Meeting MACT Head-On
A new air toxics control system helps manufacturers achieve EPA standards by handling VOC and HAP emissions
The DuPont Front Royal plant has been the leading name in the automotive refinishing
industry. DuPont Performance Coatings, formed from DuPont Automotive finishes
and DuPont's acquisition of Herberts, is the world's largest supplier of original
equipment manufacturer (OEM) and aftermarket coatings and the world's third
largest coatings company, overall. DuPont has continually reduced solid waste
generation and air emissions over the past five years. Since 2000, DuPont has
reduced solid waste by 30 percent and air emissions by more than 50 percent.
The DuPont Front Royal site is continuing its commitment to environmental excellence
by installing an additional abatement system to further reduce its air emissions
Like with many other manufacturing facilities, such as semiconductor fabricators,
miscellaneous organic chemical manufacturing, miscellaneous coating manufacturing,
reinforced plastic composites, painting processes, and others that utilize volatile
organic compounds (VOCs) and hazardous air pollutants (HAPs) in their products,
low concentration, high volume (LCHV) room ventilation exhaust becomes a major
economic issue in meeting their maximum achievable control technology MACT requirements
under the Clean Air Act.
The low-concentration, high-volume (LCHV) VOC / HAP process exhaust streams
substantially increase the capital and operating costs associated with most
air emission control equipment. As most plant manager have already realized,
the cost of natural gas has gone through the roof, and their low-efficiency
thermal oxidation equipment is burning up their production profits faster than
they can produce them. Natural-gas costs have nearly doubled in the past year,
and it is expected to continue to increase in the years to come.
To meet MACT requirements and minimize operating costs, DuPont selected a hybrid
concentrator and rotary-valve regenerative thermal oxidizer (RL RTO) system.
The hybrid system virtually eliminates the need for additional auxiliary natural
gas. The energy produced from the combustion of the high-concentration, low-volume
(HCLV) of VOCs / HAPs provides nearly all of the energy to self-sustain the
operation of the RL RTO system.
The system provides control for several areas, including manufacturing building
exhaust with concentrations less than 75 PPMv (parts per million), solvent storage
tank vents with high polar vapor component (Methanol), manufacturing process
vents with VOC / HAP, resin-manufacturing process vents with a wide vary of
components, and solvent-recovery process vents. All of the various areas have
different process flows, solvent types, solvent concentrations, and operating
schedules that required a specialized control approach in order to minimize
operating costs and maximize control performance to meet MACT requirements.
The overall system control efficiency is critical for a site to meet synthetic
emission minor status and maintain its right to operate. Therefore, to achieve
the required reduction in emissions, all of the factors from the sources must
be addressed to ensure the plant stays below MACT limits. The combined overall
destruction removal efficiency (DRE) must include equipment uptime as well as
capture and destruction efficiency (see Table 1).
|RTO Destruction efficiency
(Not Counting Planned Down Time)
(Capture X Destruction X Uptime)
This overall control efficiency must meet MACT requirements, which are based
on standards for each listed source category according to a prescribed schedule
and are based on the best demonstrated control technology or practices within
the regulated industry. Typically these sources are based on the Title III requirements
The Hybrid System
The system includes additional features to allow unexpected downtime of an individual
concentrator module and still maintain the full process flow through the remaining
system. This, along with other redundant and/or high-reliability designs, ensures
that the system will maintain the high efficiency uptime requirements necessary
for successful compliance.
The basic system operates in the following fashion. The LCHV process streams
are directed to four two-stage pre-filter systems to remove any potential dust
and dirt prior to the adsorber units. These units include monitoring and easy
access to clean and replace filters as needed without disrupting the overall
process operation. After the filters, the process then enters into the zeolite
rotating concentrator disc, where zeolite media adsorbs and removes the incoming
VOCs / HAPs, before the clean air is discharged to the atmosphere through an
induced-draft, low-horsepower fan to a common stack.
A slip stream results from the LCHV process flow after the filtration is used
to cool each concentrator zeolite disc after the higher temperature de-sorption
process. This stream is then directed to an air-to-air heat exchanger where
it is preheated by the hotter exhaust from the RL RRTO unit before it is returned
to the concentrator disc to strip the disk of collected VOCs / HAPs. The air
stream, with its concentrated VOCs / HAPs, is then sent to the rotary valve
RL RTO unit for thermal oxidation into carbon dioxide and water.
The honeycomb zeolite structure of the adsorbent media disc yields a very low
pressure drop, which minimizes electricity needed for concentrator exhaust fan
power. In addition, it provides heat transfer to minimize the overall fuel costs.
The fuel costs are also kept extremely low by the reduced size of the thermal
oxidizer -- only 5 to 20 percent of the process flow is actually sent to the
oxidizer (but this includes nearly all of the VOC / HAPs), which in most cases
will self-sustain in the RL RTO unit, requiring little or no auxiliary fuel.
The concentrated stream, along with the other non-compatible process streams,
are drawn into the inlet side of the RL RTO rotary valve and directed up through
five of the 12 heat recovery chambers, which provide 95 percent thermal energy
recovery. The HCLV stream is preheated in the ceramic heat-recovery bed to the
auto-ignition temperature of the solvents. The spontaneous combustion in the
heat recovery beds, approximately 3 percent of the lower explosive level (LEL)
or greater, will provide all the energy to complete the oxidation process. This
allows the RL RTO system to self-sustain or maintain the purification temperature
to complete the oxidation process with little or no auxiliary fuel. The hot,
purified clean-air stream is drawn down through another set of five heat recovery
chambers where the energy is recovered and stored for preheating the next cycle
of concentrated solvent air as it enters the RL RTO.
The rotary valve sequences each heat-recovery chamber from inlet to flushing,
which provides the ability to achieve more than 99 percent destruction-removal
efficiency. The RL RTO unit includes a variable energy system that provides
additional energy to the air-to-air heat exchanger system for desorption on
the concentrators. In addition, it can provide the periodic high-temperature
energy for regeneration of the concentrator disc. The HCLV or non-compatible
concentrator solvents go directly to the inlet of the RL RTO unit and are processed
with the concentrated stream from the disc.
The system includes:
- A completely negative pressure (induced-draft) system to eliminate the potential
of fugitive leakage or emissions;
- A separate control room for the complete system control;
- Four skid-mounted modular concentrator units that allows individual module shutdown
while maintaining full process flow through the rest of the system;
- Four skid-mounted modular integral two-stage filtration system that also allows
individual module shutdown while maintaining full process flow through the rest
of the system;
- Design provisions for additional future concentrator modules to provide increased
capacity and/or removal efficiency;
- An online full-flow, bake-out feature with rotary valve regenerative thermal
oxidizer (RL RTO) unit;
- An integral RL RTO dual-fuel, nature-gas, and #2 Oil combustion system is included,
which eliminates the need for a separate inline concentrator heater;
- A system design that eliminates the potential nitrogen oxide (NOx)and concentrator
media degradation from a direct fired nature gas and # 2 oil inline heater operation;
- A single process exhaust stack to facilitate a single continuous emission monitoring
DuPont selected a hybrid concentrator and rotary-valve regenerative thermal
oxidizer (RL RTO) system to meet their MACT requirements and minimize their
overall operating costs.
This type of hybrid system with thermally efficient RL RTO units, increased
reliability and up-time design, and short installation designs with shop-assembled,
skid-mounted modular units, make it possible for industry to remain in regulatory
compliance while continuing to turn a profit.
|Title III: Air Toxics
The list of source categories under this section of the
Clean Air Act must include: (1) major sources emitting 10 tons per year
of any one, or 25 tons per year of any combination of those pollutants;
and, (2) area sources (smaller sources, such as dry cleaners).
This article originally appeared in the 09/01/2006 issue of Environmental Protection.