HCl and SO2 removal of flue gas

[WATECO]

Good evening,

I have to design a wet scrubber installation for the removal of HCl and SO2 from flue gas.
In theory this can be done in a 1-step wet scrubber, neutralised with caustic, lime, soda, etc.
In practice mostly a 2-step scrubber is used.
In the first step HCl is removed (at a rather low pH of 1-2), in the second step SO2 is removed (at a pH of 6-8).

Can anyone explain me why a (more costly) 2-steps scrubber is used instead of a 1-step scrubber?

Dirk Koot

 

[25362]

HCl absorption is thermodinamically favoured over the competing SO₂ absorption.
It also depends on the actual proportions of HCl and SO₂. For example, when HCl>SO₂, HCl absorbs better at higher temperatures than SO₂.

There are several links that may be helpful, for example:

http://www.itas.fzk.de/deu/Itaslit/acri00a.pdf

http://www.bvsde.paho.org/bvsacd/cd43/msw.pdf

http://www.thermalnet.co.uk/Resourc...ng in flue gas from combustion of biomass.pdf

 

[siretb]

For many applications where HCl is present (incineration, ...) the HCl content is much higher than SO2 content.

HCl is so much soluble in water that no neutralisation reagent is needed, so this may mean big savings (beware of materials and HF). Or , because when some neutralsation is performed is the first scrubber, the reagent may be different (limestone in the first and caustic in the second)

There is another reason. HCL and SO2 are not the only pollutants. Mercury (Hg) may be present, and it is not good pratice to scrub mercury at an elevated pH (say>5)

But yes, if you have only SO2 and HCl a design with one scrubber is possible. The pH may be difficult to control, and too low a pH means little SOI2 capture and too high a pH means scaling.

 

[davefitz]

regarding the original post:
before they had developed duplex alloys ( 2205) there had been a practice of using a "pre-scrubber" to remove HCl- this would reduce the corosiveness of the slurry in the downstream SO2 scrubber and allow lower cost alloys.Now , in europe, they have lower cost plastic (PPE) and ceramic ( for valves and spray nozzles) scrubbers, lower cost than duplex metal and less corrosion- allows chlorides up to 75,000 ppa big issue is finding a hoome for all the heavy metals contained in the scrubber slurry blowdown- the less blowdown , the easier it is to install a ZLD .

 

[Armen75]

Wouldn<t a 2 step process also provide you with a back-up; As the exhaust is usually going to Atm and you are trying to minimise SO2 or HCl venting, having the second step acts as a polishing unit and as a back-up in case the absorption in the 1st step suddenly decreases.

 

[purtelas]

I designed a municipial waste incinerator therefore I may be able to help you in this topic.
usually Ca(OH)2 is used due to its cheapness and high activity. 20% mixture is created with CaCl2 to catalyze the dry reaction.

a spray dry scrubber, baghouse filter and wet scrubber is used as unit operators.

depending on the feed for combustion the SO2 and HCl ratio is changing between
SO2 concentration 300-800 ppm
HCl concentration 400-3000 ppm

effect of HCl concentration to efficiency of SO2 removal is investigated in the article by Jens Stein and Mathias Kind. Maximum SO2 removal is achieved when HCl concentration is 535 ppm. However HCl acts as inhibitor in this situation. (which is common if you burn plastic bags or PVC's).

residence time for the flue gas is 20 seconds and practically you can achieve 93% removal for HCl and 75% removal for SO2.
if yo lower the residence time to 15 seconds. 85% HCl removal and 60% removal for SO2 is achieved.

compare the remaining concentrations with the legal restrictions if needed place another device. Wet scrubber is giving better results but higher pressure drop is experienced. (also you obtain less steam).

You can read these people's articles
B.C. Meikap
modeling of removal of SO2 from FG in horizontal cocurrent g-l scrubber

Jens stein, mathias kind
Influence of HCl on SO2 absorption in SDA

also you can look at the simulation done by the Janelli and Minulla which is done for power plant.