As seen in the May 2011 issue of
Midstream Business Magazine
Covering Your Tail Gas
The discovery of new shale formations
throughout the world and the development of new drilling
technologies have increased natural gas production and prompted
government agencies to instigate tighter air-pollutioncontrol
regulations on processing operations. Unfortunately, meeting
environmental regulations is not a profit-generating endeavor.
Simply put, the time and money spent on protecting air, water and
land does not help midstream companies produce more natural gas to
ensure the nation's energy security. As a result, saving money
while meeting or exceeding regulations should be on every midstream
company's wish list.
Much like other industries, oil and gas
producers are often required by the Environmental Protection
Agency(EPA) to obtain a Title V permit, the objective of which is
to prevent untreated air pollutants from entering the atmosphere.
In addition to their harmful effects on plants and trees, these
pollutants, known as volatile organic compounds (VOCs) and
hazardous air pollutants (HAPs) are known to cause respiratory
ailments, heart conditions, birth defects, nervous system damage
and cancer in humans.
Under the Clean Air Act, most companies
with the potential to release more than 10 tons of a single VOC or
HAP during a one-year period, or 25 tons of multiple compounds,
must install either a pollution-control device or the maximum
achievable control technology (MACT). Many of the pollution
control devices currently used to abate these emissions also emit
significant amounts of carbon dioxide (CO2) and nitrous
oxides (NOX). With mandatory greenhouse gas (GHG) reporting
on the horizon, processors could soon be paying for the carbon
emissions generated by some of these pollution control systems,
adding to the capital and operating costs associated with
regulatory compliance.
Midstream's abatement history
A number of production techniques and
processes used by midstream companies are, or soon will be,
regulated as emission sources. From stationary combustion engines
to amine systems, the industry is facing some fairly strict
legislation.
One area of great concern is amine
tail-gas treatment. Amine systems are a very common and
critical component used by natural gas-processing facilities to
remove acid gases, CO2 and hydrogen sulfide
(H2S) from the well head. This is accomplished by
running the gas through a column with amine liquid flowing in the
opposite direction, stripping acids from natural gas and absorbing
them into the liquid. The natural gas is then sent for processing
while the amine is sent to be regenerated. The regeneration process
removes the acid gases from the amine solution, allowing it to be
re-used, but the process creates tail gas. This tail gas, and the
means for treating it, offer the latest opportunity for the
implementation of new technologies and increased profits.
Thermal and catalytic oxidizers are
technologies commonly used on a wide variety of applications where
VOC, HAP and odor abatement is required. They destroy harmful
emissions through the process of high-temperature
combustion. Midstream companies have historically used flares,
vapor combustors, direct-fired thermal oxidizers (TOs) or
recuperative systems for emission destruction. Applications where
these devices are applied range from amine tail-gas treatment,
nitrogen rejection units and liquefied natural gas (LNG) processes.
The temperature in these systems is maintained somewhere between
1,400°F and 1,800°F so that hydrocarbons are converted to CO2 and
water vapor, while the H2S is converted to sulfur dioxides
(SO2 and SO3).
When designed properly, these older
technologies are fairly dependable, but their effectiveness and
efficiency can spark a heated debate. In the case of flares, water
is often injected into the device to reduce visible black smoke.
This drastically reduces the destruction efficiency-and the EPA is
taking note. While TOs and vapor combustors can achieve destruction
efficiencies around 99%, they share a common negative aspect with
flares: They have a high fuel consumption rate.
Large amounts of fossil fuels are
required to bring the air toxins up to proper destruction
temperature. Rather than use the heat generated from combustion to
preheat incoming pollutants, the energy is simply released into the
atmosphere, along with CO as GHGs. Chart A demonstrates just how
significant the carbon emissions can be from the various
technologies.
Enter the RTO
All too often, production facilities
take the "no news is good news" approach to their air-pollution
control equipment when they really should be chasing the benefits
of a "company stays green and saves green" approach. A proven, more
fuel-efficient abatement technology, called the regenerative
thermal oxidizer (RTO), is now being applied to tail-gas treatment
where it was once thought impossible.
What differentiates it from other
technologies is its ability to use the proper mix of temperature,
residence time (or dwell time), turbulence and oxygen to convert
pollutants into carbon dioxide and water vapor, while reusing the
thermal energy generated to reduce operating costs. In some cases,
emission destruction can occur without any additional natural gas
or other supplemental fuel.
VOC- and HAP-laden process gas is
routed into the inlet manifold of the oxidizer, flow control or
poppet valves, which then directs this gas along with fresh air for
combustion into energy-recovery chambers where it is preheated. The
process gas and contaminants are progressively heated in the
ceramic media beds as they move toward the combustion chamber.
Once oxidized in the combustion
chamber, the hot purified acid gas releases thermal energy as it
passes through the media bed in the outlet flow direction. The
outlet bed is heated and the gas is cooled so that the outlet gas
temperature is only slightly higher than the process inlet
temperature. Poppet valves alternate the airflow direction into the
media beds to maximize energy recovery within the oxidizer.
Thermal energy recovery (TER) within an
RTO can reach 97%, reducing, and in some cases eliminating, the
auxiliary fuel requirement. Some gas plants have reported over
$500,000 in operating cost savings annually. With destruction
capability over 99%, the RTO is not only an efficient alternative
for this application, but also very effective. However, careful
consideration must be given to the design and materials of
construction to avoid corrosion, equipment failures, non-compliance
and safety issues.
Case in point
The midstream division of a large,
multinational energy corporation was operating several amine
systems around the country with tail-gas treatment. A TO at one of
these facilities in the western U. S. had numerous operational
problems and extremely high operating costs.
The company asked Anguil Environmental
Systems Inc., an oxidizer and air-pollution control systems
provider, to evaluate various replacement options. The process data
provided showed a tail-gas flow rate of about 15,000 pounds per
hour (lbs/hr) or about 2,500 standard cubic feet per minute (SCFM),
a calorific value of 6 Btu/SCF and 25 parts per million by volume
(ppmv) of H2S. After evaluating numerous oxidizer
thechnologies, including TOs and thermal recuperative oxidizers,
Anguil determined that the best solution would be an RTO.
Having designed oxidizers for similar
corrosive applications, the engineers at Anguil recommended that
the RTO be built with special materials of construction and design
considerations to combat the presence of both carbonic and sulfuric
acid.
Carbonic acid is caused by high
CO2 levels combined with a saturated process
stream. Sulfuric acid is created when H2S is oxidized
and the resulting SO3 combines with water vapor present
in the RTO exhaust gas. The amine process exhaust at this midstream
operation was inert, or lacking oxygen; therefore, fresh air was
required for oxidation.
Heat released from combustion of these
hydrocarbons can be very high, so fresh air is also added to keep
the system from an over-temperature condition. To eliminate
condensing of water vapor of the tail gas inside the RTO, this
ambient air is preheated to protect metal surfaces from the
inorganic acids condensing.
A unique system utilizing excess heat
from the combustion chamber to provide the necessary heat was
deployed on this system, further reducing operating costs. The
preheat component eliminates the need for additional equipment
(gas-fired heater, steam coil) and further minimizes auxiliary fuel
consumption. The next step in the corrosion-protection strategy is
to implement various stainless-steel alloys on critical components
and a corrosion-resistant coating on the inside of the
energy-recovery chambers and combustion chamber. The type of
stainless steel chosen depends on the presence and concentration of
H2S.
The critical components are chosen
based on their function within the RTO and their location. These
components see exhaust temperatures of up to 600°F, above the limit
of corrosion-resistant coatings. The energy recovery chambers and
combustion chamber are internally insulated with soft ceramic
refractory insulation, limiting the shell temperature (and the
maximum temperature to which the coating will be exposed) to 200°F,
well below the safe limit of the coating. With the combination of
extremely high TER, and the tail-gas calorific value of 6 Btu/SCF,
this RTO requires no auxiliary fuel to achieve 99% hydrocarbon and
H2S destruction efficiency.
By comparison, a TO or flare designed
for the same process gas would consume more than $100 per hour of
auxiliary fuel, resulting in an annual fuel cost of more than
$750,000. Also, the reduced natural gas consumption results in an
additional 2,600 lbs/hr of GHG emissions compared to the RTO.
Processing profit
With the industrial price of natural
gas at $6 per thousand cubic feet, and a one-to-one correlation
between a cubic foot of natural gas and a cubic foot of
CO2 emissions, certified carbon credits could go for
about $10 to $30 per metric ton or 1,000 kilograms. This is about
20,000 cubic feet of CO2 on a one-to-one cubic foot to
cubic foot basis, then 20,000 cubic feet of natural gas produces
one metric ton of CO2.
The credit might be worth $1 per
thousand cubic feet of natural gas, or about 15% of the cost,
meaning a reduction in natural gas consumption would not only save
operating costs but it could, in theory, produce income, assuming a
credit can be certified and traded.