Opacity Problem from Water-Based Coatings

[Wayne Pferdehirt <pferdehi@epd.engr.wisc.edu>]

P2 Colleagues:

I'm working with a hanger manufacturer on an air emissions problem and
would appreciate any suggestions might have to offer.

They recently received a notice of noncompliance for opacity violations
at the stack from their bake oven.  They use a diptank to coat
hangers, which they then bake in an oven at about 350F.  Details are
provided below.

DNR has indicated opacity readings of as high as 30%.  The stack,
which I inspected up close on the roof, has a thick, black deposit that
can be peeled from the metal.  Emissions that I saw had a blue-grey
tint.

My assessment is that the problem is a combination of suspended solids
primarily from drippage in the oven, along with organics.  The
primary fronts of attack I have recommended are reducing drippage through
a variety of means (listed below) and improving design/operation of the
oven.

My questions are:
1.  Sound like anything you've tackled before?  Anything I'm
missing?

2.  Do you know of any applications where powder coating is has
proven economically feasible on such low-cost, non-durable products as
hangers?  This would be a bigger investment than they are
anticipating, but might be the better long-term investment.  I
suspect but don't have good data to verify, that powder coating would be
too expensive for this application.

**************************************************

Background

In operation for 30 years.

Recently applied for renewal of operating permit.  DNR identified opacity as a problem on recent inspection.  Just received notice of noncompliance.  DNR says opacity is as high as 30%.

Have performed no stack testing.

DNR grabbed sample of paint for testing.

Bake oven temp:  350 F

Approx time from racking of hangers to removal:  30 min
Approx time in oven:  10 min.

Bake oven uses indirect heat.  Combustion area is under the floor of the oven.  Fired by natural gas.  Apparent main areas for introducing air to oven is at ends, where hangers enter and exit.  Oven has recirculating fans after burners.

Changed from solvent-based paints to water-based paints approx. 10 years ago.  Says they have made no significant change to operation since, except to reduce HAPs in paint.  Says smoke has always been there, but that DNR just noticed.  Said they had smoke when they used solvent-based also.

Coatings used:
        Wolke Paint Mfg.
LC HAP-Free Bakecote Blue; WC-2248-04  (mixed 5 parts paint to 1 part water)
LC HAP-free Bakecote Clear Gold; WC-2237-01  (mixed 1 part paint to 1 part water)
HAP-free Bakecote Chi-gold; WC-2255-05  (mixed 5 parts paint to 1 part water)
HAP-free Bakecote White; WC-2291-14  (mixed 6 parts paint to 1 part water)

Exhaust stack is coated with blackened hard coating that can be peeled away.  They say they remove this every few months.

They shot-blast racks to remove excess paint once per shift.

Options

Reduce drippage in oven:

§       Improve design of rack and hooks to pick up less paint in dip tank and shed paint more readily

        §       

Air
knife to blow off excess paint

§       Vibrate hangers to shed excess paint

§       Decrease line speed to reduce carryout from dip tank and increase drippage and drying before oven

§       Increase travel time between drip tank and oven

§       Adjust paint mix (viscosity and constituents) to reduce drippage

Improve design and/or operation of oven

§       Improve mixing of air within oven; place baffle(s) to eliminate short circuit between fans following burners and the intake for the exhaust duct near the entrance to the oven.

§       

Reduce
velocity / increase turbulence of air flow to reduce suspension of
particulate Possibly increase temperature to improve combustion of
organics (Laidlaw is concerned that higher temp will cause white hangers
to lose gloss)

§       

Review
size of oven exhaust fan; oversized for application?

If above options are inadequate, install stack combustion unit to
complete combustion of organics 
OR
Consider alternative coating process; e.g., powder coat

Wayne P. Pferdehirt
Director, MEPP, http://epdweb.engr.wisc.edu/mepp/
Co-Director, PNEAC  http://www.pneac.org
Specialist, SHWEC, http://www.uwex.edu/shwec/
tel 608.265.2361
fax 608.262.6250