Volume 8 [2006]

Combustion of poor quality coals is used today worldwide for energy production. The removal
of polluting compounds, i.e. S, N, Hg and Cl from them prior to combustion is important not
only for improving the coal quality, but also for meeting the strict environmental standards.
Thus, a preventive fuels pre-treatment technique, based on low temperature carbonisation,
has been developed that aims to convert ‘dirty’ fuels to cleaner ones. This study looks at the
effect of pyrolysis temperature and residence time on the reactivity of clean coals produced
by poor quality coals. Pyrolysis tests of three poor quality coals (Ptolemais, Bulgarian and
Australian) were performed in a lab-scale fixed bed reactor, under helium atmosphere and
ambient pressure. Ptolemais lignite is the main fuel for electricity production in Greece. The
effects of carbonisation temperature (200-900°C) and residence time (5-120 min) on the
properties of the obtained chars were investigated. Special attention was paid to the polluting
compounds (S, N, Hg, Cl) removal. The reactivity under combustion conditions of the
Ptolemais chars produced was also investigated. It was observed that low temperature
carbonisation could contribute to clean coal production by effectively removing the major part
of the existing polluting compounds in the coals. Thus, depending on coal type, nitrogen,
mercury and chlorine conversion continuously increase with temperature, while sulphur
removal seems to reach a plateau above 500 – 600°C. Furthermore, the prolongation of
carbonisation time above 20 min does not affect the elemental conversion of the pollutants.
Therefore, carbonisation at 500 – 600°C for ~20 min could be considered sufficient for clean
coal production from poor quality coals. The reactivity of the prepared clean coals was
evaluated by performing non-isothermal combustion tests in a TA Q600 thermobalance at
ambient pressure and 20°C min-1 heating rate. At increased pyrolysis temperatures higher
initial combustion temperatures were observed, due to the volatile reduction in char
production stage. The latter results to lower total conversions.

Μany applications in water quality management have a common key water quality parameter,
dissolved oxygen, resulting to the critical role of aeration. On the other hand, in municipal and
industrial wastewater, especially where aeration is applied, the presence of volatile organic
compounds (VOCs) causes several concerns including a direct threat to humans, partly due
to their emission from treatment tanks. pH, temperature and Henry’s Law govern VOCs’
speciation and consequently their emission characteristics. Limited data and simplifications of
available mass-transfer models pose obstacles to a realistic approach, especially in the
presence of a chemical equilibrium, for example in the case of mercaptans.
In the present study the importance of oxygen transfer and stripping of a VOC (n-butyl
mercaptan) on aeration’s overall effectiveness are examined separately. Clean water
oxygenation and stripping of mercaptan to an inert gas (nitrogen) were studied aiming to
consider mass transfer aspects and to investigate the influence of chemical equilibrium
between ionic and neutral form of the target compound in neutral and alkaline solutions.
Using appropriate mass transfer relationships (dynamic method), experimental data were
analyzed for the determination of overall mass transfer coefficient ( kOL,O2α ) of oxygen.
Correlating kOL,O2 α with the corresponding mass transfer coefficient of n-butyl mercaptan in
neutral solutions (calculated according the model proposed by Matter-Muller et al. [1]), a
value of ratio βy of 0.566 is found, close to the reported values of other VOCs with similar
values of Henry’s constant. At alkaline pH however the conventional simplified model fails to
predict realistic values of mass-transfer coefficients. A coupled differential algebraic equation
system, based on mass balances, taking into account dissociation of the compound to be
stripped and assuming chemical non-equilibrium conditions during stripping, was developed.
Reaction parameter k2 was calculated with non-linear least-squares analysis. The model
predicts satisfactorily the experimental data and it provides a useful tool for the semibatch
stripper design in situations where a reversible reaction is involved. At pH values below 8.5
mercaptan concentration falls exponentially whereas above 10.5 it tends to linearity. The
bubble equilibrates and mercaptan transferred depends upon solubility and not diffusivity.
Especially after depletion of initial neutral compound, transport depends upon neutral/ionic
form speciation. The effectiveness of stripping n-butyl mercaptan, at a given pH, is mainly
determined by a proportionality constant considered as “fugacity capacity” (removal effect on
the process) and by a reversible reaction rate constant k2 (kinetic effect on the process). The
‘’fugacity capacity” is determined by hydrophobicity (i.e. low solubility and high limiting activity
coefficient) rather than pure-component volatility (i.e. vapor pressure or boiling point). High
limiting activity coefficient promotes mercaptan emission due to established vapor-liquid
equilibrium, while the low reaction parameter k2, controls neutral compound quantity. At high
pH, where ionic form predominates, experimental data showed that stripping was almost
independent of the gas flow rate applied. A strong sensitivity of the model to uncertainty of γ∞
was found: γ∞ controls emission rate and through this the dynamic variations of neutral/ionic
concentration profiles whereas reaction rate law parameter k2 controls the neutral/ionic
transformation and it is the crucial quantity which governs the process at high pH values.

At present, chemical processes are widely used to remove phosphorus and organic matter
from water or wastewater either as the only advanced treatment method or as a pre-treatment
stage to biological treatment, and aluminium salts have been traditionally used as coagulants.
In the present study removal of orthophosphate or dissolved organic matter with the use of
two aluminium sources: alum [Al2(SO4)3•18H2O] and aluminium hydroxide [Al(OH)3(s)] is
examined for the wastewater treatment process. Amorphous aluminium hydroxide was
chosen because it is the hydrolyzed (olated) product of alum and an important adsorbent of
inorganic and organic substances in soils since bayerite, gibbsite and boehmite contain a
large portion of aluminium hydroxides. Laboratory jar test studies were carried out using
either an orthophosphate solution (10 mg l-1) or a tannic acid solution (50 mgC l-1) as
simulated wastewater and the effect of parameters such as coagulant dose (up to 15 mg Al l-1
in the case of alum and up to 90 mg Al l-1 in the case of Al(OH)3) and pH (2-12) is
investigated. The values of these parameters were based upon measurements on municipal
wastewaters from the input of the primary treatment of a wastewater plant in Athens.
Orthophosphates have been chosen as a P surrogate as they are the major portion of the
total P found in wastewater and tannic acid solution was used as a surrogate for soluble
organic matter. It was proven that alum is much more efficient in phosphorus and tannic acid
removal than aluminium hydroxide. The optimal pH values are 5-6 in both cases, alum and
aluminium hydroxide, although alum is efficient in a wider pH range (4-7) and a mechanism
was proposed to interpret these results. In pH values less than 6 the mechanism proposed
was chemical bonding between Al species and tannic acid or phosphates creating insoluble
complexes while in bigger pH values (6-8) adsorption on solid Al(OH)3. Freundlich isotherm
was proven to fit satisfactorily the experimental data for aluminium hydroxide and
orthophosphate at 250C suggesting heterogeneous sorption, with KF and N values 49,1 and
0,19 respectively. The findings of this work may not only contribute to a better understanding
of the chemistry of chemical wastewater treatment and therefore to an improvement of the
process but also on phosphorus and organics fixation in soils that contain a large portion of
aluminium hydroxides.

Coals contain elements which, although usually found in concentrations lower than 1% (trace
elements), can give rise to environmental or technological problems. After gasification most of
these elements may occur in gas phase in different proportions. In order to avoid the
problems that the presence of trace elements in gas phase can originate during coal
gasification processes, a suitable technology needs to be developed. The systems currently
being studied and developed for gas cleaning in coal gasification, focus on the removal of
sulphur, particulate matter, nitrogen, alkali metals and halogens but not on corrosive or toxic
trace elements. Nevertheless, the reduction of trace elements using solid sorbents in gas
phase at high temperatures appears to be a promising method for combustion systems.
The main objectives of this work were to determine the capacity of different solid sorbents for
retaining arsenic, selenium, cadmium and zinc species in gases from coal gasification
systems at 550 and 750 ºC and to find out how the sorbent characteristics and operational
variables (temperature and gas composition) influence retention. To attain these objectives
the sorption capacity (mg of element per g of sorbent) and the efficiency (percentage of
element retention) were determined. The study was carried out in a laboratory scale reactor,
in which the sorbent was employed as a fixed bed, using synthetic gas mixtures. At the end of
each experiment, the sorbent bed (mixture of sorbent + sand) was finely ground and
dissolved in a microwave oven with HF, HNO3 and H3BO3, and the element in solution was
determined by ICP-MS.
The results are discussed in the light of the data for combustion conditions reported in the
literature, and possible retention mechanisms are proposed. Different amounts of arsenic,
selenium, cadmium, and zinc can be retained in solid sorbents at high temperatures. It was
observed that, in a coal gasification atmosphere, limestone, fly ashes and metallic oxide
mixtures containing spinels, were the best sorbents, though in each case the retention
capacity depended on temperature and atmosphere. Retention capacities between 16-24
mg g-1 were obtained using limestone and fly ashes for arsenic retention. For selenium, the
maximum retention capacities ranging between 50-56 mg g-1 were attained using limestone.
Alumina in a gasification atmosphere containing HCl was the best sorbent for zinc removal
(52 mg g-1). The lowest retention capacities were obtained for cadmium, these being <1 mg g-1
for the different sorbents tested. Retention probably proceeds through different mechanisms,
but in most cases a chemical reaction is involved.