Aerobic biodegradation of solid waste provides an attractive option for landfill management, as the process can result in more rapid settling to create additional space for solid waste disposal. Aerobic biodegradation can be used to reduce methane emissions for landfills, particularly those that are not generating enough methane to warrant gas collection systems. The heat released by the aerobic biodegradation process can be used to treat leachate recirculated into the landfill by evaporating its water and biodegrading its organic compounds. Aerobic biodegradation can also be used for substantial in situ reduction or elimination of toxic volatile organic compound emissions from the landfill.
Aerobic biodegradation in a landfill is carried out by microorganisms that are dependent on oxygen to metabolize the organic substances in landfill solid waste. The end products are normally carbon dioxide and water. Some organic substances are very resistant to degradation (e.g. certain plastics); however, a substantial percentage of municipal solid waste (MSW) can be degraded to the normal end products. Because oxygen is a very reactive compound, the microorganisms that utilize it in their enzyme systems to metabolize solid waste, termed aerobic microorganisms, do so at rates that are one or more orders of magnitude more rapid than those microorganisms, termed anaerobic, that function without the presence of oxygen. Living organisms utilize substances for growth or energy in steps that are energetically favorable for the organisms. The overall process releases energy in the form of heat that can result in increasing the temperature of the substrate, in this case MSW, in which the microorganisms exist.
Aerobic biodegradation not only proceeds at a much faster rate than anaerobic biodegradation but also releases substantially more heat per mass of organic material consumed. Since a landfill is a high volume, low thermal conductivity structure, the heat released by biodegradation cannot be easily dissipated to the surrounding environment to cool the waste mass. The resulting increase in temperature creates a problem for both the biodegrading microorganisms and the landfill operator, since high temperature has been shown to inhibit or kill the microorganisms and create underground fires by spontaneous combustion. The critical temperature at which spontaneous combustion has been observed to start is generally 70 to 80° C., above which point one is likely to see a dramatic and possibly uncontrollable temperature rise, even to the point of combustion. Therefore, steps must be taken by the aerobic landfill operator to cool the landfill to below the critical temperature.
A second consideration is the variability of biodegradation rate with temperature. The landfill operator must target the proper temperature for maximum rate of degradation in order to be as efficient as possible during aerobic landfill operation. Studies have shown that the optimum temperature for biodegradation varies with the type of waste that is being aerated. For fresh MSW, that optimum temperature has been shown to be 60° C. In this case, there is a minimum of a ten degree window before the critical temperature is reached for uncontrolled temperature rise and the resulting damage. The landfill operator is then faced with the delicate balancing act of maintaining the waste temperature high enough to optimize the biodegradation rate, yet low enough to prevent runaway temperature rises.
MSW is typically very heterogeneous. Not only does its content vary in terms of types of biodegradable and nondegradable material, but the density, permeability, moisture content, and hydraulic conductivity of the waste mass also vary. All of these parameters contribute to a variable thermal conductivity and rate of heat generation throughout the landfill waste, resulting in variable effectiveness of cooling methods to maintain temperatures below the critical temperature. The existence of “hot spots”, locations that might reach this critical temperature, should be taken into account in any cooling system design.
Temperature in an aerobic landfill is normally controlled by forced injection and/or extraction of air through the waste mass while supplying water (and/or landfill leachate) to replace the water evaporated. The flow of air supplies oxygen for biodegradation reactions and carries away excess heat stored as latent heat in water vapor. The airflow demand for cooling exceeds that required to supply oxygen for biodegradation by an order of magnitude or more. The water evaporated can be replaced efficiently by water (and/or leachate) recharged into the landfill by pressurized drip irrigation, sprinklers, or other methods known to practitioners of the art. The optimum moisture level for biodegradation ranges from 40% to 60% moisture by weight. However, the practical application of aerobic bioreactors is limited to wastes with significantly lower moisture contents.
Three issues must be considered with regard to moisture content in a landfill. Firstly, increasing MSW moisture level to 40% to 60% increases water permeability significantly in accordance with the general principles of flow in porous media. The higher water permeability results in water management problems associated with seeps from the sides of the landfill and increasing landfill drainage to underlying soils or landfill liner. Our experience indicates that the actual percentage by weight of moisture that can be easily maintained in landfills (commonly termed “field moisture capacity”) is approximately 20 to 30%. Higher moisture contents, 40% or above, are typically found only when excessive water injection is practiced or where water is “perched” on underlying, low-water-permeability layers of residual, fine-grained sediments used for daily cover. Secondly, a moisture content as high as 40% to 60% in solid waste will fill in waste material pores, thereby reducing air permeability to levels that limit or prevent effective air flow to the waste. The reduction in the rate of air delivery caused by the significant reductions in gas permeability associated with high moisture content results in a substantial reduction in the rate of cooling. Even with enough moisture present to continually saturate the air as it passes through the waste mass, the cooling requirements will not be met if the air supply to the MSW is insufficient to carry the water vapor needed to transport the excess heat. The rate of airflow is the critical factor regarding whether the waste mass is effectively cooled. Thirdly, this reduction in air delivery to portions of the waste may reduce oxygen supply below levels required to sustain significant rates of aerobic biodegradation, even though some flow may occur via channeling through the large pores that are typical in solid waste. In summary, from a practical point of view, it is not possible to utilize the higher 40 to 60% levels of moisture that provide maximum biodegradation rates.
The goal of the invention is to optimize the aerobic biodegradation rate under the conditions existing in and around the waste mass. The moisture content of the waste must be properly maintained, and the other, landfill-specific variables familiar to those in the art must be considered in the operational design in order to keep the temperature of the waste mass within controllable limits and within the range of effective biodegradation. Controlling the temperature of the waste mass requires supplying the correct flow of air through the waste mass. Air flow must be maintained at a high enough rate to keep the waste below a temperature that may kill or unduly inhibit the microorganisms or initiate spontaneous combustion, but also must be at a low enough rate that the waste is not cooled to the point of slowing the biodegradation rate. It is important to recognize that airflow and water supply will vary significantly over the aerobic bioreactor's operational period.
In the case of an anaerobic waste mass, the goal of the invention is to reduce the internal temperature of the waste mass to levels that are not inhibitory to methanogenesis (biological generation of methane). Heat removal in anaerobic waste cannot be accomplished with airflow containing enough oxygen to be toxic to the methanogenic microbes. Oxygen would furthermore initiate aerobic biodegradation, which would accelerate heat generation. Heat generation can be controlled in such a waste mass by collecting the waste's landfill gas (which is depleted in oxygen), cooling it (reducing its water and heat content), and recirculating it through the waste mass to carry away the waste's excess heat. The cooling mechanism would be the same as that used for an aerobic waste mass; i.e., evaporation of water within the waste mass as the gas passes through, the heat being carried away by virtue of the latent heat of vaporization of water. The anaerobic waste mass would be the source of the recirculating landfill gas, which would be composed of approximately 50% methane and 50% carbon dioxide.
Airflow through a porous medium such as a waste mass is a complex physical process. However, such flow can be approximated by Darcy's Law, expressed here in the following form:
where Fv is the volumetric rate of flow (m3-sec−1), κ is the gas permeability (m2), A is the cross sectional area (m2) of the waste mass volume, L is the length (m) of the waste mass through which the air flows, μ is the dynamic viscosity (kg-m−1s−1) of the air, and ΔP is the pressure gradient (Newton-m−2) required for the given airflow. An approximated physical depiction would be a (straight) line of a plurality of equally-spaced injection wells manifolded into an air-injection blower which moves air through the waste mass towards another straight line of a plurality of equally-spaced extraction wells configured in a similar manner as the injection well system (extraction blower and manifolded wells).
We illustrate the analysis of an aerobic bioreactor landfill by choosing a conceptual landfill that we will call Example Landfill. Airflow through the Example Landfill's waste mass can be depicted for purposes of simplicity as unidirectional from one line of wells to another through a rectangular cross-sectional area. Assumed dimensions are 180 meters in length for each line of wells, a length of 30 meters between adjacent lines of wells, and an effective landfill depth of 10 meters. The distance between the wells in a line of wells is indeterminate; the wells are assumed to be evenly spaced and close enough to each other to provide substantially parallel flow lines that are perpendicular to the well lines. Given any set of values for the variables κ, L, A, and μ, we can express ΔP as proportional to Fv, with the other variables in the equation forming the proportionality constant at that temperature.
In our approximation of the Example Landfill, we recognize that as air passes through the landfill, the pressure variation in the airflow affects the values of the dynamic viscosity μ at a given temperature. The permeability κ of the landfill waste can furthermore change over time due to factors such as settling and grain size changes during conversion of the solid waste to carbon dioxide and water. We approximate the dynamic viscosity value as that at an average pressure of 1-atmosphere. Although permeability is universally acknowledged to change over time in a biodegrading landfill, there is no consensus regarding an empirical function for showing these changes and, in fact, the solid waste community differs on whether the permeability may either increase or decrease depending on site-specific conditions. We therefore assume a reasonable value for the permeability and hold it constant as we model the effect of flow increases on pressure. We also assume a constant value for the porosity. However, that parameter does not enter into the current discussion, but will be used later in the section on biodegradation of the landfill waste.
For the conceptual Example Landfill, we assume the following reasonable parameter values: κ=1.00×10−11 m2 (in our experience, a representative average horizontal permeability for landfill waste); μ=1.82×10−5 kg-m−1s−1 (viscosity of water-saturated air at 60° C., the optimum temperature for biodegradation of typical solid waste); L=30 m, and A=180 m×10 m, or 1800 m2. Then Fv=3.30×10−5 ΔP, which allows us to calculate ΔP at a given flow rate. We note that the product, Fv ΔP, has units of Newton-m/sec, or power (work per unit time) in watts. Therefore, the power requirements are easily calculated for a given flow rate through a waste mass with a known permeability, gas viscosity, flowpath length, and cross-sectional area.
Note that
The invention operates in a repetitive sequence of cooling (recycle, atmosphere off) mode and aerobic (atmosphere on) mode. In the cooling mode, the operational apparatus consists of: 1) one or more gas extraction blowers that extract the exhaust gases from a manifold of one or more extraction wells and/or trenches installed in the waste landfill; 2) a heat exchanging unit that receives and cools the extracted exhaust gases and condenses out their excess water (which may be used to resupply the landfill waste); and 3) one or more gas injection blowers that receive the exhaust gases, depleted of oxygen and reduced in temperature and excess moisture, and reinjects them into a manifold of one or more injection wells and/or trenches installed in the waste, and then back through the waste mass to assist in cooling. In the aerobic mode, the exhaust gases are extracted directly from the extraction manifold and injected into the atmosphere, before or after being processed by the heat exchanger, and the injection blower is used to inject air from the atmosphere into the manifold, then into the waste in order to sustain aerobic biodegradation. In a second configuration, extraction and injection could be accomplished by using one or more blowers operating in both extraction and injection modes while switching valves (manually or automatically, to meet the bioreactor's recycling and aerobic mode requirements. In this second configuration, passive extraction (venting) would be employed to exhaust gases during the aerobic mode. The invention can be used to regulate the temperature of the waste mass between a lower and higher temperature set point to control the biodegradation rate, which may be in either an anaerobic or aerobic state. In the case of an aerobic waste mass, the required air flow rate for temperature control is reduced, resulting in substantially lower energy costs. In the case of anaerobic waste mass, oxygen depleted air is recycled through the waste mass to provide enough cooling to maintain high anaerobic waste decay rates.
Table 1 lists the calculated data used to construct
Table 2 uses a range of estimated aerobic reaction rates at 60° C. and employs the Arrhenius equation to calculate MSW aerobic reaction rates at other temperatures from 20 to 85° C., as discussed in the detailed description of the invention.
Table 3 calculates and lists the air flow rates required to maintain the Example Landfill at a given temperature assuming its waste mass biodegradable fraction is 0.5.
Table 4A calculates airflow rates necessary for the Example Landfill to maintain the temperature of the waste mass at a temperature of 60° C.
Table 4B calculates airflow rates necessary for the Example Landfill to maintain the temperature of the waste mass at a temperature of 70° C.
Table 5 lists the calculated values for power, time, and flowrates needed to maintain the Example Landfill at a temperature of 60° C. as the biodegradable fractions varies from 0.5 to 0.005.
Table 6 calculates and lists the parameters that make up the components of the constants K1, K2, and K3 that are used for computational simplicity in the detailed description of the invention.
Table 7 is a list of properties for air and water used in the aerobic bioreactor calculations for the Example Landfill.
Table 8 contains sets of calculations for two example application of the invention using the Example Landfill given different initial operating times (recycle on; recycle off/atmospheric air on). The first example uses a minimum temperature of 50° C. and a maximum of 60° C. The second example uses a minimum temperature of 50° C. and a maximum of 70° C.
Table 9 is a tabulation of the power and time requirements to reduce the biodegradable fraction of the Example Landfill for 0.5 to 0.3 using recirculation to maintain the temperature between 50 and 60° C.
We describe the invention by first analytically calculating the air flow necessary to control the temperature using air alone without the aid of the invention. We then show that when the invention is employed, the air flow necessary to control the temperature of the waste mass may be lowered. Owing to the quadratic relation of the power required for a given airflow, lowering airflow substantially reduces both the power requirements and capital equipment costs for temperature control within the waste mass.
Definitions:
Note: A variable listed as Variable(T), for instance E(T), indicates the value of the variable for a given temperature T)
Fv=Air flow rate through the waste mass, cm3-sec−1
Ω=Volume of waste mass, cm3
E=Energy density of waste mass, erg-cm−3
Ebio=Energy generation rate from microbial action erg-sec−1-cm−3
Er=Energy released during biodegradation, erg-gm−1
fb=Biodegradable organic fraction of solid waste
Lv=Energy of vaporization for water, erg-gm−1
ΔE1=Change in energy density of waste mass during half of cycle, erg-cm−3
Δρv=ρvout−ρvin=Difference of outflow vapor density minus inflow vapor density, gm-cm−3
Δenc=encout−encin, where enc=Energy density of the noncondensable gas component of the air flow through the waste mass, erg-cm−3
Δev=evout−evin, where ev=Energy density of water vapor, erg-cm−3
Δel=Energy in water added to the waste mass to maintain constant water content, erg-cm−3
Cnc=Heat capacity of the noncondensable component of the air flow through the waste mass, erg-gm−1-° C.−1
Cv=Heat capacity of the vapor component of the air flowing through the waste mass, erg-gm−1-° C.−1
Cl=Heat capacity of the liquid water component within the waste mass, erg-gm−1-° C.−1
Cw=Heat capacity of the solid waste, erg-gm−1-° C.−1
T=Temperature, ° C.
Tmax=Maximum temperature allowed, ° C.
Tcool=Lowest temperature during recirculation phase, ° C.
Tin=Input temperature of oxygenated air when oxygen on, ° C.
Tout=Temperature of effluent air from the waste mass, ° C. (=Tmax for steady-state conditions)
Trecyc=Input temperature into waste mass of recirculated gas, ° C.
Tl=Temperature of added liquid water, ° C.
ρnc=Density of the noncondensable gas component of the air flowing through the waste mass, gm-cm−3
ρl=Density of water, gm-cm−3
ρvsat=Density of water vapor in saturated air, gm-cm−3
ρw=Density of the solid waste, gm-cm−3
kr=Biodegradation rate of waste mass, sec−1
σ=Decimal value of water saturation of waste mass (1=100%); f=(1−σ)
ε=Decimal value of porosity
RHrecycle=Decimal value of relative humidity in recycle mode
RHin=Decimal value of relative humidity for injected atmospheric air
The relationships between the definitions are as follows (some of the relationships are provided for sake of completeness):
E
(T)=(1−ε)ρw CwT+ε{σ ρlClT+f[ρncCncT+ρvsat(T)(CvT+Lv)]}
Δ1=E(Tmax)−E(Tcool)
E
bio=(1−ε)fbρwErkr(T)
Δρv=ρvout(Tout)−ρvsat(Tin)RHin
Δenc=ρncCnc(Tout−Tin)
Δev=Cv(μvsat(T)Tout−ρvsat(Tin)RHin Tin)
Δel=Cl(ρvsat(Tout)Tout−ρvsat(Tin)RHin Tin)
Ē
bio=(1−ε)fbρwEr·½(kr(Tmax)+kr(Tcool))
nc1=ρncCnc(
v1
=C
v[½(ρvsat(Tmax)Tmax+ρvsat(Tcool)Tcool)−ρvsat(Tin)RHin Tin]
l1
=C
l[½(ρvsat(Tmax)+ρvsat(Tcool))
v1=½[(ρvsat(Tmax)+ρvsat(Tcool))−ρvsat(Tin)RHin]
nc2=ρncCnc(
=½(Tmax+Tcool)
v2
=C
v[½(ρvsat(Tmax)Tmax+ρvsat(Tcool)Tcool)−ρvsat(Trecycle)RHrecycle Trecycle]
l2
=C
l[½(ρvsat(Tmax)+ρvsat(Tcool))
v2=½[(ρvsat(Tmax)+ρvsat(Tcool))−ρvsat(Trecycle)RHrecycle]
We now list a set of reasonable values for the given definitions in our analysis of airflow through a waste mass with and without use of the invention. We will use the same Example Landfill waste mass volume utilized in the discussion on power requirements, i.e.:
Ω=5.40×104 m3=5.40×10 10 cm3
Er=1.5×1011 erg-gm−1
fb=0.5
Tmax=Specified for each example.
Tcool=Specified for each example.
Tin=25° C.
Tout=Temperature of effluent air from the waste mass, ° C., equivalent to Tmax
Trecyc=35° C. We assume heat exchanger efficiency specifications are cooling to 10° C. or less above ambient air temperature (Tin).
Tl=25° C.
Cnc˜7.2×106 erg-gm−1-° C.−1
Cv≈1.3×107 erg-gm-−1-° C.−1
Cl=4.2×107 erg-gm-−1-° C.−1
Cw=8.37×104 erg-gm-−1-° C.−1
Lv=2.26×1010 erg-gm−1
ρnc˜10−3 gm-cm−3
ρl=Density of water, 1 gm-cm−3
ρvsat(T) Taken from Table 6, gm-cm−3 for each Tmax and Tcool.
ρvsat25° C.=(Table 6) 2.32×10−5 gm-cm−3; for assumed ambient (input from atmosphere) temperature.
ρvsat35° C.=(Table6) 3.99×10−5 gm-cm−3; for assumed recycle (input from heat exchanger) temperature
RHin=0.30
RHrecycle=1.0
ρw=0.4 gm-cm−3
kr60° C.=5.0×10−8 sec−1
kr50° C.=2.5×10−8 sec−1
σ=0.30; f=(1−σ)=0.70
ε=0.45
Chemical reaction rates are normally a function of temperature. A useful method of expressing the effect of temperature on a reaction rate is to compare the measured reaction rate at one temperature to the rate at a temperature 10° C. lower. This ratio is called the temperature coefficient Q10. For biological chemical reactions (such as waste degradation in landfills) that are not diffusion controlled, an approximate doubling of the reaction rate kr for each 10° C. increase is observed (i.e., Q10=2) over the narrow temperature range necessary for living organisms to metabolize. This effect holds until the temperature for the maximum rate is reached, above which point the rate will decline, generally with the same Q10, until the organisms are inactivated by temperature. The Arrhenius equation is a mathematical relationship between temperature and the rate of reaction, which we express here in integrated form:
where kr2 and kr1 are the reaction rates at absolute temperatures T2 and T1, respectively, R the gas constant, and Ea the activation energy for the reaction in cal-mol−1. The activation energy is the amount of energy required by a molecule to undergo a chemical reaction. Biological systems operate over a limited temperature range. A range of 10 to 85° C. corresponds to a range of only 283 to 358 K, with the product of T1 and T2 changing only slightly over this range. It is therefore reasonable to assume that
is constant over the temperature range of typical biological systems, allowing Equation (2) to be written as:
k
r2
=k
r1
e
φ(T
−T
) (3)
A Q10 of 2 is equivalent to a φ of 0.069, with T2>T1. For temperatures above the maximum, T1 and T2 are reversed in the exponent, giving it a negative value and showing a decrease in reaction rate as the temperature increases.
Landfill waste reaction rates have been extensively studied by the solid waste industry with evidence strongly indicating a first-order rate equation of the form
with the symbols as defined previously. The product fbΩ represents the quantity of biodegradable solids in the volume of the waste mass, and the negative product of the two indicates that this quantity decreases over time. Expressing this equation in integral form, we have
where the integration limits are from an initial fraction fb0 to a final fraction fb, and from and initial time of 0 to time t. Integration yields
which is the time for a waste mass to decay under first-order kinetics from an original biodegradable fraction to a target fraction. Note that the time depends only on the ratio of initial and final fractions of biodegradable waste. In practical terms, Equation (6) states that whatever the amount of solid waste present, the time required to go from the same ratio of original and final fractions will be the same, regardless of whether the fractions are, for example, 0.5 and 0.1 or 0.005 and 0.001. Therefore, it is to the landfill operator's advantage to identify the highest fraction that may be left in place and degraded by natural processes (such as oxygen diffusion) that attenuate methane or biodegradable waste. Otherwise costs will increase by continuing to pass air through the system unnecessarily.
Biodegradation rates at a given temperature can vary from landfill to landfill, depending on the local conditions and the composition and age of the waste. For fresh MSW the optimum rate occurs at a temperature of approximately 60° C. If we have a determination of the biodegradation rate at that temperature, we may use Equation (3) to generate values for the reaction rate at the specific site for a range of anticipated temperatures, using the reasonable set of assumptions mentioned. The optimum rates at 60° C. are believed to lie between 10−7 and 10−8 sec−1 for fresh MSW. Table 2 lists calculated reaction rates using five different maximum values at 60° C. for the temperature range 20 to 85° C.
To demonstrate the usefulness of the invention, we now develop some example comparisons. One example maintains the temperature of a waste mass at a given temperature Tmax using air alone to cool the waste mass. The second example maintains the temperature of the same waste mass between temperatures Tmax and Tcool using the invention to provide both an atmospheric air cooling mode alone and a recycle mode which recirculates deoxygenated air through the waste mass. We also demonstrate that it is possible with the invention to specify the airflow rate at which the operator chooses to maintain the waste mass temperature. We then can compare power requirements for each case to show the substantial power savings created by the invention. We consider here an aerobic landfill, but those versed in the art will recognize the applicability of the invention to other waste masses, including those that are functioning anaerobically. In the anaerobic case, the airflow must always be in the recycle mode to prevent atmospheric-source oxygen from entering the recirculating gases.
For any actively biodegrading waste mass, an energy balance equation may be written which states in mathematical terms that the energy rate of change equals the biodegradation energy rate and the net flux of energy from gases, vapor, water, and evaporation:
In the above equation, Δei=eiout−eiin
Case I: with the oxygen (from atmospheric air) always on, and the temperature maintained at a given level,
since (Δenc+Δev−Δel+Lv Δρv) is a constant for a given set of temperatures, we can represent this term as a constant, K3, reducing the expression to
F
v
=ΩE
bio
/K
3 (10)
The value for the constant K3 is given in Table 3 for different temperatures. Biodegradation of solid waste takes place over a range of temperatures. Biodegradation is appreciable at temperatures from as low as 10° C. to perhaps as high as 75° C. or possibly even higher. Therefore the operator of the landfill or other waste mass has the option of operating the invention at lower minimum and higher maximum temperatures than are given here. For example, running a system at a maximum temperature of 70° C., should such a temperature be feasible under the given circumstances, will reduce the biodegradation rate and therefore the heat generated by biodegradation. Additionally, the higher temperature will provide more efficient heat removal because, in the temperature range of 40 to 90° C., a 10° C. increase in air temperature approximately doubles the water-vapor holding capacity of the air flowing through the waste mass. Conversely, it may be advantageous to have a lower minimum temperature when recycling oxygen-depleted air through the landfill. In the anaerobic case, lower gas flow and lower temperatures may be determined to be preferable to improve gas generation rates for the specific circumstances of the waste mass. Hence the invention has potential applicability for a wide range of temperature minima and maxima.
Table 3 provides the calculated flow rates required to maintain the example landfill at a given temperatures with atmospheric air constantly injected. The values for K3 are also given for each temperature.
An important fact that demonstrates a utility of the invention is that as the biodegradable fraction of the waste is reduced, the flow rate needed to maintain the landfill at a desired temperature is also reduced. This is because the Ebio term in equations (8) through (10) is a function of both the biodegradable fraction fb and the biodegradation rate of the waste mass kr. While the rate remains constant at a given temperature, the biodegradable fraction decreases over time, and thus lowers the value of Ebio. As illustrative examples, Table 4A presents the calculated flow rates required to maintain the temperature of the waste at 60° C., the optimum biodegradation temperature for fresh MSW, as the fraction of biodegradable waste is reduced from 0.5 to 0.001. Table 4B presents the calculated flow rates for the same variables at 70° C.
Since flow rate is related to pressure and thus power requirements, and since the biodegradable fraction can be expressed in terms of the time, we can also determine the total power requirements and the time required to reach a target biodegradable fraction. This is shown in
Case II (with recycle): During a cycle, T increases from Tcool to Tmax as oxygen is on, then back to Tcool when oxygen is turned off (recycling phase). It will be shown that with a specified flow rate, temperature range, and biodegradable fraction, we can determine ton (the time oxygen is circulating through the landfill) and toff (time in the recycling phase).
During the period when oxygen is being supplied to the waste mass at a given flow rate, the expression for the waste mass to increase in temperature from an initial to a final T by a given amount over a time ton is
The first term in the curly brackets represents the amount of energy that is being supplied by biodegradation; and the second, in square brackets within the curly brackets, is the rate at which energy is being carried away, both rates per unit volume. The approximation is made that the waste mass heats up at a uniform rate, which may not be true locally, but is approximately correct using the average of values over the entire landfill. Once again, Ēbio is the term involving the biodegradable fraction, which will change over time, and the biodegradation rate, here given as (kr(Tmax)+kr(Tcool))½. This is less than the rate that biodegradation proceeds at constant temperature, since physically what is happening during the cycling is that the waste mass heats up to its target maximum, and then the oxygen is shut off to allow it to cool to its target minimum. As a result the combined average rate is lower. Since the terms (
During the recycling period, when air from the waste mass is being recirculated through the waste mass at a given flow rate, the expression showing the decrease in temperature by a given amount over a time toff is
Because no biodegradation is taking place, the expression represents only loss of heat energy over time (other than the added liquid, which is given an opposite sign). Since the terms (
Values for these constants are calculated in Table 6.
Both expressions for ton and toff represent the initial times on and off for the original biodegradable fraction. As mentioned, this changes over time. We are free to pick a flow rate; once done, if that rate of air flow is continued until target biodegradable fraction is reached, the only variable that will affect the times is the biodegradable fraction in the expressions for ton. To demonstrate conceptually the utility of the invention, we first calculate the initial ton and toff for different flow rates and initial biodegradable fractions for two different temperature ranges. We could take any number of sets of operating conditions in terms of Tcool and Tmax, but for the purposes of illustration, we select two: Tcool and Tmax at 50° C. and 60° C., respectively and Tcool and Tmax at 50° C. and 70° C., respectively.
Using the two ranges of temperatures, we now calculate the operating conditions for each range in terms of ton and toff for various flow rates in order to determine a cost-effective configuration in terms of energy usage. The flow rates needed to maintain a specified temperature for a waste mass with a given volume and biodegradable fraction, as presented in Table 3, are calculated based on the kr at that temperature. A higher flow rate will cool the operating temperature of the landfill to below the target temperature. In our model, the expression for ton which contains Ēbio uses the “effective kr” of ½ (kr(Tmax)+kr(Tcool)) rather than the normal Ebio used in the equation calculating the flow necessary to maintain the waste mass at a specified temperature. This average value is used because the waste mass is heating up from an initially cooler temperature, which affects the reaction rate over the time the waste mass heats to the target maximum temperature.
Table 8 lists the initial operational ton and toff for a range of selected flow rates for the Example Landfill waste mass with the minimum and maximum temperatures as above. It also lists the time required to completely degrade the landfill under a given set of operating conditions, and a specific biodegradable fraction.
Numerous scenarios may be chosen by the landfill operator regarding how best to apply the invention. The choice depends on the optimal cost combination of capital, operating, and maintenance costs. These are dependent on site-specific conditions. Our analysis points toward a general approach of determining the various cost factors and then choosing the lowest practical initial flowrate. This approach will reduce the power and capital costs during the beginning phase of the aerobic treatment of the landfill. At some predetermined time, when the biodegradable fraction has reached a specified value, the landfill operator can switch to a continuous atmospheric air flow mode and take advantage of the higher biodegradation rate that occurs, thus reducing the remaining operational time while avoiding the higher initial power and capital costs. Such choices would need to be determined by numerical simulations as a preliminary engineering design step. Practitioners of the art will recognize such prior considerations, as well as other possible approaches, to the application of the invention.
We then present a concrete example using a specified flow rate and initial biodegradable fraction to show the savings in power and capital costs that the invention can provide when compared to a system that uses continuous air flow throughout. As our example, we compare the flow requirements and power required for two systems, using the Example Landfill. Both start with a biodegradable fraction of 0.5 and the target fraction is selected to be 0.005. One system uses continuous air flow to cool the waste mass and maintain the temperature at 60° C. The other system (the invention) uses a recycling mode to reduce the biodegradable fraction 0.5 to 0.3. It then switches to continuous air flow. From that point on, both systems use the same airflow and power to reach the target fraction of 0.005.
The system with continuous airflow must start with a higher flow rate in order to maintain the temperature at 60° C. The bioreaction rate is 5.0×10−8 sec−1. As shown in Table 4A and
For the invention, we choose an initial flowrate of 16,400 CFM. Table 9 indicates an initial time on of 22.4 minutes and an initial time off of 18.1 minutes. The power requirements to reach the 0.3 biodegradable fraction target are 7.58×106 kilowatt-hours and the time to accomplish this is 0.476 years. At this point the continuous airflow process begins. To maintain a temperature of 60° C. from this point forward, we must increase the flowrate, since the bioreaction rate increases at a constant temperature of 60° C. We now follow the same power curve as the first system (Table 4A), going from an initial biodegradable fraction of 0.3 to the target fraction of 0.005. The reaction rate now becomes 5.0×10−8 sec−1; identical to the first system. The flowrate must be adjusted to 20,000 CFM initially to maintain 60° C. The 0.005 target is reached by using an additional 7.40×106 kilowatt-hours and 2.71 years. The total power consumption is then 1.50×107 kilowatt-hours using the invention versus 2.13×107 kilowatt-hours and 3.06 years versus 3.19 years for the two systems, respectively.
We believe this shows clearly by example the potential cost saving by using the invention in aerobic treatment of a waste mass. The lower capital costs are attained through reduction in needed blower capacity (20,000 CFM rather than 33,400 CFM) and savings in power costs, 6.30×106 kilowatt-hours, while operating time is increased by less than two months.
A diagram of a preferred configuration of the invention apparatus in a landfill injection/extraction system is given in
After passing through the heat exchanger the air is directed to an apparatus, in this case a 4-port, 2-position solenoid-activated valve, which is actuated to either vent the exhaust gas to the atmosphere or to a processing unit such as activated carbon, or to direct the cooled exhaust back into the landfill via an injection blower. The injected exhaust may be completely or partially deoxygenated as a result of passing through the biodegrading landfill waste, thus either slowing or stopping completely the biodegradation taking place in the landfill waste. If the exhaust is vented to a location outside of the waste mass, fresh air is concurrently injected into the waste mass by the injection blower. Those who are versed in the art will recognize that a more sophisticated valving system can be used here to bleed fresh air into any exhaust gas reinjected into the landfill waste, thereby creating a mixture of fresh air and exhaust gases, should it be deemed necessary to add oxygen below its normal concentration in fresh air. They will also recognize that the recycling flow rate need not be the same as the injected air flow rate, in case a faster cooling rate is desired.
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This application is related to and claims priority from the U.S. Provisional Application Ser. No. 61/041,859, filed Apr. 2, 2008, which provisional application is incorporated by reference herein.
Number | Date | Country | |
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61041859 | Apr 2008 | US |