19

CHAPTER 3

Sulfur Burning

70% of sulfuric acid is made from elemental sulfur. The elemental sulfur is:

(a) received molten or melted with pressurized steam (sulfur melting point--390 K)
(b) atomized in a hot (1400 K) furnace
(c) burnt in the fiLrnace with excess dry air to form hot SO2, 02, N2 gas.

Sulfuric acid is then made from step (c)'s gas by:

(d) cooling the gas in a boiler and steam superheater
(e) catalytically reacting its SO2(g) and O2(g) to form SO3(g)
(f) contacting step (e)'s product gas with strong sulfuric acid to make H2SO4 by the
reaction SO3(g) + H 2 0 ( e ) i n acid --9, H2SO4(e)i n strengthened acid.
Steps (b) to (0 are cominuous.

This chapter describes steps (a) to (d), Fig. 3.1. Steps (e) and (f) are described in
Chapters 7, 8 and 9.

3.1 Objectives

The objectives of this chapter are to describe:

(a) the physical and chemical properties of elemental sulfur

(b) transportation of elemental sulfur to the sulfur burning plant

(c) preparation of elemental sulfur for combustion

(d) sulfur burners and sulfur burning furnaces

(e) control of sulfur burning offgas composition, temperature and volume.

20

molten sulfur (410 K) delivered

molten or delivered solid and
steam-melted on site

steam
,A
sulfur burning
furnace iI
.. ~4-,~,';ii':"1400 K i : ~ : -:
clean, dry ,/3r J ."::::: /l~oiler & steam
air, 390 K, superheater
1.4 bar

11 volume% SO2, 10 volume% O2,

79 volume% N2 gas (700 K) to catalytic
SO2 oxidation and H2SO4 making

Fig. 3.1. Sulfur buming flowsheet - molten sulfur to clean dry 700 K SO2, 02, N2 gas. The
fumace is supplied with excess air to provide the 02 needed for subsequent catalytic oxidation of
SO2, to SO3. Table 3.1 gives industrial sulfur burning data.

3.2 Sulfur

The elemental sulfur used for making sulfuric acid is virtually all a byproduct of natural
gas and petroleum refining. It contains 99.9+% S. Its main impurity is carbon from
natural gas or petroleum.

Its melting point is 388 - 393 K, depending on its crystal structure. It is easily melted
with pressurized steam pipes.

3.2.1 Viscosity

The viscosity of molten sulfur is described in Fig. 3.2. Its key features are a viscosity
minimum at 430 K and a ten thousand-fold viscosity increase just above 430 K.

Sulfur burners are fed with---410 K molten sulfur, near the viscosity minimum but
safely below the steep viscosity increase. Sulfur temperature is maintained by
circulating 420 K steam through sulfur storage tank steam pipes just ahead of sulfur
burning. Below ground or insulated above ground storage tanks are used.

Sulfur's huge increase in viscosity just above 430 K is due to a transition from $8 ring
molecules to long interwoven S chain molecules (Dunlavy, 1998).

3.3 Molten Sulfur Delivery

Elemental sulfur is produced molten. It is also burnt molten.

 21

100.000

10.000

~E 1.000
==

~o 0.100

0.010

0.001 , , t

360 400 440 480 520

Temperature, K

Fig. 3.2. Molten sulfur viscosity as a function of temperature (Tuller, 1954). The viscosity
minimum at 430 K and the enormous viscosity increase just above 430 K are notable.

Where possible, therefore, sulfur is transported molten from sulfur making to sulfur
burning. It is mainly shipped in double walled, steam heatable barges and railway tank
cars. This gives easy handling at both ends of the journey. Even if the sulfur solidifies
during the journey, it is easily melted out with 420 K steam to give a clean, atomizable
raw material. Short distance deliveries are sometimes made in single walled tanker
trucks.

Sulfur that is shipped this way is ready for burning. Sulfur that is shipped as solidified
pellets or flakes picks up dirt during shipping and storage. This sulfur is melted and
filtered before being burnt (Sander et. al., 1984, p 174, Sparkler, 2004).

Sulfur is shipped solid when there are several intermediate unloading-loading steps
during its journey, e.g. train-ship-train. An example of this is shipment of solid sulfur
from interior Canada to interior Australia.

3.3.1 Sulfur pumps and pipes

Molten sulfur has a viscosity (-0.01 kg m 1 s1, 400-420 K, Fig. 3.2) about ten times that
of water (-0.001 kg m -~ sl, 293 K). Its density i s - 1 . 8 kg/m 3. It is easily moved in
steam jacketed steel pipes (Jondle and Hornbaker, 2004). Steam heated pumps much
like that in Fig. 9.2 are used. Molten sulfur is an excellent lubricant at 410 K. Sulfur
pump impellers need no additional lubrication.
22

3.4 Sulfur Atomizers and Sulfur Burning Furnaces

Sulfur burning consists of."

(a) atomizing molten sulfur and spraying the droplets into a hot furnace, Fig. 3.3
(b) blowing clean, dry 390 K air into the furnace.
The tiny droplets and warm air give:
(c) rapid vaporization of sulfur in the hot furnace
(d) rapid and complete oxidation of the sulfur vapor by 02 in the air.
Representative reactions are"

boiling point,
718K
S(Q --~ S(g) (3.1)

S(g) + O2(g) -+ SO2(g) + heat (3.2).

in air in SO2,02, N2 gas

The combined heat of reaction for Reactions (3.1) and (3.2) is --- -300 MJ per kg-mole of
s(t).

Fig. 3.3. Burner end of sulfur burning furnace. Atomized molten sulfur droplets are injected into
the furnace through steam-cooled lances. Dry combustion air is blown in through the circular
openings behind. The sulfur is oxidized to SO2 by Reactions (3. l) and (3.2). Atomization is done
by spiral or fight angle flow just inside the burner tip.
 23

3.4.1 Sulfur atomizers

Molten sulfur spraying is done with:

(a) a stationary spray nozzle at the end of a horizontal lance, Fig. 3.3

(b) a spinning cup sulfur atomizer, Fig. 3.0 (Outokumpu, 2005)

In both cases, molten sulfur is pumped into the atomizers by steam jacketed pumps.

The stationary spray nozzle has the advantage of simplicity and no moving parts. The
spinning cup atomizer has the advantage of lower input pressure, smaller droplets, more
flexible downturn and a shorter furnace.

Fig. 3.4. Entrance to fire tube boiler tubes after Fig. 3.3's sulfur burning furnace. 1400 K gas (-11
volume% SO2, 10 volume% O2, 79 volume% N2) leaves the furnace and enters the boiler. It turns 90~ in
the boiler and flows into the tubes. The tubes are surrounded by water. Heat is transferred from the hot
gas to the water - cooling the gas and making (useful) steam. The tubes are typically 0.05 m diameter.
Table 3.1 gives industrial furnace data. Sulfur furnace boilers are discussed by Roensch (2005).

3.4.2 Dried air supply

Air for sulfur buming is filtered through fabric and dried. It is then blown into the
sulfur burning fumace. It is blown in behind the sulfur spray to maximize droplet-air
contact.

The drying is done by contacting the air with strong sulfuric acid, Chapter 6. This
removes H20(g) down to --0.05 grams per Nm 3 of air. Drying to this level prevents
accidental HzSO4(g) formation and corrosion after catalytic SO3(g) production.
24

Table 3.1. Details of 3

Plant S1
startup date
acid production, tonnes H2SO4/day 4400

Sulfur
source imported pastel
impurities, parts per million by mass
carbon
inorganic oxides
other
sulfur filtration method three 35 leaf pressure
leach filters, 2 online

Sulfur burning furnace data

number of furnaces 1
shell length x diameter, m 18.36 x 6.0
refractory types 0.23 m HB fire brick &
0.115 m insulting brick
sulfur bumers
spinning cup or spray guns spray guns
number of burners per furnace 7
sulfur burning rate, tonnes/hour 60.1
temperatures, K
dry air into furnace 416
molten sulfur into furnace 405
gas out of furnace 1445

Boiler
type fire tube
number 2
length x diameter, m 7.85 • 3.505 (each)
number of tubes 1550 each
tube diameter, m 0.046 ID
tube material SA-178-A
number of superheaters 2
number of economizers 3
gas temperatures, K
into boiler 1444
out of economizer 696
steam production, 3.88
tonnes of steam per tonne of sulfur
pressure, bar 63.8
temperature, K 554 (753 aider super
heaters)

Product gas
flowrate, thousand Nm3/hour 356
composition, volume%
SO3 0.184
SO2 11.6
02 9.06
N2 79.1
 25

sulfur buming fumace operations.

M1 $2
1974 1965
270 1800
including from molybdenum
sulfide roaster gas
liquid
< 1000
<200

none none

1 1
10.4 x 3.2 18.3 x 5.5 inside shell
0.23 m hard (high T) brick
0.11 m insulating brick

spray gun spray guns

1 4
2.0 max 24.7

714 363
408 411
870-1260 1362

fire tube fire tube

1 2
6.4 • 5 x 3.5; 2.9 x 2.7
600 1860; 1210
0.05 0.05
carbon steel carbon steel
0 1
0 1

870-1280 1362
810-950 700
1.64

17 32
517

50 210

4.7-5.6 8.38
13.6 12.6
79.02
26

3.4.3 Main blower

The dried air is blown into the sulfur burning furnace by the acid plant's main blower.

The blower is a steam or electricity driven centrifugal blower (Jacoby, 2004). It blows
air into the sulfur burning furnace- and the furnace's offgas through the remainder of
the acid plant. 0.3 to 0.5 bar pressure is required.

A 2000 tonnes of H2S04 per day sulfur burning acid plant typically requires a 4000 to
4500 kW main blower.

3.4.4 Furnace

Sulfur burning furnaces are 2 cm thick cylindrical steel shells lined internally with 30 to
40 cm of insulating refractory, Fig. 3.3. Air and atomized molten sulfur enter at one
end. Hot SO2, O2, N2 gas departs the other into a boiler and steam superheater (Fig.
3.4). Some furnaces are provided with internal baffles. The baffles create a tortuous
path for the sulfur and air, promoting complete sulfur combustion. Complete sulfur
combustion is essential to prevent elemental sulfur condensation in downstream
equipment.

3.5 Product Gas

Sulfur burning is operated to produce 1400 K gas containing:

-~11 volume% $02

-10 volume% 02
79 volume% N2

This product has enough SO 2 and a high enough 02/S02 ratio for subsequent catalytic
SO2 + 89 ~ SO3 oxidation. It is also cool enough for its heat to be recovered as
steam in a simple fire-tube boiler (Thermal Ceramics, 2005) and steam superheater.

The gas contains only 0.1 or 0.2 volume% SO3 despite its high 02 content. This is
because the equilibrium constant for SO2 + 89 ~ SO3 oxidation is small (0.06) at
1400 K, Fig. 7.3.

3.5.1 Gas destination

Product gas departs the sulfur burning furnace/boiler/superheater into:

(a) a catalytic SO2 oxidation 'converter'

then to:

(b) SO3(g ) +H2O(~)in sulfuricacid ~ H2SO4(g)in strengthenedacid acidmaking.

 27

The boiler and superheater cool the gas to ,--700 K, the usual temperature for catalytic
SO2 oxidation. They also produce steam for the acid plant main blower and for making
electricity.

3.5.2 Composition and temperature control

The composition and temperature of sulfur burning's product gas are controlled by
adjusting the sulfur burning furnace's:

input air
input sulfur

ratio, Figs. 3.5 and 3.6.

As the figures show, raising a furnace's air/sulfur ratio:

(a) increases product gas 0 2 concentration, Fig. 3.5

(b) decreases product gas SO 2 concentration, Fig. 3.5

(c) decreases product gas temperature, Fig. 3.6.

These relationships allow simple automatic control of product gas composition and
temperature. Note, however, that composition and temperature are not independent
variables.

Replacement of some of the sulfur burner's input air with oxygen can be used to give
independent temperature control (Miller and Parekh, 2004). Raising the oxygen/air
ratio increases offgas temperature because less N2 has to be heated by the S(g) + O2
SO2 reaction. Lowering the oxygen/air ratio has the opposite effect.

3.5.3 Target gas composition

The Section 3.5 gas (11 volume% SO2, 10 volume% 02, 79 volume% N2) is chosen to
give efficient downstream catalytic SO/+ 89 --~ SO3 oxidation. A requirement for
this is a volume% O2/volume% SO2 ratio around one.

In recent years there has been a tendency to increase volume% SO2 in sulfur burning
gas by lowering the input air/sulfur ratio, Fig. 3.5. An increase in SO2 concentration
lowers the volume of gas that must be blown through the acid plant per tonne of product
HzSO4. It thereby lowers:

(a) blowing energy cost

(b) equipment size requirements, hence capital cost.

28

r 12
.m 0r )

dg
r

O. =
_
cO E
ID 10
E~
>

., I i .,
#
7 8 9 10
Sulfur burning furnace 'input mass air/input mass sulfur' ratio

Fig. 3.5. Volume% SO2 and 02 in gas produced by burning S with excess dry air (calculated by
means of S, O and N molar balances). N 2 concentration is 79 volume% at all ratios, not shown.
This is because consumption of one kg-mole of 02 produces one kg-mole of SO2. (# For
example, 7 kg of input air for every 1 kg of input sulfur.)

Unfortunately, decreasing the input air/input sulfur ratio also decreases the O 2 / S O 2 ratio
of the gas (Fig. 3.5), potentially lowering catalytic oxidation efficiency.

An alternative way to increase S O 2 concentration (and decrease furnace exit gas

volume) is to feed less N2 to the sulfur burning furnace - by replacing some air with
oxygen (Miller and Parekh, 2004).

3.5.4 Target gas temperature

Decreasing sulfur burning's air/sulfur ratio raises product gas temperature, Fig. 3.6. If
carried too far (i.e. to raise % SO2-in-gas), this may damage the sulfur burning furnace
or boiler.

11-12 volume% 802, 1400 K sulfur burning gas seems optimum.

3.6 Summary

70% of the world's sulfuric acid is made from elemental sulfur. Virtually all of this
sulfur is the byproduct of natural gas and petroleum refining.

Elemental sulfur melts at---390 K. It is easily melted with pressurized steam pipes and
pumped molten around the sulfur burning plant.
 29

1600
410 K input liquid sulfur I
390 K input dried air
I
x 1500

e-
l...

..Q

= 1400

~I,_ 1300

1200 #
I

7 8 9 10
Sulfur burning furnace 'input mass air/input mass sulfur' ratio

Fig. 3.6. Temperature of offgas from burning sulfur with excess air (calculated by means of S,
O, N and enthalpy balances). Offgas temperature is decreased by raising input air/input sulfur
ratio. This is because (i) excess air in offgas increases with an increasing input air/input sulfur
ratio and because (ii) this excess air absorbs sulfur oxidation heat. (# For example, 7 kg of
input air for every 1 kg of input sulfur.)

Sulfur burning is the first step in making sulfuric acid from elemental sulfur. It entails:

(a) atomizing molten sulfur in a hot furnace and burning it with excess dried air

(b) cooling the product gas in a boiler and steam superheater.

The product is--11 volume% SO2, 10 volume% 02, 79 volume% N2 gas (700 K),
perfect for subsequent catalytic SO2 + V202 ~ SO3(g) oxidation and H2S04
manufacture.

Sulfur burning's product gas composition and temperature are readily controlled by
adjusting the sulfur furnace's input air/input sulfur ratio. Replacement of some of the
input air with oxygen gives the process independent 02/S02, temperature and volume
control.

References

Dunlavy, D. (1998) An animated view of the polymerization of sulfur.

www.molecules, org/experiments/Dunlavy/Dunlavy.html
30

Jacoby, K. (2004) Main blowers in acid plants. Preprint of paper presented at Sulphur 2004
conference, Barcelona, October 27, 2004, 249 260. www.agkkk.de

Jondle, J. and Hombaker, D. (2004) Handling molten sulphur in refineries. Sulfur, 292, May-
June, 2004, 43 47. www.britishsulphur.com

Miller, D. and Parekh, U. (2004) Upgrading virgin acid plants using oxygen. Sulfur, 290,
January-February, 2004, 43 47. www.britishsulphur.com

Outokumpu (2004) Sulphuric Acid Plants (Sulphur Combustion Section). Brochure distributed at
Sulphur 2004 conference, Barcelona, October 24-27, 2004. www.outokumpu.com

Outokumpu (2005) Latest Developments in Sulfur Burning Sulfuric Acid Plants. Brochure
distributed at 29th Annual Clearwater Conference (AIChE), Clearwater, Florida, June 3 and 4,
2005 (also presented as paper by Bartlett, C. and Rieder, J., Outokumpu Technology GmbH)
www.outokumpu.com

Roensch, L. F. (2005) Steam and boiler water treatment for the modem sulfuric acid plant, paper
presented at 29th Annual Clearwater Conference (AIChE), Clearwater, Florida, June 3, 2005
www.chemtreat.com Also, Fahrer, N.D. and Roensch, L.F. (2005) Steam and boiler water
treatment technologies for the modem sulfuric acid plant, paper distributed at 29 th Annual
Clearwater Conference (AIChE), Clearwater, Florida, June 3 and 4, 2005. www.chemtreat.com

Sander, U.H.F., Fischer, H., Rothe, U., Kola, R. and More, A.I. (1984) Sulphur, Sulphur Dioxide,
Sulphuric Acid, British Sulphur Corporation Ltd., London. www.britishsulphur.com

Sparkler (2004) Vertical Plate Filters. Brochure distributed at Sulphur 2004 meeting, Barcelona,
October 24-27, 2004. www.sparkler.nl

Thermal Ceramics (2005) Fire tube boiler.

www. thermal ceramics, co m/products/firetubeboiler.asp

Tuller, W.N. (1954) The Sulphur Data Book, McGraw-Hill, New York, 5 7.
www.mcgraw-hill.com