Agriculture Reference
In-Depth Information
3.3.2.2
Modeling and Simulation
Various research teams have achieved important advances in the mathematical
modeling and simulation of composting, usually with an emphasis on thermal ef-
fects (Barneto et al.
2010
; Fontenelle et al.
2011
; Zambra et al.
2011
,
2012
; Zhang
et al.
2012
). Vidal-Beaudet et al. (
2012
) carried out related modeling work to predict
long term effects after compost has been added to soil.
Barneto et al. (
2010
) employed an autocatalytic kinetic model to represent a
composting process. Model predictions were compared with results obtained by
thermo-gravimetric analysis and calorimetry. The main components, cellulose,
hemicellulose, and lignin, were assigned different parameters. Water and carbon
di-oxide flows, as well has hydrogen production, were modeled. The positive effect
of the lignin component on the production of hydrogen during composting was suc-
cessfully predicted.
Fontenelle et al. (
2011
) developed a mathematical model incorporating empir-
ically-determined rate expressions. The effects of yeast, bacteria, and fungi were
separately modeled for a mixture including sugars, starches, hemicelluloses, and
cellulose with the uptake of oxygen and production of heat. The dynamics of
composting were faithfully captured by the model. The model was able to predict
trends of temperature with time and also oxygen levels at various locations in
the pile.
The principle contribution of Zambra et al. (
2011
,
2012
) was to capture, in their
model, the three-dimensional aspects of a compost pile, such as generation of heat
in the core and dissipation of heat at the surfaces. A nonlinear model based on dif-
fusion rates was used to predict the effects of aerobic bacteria and their metabolism
during decomposition of biomass. As noted earlier, their model was unique in its
ability to predict conditions leading to combustion due to excessive temperature and
insufficient moisture in the core of a large compost pile.
The model of Zhang et al. (
2012
) was based on “biochemical fractionation”,
in which different rate constants were applied to different components, effectively
dividing the sample into fractions. Observed versus predicted proportions of differ-
ent fractions were compared for a set of contrasting experiments. Variables such as
temperature and oxygen content were inserted as constants. As noted by the authors,
in future work it might be possible to achieve better calibration between such a
model and experimental outcomes by including the evolution of temperature in the
simulation.
3.3.3
Aeration during Composting
The cycling of air through a compost pile has a complex relationship to tempera-
tures and moisture. On the one hand, introduction of fresh air, which is usually
cooler, is necessary to support the ongoing metabolic processes. But on the other
hand, excess fresh air, beyond what is needed for respiration, can cool the pile.
