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F. Viganò – A practical method to calculate the R1 index of Waste-to-Energy facilitiesSupplementary material

APPENDIXES: Table of contents

APPENDIX A: “specific productions” and LHVs of various substances.......................................1A.1 Natural gas............................................................................................................................1A.2 Natural gas + stoichiometric air (i.e. natural gas burners)....................................................2A.3 Light fuel oil / Diesel fuel + stoichiometric air (i.e. diesel-fired burners)............................3A.4 Dolomitic sorbent.................................................................................................................4A.5 Gaseous ammonia for de-NOX SCR and SNCR systems.....................................................5A.6 Liquid water..........................................................................................................................7A.7 Water solution of ammonia for de-NOX SCR and SNCR systems.......................................8A.8 Solid urea for de-NOX SNCR systems................................................................................10A.9 Liquid water-urea solution for de-NOX SNCR systems.....................................................12A.10 Sodium bicarbonate in dry scrubbing systems.................................................................13A.11 Calcium hydroxide in dry scrubbing systems...................................................................14A.12 Milk of lime in semi-dry scrubbing systems....................................................................16A.13 Wet scrubbing systems.....................................................................................................18A.14 Unburnt C in bottom and fly ashes...................................................................................19A.15 References.........................................................................................................................20

APPENDIX B: Sources of the data in Figure 4.............................................................................21APPENDIX C: Using hourly averaged values for calculations....................................................22

APPENDIX A: “specific productions” and LHVs of various substances

This appendix reports the sources (calculations and / or references) of the values of “specific

productions” and LHVs reported in Table 1.

A.1 Natural gas

The MatER guidelines refer to a typical North-Italian natural gas:

[CH4] = 97.10%Vol.

[C2H6] = 1.41%Vol.

[C3H8] = 0.56%Vol.

[C4H10] = 0.34%Vol.

1


[CO2] = 0.01%Vol.

[N2] = 0.58%Vol.

The stoichiometric combustion in air (3.77 moles of other species per mole of oxygen) leads to the

following specific productions:

dea = −9.744 mN3/mN

3 = −9.236 mN3/mS

3

dsfg = 8.738 mN3/mN

3 = 8.282 mN3/mS

3

wv = 2.024 mN3/mN

3 = 1.918 mN3/mS

3

solids = 0.000

The LHV is determined according to EN ISO 6976 by considering butane as 50% normal and 50%

iso. The result is 36.675 MJ/mN3 = 34.751 MJ/mS

3.

A.2 Natural gas + stoichiometric air (i.e. natural gas burners)

The specific production figures given for natural gas are correct only in the case natural gas alone is

introduced into flue gas and the combustion takes place by consuming the DEA already present in

flue gas. The burners normally used in FGTS (as well as those used in combustion chamber)

introduce both the fuel and the required ambient air. Actually, also some excess air is introduced,

but its contribution must be accounted separately (as “false air” in FGTS, or in the closure of the

mass balance for the boiler).

With the use of burners, no consumption of the flue gas content of DEA takes place, moreover, in

addition to the DSFG and WV produced by the combustion, also the WV (i.e. humidity) associated

with the consumed ambient air is left in the flue gas.

2


Therefore, the “specific productions” for the use of natural gas-fired burners are:

dea = 0.000

dsfg = 8.738 mN3/mN

3 = 8.282 mN3/mS

3

wv = (2.024 + 9.744 ∙ ) mN3/mN

3 = (1.918 + 9.236 ∙ ) mN3/mS

3

solids = 0.000

Where “” is the content of WV (i.e. humidity) in ambient air defined by equation 12.

The LHV associated with natural gas is reported in A.1.

A.3 Light fuel oil / Diesel fuel + stoichiometric air (i.e. diesel-fired burners)

Data for light fuel oil / diesel fuel are taken from Jungbluth (2012).

The stoichiometric combustion reaction for the brute formula CH1.83 gives the following specific

productions:

dea = 0.000

dsfg = 10.516 mN3/kg = 8.833 mN

3/l

wv = (1.482 + 11.257 ∙ ) mN3 kg-1 = (1.245 + 9.456 ∙ ) mN

3 l-1

solids = 0.000

3


For the conversion from kilogram (“kg”) to litre (“l”) basis, the volume mass of 0,84 kg l -1 is

considered.

The corresponding LHV is 42.8 MJ kg-1 = 35.952 MJ l-1.

A.4 Dolomitic sorbent

Different calcium-based sorbent can be used for the neutralization of the acid compounds found in

the flue gas of waste combustion (mainly HCl). In recent years, a dolomitic sorbent for high

temperature abatement has been adopted by several WtE plants in Italy (Biganzoli et al., 2015). It is

injected directly into the combustion chamber of WtE boilers both to achieve a first partial

reduction of acid gases concentrations as well as to shave their peaks.

From the technical specifications sheet of the sorbent, the technical formula results Ca(OH)2∙MgO

for about 97% by mass, with the remaining 3% being mainly CO2.

Therefore, 1 kg of sorbent comprises:

0.628 kg of Ca(OH)2 = 8.479 mol;

0.342 kg of MgO = 8.479 mol;

0.030 kg of CO2 = 0.682 mol.

The experimental results reported by Biganzoli et al. (2015) show that, on average basis, MgO ends

up into boiler ash without significantly reacting with acid species, whereas calcium reacts abating

about 2 mols of HCl (the predominant species, considered representative of all acid compounds) per

kilogram of injected sorbent. The abatement of HCl produces CaCl2. The dehydratation of Ca(OH)2

at high temperature generates a large excess of very fine CaO, which is exposed, along the boiler, at

medium-high temperature, for a long time, to a significant concentration of CO2, hence ending up

mostly into CaCO3 by absorbing some CO2. In consideration that, as reported in the literature, the

carbonation of CaO to CaCO3 forms a layer of CaCO3 on the particles that does not allow a full

carbonation of the CaO particle cores, it is assumed that 1/3 of the excess CaO is left unreacted

inside the particles. Therefore, the overall reaction for 1 kg of sorbent is:

4


(8.479 Ca(OH)2∙MgO + 0.682 CO2) + 2 HCl + 4.304 CO2 →

CaCl2 + 8.479 MgO + 4.986 CaCO3 + 2.493 CaO + 9.479 H2O

The corresponding specific productions are:

dea = 0.000

dsfg = (-4.304 – 2.000) ∙ 22.414 mN3 Mg-1 = -0.1413 mN

3 kg-1

wv = 9.479 ∙ mN3 Mg-1 = 0.2125 mN

3 kg-1

solids = (1.000 ∙ 110.98 + 0.342 + 4.986 ∙ 100.09 + 2.493 ∙ 56.077) kg Mg-1 = 1.092 kg kg-1

Since the LHV is equal to minus the heat of reaction, it can be evaluated also for this substance,

with reference to the reaction reported above. By using the enthalpies of formation for pure

substances reported by McBride et al. (2002), the result is:

LHV = (8.479 ∙ -985.9 + 4.986 ∙-393.510 + 2.000 ∙-92.310 – 1.000 ∙ 795.800 +

– 4.986 ∙ 1206.600 – 2.497 ∙ -634.920 – 9.479 ∙ -241.826) kJ kg-1 = 0.1809 MJ kg-1

The reaction is slightly exothermic, since LHV > 0.

A.5 Gaseous ammonia for de-NOX SCR and SNCR systems

Ammonia based reactants are largely used in WtE plants as reducing agent for NOX, in SCR and

SNCR systems. Gaseous ammonia is always present in all the NOX reduction reactions that take

5


place in these systems.

The dosage of ammonia is typically exceeding the stoichiometric requirement by a certain factor ,

depending on the type of system and its settings. The operating data of the analysed plants, show

that lies in between 1.1-1.5 for tail end SCR systems, and in between 2.0 – 2.4 for high dust SCR

systems. The excess of ammonia ends up into molecular nitrogen and water vapour.

Therefore, by a conversion efficiency of NOX equal to , the overall reaction is:

NO2 + NH3 → ( + ) / 2 N2 + 3/2 H2O + ( – ¾ ) O2 + (1 – ) NO2

The corresponding specific productions are:

dea = ( – ¾ ) / / 20.95% ∙ 22.414 mN3 kmol-1 = (106.99 / – 80.25) mN

3 kmol-1 =

= (6.282 / – 4.712) mN3 kg-1

dsfg = [( + ) / 2 – ( – ¾ ) ∙ (1 / 20.95% – 1) – ] / ∙ 22.414 mN3 kmol-1 =

= (74.64 – 95.78 / ) mN3 kmol-1 = (4.383 – 5.624 / ) mN

3 kg-1

wv = 3/2 ∙ mN3 kmol-1 = 33.621 mN

3 kmol-1 = 1.974 mN3 kg-1

solids = 0.000

Based on the enthalpies of formation for pure substances reported by McBride et al. (2002), the

LHV results:

LHV = [ ∙ 34.193 + ∙ -45.940 – ( + ) / 2 ∙ 0.000 – 3/2 ∙ -241.826 +

– ( – ¾ ) ∙ 0.000] / / 17.031 MJ kg-1 = (18.602 + 2.008 / ) MJ kg-1

6


Also this reaction is exothermic, since LHV > 0.

Average values of and for SNCR systems are respectively 2.2 and 0.65, whereas for modern

tail-end SCR systems these values are 1.1 - 1.2 (consider 1.15) and 0.8 - 0.9 (consider 0.85).

Therefore, mean values for the various properties referred to the two systems are reported in the

following Table.

dea dsfg wv solids LHVSubstance MU(a) mN

3 MU-1 mN3 MU-1 mN

3 MU-1 kg MU-1 MJ MU-1

Gaseous ammonia for de-NOX SCR and SNCR systems kg -4.712

+6.282 /+4.383

-5.624 / +1.974 0.000 +18.602+2.008 /

Gaseous ammonia for de-NOX SCR and SNCR systems (e.g. =2.2, =0.65) kg -2.856 +2.721 +1.974 0.000 +19.195

Gaseous ammonia for de-NOX SCR systems (e.g. =1.15, =0.85) kg -0.0683 +0.226 +1.974 0.000 +20.086

(a) MU = Measurement Unit.

A.6 Liquid water

Liquid water is often injected into flue gas either alone, for temperature control / cleaning purposes,

or together with other reactants (like ammonia for de-NOX systems).

The “reaction” that liquid water undergoes when injected into hot flue gas is simply evaporation.

Therefore, the specific productions of dea, dsfg and solids are all null, whereas for wv:

wv = / 18.016 mN3 kg-1 = 1.244 mN

3 kg-1

The corresponding LHV is equal to minus the enthalpy of evaporation of water at 25 °C (reference

temperature for the definition of LHVs). Such a value is well-known, and can be determined also as

the difference between the enthalpy of formation of water vapour and that of liquid water. The data

reported by McBride et al. (2002) give:

7


LHV = -2.4425 MJ kg-1

The evaporation of water is, as known, endothermic, as LHV < 0.

A.7 Water solution of ammonia for de-NOX SCR and SNCR systems

Most SCR and SNCR systems use a water solution of ammonia as reducing agent. Commercially, it

is available in different qualities, even if the most common are those with NH3 content by mass of

24.5% and 40.0%. The first being more common, since less hazardous.

From the point of view of the mass balance, the injection of ammonia solution is equivalent to the

separate injections of the corresponding water and ammonia. Therefore, the specific productions are

determined as weighted averages of those of liquid water and gaseous ammonia. With reference to

the most common 24.5%w NH3 solution:

dea = [(-4.712 + 6.282 / ) ∙ 24.5% + 0.000 ∙ (1.000 – 24.5%)] mN3 kg-1 =

= (-1.154 + 1.539 / ) mN3 kg-1

dsfg = [(4.383 – 5.624 / ) ∙ 24.5% + 0.000 ∙ (1.000 – 24.5%)] mN3 kg-1 =

= (1.074 – 1.378 / ) mN3 kg-1

wv = [1.974 ∙ 24.5% + 1.244 ∙ (1.000 – 24.5%)] mN3 kg-1 = 1.423 mN

3 kg-1

solids = [0.000 ∙ 24.5% + 0.000 ∙ (1.000 – 24.5%)] = 0.000

Since the solution is strongly non-ideal, the determination of the LHV requires taking into account

also the enthalpy of mixing. With reference to the results given by the excess free enthalpy method

reported by Mejbri and Bellagi (2006), the mixing enthalpy of the liquid solution of water-ammonia

8


can be expressed, at reference conditions (101,325 Pa and 298.15 K), as function of the ammonia

mass fraction by means of the polynomial form:

hmix = (173.05 x3 + 723,55 x2 – 897,37 x + 83.32) kJ kg-1

For the 24.5%w solution, the result is hmix = -729.05 kJ kg-1.

The enthalpy of evaporation of ammonia at 25 °C (reference temperature for the definition of

LHVs) can be determined as the difference between the enthalpy of formation of ammonia vapour

and that of liquid ammonia. The calculation by Refprop v. 9.0 (Lemmon et al., 2010) gives hev,NH3

= 1,165.2 kJ kg-1.

With these data and the LHVs of gaseous ammonia and liquid water, the heat released by the

reaction of 1 kg (i.e. the LHV) of the 24.5%w water-ammonia solution can be determined as:

LHV = 24.5% ∙ (LHVNH3(g) – hev,NH3) + (1.000 – 24.5%) ∙ LHVH2O(L) + hmix =

= [24.5% ∙ (18.602 + 2.008 / – 1.1652) + (1.000 – 24.5%) ∙ -2.4452 + -0.7291] MJ kg-1 =

= (1.699 + 0.492 / ) MJ kg-1

Also the reaction of this substance is exothermic, since LHV > 0.

With the same values of and previously considered, which are representative of SNCR

(respectively 2.2 and 0.65) and SCR systems (respectively 1.15 and 0.85), the mean values for the

various properties are reported in the following Table.

Dea dsfg Wv solids LHV

9


Substance MU(a) mN3 MU-1 mN

3 MU-1 mN3 MU-1 Mg MU-1 MJ MU-1

Liquid water-ammonia (24.5%w) for de-NOX SCR and SNCR systems kg -1.154

+1.539 /1.074

-1.378 / +1.423 0.000 +1.699+0.492 /

Liquid water-ammonia (24.5%w) for de-NOX systems (e.g. =2.2, =0.65) kg -0.700 +0.667 +1.423 0.000 +1.844

Liquid water-ammonia (24.5%w) for de-NOX systems (e.g. =1.15, =0.85) kg -0.0167 +0.0553 +1.423 0.000 +2.062


A.8 Solid urea for de-NOX SNCR systems

Some SNCR systems use solid urea as reducing agent. It is injected under the form of small grains

in the post-combustion zone of the boiler, by means of a pneumatic system.

From the point of view of mass and energy balance, the overall reaction of NOX abatement can be

considered as composed of two steps. In the first one, the urea reacts with water vapour to form a

gaseous ammonia reactant. In the second step, the reaction of gaseous ammonia takes place in

accordance with the specifications given in A.5.

Therefore, the first reaction step is:

(NH2)2CO + H2O → 2 NH3 + CO2

The contributions of this first reaction to the overall specific productions are:

NH3_prod = 2 ∙ (17.031 kg kmol-1) / (60.056 kg kmol-1) = 0.567 kg kg-1

dea = 0.000

dsfg = 1 ∙ 22.414 mN3 kmol-1 / 60.056 kg kmol-1 = 0.3731 mN

3 kg-1

wv = -1 ∙ 22.414 mN3 kmol-1 / 60.056 kg kmol-1 = -0.3731 mN

3 kg-1

10


solids = 0.000

According to Koebel and Strurz (2003), the considered reaction is endothermic, hence it gives a

contribution to the overall LHV (equal to minus the heat of reaction at standard conditions) of -86.9

MJ per kmol of urea. In mass basis, it results:

LHV = -86.9 MJ kmol-1 / 60.056 kg kmol-1 = -1.447 MJ kg-1

The overall NOX abatement reaction features the following specific productions and LHV:

dea = deaNH3 ∙ NH3_prod + dea = [(6.282 / – 4.712) ∙ 0.567 + 0.000] mN3 kg-1 =

= (3.563 / – 2.672) mN3 kg-1

dsfg = dsfgNH3 ∙ NH3_prod + dsfg = [(4.383 – 5.624 / ) ∙ 0.567 + 0.373] mN3 kg-1 =

= (2.859 – 3.190 / ) mN3 kg-1

wv = wvNH3 ∙ NH3_prod + wv = [1.974 ∙ 0.567 – 0.373] mN3 kg-1 = 0.746 mN

3 kg-1

solids = solidsNH3 ∙ NH3_prod + solids = [0.000 ∙ 0.567 + 0.000] = 0.000

LHV = LHVNH3 ∙ NH3_prod + LHV = [(18.602 + 2.008 / ) ∙ 0.567 – 1.447] MJ kg-1 =

= (9.103 + 1.139 / ) MJ kg-1

The overall reaction is exothermic, since LHV > 0.

A.9 Liquid water-urea solution for de-NOX SNCR systems

11


Urea, as a reducing agent for de-NOX SNCR systems, is more commonly used in liquid water

solution. Solutions with different urea concentration are commercially available, even if the most

common one, typically used in WtE plants, features 45% by mass urea concentration.

The specific productions can be determined as weighted averages of those referred to solid urea and

liquid water. Therefore:

dea = [(3.563 / – 2.672) ∙ 45.0% + 0.000 ∙ (1.000 – 45.0%)] mN3 kg-1 =

= (1.603 / – 1.203) mN3 kg-1

dsfg = [(2.859 – 3.190 / ) ∙ 45.0% + 0.000 ∙ (1.000 – 45.0%)] mN3 kg-1 =

= (1.286 – 1.435 / ) mN3 kg-1

wv = [1.974 ∙ 45.0% + 1.244 ∙ (1.000 – 45.0%)] mN3 kg-1 = 1.020 mN

3 kg-1

solids = [0.000 ∙ 45.0% + 0.000 ∙ (1.000 – 45.0%)] = 0.000

Since the solution is strongly non-ideal, the determination of the LHV requires taking into account

also the enthalpy of mixing. Koebel and Strutz (2003) report the following expression for the

integral enthalpy of mixing of solid urea into water at reference conditions:

hmix = (15.305 – 3.512∙10-1 m + 2.320∙10-2 m2 – 1.008∙10-3 m3 + 1.880∙10-5 m4) MJ kmol-1

Where m is the molality of the solution, i.e. mols of urea per kilogram of water.

For the 45.0%w solution, the molality is 13.624 and the corresponding hmix = 15.305 MJ per

kilomole of urea in the solution. hmix > 0 means that the mixing is endothermic, hence it gives a

positive contribution to LHV (during the reaction, the energy that has been stored will be released).

12


With this datum and the LHVs of solid urea and liquid water, the heat released by the reaction of 1

kg (i.e. the LHV) of the 45.0%w water-urea solution can be determined as:

LHV = 45.0% ∙ LHV(NH2)2CO(s) + (1.000 – 45.0%) ∙ (LHVH2O(L) + hmix ∙ 13.624/1000 kmol kg-1) =

= [45.0% ∙ (9.103 + 1.139 / ) + (1.000 – 45.0%) ∙ (-2.4452 + 15.315 ∙ 13.624/1000)] MJ kg-1 =

= (2.868 + 0.512 / ) MJ kg-1

Also the reaction of this substance is exothermic, since LHV > 0.

A.10 Sodium bicarbonate in dry scrubbing systems

Finely powdered sodium bicarbonate is injected at low temperature in FGTS to neutralize and abate

acid gases. As for the case of high temperature abatement, HCl, the predominant acid species, can

be considered representative of all acid compounds. The corresponding overall reaction, which

considers also the presence of excess reactant and the conversion from bicarbonate to carbonate, is:

HCl + NaHCO3 → NaCl + ( – 1) / 2 Na2CO3 + ( + 1) / 2 H2O + ( + 1) / 2 CO2

By assuming a mean purity of the reactant of 98.5% (being the complemental 1.5% inert solid

species), the corresponding specific productions are:

dea = 0.000

dsfg = (– 1) / 2 / ∙ 22.414 mN3 kmol-1 / (84.007 kg kmol-1) ∙ 98.5% =

= (0.1314 – 0.1314 / ) mN3 kg-1

wv = (+ 1) / 2 / ∙ mN3 kmol-1 / (84.007 kg kmol-1) ∙ 98.5% =

13


= (0.1314 + 0.1314 / ) mN3 kg-1

solids = [58.442 + (– 1) / 2 ∙ 105.989] / kg kmol-1 / (84.007 kg kmol-1) ∙ 98.5% + 1.5% =

= (0.6364 + 1.307 / ) kg kg-1

Based on the enthalpies of formation for pure substances reported by McBride et al. (2002), the

LHV results:

LHV = [-92.310 + ∙ -950.850 – -411.260 – ( – 1) / 2 ∙ -1,129.190 – ( + 1) / 2 ∙ -241.826 +

– ( + 1) / 2 ∙ -393.510] / / 84.007 ∙ 98.5% MJ kg-1 = (0.844 / – 0.804) MJ kg-1

Since > 1, this reaction is endothermic, with LHV < 0.

Typical values of adopted in these systems range between 1.2 and 1.4 (consider 1.3). Therefore,

the following table summarizes all the properties.


3 MU-1 mN3 MU-1 mN


Sodium bicarbonate in dry scrubbing systems kg 0.000 +0.1314

-0.1314/+0.1314

+0.1314/+0.6364-1.307/

-0.804+0.844/

Sodium bicarbonate in dry scrubbing systems (typical values for = 1.3) kg 0.000 +0.0303 +0.232 +0.686 -0.155


A.11 Calcium hydroxide in dry scrubbing systems

Finely powdered calcium hydroxide is injected at low temperature in FGTS to neutralize and abate

acid gases.

As for previous acid gas abatement processes, HCl, the predominant acid species, can be considered

representative of all acid compounds. The corresponding overall reaction, considering CaCl2 as the

final product, is:

14


2 HCl + Ca(OH)2 → CaCl2 + 2 H2O

However, several studies (Bodénan and Deniard, 2003, Partanen et al., 2005) have shown that the

largely predominant product of this type of processes is calcium hydroxychloride (CaOHCl) rather

than calcium chloride (CaCl2). In the light of this evidence, a more representative overall reaction,

which takes into account also the excess of reactant, is:

2 HCl + Ca(OH)2 → 2 CaOHCl + ( – 2) Ca(OH)2 + 2 H2O

Such a reaction can actually take place with dosage ≥ 2. This condition is usually verified, since

these systems normally work with 3 – 4 times the reactant required by the complete reaction, which

is commonly considered the reference for defining the dosage factor . In addition to the large

excess of fresh reactant, these systems normally adopt the recirculation of a significant fraction of

the abatement products, since they still contain unreacted calcium hydroxide.

By assuming a mean purity of the reactant of 96% (being the complemental 4% inert solid species),

the corresponding specific productions are:

dea = 0.000

dsfg = -2 / ∙ 22.414 mN3 kmol-1 / (74.093 kg kmol-1) ∙ 96% =

= -0.581 / mN3 kg-1

wv = 2 / ∙ 22.414 mN3 kmol-1 / (74.093 kg kmol-1) ∙ 96% =

= 0.581 / mN3 kg-1

15


solids = [2 ∙ 92.538 + (–2) ∙ 74.093] / kg kmol-1 / (74.093 kg kmol-1) ∙ 96% + 4% =

= (1.000 + 0.478 / ) kg kg-1

By considering the enthalpies of formation for the various pure species reported by McBride et al.

(2002), except for that of CaOHCl, which is reported by Partanen et al. (2005), the LHV results:

LHV = (2 ∙ -92.31 + 2 ∙ -985.90 – 2 ∙ -540.10 – 2 ∙ -241.826) / / 74.093 ∙ 96% MJ kg-1 =

= -7.678 / MJ kg-1

This reaction is endothermic, since LHV < 0.

With the average value of = 3.5, representative of most of these systems, the values of the various

properties are reported in the following Table.


3 MU-1 mN3 MU-1 mN


Calcium hydroxide in dry scrubbing systems kg 0.000 -0.581/ +0.581/ +1.000

+0.478/ -7.678/

Calcium hydroxide in dry scrubbing systems (e.g. = 3.5) kg 0.000 -0.1660 +0.1660 +1.137 -2.194


A.12 Milk of lime in semi-dry scrubbing systems

Finely powdered calcium hydroxide is often dissolved into water to produce the milk of lime, a

liquid reactant used in semi-dry acid gases abatement processes. It is an oversaturated solution of

calcium hydroxide in water, since the solubility is very limited. Even if more a mixture than a

solution, the fine particles of solid remain suspended for a long time, even without agitation. It is

also called more properly “lime slurry”.

This reactant is injected in a device usually called spray-drier, where the water content of the

solution evaporates completely leaving in the flue gas stream the products of acid gas

16


neutralization, as well as the excess of reactant, under the form of finely dispersed solids.

The presence of the liquid droplets enhances the reaction speed and the abatement requires a

reduced excess of reactant with respect to the dry process.

The reaction of the milk of lime leads to the same reaction previously considered for the powdered

calcium hydroxide. Therefore, from the point of view of the mass balance, the specific productions

can be determined as weighted averages of those pertaining to solid calcium hydroxide and liquid

water.

The milk of lime composition varies from plant to plant, even if a very common recipe is to add

0.15 kg of calcium hydroxide to 1 kg of water, producing a slurry with a content of lime of 13.043%

by mass. The corresponding specific productions are:

dea = [13.043% ∙ 0.000 + (1.000 – 13.043%) ∙ 0.000] = 0.000

dsfg = [13.043% ∙ -0.5808 / + (1.000 – 13.043%) ∙ 0.000] mN3 kg-1 =

= -0.0758 / mN3 kg-1

wv = [13.043% ∙ 0.5808 / + (1.000 – 13.043%) ∙ 1.244] mN3 kg-1 =

= (1.082 + 0.0758 / ) mN3 kg-1

solids = [13.043% ∙ (1.000 + 0.478 / ) + (1.000 – 13.043%) ∙ 0.000] =

= (0.1304 + 0.0623 / ) kg kg-1

Since the milk of lime is not a solution, but a suspension of solid calcium hydroxide in water, the

enthalpy of mixing even if present is very limited and can be neglected. Therefore, also the LHV of

the mixture is determined as weighted average of the LHVs of solid calcium hydroxide and water:

17


LHV = [13.043% ∙ -7.678 / + (1.000 – 13.043%) ∙ -2.4425] MJ kg-1 =

= (-2.124 – 1.001 / ) MJ kg-1

Also this reaction is endothermic, since LHV < 0.

Typical values of used in these systems lie in the range 2.2 – 2.6. The following Table reports the

properties corresponding to = 2.4.


3 MU-1 mN3 MU-1 mN


Milk of lime(b) in semi-dry scrubbing systems kg 0.000 -0.0758/ +1.082

+0.0758/+0.130

+0.0623/-2.124

-1.001/Milk of lime(b) in semi-dry scrubbing systems (e.g. = 2.4) kg 0.000 -0.0316 +1.113 +0.156 -2.541

(a) MU = Measurement Unit. (b) 0.15 kg of calcium hydroxide in 1 kg of water.

A.13 Wet scrubbing systems

In wet scrubbing systems, the main effect to be considered from the point of view of mass and

energy balances is the evaporation / condensation of possible large amounts of water. Normally, the

effect of chemical reactions due to the presence of reactants is only marginal.

To consider properly the effects of these systems, a number of operating data is required: inlet and

outlet temperature of flue gas; amount of water added / removed; production of sludge and its

content of moisture.

The definition of the addition / subtraction of flue gas components typically requires drawing mass

and energy balances on the piece of equipment involved. Therefore, it is a case specific operation.

A.14 Unburnt C in bottom and fly ashes

The unburnt carbon (“C”) that ends up into bottom and fly ashes influences the mass and energy

balances of the combustor / boiler system, since the production of unburnt C brings along some

reduction in flue gas production and air consumption, as well as the production of additional solid

18


residues.

If the unburnt C is considered as an output flow of the control volume (as it is natural), the overall

reaction the describes the process is:

C(s) + O2 → CO2

Therefore, the reaction is associated with a production of DSFG and consumptions (i.e. a negative

specific productions) of DEA and Solids:

dea = (-1 / 20.95% ∙ 22.414 / 12.011) mN3 kg-1 = -8.908 mN

3 kg-1

dsfg = [1.000 + (1.000 / 20.95% – 1.000)] ∙ 22.414 / 12.011 mN3 kg-1 =

= 8.908 / mN3 kg-1

wv = 0.000

solids = -1.000 kg kg-1

For the purpose of determining the LHV of the solid unburnt C in bottom and fly ashes, it can be

considered as graphite, in accordance with the EN 12952-15 norm. Therefore, it results:

LHV = 33.000 MJ kg-1

The reaction is, of course, exothermic (LHV > 0).

A.15 References

19


Biganzoli, L., Racanella, G., Marras, R., Grosso, M., 2015. High temperature abatement of acid

gases from waste incineration. Part I: Experimental tests on full scale plants. Waste Management

36, 98-105.

Bodénan, F. and Deniard, Ph., 2003. Characterization of flue gas cleaning residues from European

solid waste incinerators: assessment of various Ca-based sorbent processes. Chemosphere 51, 335–

347.

CEN (Comité Européen de Normalisation), 2003. EN 12952-15: Water-tube boilers and auxiliary

installations - Part 15: Acceptance tests.

Koebel, M. and Strutz, E. O., 2003. Thermal and Hydrolytic Decomposition of Urea for

Automotive Selective Catalytic Reduction Systems: Thermochemical and Practical Aspects. Ind.

Eng. Chem. Res. 42, 2093–2100.

Jungbluth, N., 2012. Characteristics of the Products. English version of the Ecoinvent report No. 6-

IV “Erdöl”, Chapter 3. Data v2.0 (2007). Translation Peter, F.. Villigen & Duebendorf, June 2012.

Lemmon E. W., Huber, M. L., McLinden, M. O., 2010. REFPROP: Reference Fluid

Thermodynamic and Transport Properties. NIST Standard Reference Database 23, Version 9.0.

McBride, B. J., Zehe, M. J., Gordon, S., 2002. NASA Glenn Coefficients for Calculating

Thermodynamic Properties of Individual Species. NASA/TP—2002-211556, September 2002.

Mejbri Kh., Bellagi A., 2006. Modelling of the thermodynamic properties of the water–ammonia

mixture by three different approaches. International Journal of Refrigeration 29, 211–218.

Partanen, J., Backman, P., Backman, R., Hupa, M., 2005. Absorption of HCl by limestone in hot

flue gases. Part II: importance of calcium hydroxychloride. Fuel 84, 1674–1684.

APPENDIX B: Sources of the data in Figure 4

Data in Figure 4, i.e. dry air consumption in normal cubic meter per kilogram of combustible matter

vs. Dry Stoichiometric Flue Gas (DSFG) production in normal cubic meter per kilogram of

20


combustible matter, are calculated based on the elemental compositions reported by the following

sources:

Ref. 1 Yin, C. Y., 2011. Prediction of higher heating values of biomass from proximate and

ultimate analyses. Fuel 90, 1128-1132.

Ref. 2 Komilis, D., Evangelou, A., Giannakis, G., Lymperis, C., 2012. Revisiting the elemental

composition and the calorific value of the organic fraction of municipal solid wastes.

Waste Management 32, 372-381.

Ref. 3 Walters, R. N., Hackett, S. M., Lyon, R. E., 2000. Heats of Combustion of High

Temperature Polymers. Fire and Materials 24, 245-252.

Ref. 4 Niessen, W.R., 2002. Combustion and Incineration Processes: Applications in

Environmental Engineering, Third Edition. CRC Press.

Ref. 5 Ahmad, M., and Subawi, H., 2013. New Van Krevelen diagram and its correlation with

the heating value of biomass. Research Journal of Agriculture and Environmental

Management 2, 295-301.

Ref. 6 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 22, 319-329.

Ref. 7 Otero, M., Gómez, X., García, A.I., Morán, A., 2007. Effects of sewage sludge blending

on the coal combustion: A thermogravimetric assessment. Chemosphere 69, 1740-1750.

Ref. 8 Folgueras, M.B., Díaz, R.M., Xiberta, J., Prieto, I., 2003. Thermogravimetric analysis of

the co-combustion of coal and sewage sludge. Fuel 82, 2051-2055.

Ref. 9 United States. Department of Energy. National Renewable Energy Laboratory (U.S.).

Office of Scientific and Technical Information, 1986. Thermodynamic Data for Biomass

Conversion and Waste Incineration. United States. Department of Energy.

Ref. 10 Parikh, J., Channiwala, S.A., Ghosal, G.K., 2007. A correlation for calculating elemental

composition from proximate analysis of biomass materials. Fuel 86, 1710-1719.

Ref. 11 Zhou, H., Long, Y. Q., Meng, A. H., Li, Q. H., Zhang, Y. G., 2015. Classification of

21


municipal solid waste components for thermal conversion in waste-to-energy research.

Fuel 145, 151-157.

Each source reports data regarding different types of materials, which are indicated in the graph

with different colours depending on fourteen categories (see the legend of colours in the graph).

APPENDIX C: Using hourly averaged values for calculations

As stated in the article, the calculations to evaluate the amount of “false air” which enters the FGTS

must be carried out at least on hourly basis. This is to avoid introducing excessive errors due to the

averaging processes involved.

An example showing the situation is the dependence of the amount of DEA on either the flue gas

flowrate or the oxygen concentration (Eq. 4), which is nearly quadratic because of the behaviour of

the combustion control systems adopted in WtE plants. In fact, when oxygen content is above its

setpoint the control system acts to reduce it by decreasing the excess air and, consequently, the flue

gas flowrate. The inverse situation takes place when the oxygen concentration is below its setpoint.

Therefore, the average of the product between flue gas flowrate and oxygen concentration is

different from the corresponding product of the two averages.

From the theoretical point of view, it is rather easy to demonstrate that the average of a squared

variable is always greater than the square of its average value: it is the relationship between

variance and first and second momentum of a statistical distribution. With a common statistical

notation, it reads:

σ 2=1n∑i

( x i−x )2=1n∑i

xi2+ 1

n∑ix2−2

n∑ix x i=

1n∑i

x i2−x2

⇒ 1n∑i

x i2=x2+σ 2

22


Therefore, as the averaging period gets longer, more and more contributions to the variance is lost,

leading to a systematic underestimation of the DEA in flue gas.

The typical situation of WtE plant is depicted in the following graph, where the systematic errors in

evaluating the three gaseous components of flue gas are reported vs. the duration of the averaging

period. The starting point was the averaged values on one-minute basis.

As it can easily be seen, with one-hour averaging time, the maximum systematic error in this graph

is of the order of 0.5%. However, larger errors can be found. The largest error found over short

periods (few weeks) for the plant so far analysed was in between 3 and 4%. Therefore, on annual

basis the hourly averaged values warrant limiting the error to a maximum of 4%.

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

5 min 10 min 20 min 30 min 1 h 2 h 4 h 8 h 16 h 24 h

Syst

emati

c err

or, %

WV DEA DSFG

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