TY - BOOK
T1 - Experimental and Theoretical Investigations of Wet Flue Gas Desulphurisation
AU - Kiil, Søren
PY - 1998
Y1 - 1998
N2 - This thesis describes experimental and theoretical investigations
of wet flue gas desulphurisa-tion (FGD). A review of the current
knowledge of the various rate determining steps in wet FGD plants
is presented. The experimental work covers laboratory studies as
well as pilot- and full-scale experiments. In the theoretical part
of the work, the laboratory and pilot plant observations are
investigated using mathematical modelling.The mechanism underlying
the rate of dissolution of finely grinded limestone particles was
examined in a laboratory batch apparatus using acid titration.
Three Danish limestones of dif-ferent origin were tested. A
transient, mass transport controlled, mathematical model was
de-veloped to describe the dissolution process. Model predictions
were found to be qualitatively in good agreement with experimental
data. Deviations between measurements and simulations were
attributed primarily to the particle size distribution (PSD)
measurements of the limestone particles, which were used as model
inputs. The measured PSD was probably not representa-tive of a
given limestone sample because of agglomeration phenomena taking
place in the dis-perser, preventing a stable and accurate
measurement. Other factors, such as convective mass transfer,
porosity and porosity changes, and perhaps surface reaction, may
also influence the rate of dissolution. However, those effects
could not, due to the uncertainty of the PSD, be confirmed.
Empirical correlations for the dimensionless mass transfer
coefficients in a pilot plant (falling- film column) were
determined. The correlations are valid at gas phase Reynolds
numbers from 7500 to 18,300 and liquid phase Reynolds numbers from
4000 to 12,000, conditions of industrial relevance. The presence
of inert particles in the liquid phase was found to decrease the
rate of gas phase mass transport with up to 15 %, though the
effect could not be correlated.A detailed model for a wet FGD
pilot plant, based on the falling film principle, was devel-oped.
All important rate determining steps, absorption of SO2, oxidation
of HSO3-, dissolution of limestone, and crystallisation of gypsum
were included. Model predictions were compared to experimental
data such as gas phase concentration profiles of SO2, slurry
pH-profiles, sol-ids contents of the slurry, liquid phase
concentrations, and residual limestone in the gypsum. Simulations
were found to match experimental data for the two Danish limestone
types (Faxe Bryozo and a chalk, Mikrovit) investigated. Gas phase
mass transport was found to be the dominating rate determining
step, though the liquid phase mass transport resistance could not
be neglected. Simulations and experimental data both showed the
same degree of desulphuri-sation and absorber pH profile for the
two limestone types using a holding tank pH of 5.5, but the
residual limestone in the gypsum was significantly lower for the
chalk. Furthermore, simulations showed that between 10 and 30 % of
the limestone dissolves in the absorber de-pending on the process
conditions. A typical holding tank pH of 5-5.5 (also used in
full-scale wet FGD packed towers) was found to be a reasonable
compromise between residual lime-stone in the gypsum and the
degree of desulphurisation. Simulations were only slightly
sensi-tive to the temperature in the interval 313 - 333 K,
pertinent for full-scale wet FGD packed towers. The possibility of
co-firing straw and coal was investigated in a full-scale power
plant. No ef-fects on the overall performance of the wet FGD plant
were observed, though laboratory ex-periments with fine dust and
fly ash from the full-scale experiments showed a decrease in
limestone reactivity. However, the test period was only about one
week, probably not allowing the FGD plant to reach steady state
operation. Pilot-scale experiments were initiated to investigate
the possibility of oxidising spray dry scrubber by-products (TASP)
to gypsum in wet FGD plants. The investigations showed that it was
possible to oxidise the TASP at concentrations of up to 300
g/litre (feed tank basis). The residual Ca(OH)2 in the TASP could
be exploited for desulphurisation resulting in a reduced limestone
consumption and an increased removal rate of SO2. The simultaneous
presence of Al3+ and F- did not, as reported in the literature,
inhibit the rate of dissolution.The results of this work
demonstrate that mathematical modelling can be a powerful tool in
the process of obtaining a detailed understanding of the complex
interactions between chemi-cal reactions and mass transport
phenomena in wet FGD plants.
AB - This thesis describes experimental and theoretical investigations
of wet flue gas desulphurisa-tion (FGD). A review of the current
knowledge of the various rate determining steps in wet FGD plants
is presented. The experimental work covers laboratory studies as
well as pilot- and full-scale experiments. In the theoretical part
of the work, the laboratory and pilot plant observations are
investigated using mathematical modelling.The mechanism underlying
the rate of dissolution of finely grinded limestone particles was
examined in a laboratory batch apparatus using acid titration.
Three Danish limestones of dif-ferent origin were tested. A
transient, mass transport controlled, mathematical model was
de-veloped to describe the dissolution process. Model predictions
were found to be qualitatively in good agreement with experimental
data. Deviations between measurements and simulations were
attributed primarily to the particle size distribution (PSD)
measurements of the limestone particles, which were used as model
inputs. The measured PSD was probably not representa-tive of a
given limestone sample because of agglomeration phenomena taking
place in the dis-perser, preventing a stable and accurate
measurement. Other factors, such as convective mass transfer,
porosity and porosity changes, and perhaps surface reaction, may
also influence the rate of dissolution. However, those effects
could not, due to the uncertainty of the PSD, be confirmed.
Empirical correlations for the dimensionless mass transfer
coefficients in a pilot plant (falling- film column) were
determined. The correlations are valid at gas phase Reynolds
numbers from 7500 to 18,300 and liquid phase Reynolds numbers from
4000 to 12,000, conditions of industrial relevance. The presence
of inert particles in the liquid phase was found to decrease the
rate of gas phase mass transport with up to 15 %, though the
effect could not be correlated.A detailed model for a wet FGD
pilot plant, based on the falling film principle, was devel-oped.
All important rate determining steps, absorption of SO2, oxidation
of HSO3-, dissolution of limestone, and crystallisation of gypsum
were included. Model predictions were compared to experimental
data such as gas phase concentration profiles of SO2, slurry
pH-profiles, sol-ids contents of the slurry, liquid phase
concentrations, and residual limestone in the gypsum. Simulations
were found to match experimental data for the two Danish limestone
types (Faxe Bryozo and a chalk, Mikrovit) investigated. Gas phase
mass transport was found to be the dominating rate determining
step, though the liquid phase mass transport resistance could not
be neglected. Simulations and experimental data both showed the
same degree of desulphuri-sation and absorber pH profile for the
two limestone types using a holding tank pH of 5.5, but the
residual limestone in the gypsum was significantly lower for the
chalk. Furthermore, simulations showed that between 10 and 30 % of
the limestone dissolves in the absorber de-pending on the process
conditions. A typical holding tank pH of 5-5.5 (also used in
full-scale wet FGD packed towers) was found to be a reasonable
compromise between residual lime-stone in the gypsum and the
degree of desulphurisation. Simulations were only slightly
sensi-tive to the temperature in the interval 313 - 333 K,
pertinent for full-scale wet FGD packed towers. The possibility of
co-firing straw and coal was investigated in a full-scale power
plant. No ef-fects on the overall performance of the wet FGD plant
were observed, though laboratory ex-periments with fine dust and
fly ash from the full-scale experiments showed a decrease in
limestone reactivity. However, the test period was only about one
week, probably not allowing the FGD plant to reach steady state
operation. Pilot-scale experiments were initiated to investigate
the possibility of oxidising spray dry scrubber by-products (TASP)
to gypsum in wet FGD plants. The investigations showed that it was
possible to oxidise the TASP at concentrations of up to 300
g/litre (feed tank basis). The residual Ca(OH)2 in the TASP could
be exploited for desulphurisation resulting in a reduced limestone
consumption and an increased removal rate of SO2. The simultaneous
presence of Al3+ and F- did not, as reported in the literature,
inhibit the rate of dissolution.The results of this work
demonstrate that mathematical modelling can be a powerful tool in
the process of obtaining a detailed understanding of the complex
interactions between chemi-cal reactions and mass transport
phenomena in wet FGD plants.
M3 - Book
SN - 87-90142-30-6
BT - Experimental and Theoretical Investigations of Wet Flue Gas Desulphurisation
PB - Tekst & Tryk A/S
CY - Vedbæk
ER -