e r t Department of Applied Energy, Cranfield Institute of Technology, Bedford MK43 0AL, UK
ABSTRACT The rate of production of landfill gas (LFG) varies considerably from site to site. A major obstacle in determining thefinancial viabilities of proposed LFG utilisation schemes, for sites which are producing appreciable quantities of gas, is the current inability to forecast accurately the considered site's gasproduction rate. In essence there are two techniques for determining a site's gas-production rate:
(1) The site's behaviour is modelled mathematically and the LFG production rate is predicted from the composition of the deposited waste and an understanding of how quickly the organic material decays to form LFG. (2) Gas-extraction flows and associated gas-pressure measurements are taken at the site. The second technique is preferable because more-accurate estimates can be made as to the mass of LFG likely to be produced per day over the next decade. The present paper discusses two approachesfor the implementation of the second technique, and makes recommendations with respect to procedure.
NOTATION A C h K
Characteristic constant in eqn (1) The site's LFG-collection efficiency (%) The total thickness of the buried refuse (m) Site's gas conductance (m2/Pa/h) 21 Applied Energy 0306-2619/89/$03-50 © 1989 Elsevier Science Publishers Ltd, England Printed in Great Britain
22 m i
m s
P Pa Pe
Po Pw
r rb
R S
N. Gardner, S. D. Probert
The error arising from the overall inaccuracy of the site measurements The estimated error which accounted for the spread in the calculated results for Differential pressure within the landfill, at a horizontal distance r from the well (Pa or m m WG) Unperturbed mean static pressure in the landfill refuse (Pa) Estimated mean static differential pressure within the landfill at the site (Pa or m m WG) Estimated differential pressure at the vertical borehole (Pa or m m
WG) Suction pressure applied at the well head (Pa) Gas extraction rate (m3/h) Gas flux (m3/mZ/h) Horizontal distance from the well-head's vertical borehole (m) Radius of the well's casing (m) Radius of influence within the deposited refuse for a particular suction applied at the well head (m) A constant which determines how quickly the mean static differential landfill pressure (Pc) is attained (Pa/m or m m WG/m) Total gas production rate (m3/m 3 of site/h)
ABBREVIATIONS LFG mm WG rope NTP tce
Landfill gas (i.e. it consists principally of methane, carbon dioxide and nitrogen) Unit of differential pressure, equivalent to that due to a column of water whose height is expressed in millimetres The overall most probable error in the estimated gas-production rate for a site Normal temperature and pressure Tonnes of coal equivalent
GLOSSARY
A gas-well's performance characteristic relates the quantity of L F G which is collected per h o u r to the well-head suction. A site's gas-collection efficiency, C, is defined as the ratio of the collection to production rates of LFG, and usually expressed as a percentage.
Combustible-gas production from refuse in landfill sites
23
A site's gas conductance (K) can be defined by Darcy's law: Of = K-~r For a location in a site having a gas conductance of 1 m2/pa/h and a pressure gradient of 1 Pa/m, the resulting flux of gas along this pressure gradient, across an element of 1-m 2 area, would be 1 m3/h. The darcy is the unit in which the permeability coefficient of the deposited refuse is expressed: 1 d a r c y - 9.87 × 10-13m 2 The error m I arises from the overall inaccuracies in measuring the pressures in the probes, their exact locations relative to the LFG-extraction well, and the site's gas conductance. The estimated error m s indicates the spread in the calculated results for ~. The overall most probable error (mpe) in determining a site's gas production rate, ~, was estimated from a knowledge of mt and m s, via mpe = (m~ + m~) 1/2 Note
Where pressures have been referred to in this text, they are differential pressures measured relative to atmospheric pressure.
B U R N I N G OR BIOGAS STRATEGIES FOR SOLID WASTES? National policies, and the legislation resulting therefrom, are increasingly aimed at reducing environmental pollution, and so the combustion and dumping of refuse are being subjected to growing scrutiny and criticism. The growing interest in pollution prevention (especially if associated with energy conservation) is particularly desirable for the UK, which is heavily industrialised and has such a high population density that intensive uses of land and water ensue. This trend is likely to continue despite the fickle energy market, i.e. large fluctuations in the unit price of crude oil. It is also likely that the polluter will increasingly be required to pay for the damage so produced, thereby encouraging the introduction of pollution-prevention facilities. Another longer-term trend of a sustainable society is that, each year, the rubbish produced during the year will be fed directly to biogas production plants and used that year.
24
N. Gardner, S. D. Probert
At present only about 5% of Europe's solid refuse (~400 x 106tonnes/ year) is incinerated, the rest being dumped or used for landfilling. In theory, combustion offers better prospects of a greater percentage of energy recovery from the solid wastes, but in practice there are numerous problems, e.g. having to 'size' the waste appropriately before presentation to the burners, and the dangers of inadequate combustion leading to the production of dioxins and other harmful by-products. (Possible links of dioxins in the atmosphere with cases of cancer of the larynx have been suggested.) A recent survey by the National Society for Clean Air showed that about one-quarter of the 1700 boilers and incinerators protected by Crown immunity in the UK, including hospital incinerators, were regarded as sources of air pollution. Thirty-nine environmental health officers in the UK also expressed concern about the levels of dioxin present in the environment. Thus the dumping of refuse as landfill appears likely to continue for the foreseeable future, though several new totally integrated solid-waste-to-energy plants (e.g. as designed by Kruga and Alpha Technik, Denmark) will be built during the next 5 years. 1 However, there are drawbacks to the dumping of refuse, e.g. leaching possibly resulting in the contamination of soil (well away from the landfill site) and groundwater supplies, as well as the decreasing number of suitable, acceptable and available sites near the major conurbations rejecting the refuse. Also landfill sites produce landfill gas (LFG) which, because of its smell, can be highly objectionable when allowed to pollute the local atmosphere, and it can cause fires or explosions if permitted to migrate laterally from the site into nearby buildings. Thus pumped extraction of the LFG from the site may be necessary in order to avoid these problems: the LFG being produced may then also be used as an energy supply. If no customers for large amounts of LFG exist close to the landfill site, then electricity generation using the LFG could be an acceptable option. However the combustion products of some trace gases (e.g. mercaptans and halocarbons) that arise when LFG is burned can be highly corrosive to the engines or turbines involved in the electricity generation. Thus LFG cleanup (prior to combustion) would probably be required so increasing the payback period (i.e. the required capital investment divided by the financial savings achieved per year as a result of that investment). However, the easiest and most financially attractive option is the direct combustion of LFG using appropriately designed burners. This will reduce the combustion of coal, oil or natural gas that would otherwise occur: the pay-back period may then be less than 1 year. An example of such an application is the use of LFG for firing in local brick or cement kilns. The yield and longevity of supply of LFG from a particular landfill site are influenced by: (a) the deposited refuse's composition; (b) the packing density;
Combustible-gas production from refuse in landfill sites
25
(c) the moisture content; (d) the site's geometry and hydrogeology; (e) the cover material; (f) the local climate (especially rainfall); and (g) the period elapsed from the time of deposition of the refuse. The lack of control or knowledge of some of these influential parameters makes the theoretical prediction of the likely rate of yield of LFG versus time characteristic for a particular site almost impossible. Thus we have to resort to measurements of LFG amounts yielded for various applied suctions at the gas-well head in order to predict the likely total yield per tonne of refuse at that site. Up to 350 of the 669 licensed landfill sites in the U K are, or have the potential to become, economically viable LFG producers. 2 The most financially attractive of the U K sites occur in a broad band stretching from the southeast of England up to the northwest. A typical yield of approximately 2 therms per tonne of biodegradable refuse per annum (,,~ 11 m 3 LFG per tonne per year) over a period of 13 years, may be expected. 2 The annual total output from all LFG installations in the U K amounted, by the end of 1987, to 1.2 x 105 tce. By AD 2000, it is anticipated that the output will rise to 106 tce per year and eventually to 3 x 106 tce per year. The present study discusses two techniques for estimating the combustible-gas production rate from a landfill site. Consideration is given as to how a more appropriate choice of equation relating the variables can be obtained, so resulting in predictions of greater reliability.
APPROACH 1 This was first introduced by Kunz & Lu. 3 It requires solving a Poisson-type equation, which describes the steady-state LFG pressure-distribution around a gas-extraction well, when a constant suction is applied at the well head. Generally, q
P = A + 2-~
0~ 2
(ln r) - ~ r
(1)
where the site's gas conductance, K, can be determined by taking measurements at the gas-extraction well head (i.e. monitoring the gascollection rate for each suction pressure applied to the well). By knowing the well's performance-characteristic, the following equation, introduced by Hoeks, 4 can be used to determine K: P I - - P a = 47rKh
1 - In
(2)
26
N. Gardner, S. D. Probert
TABLE 1 LFG Production Rates Year of trial
Gas production +_rope
Well of site studied
(m3 per year of LFG at atmospheric pressure per tonne of deposited refuse)
1982 1982 1988
Stewartby D6 Stewartby D7 Calvert 16
100 + 19 80 + 12 350 + 119
If a value for 0t (say in the range 5 - , 30 ma/tonne/year) is assumed, K can be estimated typically to within 15% accuracy. Then a set of two simultaneous equations o f the form o f e q n (1) (with A and 0cunknown) can be solved for 0~.
Use of approach I This has been applied at the Stewartby (Bedfordshire) and Calvert (Buckinghamshire) landfills operated by Shanks and M c E w a n (Southern) plc. After much data sampling, the results of Table 1 were deduced. These predicted results are unrealistically high: the average cumulative gas production over the w h o l e lifetime o f a site is at m a x i m u m about 400 m 3 per tonne o f buried domestic refuse. K u n z & Lu 3 made no mention o f such a shortcoming with respect to their use of this method, and equally they provided no estimate as to the accuracy of using this approach.
Errors and uncertainties (see Table 2) Evidently m i, arising from the overall inaccuracy of each o f the measurements, makes a significant contribution to the mpe. Surprisingly, the
TABLE 2 Uncertainties in Determining a Site's LFG Production Rate by the Use of Approach 1 Well
mi (%)~
ms (%)b
rope (%)b
Calvert 16 Stewartby D6 Stewartby D7
32 12 13
10 15 7
34 19 15
aPercentage error in the LFG production rate arising from predicted rates determined from daily tests. bPercentage error in the LFG production rate due to uncertainties in the input data.
Combustible-gas production from refuse in landfill sites
27
TABLE 3 Characteristics of Various UK Landfill Sites
Landfill site, well and year Brogborough 8 (1988) Calvert 16 (1988) Stewartby B8 (1988) Stewartby D6 (1982) Stewartby D7 (1982)
Conductance (m2/pa/h) 0.001 0-0.001 4 0.002 44).003 5 0"030--0'032 0"002 4-0"003 1 0.006 2-0.007 6
Permeability (darcys) 4-5 9-13 107-114 8-11 22-27
value of m t for the tests at the Calvert site was relatively high compared with those for the tests at the Stewartby site. At Calvert, a closely spaced 'line' of vertical probes (2 ~ 3 m apart) was used, whereas the arrangement at Stewartby consisted of vertical probes located every 10m in a plane extending 50 m horizontally from the gas-extraction well head.
Published results, obtained using approach 1 Kunz & L u 3 appeared to be confident of the worthwhileness of this approach. For the Fresh Kills landfill site at Staten Island, New York, USA, the following results were predicted: gas-production rate 12.2m 3 per tonne/year; site's gas conductance 0.054m 2 per Pa per hour; and site permeability 198 darcys. (For a permeability of 1 darcy and for L F G with a dynamic viscosity of ~ 1 3 x 10-6pas, the gas conductance is 2.7 x 10 -4 m2/pa/h.) The permeability of the Fresh Kills landfill is much higher than the estimated permeabilities of the three UK sites--see Table 3.
Assumptions made in approach 1 (a) The L F G pressure-distribution around the gas-extraction well obeys a Poisson-type equation. (b) An average gas conductance for the influenced area around the gasextraction well is meaningful and applicable under the conditions likely to be encountered. (c) No account of the variations of the pressure profiles with depth has been taken: in this respect, it should be remembered that solving eqn (2) will yield an estimate for ~ for a particular depth at the site. Utilising pressure probes, which extend radially to at least 30 m from the well, will tend to reduce the chance of predicting too local a value of ~. For this approach to be successful, it is essential that assumption (a) is
28
N. Gardner, S. D. Probert
valid. However a closer examination of the predictions obtained using eqn (1) revealed that it was truly representative over only a small portion of the well's total area of influence. Tests highlighted that this equation was not always reliable in predicting pressures, especially for those locations more than 40 m from the well. Determining an average LFG conductance of a site over the volume of refuse influenced by a gas-extraction well did not pose any problem. However, is substituting an average value of K into eqn (1) justified? Although the likely magnitude of the uncertainty in determining K is relatively large and this influences significantly the overall accuracies of the predictions obtained via approach 1, the major limitation of approach 1 is the inability of a Poisson-type equation to model a gas-well's pressureprofile consistently and adequately. Assumption (c) is a limitation which is common to all existing pressuredependent techniques for determining ~. A more useful value for ct could be obtained from pressure measurements taken with lines of vertical probes, each line extending to a different depth, and from data for other gas wells at the site.
APPROACH2 This is identical to the first approach, except that the Poisson equation is replaced by an exponential equation to describe the pressure distribution in the dumped refuse. The following equation was regarded as a reasonable approximation to reality: two of its constants are the pressure po at the well head and the distant landfill pressure (or mean static landfill pressure)Pc. Then
P--Pe _ ( p ~ - po) exp ( _ S rr)
(3)
The advantages that this equation offers can be summarised as follows: (1) It appears successful in modelling the behaviours of sites with small landfill differential pressures ( < 10 mm WG static pressure); (2) It produces more-accurate predictions for distant landfill pressures even when the constants for eqn (3) are deduced from data taken entirely near the borehole (which are easier to measure). The main drawback with this approach is that it is necessary to introduce a criterion for defining the radius of influence for the suction applied at the well head. For the present investigation, this was estimated from information provided in a study performed at the Stewartby landfill site in
Combustible-gas production from refuse in landfill sites
29
1982 by Campbell et al. 5 The criterion was developed by examining the distant piezometer data for different borehole suctions. From such an examination, it was possible to estimate the radius of influence to within _ 5 m. This enabled one to predict the pressure-gradient at the radius of influence, i.e. outside this zone the well's influence was insignificant. The limiting criterion turned out to be ~-r, = R = 0" 130 m m W G / m
(4)
Uncertainties with approach 2 The gas production rate, ~, can be estimated using the following equation: o~ = ~ h R 2
(5)
where the radius of influence R can usually be determined to better than 10% accuracy. For some sites, the uncertainty in the value of h has posed more difficulties, e.g. at the Calvert site it is known that approximately 30% of its gas wells are relatively ineffective because of flooding. Also the surrounding and more distant refuse may be near to moisture saturation, but may not necessarily have the same level of liquid present. Therefore, particularly for the Calvert site, defining a single value for the effective depth of refuse, h, is open to criticism. Of the 24.5-m depth of refuse around well 16, it was decided on the basis of experimental tests to use a 20-m depth of refuse in the calculations: this allowed for the lower 4.5 m of the site being flooded. Together, these two uncertainties lead to a total uncertainty of approximately 25% for the estimated L F G production rate.
Difficulties with approach 2 During tests at the Calvert landfill site, where several radial lines of vertical probes were employed, the suctions used throughout the trial tended to be excessive. This did not cause any difficulty with respect to the mathematical modelling. However, the predictions for the 'distant' gauge-pressures were then sometimes slightly negative, which obviously caused difficulties in assigning a meaningful value for the radius of influence. This approach yields an estimate for each well's radius of influence for a particular depth in the site. On examination of Fig. 1, it can be seen that, provided that the pressure-measuring probes are placed as deep as the perforated well-casing, a reasonably representative estimate of the value of R can be obtained. However, it is also apparent that the radius of influence
30
N. Gardner, S. D. Probert lOre GAS WELL PIPE(THELOWER~,'SmOF ITS LENGTH iS PERFORATEO) .
! i
i
i
i
.
k/
.
.
.
.
p
', .
.
\
I
.
/:
11rn
, \
i
22m
33m
- - 0.10- "NEGATIVE"PRESSURE=0-10 INCHESWG
~
Z0NE IN WHICH TI"IE PIEZ0HETERSARE PLAEED
Fig. 1. Predicted constant-pressure contours for an LFG extraction rate of 340 m3/h from a 23-m thick bed of buried refuse. (From Ref. 6.)
changes with depth. It therefore follows that the gas well's mean radius of influence will be smaller than the estimate derived from probes placed in the region indicated by the shaded area of Fig. 1.
C O M P A R I S O N OF T H E TWO A P P R O A C H E S If the constants o f e q n (1) are known or can be determined, it is possible to use this equation to predict the pressure near the well head (i.e. at 0.5 m from the well) and distant pressures (e.g. at 4 0 m from the well). Then an appreciation can be gained as to how well the equation describes the pressure distribution over the borehole's possible range of influence. Table 4 illustrates the mismatch between predictions and observations that arise. Evidently there are arrangements where the Poisson-type equation provides a better description than that achieved via approach 2. Approach 1 appears to give more-accurate predictions near the well for a widely spaced series of probes. However, these predictions were derived from sites whose static landfill differential pressures were high (> 15 mmWG), and above atmospheric pressure. Consequently, it was relatively easy to determine the site's differential-pressure distribution as far out as 40 m from the borehole. Conversely, when the site's suction pressures (or static gauge pressures within the landfill) are small, approach 1 seems to be less capable of producing accurate results than approach 2. For the prediction of a site's gas-production rate, it is essential that an
Combustible-gas production from refuse in landfill sites
31
TABLE 4 T h e Differences in t h e M e a s u r e d P r e s s u r e s A r o u n d a G a s E x t r a c t i o n Well a n d t h e P r e d i c t e d Values R e s u l t i n g f r o m U s i n g the T w o Different A p p r o a c h e s D e s c r i b e d in this I n v e s t i g a t i o n
Site and well designation
Humberfield Humberfield Stewartby well A Stewartby well A Calvert well B
Distancesof the vertical probes radially from the gas well (m)
Applied suction at the well head 'Distant" gauge pressure in the buried (ram WG) refuse (ram WG) Actual
1,2 or 8.5 1,2 or 8.5
°Est. 1 at 0.5m
-7.2 -13.0
best.2 at well
-5.7 -8'5
-5.9 -8.8
Furthest aEst.1 at best. 2 at R measured 40m (which is pressure estimated or static using eqn 4 pressure as the criterion) 1.1-4.2 1.1-4-5
69.6 76.9
5.8 4.6
8, 18 or 30
-418
-251
- 147
92
92
98
8,30 or 41 0-94,2.24 or 14.78
-170 -380
-141 -216
-80 -224
107 - 16
95 225
111 -7.6
a Est. 1 gives the results obtained from using a Poisson-type equation (i.e. approach 1). b Est. 2 indicates results obtained from using an exponential-modelling function (i.e. approach 2).
TABLE 5 P r e d i c t e d Radii o f Influence a n d L F G P r o d u c t i o n R a t e s for V a r i o u s U K Landfill Sites
Site/well
Depth of Distance refuse of probes (m) from well (m)
Date of trial
Humberfield
20
1, 2, 8.5
25/7/88 27/7/88
Calvert well 16
20
5, 10, 15
19/2/88 21/2/88 15/4/88 14/5/82 13/5/82 23/5/82
0.94, 2.24, 14.78 Stewartby well 7b
13.5 8,30,41
Suction Flow Gas applied (m3/h) production (ram WG) rate (m3/ tonne/year) - 13 - 7.2
CH4 (%)
Valueof R deducedby applying eqn (4) (m)
116 80
19.0 12.7
22 30
19.4 23,0
-400 -257 - 380
133 105 140
21.9 19.3 34.4
56 56 51
32,5 30-8 26-6°
- 170 - 163 -418
99 121 156
9.9 11.4 14-3
62 61 52
51.7 53.3 54.0
The gas-well head had been maintained at a high suction for many days--apparently inducing a negative 'distant' pressure (according to approach 2), which consequently gave speculative estimates for both the radius o f influence and the gas production rate. bHigh static differential pressures were encountered during this study. It was therefore possible to measure readily the landfill pressures at distances of 40 m or more from the well head.
32
N. Gardner, S. D. Probert
accurate estimate of the radius of influence can be obtained. This is because the solution ofeqn (1) (undertaken in the hope that an accurate estimate for c¢ can be made) has proved unreliable as indicated earlier. The accurate determination of differential pressures within the landfill refuse near the well in addition to the more distant regions (i.e. more than 15 m from the well head) are important in modelling the well's behaviour. However, when the differential pressures of the L F G within the site are small, these 'distant' pressures cannot be measured with adequate accuracy. So what is a truly representative modelling equation and how should its applicability be defined? Because our main interest lies in the more distant region from the well, the only indication we have as to how well the modelling equation applies, is how the predicted distant pressures correspond with the static pressures measured at the site. To achieve this, it is necessary either to obtain a mean site static pressure or to determine the approximate static pressure distribution in a particular direction from the gas well. In view of the above comments and because in general it is easier to measure accurately the landfill pressures near the well, it appears that a better estimate of the well's influence is obtained by modelling the site data with an exponential equation (i.e. using approach 2)---see Table 5.
C O M P A R I S O N OF L F G - P R O D U C T I O N RATE P R E D I C T I O N S W I T H ESTIMATES D E R I V E D F R O M SITES W H I C H EMPLOY EXTENSIVE G A S - C O N T R O L OR U T I L I S A T I O N STRATEGIES For example, an estimate for the gas collected per tonne of refuse deposited at the Calvert site was made from the hourly yields of L F G collected. Allowance was included for the percentage of the site which was not active because of flooding. The final estimate gave ,~ 18 m 3 (at NTP) L F G per tonne-year. This result corroborates the average of the annual gasproduction predicted rates listed in Table 5. The collection efficiency for the Calvert site was estimated to be nearly 90%.
RECOMMENDATIONS
(1) Excessive suctions (i.e. > 250 m m WG) should not be used during the trials because it is undesirable to create high negative differentialpressures at large distances from the well, thereby drawing air into the site; this depends upon the capping effectiveness. A suitable
Combustible-gas production from refuse in landfill sites
33
choice of relatively low sunction pressure will also minimise the ingress. (2) In order to generate a useful pressure profile, the probes should be located at up to 30m from the gas well. Probes at 5, 10 and 30m would be suitable. For sites where the static differential pressures are small ( < 5 m m WG), it will probably be necessary to use a closer arrangement. To provide a measurable response at each of the probes, a suction of up to 150mm WG at the well-head is recommended. (3) Ensure that the lower ends of the vertical probes are placed at a depth at which the gas well has perforations. If located 1 m higher, a lower but probably adequate accuracy would be achieved. For sites without a clay cap, it is recommended that the probes are inserted to at least a 3-m depth, though deeper still is desirable. (4) Recent tests have illustrated that small changes in suction at the wellhead have caused almost immediate small changes in the differential pressures observed at the probes. Since such small changes in the suction at the well-head will inevitably persist long after 3 hours of gas extraction, site data obtained in under 3 hours, are usually adequate. CONCLUSIONS The exponential modelling equation offers greater reliability throughout the range of site pressures that are likely to be encountered. The estimated gasproduction rate for the Calvert landfill site is reasonable when compared with the actual gas-collection rate, so suggesting the worthwhileness of this new approach. The application of the listed recommendations should lead to more coherent sets of deductions for L F G sites.
A C K N O W L E D G E M ENTS The authors wish to thank Shanks & McEwan plc, Bedford, and Environmental Resources Ltd, London, for their interest in this project.
REFERENCES 1. Thompson, D., Biotechnology for energy conservation and a cleaner environment. Process Engineering, 69(12) (1988) 39--41. 2. Richards, K. M., Landfill gas--the technology and economics for the UK, In Energy Worm Yearbook, 4th edn, Institute of Energy, 1988, pp. 58-61.
34
N. Gardner, S. D. Probert
3. Kunz, C. O. & Lu, A. H., Methane production rate studies and gas-flow modelling for the fresh kills landfill. Paper prepared for the New York State Energy Research and Development Authority, 6/ET-FUC/79, ERDA Report 80-21, November 1980. 4. Hoeks, J., Significance of biogas production in waste tips. Waste Management and Research, 1 (1983) 323-35. 5. Campbell, D. J. V., Moss, H. D. T. & Rees, J., Landfill-gas abstraction programme, Stewartby. Final Report, Shanks & McEwan (Southern), Bromham Road, Bedford, UK, 1983. 6. Esmaili, H., Control of Gas Flows from Sanitary Landfills, Journal of Environmental Engineering, 101(4) (1975) 555-66.
Combustible-Gas Production from Domestic, Municipal and Industrial Refuse Deposited in Landfill Sites N. G a r d n e r & S. D. P r o b e r t Department of Applied Energy, Cranfield Institute of Technology, Bedford MK43 0AL, UK
ABSTRACT The rate of production of landfill gas (LFG) varies considerably from site to site. A major obstacle in determining thefinancial viabilities of proposed LFG utilisation schemes, for sites which are producing appreciable quantities of gas, is the current inability to forecast accurately the considered site's gasproduction rate. In essence there are two techniques for determining a site's gas-production rate:
(1) The site's behaviour is modelled mathematically and the LFG production rate is predicted from the composition of the deposited waste and an understanding of how quickly the organic material decays to form LFG. (2) Gas-extraction flows and associated gas-pressure measurements are taken at the site. The second technique is preferable because more-accurate estimates can be made as to the mass of LFG likely to be produced per day over the next decade. The present paper discusses two approachesfor the implementation of the second technique, and makes recommendations with respect to procedure.
NOTATION A C h K
Characteristic constant in eqn (1) The site's LFG-collection efficiency (%) The total thickness of the buried refuse (m) Site's gas conductance (m2/Pa/h) 21 Applied Energy 0306-2619/89/$03-50 © 1989 Elsevier Science Publishers Ltd, England Printed in Great Britain
22 m i
m s
P Pa Pe
Po Pw
r rb
R S
N. Gardner, S. D. Probert
The error arising from the overall inaccuracy of the site measurements The estimated error which accounted for the spread in the calculated results for Differential pressure within the landfill, at a horizontal distance r from the well (Pa or m m WG) Unperturbed mean static pressure in the landfill refuse (Pa) Estimated mean static differential pressure within the landfill at the site (Pa or m m WG) Estimated differential pressure at the vertical borehole (Pa or m m
WG) Suction pressure applied at the well head (Pa) Gas extraction rate (m3/h) Gas flux (m3/mZ/h) Horizontal distance from the well-head's vertical borehole (m) Radius of the well's casing (m) Radius of influence within the deposited refuse for a particular suction applied at the well head (m) A constant which determines how quickly the mean static differential landfill pressure (Pc) is attained (Pa/m or m m WG/m) Total gas production rate (m3/m 3 of site/h)
ABBREVIATIONS LFG mm WG rope NTP tce
Landfill gas (i.e. it consists principally of methane, carbon dioxide and nitrogen) Unit of differential pressure, equivalent to that due to a column of water whose height is expressed in millimetres The overall most probable error in the estimated gas-production rate for a site Normal temperature and pressure Tonnes of coal equivalent
GLOSSARY
A gas-well's performance characteristic relates the quantity of L F G which is collected per h o u r to the well-head suction. A site's gas-collection efficiency, C, is defined as the ratio of the collection to production rates of LFG, and usually expressed as a percentage.
Combustible-gas production from refuse in landfill sites
23
A site's gas conductance (K) can be defined by Darcy's law: Of = K-~r For a location in a site having a gas conductance of 1 m2/pa/h and a pressure gradient of 1 Pa/m, the resulting flux of gas along this pressure gradient, across an element of 1-m 2 area, would be 1 m3/h. The darcy is the unit in which the permeability coefficient of the deposited refuse is expressed: 1 d a r c y - 9.87 × 10-13m 2 The error m I arises from the overall inaccuracies in measuring the pressures in the probes, their exact locations relative to the LFG-extraction well, and the site's gas conductance. The estimated error m s indicates the spread in the calculated results for ~. The overall most probable error (mpe) in determining a site's gas production rate, ~, was estimated from a knowledge of mt and m s, via mpe = (m~ + m~) 1/2 Note
Where pressures have been referred to in this text, they are differential pressures measured relative to atmospheric pressure.
B U R N I N G OR BIOGAS STRATEGIES FOR SOLID WASTES? National policies, and the legislation resulting therefrom, are increasingly aimed at reducing environmental pollution, and so the combustion and dumping of refuse are being subjected to growing scrutiny and criticism. The growing interest in pollution prevention (especially if associated with energy conservation) is particularly desirable for the UK, which is heavily industrialised and has such a high population density that intensive uses of land and water ensue. This trend is likely to continue despite the fickle energy market, i.e. large fluctuations in the unit price of crude oil. It is also likely that the polluter will increasingly be required to pay for the damage so produced, thereby encouraging the introduction of pollution-prevention facilities. Another longer-term trend of a sustainable society is that, each year, the rubbish produced during the year will be fed directly to biogas production plants and used that year.
24
N. Gardner, S. D. Probert
At present only about 5% of Europe's solid refuse (~400 x 106tonnes/ year) is incinerated, the rest being dumped or used for landfilling. In theory, combustion offers better prospects of a greater percentage of energy recovery from the solid wastes, but in practice there are numerous problems, e.g. having to 'size' the waste appropriately before presentation to the burners, and the dangers of inadequate combustion leading to the production of dioxins and other harmful by-products. (Possible links of dioxins in the atmosphere with cases of cancer of the larynx have been suggested.) A recent survey by the National Society for Clean Air showed that about one-quarter of the 1700 boilers and incinerators protected by Crown immunity in the UK, including hospital incinerators, were regarded as sources of air pollution. Thirty-nine environmental health officers in the UK also expressed concern about the levels of dioxin present in the environment. Thus the dumping of refuse as landfill appears likely to continue for the foreseeable future, though several new totally integrated solid-waste-to-energy plants (e.g. as designed by Kruga and Alpha Technik, Denmark) will be built during the next 5 years. 1 However, there are drawbacks to the dumping of refuse, e.g. leaching possibly resulting in the contamination of soil (well away from the landfill site) and groundwater supplies, as well as the decreasing number of suitable, acceptable and available sites near the major conurbations rejecting the refuse. Also landfill sites produce landfill gas (LFG) which, because of its smell, can be highly objectionable when allowed to pollute the local atmosphere, and it can cause fires or explosions if permitted to migrate laterally from the site into nearby buildings. Thus pumped extraction of the LFG from the site may be necessary in order to avoid these problems: the LFG being produced may then also be used as an energy supply. If no customers for large amounts of LFG exist close to the landfill site, then electricity generation using the LFG could be an acceptable option. However the combustion products of some trace gases (e.g. mercaptans and halocarbons) that arise when LFG is burned can be highly corrosive to the engines or turbines involved in the electricity generation. Thus LFG cleanup (prior to combustion) would probably be required so increasing the payback period (i.e. the required capital investment divided by the financial savings achieved per year as a result of that investment). However, the easiest and most financially attractive option is the direct combustion of LFG using appropriately designed burners. This will reduce the combustion of coal, oil or natural gas that would otherwise occur: the pay-back period may then be less than 1 year. An example of such an application is the use of LFG for firing in local brick or cement kilns. The yield and longevity of supply of LFG from a particular landfill site are influenced by: (a) the deposited refuse's composition; (b) the packing density;
Combustible-gas production from refuse in landfill sites
25
(c) the moisture content; (d) the site's geometry and hydrogeology; (e) the cover material; (f) the local climate (especially rainfall); and (g) the period elapsed from the time of deposition of the refuse. The lack of control or knowledge of some of these influential parameters makes the theoretical prediction of the likely rate of yield of LFG versus time characteristic for a particular site almost impossible. Thus we have to resort to measurements of LFG amounts yielded for various applied suctions at the gas-well head in order to predict the likely total yield per tonne of refuse at that site. Up to 350 of the 669 licensed landfill sites in the U K are, or have the potential to become, economically viable LFG producers. 2 The most financially attractive of the U K sites occur in a broad band stretching from the southeast of England up to the northwest. A typical yield of approximately 2 therms per tonne of biodegradable refuse per annum (,,~ 11 m 3 LFG per tonne per year) over a period of 13 years, may be expected. 2 The annual total output from all LFG installations in the U K amounted, by the end of 1987, to 1.2 x 105 tce. By AD 2000, it is anticipated that the output will rise to 106 tce per year and eventually to 3 x 106 tce per year. The present study discusses two techniques for estimating the combustible-gas production rate from a landfill site. Consideration is given as to how a more appropriate choice of equation relating the variables can be obtained, so resulting in predictions of greater reliability.
APPROACH 1 This was first introduced by Kunz & Lu. 3 It requires solving a Poisson-type equation, which describes the steady-state LFG pressure-distribution around a gas-extraction well, when a constant suction is applied at the well head. Generally, q
P = A + 2-~
0~ 2
(ln r) - ~ r
(1)
where the site's gas conductance, K, can be determined by taking measurements at the gas-extraction well head (i.e. monitoring the gascollection rate for each suction pressure applied to the well). By knowing the well's performance-characteristic, the following equation, introduced by Hoeks, 4 can be used to determine K: P I - - P a = 47rKh
1 - In
(2)
26
N. Gardner, S. D. Probert
TABLE 1 LFG Production Rates Year of trial
Gas production +_rope
Well of site studied
(m3 per year of LFG at atmospheric pressure per tonne of deposited refuse)
1982 1982 1988
Stewartby D6 Stewartby D7 Calvert 16
100 + 19 80 + 12 350 + 119
If a value for 0t (say in the range 5 - , 30 ma/tonne/year) is assumed, K can be estimated typically to within 15% accuracy. Then a set of two simultaneous equations o f the form o f e q n (1) (with A and 0cunknown) can be solved for 0~.
Use of approach I This has been applied at the Stewartby (Bedfordshire) and Calvert (Buckinghamshire) landfills operated by Shanks and M c E w a n (Southern) plc. After much data sampling, the results of Table 1 were deduced. These predicted results are unrealistically high: the average cumulative gas production over the w h o l e lifetime o f a site is at m a x i m u m about 400 m 3 per tonne o f buried domestic refuse. K u n z & Lu 3 made no mention o f such a shortcoming with respect to their use of this method, and equally they provided no estimate as to the accuracy of using this approach.
Errors and uncertainties (see Table 2) Evidently m i, arising from the overall inaccuracy of each o f the measurements, makes a significant contribution to the mpe. Surprisingly, the
TABLE 2 Uncertainties in Determining a Site's LFG Production Rate by the Use of Approach 1 Well
mi (%)~
ms (%)b
rope (%)b
Calvert 16 Stewartby D6 Stewartby D7
32 12 13
10 15 7
34 19 15
aPercentage error in the LFG production rate arising from predicted rates determined from daily tests. bPercentage error in the LFG production rate due to uncertainties in the input data.
Combustible-gas production from refuse in landfill sites
27
TABLE 3 Characteristics of Various UK Landfill Sites
Landfill site, well and year Brogborough 8 (1988) Calvert 16 (1988) Stewartby B8 (1988) Stewartby D6 (1982) Stewartby D7 (1982)
Conductance (m2/pa/h) 0.001 0-0.001 4 0.002 44).003 5 0"030--0'032 0"002 4-0"003 1 0.006 2-0.007 6
Permeability (darcys) 4-5 9-13 107-114 8-11 22-27
value of m t for the tests at the Calvert site was relatively high compared with those for the tests at the Stewartby site. At Calvert, a closely spaced 'line' of vertical probes (2 ~ 3 m apart) was used, whereas the arrangement at Stewartby consisted of vertical probes located every 10m in a plane extending 50 m horizontally from the gas-extraction well head.
Published results, obtained using approach 1 Kunz & L u 3 appeared to be confident of the worthwhileness of this approach. For the Fresh Kills landfill site at Staten Island, New York, USA, the following results were predicted: gas-production rate 12.2m 3 per tonne/year; site's gas conductance 0.054m 2 per Pa per hour; and site permeability 198 darcys. (For a permeability of 1 darcy and for L F G with a dynamic viscosity of ~ 1 3 x 10-6pas, the gas conductance is 2.7 x 10 -4 m2/pa/h.) The permeability of the Fresh Kills landfill is much higher than the estimated permeabilities of the three UK sites--see Table 3.
Assumptions made in approach 1 (a) The L F G pressure-distribution around the gas-extraction well obeys a Poisson-type equation. (b) An average gas conductance for the influenced area around the gasextraction well is meaningful and applicable under the conditions likely to be encountered. (c) No account of the variations of the pressure profiles with depth has been taken: in this respect, it should be remembered that solving eqn (2) will yield an estimate for ~ for a particular depth at the site. Utilising pressure probes, which extend radially to at least 30 m from the well, will tend to reduce the chance of predicting too local a value of ~. For this approach to be successful, it is essential that assumption (a) is
28
N. Gardner, S. D. Probert
valid. However a closer examination of the predictions obtained using eqn (1) revealed that it was truly representative over only a small portion of the well's total area of influence. Tests highlighted that this equation was not always reliable in predicting pressures, especially for those locations more than 40 m from the well. Determining an average LFG conductance of a site over the volume of refuse influenced by a gas-extraction well did not pose any problem. However, is substituting an average value of K into eqn (1) justified? Although the likely magnitude of the uncertainty in determining K is relatively large and this influences significantly the overall accuracies of the predictions obtained via approach 1, the major limitation of approach 1 is the inability of a Poisson-type equation to model a gas-well's pressureprofile consistently and adequately. Assumption (c) is a limitation which is common to all existing pressuredependent techniques for determining ~. A more useful value for ct could be obtained from pressure measurements taken with lines of vertical probes, each line extending to a different depth, and from data for other gas wells at the site.
APPROACH2 This is identical to the first approach, except that the Poisson equation is replaced by an exponential equation to describe the pressure distribution in the dumped refuse. The following equation was regarded as a reasonable approximation to reality: two of its constants are the pressure po at the well head and the distant landfill pressure (or mean static landfill pressure)Pc. Then
P--Pe _ ( p ~ - po) exp ( _ S rr)
(3)
The advantages that this equation offers can be summarised as follows: (1) It appears successful in modelling the behaviours of sites with small landfill differential pressures ( < 10 mm WG static pressure); (2) It produces more-accurate predictions for distant landfill pressures even when the constants for eqn (3) are deduced from data taken entirely near the borehole (which are easier to measure). The main drawback with this approach is that it is necessary to introduce a criterion for defining the radius of influence for the suction applied at the well head. For the present investigation, this was estimated from information provided in a study performed at the Stewartby landfill site in
Combustible-gas production from refuse in landfill sites
29
1982 by Campbell et al. 5 The criterion was developed by examining the distant piezometer data for different borehole suctions. From such an examination, it was possible to estimate the radius of influence to within _ 5 m. This enabled one to predict the pressure-gradient at the radius of influence, i.e. outside this zone the well's influence was insignificant. The limiting criterion turned out to be ~-r, = R = 0" 130 m m W G / m
(4)
Uncertainties with approach 2 The gas production rate, ~, can be estimated using the following equation: o~ = ~ h R 2
(5)
where the radius of influence R can usually be determined to better than 10% accuracy. For some sites, the uncertainty in the value of h has posed more difficulties, e.g. at the Calvert site it is known that approximately 30% of its gas wells are relatively ineffective because of flooding. Also the surrounding and more distant refuse may be near to moisture saturation, but may not necessarily have the same level of liquid present. Therefore, particularly for the Calvert site, defining a single value for the effective depth of refuse, h, is open to criticism. Of the 24.5-m depth of refuse around well 16, it was decided on the basis of experimental tests to use a 20-m depth of refuse in the calculations: this allowed for the lower 4.5 m of the site being flooded. Together, these two uncertainties lead to a total uncertainty of approximately 25% for the estimated L F G production rate.
Difficulties with approach 2 During tests at the Calvert landfill site, where several radial lines of vertical probes were employed, the suctions used throughout the trial tended to be excessive. This did not cause any difficulty with respect to the mathematical modelling. However, the predictions for the 'distant' gauge-pressures were then sometimes slightly negative, which obviously caused difficulties in assigning a meaningful value for the radius of influence. This approach yields an estimate for each well's radius of influence for a particular depth in the site. On examination of Fig. 1, it can be seen that, provided that the pressure-measuring probes are placed as deep as the perforated well-casing, a reasonably representative estimate of the value of R can be obtained. However, it is also apparent that the radius of influence
30
N. Gardner, S. D. Probert lOre GAS WELL PIPE(THELOWER~,'SmOF ITS LENGTH iS PERFORATEO) .
! i
i
i
i
.
k/
.
.
.
.
p
', .
.
\
I
.
/:
11rn
, \
i
22m
33m
- - 0.10- "NEGATIVE"PRESSURE=0-10 INCHESWG
~
Z0NE IN WHICH TI"IE PIEZ0HETERSARE PLAEED
Fig. 1. Predicted constant-pressure contours for an LFG extraction rate of 340 m3/h from a 23-m thick bed of buried refuse. (From Ref. 6.)
changes with depth. It therefore follows that the gas well's mean radius of influence will be smaller than the estimate derived from probes placed in the region indicated by the shaded area of Fig. 1.
C O M P A R I S O N OF T H E TWO A P P R O A C H E S If the constants o f e q n (1) are known or can be determined, it is possible to use this equation to predict the pressure near the well head (i.e. at 0.5 m from the well) and distant pressures (e.g. at 4 0 m from the well). Then an appreciation can be gained as to how well the equation describes the pressure distribution over the borehole's possible range of influence. Table 4 illustrates the mismatch between predictions and observations that arise. Evidently there are arrangements where the Poisson-type equation provides a better description than that achieved via approach 2. Approach 1 appears to give more-accurate predictions near the well for a widely spaced series of probes. However, these predictions were derived from sites whose static landfill differential pressures were high (> 15 mmWG), and above atmospheric pressure. Consequently, it was relatively easy to determine the site's differential-pressure distribution as far out as 40 m from the borehole. Conversely, when the site's suction pressures (or static gauge pressures within the landfill) are small, approach 1 seems to be less capable of producing accurate results than approach 2. For the prediction of a site's gas-production rate, it is essential that an
Combustible-gas production from refuse in landfill sites
31
TABLE 4 T h e Differences in t h e M e a s u r e d P r e s s u r e s A r o u n d a G a s E x t r a c t i o n Well a n d t h e P r e d i c t e d Values R e s u l t i n g f r o m U s i n g the T w o Different A p p r o a c h e s D e s c r i b e d in this I n v e s t i g a t i o n
Site and well designation
Humberfield Humberfield Stewartby well A Stewartby well A Calvert well B
Distancesof the vertical probes radially from the gas well (m)
Applied suction at the well head 'Distant" gauge pressure in the buried (ram WG) refuse (ram WG) Actual
1,2 or 8.5 1,2 or 8.5
°Est. 1 at 0.5m
-7.2 -13.0
best.2 at well
-5.7 -8'5
-5.9 -8.8
Furthest aEst.1 at best. 2 at R measured 40m (which is pressure estimated or static using eqn 4 pressure as the criterion) 1.1-4.2 1.1-4-5
69.6 76.9
5.8 4.6
8, 18 or 30
-418
-251
- 147
92
92
98
8,30 or 41 0-94,2.24 or 14.78
-170 -380
-141 -216
-80 -224
107 - 16
95 225
111 -7.6
a Est. 1 gives the results obtained from using a Poisson-type equation (i.e. approach 1). b Est. 2 indicates results obtained from using an exponential-modelling function (i.e. approach 2).
TABLE 5 P r e d i c t e d Radii o f Influence a n d L F G P r o d u c t i o n R a t e s for V a r i o u s U K Landfill Sites
Site/well
Depth of Distance refuse of probes (m) from well (m)
Date of trial
Humberfield
20
1, 2, 8.5
25/7/88 27/7/88
Calvert well 16
20
5, 10, 15
19/2/88 21/2/88 15/4/88 14/5/82 13/5/82 23/5/82
0.94, 2.24, 14.78 Stewartby well 7b
13.5 8,30,41
Suction Flow Gas applied (m3/h) production (ram WG) rate (m3/ tonne/year) - 13 - 7.2
CH4 (%)
Valueof R deducedby applying eqn (4) (m)
116 80
19.0 12.7
22 30
19.4 23,0
-400 -257 - 380
133 105 140
21.9 19.3 34.4
56 56 51
32,5 30-8 26-6°
- 170 - 163 -418
99 121 156
9.9 11.4 14-3
62 61 52
51.7 53.3 54.0
The gas-well head had been maintained at a high suction for many days--apparently inducing a negative 'distant' pressure (according to approach 2), which consequently gave speculative estimates for both the radius o f influence and the gas production rate. bHigh static differential pressures were encountered during this study. It was therefore possible to measure readily the landfill pressures at distances of 40 m or more from the well head.
32
N. Gardner, S. D. Probert
accurate estimate of the radius of influence can be obtained. This is because the solution ofeqn (1) (undertaken in the hope that an accurate estimate for c¢ can be made) has proved unreliable as indicated earlier. The accurate determination of differential pressures within the landfill refuse near the well in addition to the more distant regions (i.e. more than 15 m from the well head) are important in modelling the well's behaviour. However, when the differential pressures of the L F G within the site are small, these 'distant' pressures cannot be measured with adequate accuracy. So what is a truly representative modelling equation and how should its applicability be defined? Because our main interest lies in the more distant region from the well, the only indication we have as to how well the modelling equation applies, is how the predicted distant pressures correspond with the static pressures measured at the site. To achieve this, it is necessary either to obtain a mean site static pressure or to determine the approximate static pressure distribution in a particular direction from the gas well. In view of the above comments and because in general it is easier to measure accurately the landfill pressures near the well, it appears that a better estimate of the well's influence is obtained by modelling the site data with an exponential equation (i.e. using approach 2)---see Table 5.
C O M P A R I S O N OF L F G - P R O D U C T I O N RATE P R E D I C T I O N S W I T H ESTIMATES D E R I V E D F R O M SITES W H I C H EMPLOY EXTENSIVE G A S - C O N T R O L OR U T I L I S A T I O N STRATEGIES For example, an estimate for the gas collected per tonne of refuse deposited at the Calvert site was made from the hourly yields of L F G collected. Allowance was included for the percentage of the site which was not active because of flooding. The final estimate gave ,~ 18 m 3 (at NTP) L F G per tonne-year. This result corroborates the average of the annual gasproduction predicted rates listed in Table 5. The collection efficiency for the Calvert site was estimated to be nearly 90%.
RECOMMENDATIONS
(1) Excessive suctions (i.e. > 250 m m WG) should not be used during the trials because it is undesirable to create high negative differentialpressures at large distances from the well, thereby drawing air into the site; this depends upon the capping effectiveness. A suitable
Combustible-gas production from refuse in landfill sites
33
choice of relatively low sunction pressure will also minimise the ingress. (2) In order to generate a useful pressure profile, the probes should be located at up to 30m from the gas well. Probes at 5, 10 and 30m would be suitable. For sites where the static differential pressures are small ( < 5 m m WG), it will probably be necessary to use a closer arrangement. To provide a measurable response at each of the probes, a suction of up to 150mm WG at the well-head is recommended. (3) Ensure that the lower ends of the vertical probes are placed at a depth at which the gas well has perforations. If located 1 m higher, a lower but probably adequate accuracy would be achieved. For sites without a clay cap, it is recommended that the probes are inserted to at least a 3-m depth, though deeper still is desirable. (4) Recent tests have illustrated that small changes in suction at the wellhead have caused almost immediate small changes in the differential pressures observed at the probes. Since such small changes in the suction at the well-head will inevitably persist long after 3 hours of gas extraction, site data obtained in under 3 hours, are usually adequate. CONCLUSIONS The exponential modelling equation offers greater reliability throughout the range of site pressures that are likely to be encountered. The estimated gasproduction rate for the Calvert landfill site is reasonable when compared with the actual gas-collection rate, so suggesting the worthwhileness of this new approach. The application of the listed recommendations should lead to more coherent sets of deductions for L F G sites.
A C K N O W L E D G E M ENTS The authors wish to thank Shanks & McEwan plc, Bedford, and Environmental Resources Ltd, London, for their interest in this project.
REFERENCES 1. Thompson, D., Biotechnology for energy conservation and a cleaner environment. Process Engineering, 69(12) (1988) 39--41. 2. Richards, K. M., Landfill gas--the technology and economics for the UK, In Energy Worm Yearbook, 4th edn, Institute of Energy, 1988, pp. 58-61.
34
N. Gardner, S. D. Probert
3. Kunz, C. O. & Lu, A. H., Methane production rate studies and gas-flow modelling for the fresh kills landfill. Paper prepared for the New York State Energy Research and Development Authority, 6/ET-FUC/79, ERDA Report 80-21, November 1980. 4. Hoeks, J., Significance of biogas production in waste tips. Waste Management and Research, 1 (1983) 323-35. 5. Campbell, D. J. V., Moss, H. D. T. & Rees, J., Landfill-gas abstraction programme, Stewartby. Final Report, Shanks & McEwan (Southern), Bromham Road, Bedford, UK, 1983. 6. Esmaili, H., Control of Gas Flows from Sanitary Landfills, Journal of Environmental Engineering, 101(4) (1975) 555-66.