Traditionally, measures designed to
reduce localized ground-level concentrations of sulfur oxides (SOx) used
high-level dispersion. Although these measures reduced localized health
impacts, it is now realized that sulfur compounds travel long distances in the
upper atmosphere and can cause damage far from the original source. Therefore the
objective must be to reduce total emissions. The extent to which SOx emissions
harm human health depends primarily on ground-level ambient concentrations, the
number of people exposed, and the duration of exposure. Source location can
affect these parameters; thus, plant siting is a critical factor in any SOx management
strategy. The human health impacts of concern are
short-term exposure to sulfur
dioxide (SO2) concentrations above 1,000 micrograms per cubic meter, measured
as a 10-minute average. Priority therefore must be given to limiting exposures to
peak concentrations. Industrial sources of sulfur oxides should have emergency
management plans that can be implemented when concentrations reach
predetermined levels. Emergency management plans may include actions such as using
alternative low-sulfur fuels.
Traditionally, ground-level ambient
concentrations of sulfur dioxide were reduced by emitting gases through tall
stacks. Since this method does not address the problem of long-range transport and
deposition of sulfur and merely disperses the
pollutant, reliance on this strategy
is no longer recommended. Stack height should be designed in accordance with
good engineering practice.
Approaches
for Limiting Emissions
The principal approaches to controlling SOx emissions include use of low-sulfur fuel; reduction or removal
of sulfur in the feed; use of appropriate combustion technologies; and
emissions control technologies such as sorbent injection and flue gas
desulfurization (FGD).
Choice of Fuel
Since sulfur emissions are
proportional to the sulfur content of the fuel, an effective means of reducing
SOx emissions is to burn low-sulfur fuel such as
natural gas, low-sulfur oil, or low-sulfur coal. Natural gas has the added
advantage of emitting no particulate matter when burned.
Fuel Cleaning
The most significant option for
reducing the sulfur content of fuel is called beneficiation. Up to 70% of the
sulfur in high-sulfur coal is in pyritic or mineral sulfate form, not
chemically bonded to the coal. Coal beneficiation can remove 50%
of pyritic sulfur and 20–30% of
total sulfur. (It is not effective in removing organic sulfur.) Beneficiation
also removes ash responsible for particulate emissions. This approach may in
some cases be cost-effective in controlling emissions of sulfur oxides, but it
may generate large quantities of solid waste and acid wastewaters that must be
properly treated and disposed of. Sulfur in oil can be removed through chemical
desulfurization processes, but this is not a widely used commercial technology
outside the petroleum industry.
Selection of Technology and Modifications
Processes using fluidized-bed
combustion (FBC) reduce air emissions of sulfur oxides. A lime or dolomite bed
in the combustion chamber absorbs the sulfur oxides that are generated.
Emissions Control Technologies
The two major emissions control methods
are sorbent injection and flue gas desulfurization:
1.
Sorbent injection involves adding an
alkali compound to the coal combustion gases for reaction with the sulfur
dioxide. Typical calcium sorbents include lime and variants of lime. Sodium-based
compounds are also used. Sorbent injection processes remove 30–60% of sulfur
oxide emissions.
2.
Flue gas desulfurization may be
carried out using either of two basic FGD systems: regenerable and throwaway.
Both methods may include wet or dry processes. Currently, more than 90% of
utility FGD systems use a wet throwaway system process.
Throwaway systems use inexpensive
scrubbing mediums that are cheaper to replace than to regenerate. Regenerable
systems use expensive sorbents that are recovered by stripping sulfur oxides
from the scrubbing medium. These produce useful by-products, including sulfur, sulfuric
acid, and gypsum. Regenerable FGDs generally have higher capital costs than
throwaway systems but lower waste disposal requirements and costs. In wet FGD
processes, flue gases are scrubbed in a liquid or liquid/solid slurry of lime
or limestone. Wet processes are highly efficient and can
achieve SOx removal of 90% or more. With dry scrubbing,
solid sorbents capture the sulfur oxides. Dry systems have 70–90% sulfur oxide
removal efficiencies and often have lower capital and operating costs, lower
energy and water requirements, and lower maintenance requirements, in addition
to which there is no need to handle sludge. However, the economics of the wet
and dry (including “semidry” spray absorber) FGD processes vary considerably
from site to site. Wet processes are available for producing gypsum as a byproduct.
Table 1 compares removal efficiencies and capital costs of systems for
controlling SOx
emissions.
Monitoring
The three types of SOx monitoring systems are continuous
stack monitoring, spot sampling, and surrogate monitoring. Continuous stack
monitoring (CSM) involves sophisticated equipment that requires trained
operators and careful maintenance. Spot sampling is performed by drawing gas
samples from the stack at regular intervals. Surrogate monitoring uses
operating parameters such as fuel sulfur content.
Recommendations
The traditional method of SOx dispersion through high stacks is not
recommended, since it does not reduce total SOx loads in the environment. Natural gas is the preferred fuel
in areas where it is readily available and economical to use. Methods of
reducing SOx
generation, such as fuel cleaning systems and combustion modifications, should
be examined. Implementation of these methods may avoid the need for FGD
systems. Where possible and commercially feasible, preference should be given
to dry SOx
removal systems over wet systems.
