Estimation of maximum LFG yields

University of Padua

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The reaction representing the overall methane fermentation process for organics in solid waste can be represented by the following by the following equation:

 

(1)

         The fraction of the organic matter that is converted to biomass, considering an infinite retention time in the system, is about 4% (EMCON, 1980). Therefore, for the practical evaluation of the maximum theoretical LFG yield, cell conversion of organic matter can be neglected, and equation (1) becomes:

      

 

(2)

Equation (2) states that 1 mol of organic carbon is bioconverted to 1 mol of landfillgas.

 

1 mol C org. = 1 mol (CH₄ + CO₂)

at 0° C and 1 atm:

1 mol C in organic matter = 22,4 l (CH₄ + CO₂)

on a weight basis:

1 g C in organic matter = 1,867 l (CH₄ + CO₂)

maximum theoretical yield of landfill gas

            CH₄ = 0,55-0,6  1,867  l

 

Content of biodegradable organic carbon:

 

(OC_b)_i = OC_i(f_b)_i • (1-u_i) • p_i

 

The fraction of bioconvertible carbon to landfill gas depends upon the temperature within the landfill:

 

(OCb)_i = OC_i (0,014 T + 0,28)

 

Temperature, however, primarily affects generation rates, which are strictly associated with the biological activity within the landfill.

 

LFG specific yield:

 

LFG generation rates:

The general equation that rules the biogas production is:

 

The greatest absolute exponent n of the dependent variable  (C) is called the order

of the model (i.e. order of kinetics). A zero-order kinetics means that a small increment (positive or negative) of C does not influence the rate of substrate decay or biogas production. According to some authors, many landfills have a biogas  production that follows a zero-order kinetics, especiallly during the periods of highly active gas generation.

The majority of LFG production models follow a first-order kinetics, which means

that the limiting factors is the remaining amount of substrate or the amount of

biogas already produced.

 

Generation time:

Satisfasfactory information concerning generation time is provided by the half

time (t_½), the time over which the gas generation equals half of the estimated yield.

By definition the t_½ is such that the area under the production curve is the same on

both sides. The range of values proposed for t_½ is very wide, from 2-5 years foe wet

to 10-25 years for dry climates (Augenstein and Pacey, 1991).

The half time can be also calculated in first-order kinetic models by the following

expression:

 

Lag time:

Is the time that passes from the placement of waste to the beginning of  significant gas production. Lag time can vary from a few weeks and months to 1 year and more.

 

 

LFG for how long?

 

(OC_b)_i = biodegradable organic carbon in the ith component of waste (kg biodegradable carbon/kg wet MSW)

OC_i = organic carbon content in the dry ith component of waste (kg carbon/kg dry i component)

(f_b)_i = biodegradable fraction of OC_i  (kg biodegradable carbon/kg carbon)

u_i = moisture content of the ith component of waste (kg water/kg wet i component)

p_i = wet weight of the ith component of waste (kg i component/kg MSW)

(f_b)_i = 0,83 – 0,028 LC   (on a volatile solids basis)

LC = lignin content of the volatile solids expressed as % of dry weight

 

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