POWER February 2016 - 29

COMBINED HEAT & POWER
4. Distribution of sugar factories in Cuba (2015). Source: Oficina Nacional de
Estadísticas e Información (ONEI)
0 ■ 1 - 2 ■ 3 - 4 ■ 5 - 7 ■ 7 - 8
in several industrial applications, but in Cuba
there are very few distilleries that recover it,
and then only in small quantities, thus the
bulk is released to ambient air.
The operation of bagasse-fired boilers
mills will also increase the odds of reaching
that goal.
The implementation of efficient cogeneration
systems will bring the following advantages
to the Cuban economy:
■ Reduction of ethanol and sugar production
costs.
■ Reduction or elimination of fossil fuels
dependence.
■ More secure, diverse, reliable, and
widespread supply of electricity for local
consumers.
■ A decrease in electricity transmission and
distribution losses.
■ Jobs generation for local populations.
■ Added value to agricultural waste.
■ Lower emissions of carbon dioxide (CO2)
and other gases than from conventional
fossil fuel generation.
Cuba is considered the country with the
highest potential to use bagasse-fired cogeneration
in its energy production balance,
reaching a value as high as 25%, according
to a 2004 study by the World Alliance for Decentralized
Energy (WADE).
Biomass Fuel Blending
As mentioned above, in recent years, several
biomass residues have been studied
or evaluated in order to be used alone or
mixed with sugarcane bagasse as a fuel for
cogeneration in Cuban sugar mills. One of
the most important biomass sources considered
for this purpose is the marabou
plant. This woody shrub-considered an
agricultural plague in Cuba-has an average
net heat value (NHV) of 15,700 kJ/kg,
which is about 2.1 times higher than the
NHV of sugarcane bagasse. Several studies
have shown that the steam generation
rate of a standard Cuban sugar mill will increase
by three times if marabou is used as
the main fuel, as compared with the use of
bagasse under the same operational conditions.
Recently, reports have recommended
mixing marabou biomass with bagasse (to
make a bagasse-marabou fuel blend) in orFebruary
2016 | POWER
der to avoid several operational issues during
combustion in the boiler, such as slag
deposition problems, reduced burning efficiency,
and increased fuel compactness.
Several other biomass sources are being
used or studied as fuel additives for bagasse
in selected sugar mills around the country.
The most important are rice husk (NHV
~14,000 kJ/kg) and sugarcane straw (13,000
kJ/kg).
Finally, the use of an oil sludge-fuel oil
mixture as a fuel in bagasse-fired boilers has
shown promising results to date. Some reports
have demonstrated that the use of a fuel
mixture containing 35% sludge oil and 65%
fuel oil will provide between 30% and 38%
cost savings in terms of fuel expenses, without
affecting significantly several important
issues like steam generation rate, pollutant
emissions, and boiler thermal efficiency. This
oil sludge is obtained as the heavier residue
from clarification (by a centrifugation process)
of " raw, " untreated fuel oil, prior to its
being burned in fossil fuel plants.
Emissions from the Cuban Sugarcane
Industry
The most important sources of gaseous pollutants
found in the sugarcane industry are
burning sugar cane prior to harvest operations,
fermentation processes in distilleries,
and bagasse-fired boilers' operation to produce
steam.
The practice of burning sugar cane prior
to being harvested is prohibited in Cuba because
of its negative environmental impact.
Although the practice has decreased to
10% to date, it's still taking place in some
areas either by accident or due to negligent
actions. Burning a sugarcane field releases
considerable amounts of CO2, with estimates
varying from 2,600 kg to 4,500 kg
CO2/hectare.
In conventional distilleries, great amounts
of gaseous CO2 are produced during the alcoholic
fermentation processes. A typical Cuban
distillery that produces 600 hectoliters of
ethanol per day will generate about 45 tons of
CO2. This CO2 could be recovered and used
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for steam generation is currently the most
important source of gaseous contaminants
in the Cuban sugarcane industry, and it's
closely related to several factors such as
bagasse properties (including humidity and
ash content), bagasse pretreatment processes,
furnace/boiler characteristics and
operation, as well as the type of combustion
system used. Particulate matter, combustion
products, and volatile organic compounds
are the primary pollutants emitted from the
sugarcane processing industry. Combustion
products include nitrogen oxides, carbon
monoxide, CO2, and sulfur oxides. Actually,
there aren't treatment systems and technologies
installed in Cuban sugar mills (such
as the one shown in the opening photo) to
control gaseous emissions. This is a topic
that is gaining increased attention due to recently
released environmental regulations,
and several projects (mechanical collectors,
cyclones, and wet scrubbers) are being designed
or are under development in some
selected sugar factories for gaseous emissions
control.
It must be said that sugarcane plantations
may act like efficient absorbent materials, as
they can absorb high amounts of CO2 and
transforms it into oxygen, resulting in the
so-called " forest effect. " It's calculated that
1 hectare of sugarcane can absorb more than
60 tons of CO2 and produce nearly 40 tons of
pure oxygen.
Bagasse combustion is environmentally
friendly because it boasts low particulate and
CO2 emissions. The CO2 produced during
bagasse combustion is the same amount that
the sugarcane absorbs when it grows, which
means that there is an environmental balance
between CO2 generation and utilization-
that is, CO2 is cycled back to be re-fixed by
the photosynthetic process in the next crop.
Bagasse-based cogeneration is considered
today one of the main advantages of the
Cuban sugarcane agroindustry, especially in
light of worldwide energy changes and the
need to consolidate the Cuban power mix.
Today, this industry is self-supporting from
the energy point of view, but its power contribution
is not enough to face future demands
from other dynamic sectors such as construction.
Therefore, the need to keep optimizing
it and increasing its application in other areas
of the country's economy remains. ■
-Amaury Perez Sanchez (amaury.
psanchez@reduc.edu.cu) is a chemical
engineer in the Chemistry Department,
University of Camagüey, Cuba.
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POWER February 2016

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