Using Lime For Flue
Gas Treatment
Lime plays a key role in many air pollution control applications. Lime is
used to remove acidic gases, particularly sulfur dioxide (SO2) and hydrogen
chloride (HCl), from flue gases. Lime-based technology is also being
evaluated for the removal of mercury. We are manufacturers and wholesale
suppliers of Calcium Hydrate and other Lime Chemicals.
Lime is more reactive than limestone, and requires less capital equipment.
SO2 removal efficiencies using lime scrubbers range from 95 to 99 percent
(at electric generating plants). HCl removal efficiencies using lime range
from 95 to 99 percent (at municipal waste-to-energy plants).
There are two main methods for the removal of acidic gases: dry scrubbing
and wet scrubbing. Both methods are used for cleaning flue gases from the
combustion of coal to produce electric power. Dry scrubbing is also used at
municipal waste-to-energy plants and other industrial facilities, primarily
for HCl control. Lime is used in both systems
Dry Lime Scrubbing
In dry scrubbing, lime is injected directly into flue gas to remove SO2 and
HCl. There are two major dry processes: “dry injection” systems
inject dry hydrated lime into the flue gas duct and “spray dryers”
inject an atomized lime slurry into a separate vessel.
A spray dryer is typically shaped like a silo, with a cylindrical top and a
cone bottom. Hot flue gas flows into the top. Lime slurry is sprayed through
an atomizer (e.g., nozzles) into the cylinder near the top, where it absorbs
SO2 and HCl. The water in the lime slurry is then evaporated by the hot gas.
The scrubbed flue gas flows from the bottom of the cylindrical section
through a horizontal duct. A portion of the dried unreacted lime and its
reaction products fall to the bottom of the cone and are removed. The flue
gas then flows to a particulate control device (e.g., a baghouse) to remove
the remainder of the lime and reaction products.
Both dry injection and spray dryers yield a dry final product, collected in
particulate control devices. At electric generating plants, dry scrubbing is
used primarily for low-sulfur fuels. At municipal waste-to-energy plants,
dry scrubbing is used for removal of SO2 and HCl. Dry scrubbing is also used
at other industrial facilities for HCl control. Dry scrubbing methods have
improved significantly in recent years, resulting in excellent removal
efficiencies.
Using Lime To
Treat Biosolids And Sludges
Lime can be used for effective treatment of sewage biosolids, as well as
industrial sludges and petroleum wastes.
Sewage Biosolids
Quicklime and calcium hydroxide (hydrated lime) have been used to treat
biological organic wastes for more than 100 years. The treatment of human
wastewater sludges (i.e., biosolids) by lime treatment is specifically
prescribed in U.S. EPA regulations (40 C.F.R. 503). There are many examples
of wastewater treatment systems using lime stabilization.
Industrial Sludges and Petroleum
Quicklime and hydrated lime can be used in the treatment of many industrial
sludges by correcting pH for further treatment, neutralizing acidic wastes,
and removing or immobilizing contaminants. Specific examples include
sulfite/sulfate sludges and petroleum waste.
Using Lime
To Treat Animal Wastes
The Animal Waste Problem
An emerging issue in the U.S. is the growing environmental threat caused by
animal wastes. Current management practices have begun to create
environmental problems because of the consolidation of the livestock
industry into much larger facilities, and the resulting concentration of
waste-producing activities. Concentrated animal feeding operations (“CAFOs”)
for beef cattle, swine, and poultry can create numerous problems, including
excess nutrient loading of agricultural land, eutrophication of surface
waters, groundwater contamination, pathogen release, and offensive odors.
There have been a number of incidents in which large numbers of people have
been sickened by water or food contaminated by animal wastes. These problems
will only get worse—the amount of animal manure produced annually is
estimated to be 10 times the amount of municipal sewage—and much of
that manure currently receives little or no treatment. In addition to solid
animal manure, there are large amounts of other animal wastes, such as
poultry bedding, urine, and carcasses which also are environments problems
and are estimated to total up to 100 times the amount of human wastewater
biosolids.
EPA’s CAFO rule
The Environmental Protection Agency is in the process of developing a new
rule to regulate concentrated animal feeding operations. If the final rule
resembles the proposed rule, many more of these 40,000 facilities will be
required to institute effective treatment of animal wastes than presently
do. When this happens, the need for cost-effective treatment methods will
become acute.
Lime Treatment for Animal Wastes
Lime treatment is a multi-functional, cost-effective, politically
acceptable option with respect to many of the challenges posed by animal
wastes, just as it has played an important role in biosolids (sewage)
treatment.
Using Lime
To Treat Wastewater
Lime is extensively used in the treatment of
municipal wastewaters, as well as the treatment of industrial liquid wastes.
Municipal Wastewater Treatment
In advanced wastewater treatment plants, lime precipitation is employed in
tertiary processes in which phosphorus is precipitated as complex calcium
phosphates along with other suspended and dissolved solids. Due to the high
pH of 10.5-11.0 maintained by lime, the stripping of nitrogen, another
nutrient, is facilitated. Thus, the removal of phosphorus and nitrogen helps
prevent eutrophication (algae build-up) in surface waters.
When alum and ferric chloride are employed for coagulation, lime is used to
counteract the low pH induced by these acid salts and to provide the
necessary alkalinity for efficient nitrogen removal.
In sewage plants where sewage sludge is removed by vacuum or pressure
filtration, lime and ferric chloride are employed as filter aids in the
conditioning of the sludge and for final clarification of the effluent.
Industrial Wastewater
Lime has numerous applications in treating industrial wastewaters,
especially where neutralization of acidic wastes is required. In steel
plants, sulfuric acid-based waste pickle liquors are neutralized with lime
in which the iron salts are precipitated. Lime is also a neutralizer and
precipitant of chrome, copper, and heavy metals in processes for treating
discharges from plating plants.
Lime is used to neutralize sulfuric acid wastes from rayon plants and to
neutralize and precipitate dissolved solids from wastes of cotton textile
finishing plants (dye works).
Vegetable and fruit canning wastes can be clarified with lime alone or with
supporting coagulants as an alternate to lagooning of the liquid waste. In
citrus canning, lime assists in clarifying wastewaters and in the processing
of citrus pulp by-products.
Using Lime To
Treat Hazardous Wastes
Lime is widely used to treat hazardous wastes both currently generated
process wastes and previously disposed or abandoned materials. Lime
stabilizes most metals by converting them to more chemical stable forms that
are less likely to leach. In addition, lime can react with soils to solidify
materials, further reducing the leaching of hazardous wastes. Lime can also
be used to neutralize acidic materials.
Under the U.S. EPA's land disposal restrictions regulations, currently
generated hazardous wastes that are to be land disposed must be pretreated
using the Abest demonstrated available technology. For hazardous wastes
containing metals, metals stabilization or metals precipitation is
frequently required, and lime is identified by EPA as suitable to treat
these wastes
EPA also endorses lime stabilization as a key technology for hazardous
waste site cleanups (see, e.g., Handbook for Stabilization/Solidification of
Hazardous Wastes (EPA/540/2-86/001, June 1986). In 1997, for example, EPA
announced a proposed cleanup plan as part of the Anaconda Regional Water,
Waste, and Soils Project for 14,000 acres in Anaconda, Montana. A key
element of the plan is to treat arsenic-containing soils with lime and
organics. Copper mining created environmental contamination in the 300
square mile area and concern about potential human exposures. EPA
recommended in-place lime treatment over the option of excavating and
treating the tailings and contaminated groundwater. (Nearby, the Warm
Springs Pond is already being used to capture and treat water contaminated
with metals (copper, zinc, and arsenic) that threaten the Clark Fork River.
The contaminated waters are treated with a lime solution.)
