Incineration of biowastes: a TG-MS approach
Marta Otero
CESAM (Centre for
Environmental and Marine Studies) & Department of Chemistry
University of Aveiro, Portugal
in collaboration
with
IRENA (Natural Resources Institute), University of
León, Spain
Since sludge production is intrinsic to the
water treatment process, an integral approach to addressing the issue of
wastewater must necessarily entail the rational management of sludge.
Application of urban wastewater sludge to land provides a source of
slow-release nutrients and microelements, acting as agricultural soil
conditioner. Although this could ideally be the most environmentally attractive
option, difficulties may arise for the practice. Management
of any waste must always account for the properties of the specific waste and
the local, geographical and economical conditions as there is not a universally
valid option for a certain type of waste.
Sewage
sludge is a biowaste which complete combustion, if properly designed and
operated, can provide energy, carbon dioxide, water and sulphur dioxide as the
main products. Techniques are nowadays available to control gaseous emissions
and, in some specific cases, combustion may be the only solution to the
management of sludge. From an economic point of view, the possibility of a
joint combustion of sludge and coal in power plants can be an interesting
option, since it allows for the use of existing infrastructures, already
equipped with appropriate devices for gaseous emission control and staffed with
qualified personnel. The energy policy promotes, in principle, the use of
biomass and locally generated waste as fuel in order to support the carbon
economy. Although urban sewage sludge has high carbon content, the fact of
being seen as a waste has constrained its energetic valorisation. Thus,
research and information are very important to change public opinion on sewage
sludge and to support the most appropriate management choice in each case.
Thermogravimetric analysis (TGA) has been used to characterize the
thermal decomposition of coal. The advantage of this kind of analysis is that
they give a rapid assessment of the fuel value, the temperatures at which
combustion starts and ends and other characteristics such as maximum reactivity
temperature or total combustion time. This led us to use this analytic technique
to the study the combustion of sludge (Fig. 1) and its co-combustion with coal.
Simultaneously with TGA and to gain more information on the process,
Differential Scanning Calorimetric analysis (DSC) and Differential
Thermogravimetry (DTG) burning profiles were carried out together with gaseous
emission analysis by mass spectrometry (MS). Furthermore, the kinetics of the
combustion of sludge by a simple set of TGA has been assessed.
Figure
1.
Sewage sludge as received from the municipal water treatment plant.
It has been found that the TG, DTG, DSC and
MS profiles corresponding to sewage sludge and a semianthracite coal combustion
are quite different, which is consequence of their different properties,
determined by elementary and proximate analysis. However these differences,
analysis corresponding to the temperature programmed combustion of coal-sludge
blends (sewage sludge ≤10%) are analogous to those related to coal
combustion (Fig. 2). With regard to the kinetics, although the activation energy
corresponding to the combustion of the semianthracite coal was lower than that
corresponding to sewage sludge, blends have activation energies the same order
than coal. Also, for the blends combustions, emission of CO2 is very
alike to that corresponding to the coal. Differences between coal and sludge
combustion are significant but they get unremarkable for their blends, even for
sludge of different origins.
Figure
2.
A) Combustion and co-combustion DTG curves for coal (C), sludge (SSL) and their
blends. B) Combustion and co-combustion DSC curves for coal (C), sludge (SSL)
and their blends.
At present this kind of approach is been used to study the incineration
and co-incineration of biowastes, such as animal manure and municipal solid
wastes, tyres and plastic residues.
Selected publications:
1. Otero M,
Díez C., Calvo L. F., García A.I., Morán
A. (2002) Analysis of the co-combustion of sewage sludge and coal by
TG-MS. Biomass and Bioenergy. http://dx.doi.org/10.1016/S0961-9534(02)00012-0
2. Calvo
L.F., Otero M, Jenkins B.M, García A.I., Morán A. (2004). Heating
Process Characteristics and Kinetics of Sewage Sludge in Different Atmospheres.
Thermochimica Acta. http://dx.doi.org/10.1016/S0040-6031(03)00359-9
3. Otero
M., Gomez X., García A.I., Morán A. (in press) Non-isothermal
thermogravimetric analysis of the combustion of two different carbonaceous
materials: coal and sewage sludge. Journal of Thermal Analysis and Calorimetry.
http://dx.doi.org/10.1007/s10973-007-8415-y
4. Otero
M., Calvo L.F., Gil M.V., García A.I., Morán A. (in press) Co-combustion of
different sewage sludge and coal: a non-isothermal thermogravimetric kinetic
analysis (accepted 1-12-2007). Bioresource Technology. http://dx.doi.org/10.1016/j.biortech.2007.12.011
5. Otero
M., Gomez X., García A.I., Morán A. (2007) Effects of sewage sludge
blending on the coal combustion: a thermogravimetric assessment. Chemosphere. http://dx.doi.org/10.1016/j.chemosphere.2007.05.077
6. Otero
M., Sanchez M. E., García A., Morán A. (2006) Simultaneous
thermogravimetric-mass spectrometric study on the co-combustion of coal and
sewage sludges. Journal of Thermal Analysis and Calorimetry. http://dx.doi.org/10.1007/s10973-005-7367-3
Marta
Otero got a degree in Marine Sciences at the
University of Vigo (Spain) and carried out doctorate studies at the University
in León (Spain), where she got a PhD. She got a Marie Curie fellowship to
undergo postdoctoral research at the FEUP (