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Technology - NOx
July 5, 2003
Introduction
The trend to smelting
processes utilizing oxygen enrichment has resulting in more energy efficient smelting
operations, higher throughputs and smaller downstream gas handling equipment. Higher smelter operating temperatures result from
the smaller quantity of nitrogen present in the gas which acted as a heat sink. The higher operating temperatures result in an
increase in nitrogen fixation according to the following reactions:
N2
+ O2 <-> 2NO
2NO + O2
<-> 2NO2
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Thermal NOx Formation |
Nitrogen oxides formed
in this manner are referred to as thermal NOx. In
addition to temperature, the formation of NOx is governed by residence time and oxygen
concentration. High temperatures favour the
formation of nitrogen monoxide (NO) but as the gas cools, NO2
is formed. Increasing residence time and
oxygen will also result in the formation of more NOx.
The majority of the nitrogen oxides present in the gas will be the result of
thermal NOx.
Nitrogen oxides may also
be present in the fuel that is used in the smelter to maintain operating temperatures. The combustion of the fuel leads to the formation
of chemical NOx.
A third source of
nitrogen oxides results from the operation of the wet electrostatic precipitators
(WESP’s). Electrical arcing in the
WESP’s causes the formation of nitrogen oxides.
The nitrogen oxides pass
from the smelter into the downstream acid plant where it ultimately contaminates the
product acid. NO is insoluble in sulphuric
acid so no NO enters the acid through the Drying Tower.
Instead, NO is further oxidized by the oxygen in the gas according to the
following reaction:
2NO + O2
<-> 2NO2
This reaction occurs
rapidly in the catalyst beds.
NO and NO2 in
equal molar proportions behaves as N2O3 which is readily soluble in
sulphuric acid:
NO + NO2
+ 2H2SO4 <-> 2NO•HSO4 + H2O
or
N2O3
+ 2H2SO4 <-> 2NO•HSO4 + H2O
Nitrosylsulphuric acid
(NO•HSO4) is the dominant nitrogen-containing species
that contaminates the acid and is commonly referred to as ‘nitrates’.
The nitrogen oxides in
the gas are preferential absorbed by the submircon acid mist particles which have a high
surface area to volume ratio. Thus, most of
the nitrates present in the acid plant are concentrated in the acid draining from the high
efficiency candle mist eliminators.
Nitrates in the acid
accelerates corrosion of steel equipment and discolours the acid reducing the quality of
the acid. Some acid consumers require low or
zero nitrate levels in the acid. If nitrates
are present in acid used in the sulphonation process, a reaction will occur with the
benzene ring to produce a greenish black slurry.
If Fe+2 is
present, the acid will appear pink in colour. The
following reaction occurs:
2FeSO4
+ HNO3 + H2SO4 -> HNO2 + Fe2(SO4)3
+ H2O
The pink colour is
probably caused when N2O3 is
dissolved in sulphuric acid and NO+ is formed which reacts with Fe2+ to form a
co-ordination complex. Aerating or heating
the acid will oxidize HNO2 to HNO3 which breaks the co-ordination
complex causing the pink colour to disappear. Unfortunately,
the reaction is reversible and the pink colour will return.
Treatment
The nitrate problem in
an acid plant can be treated either in the gas phase or the liquid phase. The quantity of nitrogen oxides formed in the
smelter can be minimized by ensuring the process operates steadily and minimizing local
hot spots in the furnace, however, the formation of NOx cannot be totally eliminated.
Gas Phase Treatment
Treating the NOx as far
upstream in the process is the most desirable route for any contaminate. Treatment of NOx in the gas phase involves
reducing the nitrogen oxides to nitrogen gas. In
other industries, NOx is treated in the gas phase by Selective Catalytic Reduction (SCR),
Selective Non-Catalytic Reduction (SNCR) and various scrubber technologies.
The SCR process involves
the reaction of NO and NO2 with ammonia or urea in the
presence of a catalyst to form nitrogen and water.
4NO + 4NH3
+ O2 -> 4 N2 + 6H2O
6NO2
+ 8NH3 -> N2 + 12H2O
Excess ammonia will
react with oxygen to ultimately form nitrogen and water.
4NH3
+ 5O2 -> 4NO + 6H2O
4NH3
+ 3O2 -> 2N2 + 6H2O
The reaction occurs in
the presence of the catalyst at a temperature between 300°C and 425°C. The ideal location for the treatment of NOx by
SCR is immediately prior to the first catalyst pass which is well upstream of the point
where NOx will enter the acid system. The
operating temperature at this location falls into the operating range of the SCR catalyst
so no additional heat or cooling of the process gas is required. As well, no SO3 is
present which would react with ammonia to produce ammonia sulphate.
In addition to nitrogen,
the other product from the SCR reactions is water. The
presence of water in the process gas has always been a concern otherwise there
wouldn’t be a drying tower designed to dry the gas coming from the gas cleaning
system. The water formed in the reaction will
react with the SO3 formed to produce sulphuric acid and
remain as a gas through most of the plant except where the gas is cooled close to its
dewpoint prior to entering the absorber towers. Increased
corrosion of the heat exchangers may occur if condensation
of acid is not monitored.
The technology has been
successfully implemented at a zinc smelter operated by Budel Zink in The Netherlands. With inlet NOx concentration in the range of
150-200 ppm, outlet concentrations of 4 to 6 ppm have been achieved. Stack emission are typically 6 ppm and product
acid nitrate levels meet the quality specification of less than 9 mg/L. To date, no feedback is available on the affect
the moisture in the gas has on the corrosion of the equipment.
Liquid Phase Treatment
Treating nitrates in the
acid begins by segregating the acid draining from the high efficiency candle mist
eliminators to avoid contaminating the main acid stream.
This accounts for approximately 50% of the NOx entering the acid plant. The remaining NOx will be absorbed into the acid
or exit in the plant stack.
The method used to
segregate the candle drainings will depend on whether or not the candles are the standing
or the hanging type. For standing candles,
the majority of the acid will collect on the candle tubesheet and is easily pipe outside
of the tower. For hanging candles, each
candle is equipped with a drain which must be piped individually to header inside the
tower and then taken outside the tower.
The acid from the
candles can be disposed of or treated to try and recover the acid values. Disposal will involve the neutralization of the
acid but this creates a hazard to personnel because NO and NO2 will be
liberated when the acid is diluted. This fact
forms the basis for a system to remove NOx and recover the acid.
When acid containing
nitrosylsulphuric acid is diluted to about 70% H2SO4 and the
temperature of the acid is maintained above 50°C, the following reaction takes place
liberating NO and NO2 as a gas.
2NO•HSO4
+ H2O -> NO + NO2 + 2H2SO4
The removal of
nitrosylsulphuric acid can be enhance by using air or steam to strip the acid to produce
an acid stream than can be blended back into the main acid stream.
The NO and NO2
gas stripped from the acid can be sent up the stack but this results in a brown plume
which may not be desirable. An alternative to
this method of disposal is to absorb the NO and NO2 in water
to form a weak nitric acid solution. The weak
nitric acid solution can then be neutralized without the formation of NO or NO2.
The equipment required
to treat the acid in this manner consists of small diameter packed columns. The flow of acid draining from the candles is not
always constant so a surge tank is recommended to collect the candle drainings in order to
smooth out the flow to the stripping column. Controls
are required to regulate the flow of acid and dilution water. Operating temperatures in the column should also
be monitored.
Segregating and treating
the acid draining from the candles will prevent the majority of the nitrogen oxides from
entering the product acid stream. To further
reduce and eliminate the nitrates in the acid the addition of a strong reducing agent is
required. The most common reducing agent used
is hydrazine (H2N4), hydrazine hydrate or hydrazine sulphate. Other reducing agents are urea and hydroxylamine. The reaction between hydrazine hydrate and
nitrosylsulphuric acid is as follows:
3N2H4•H2O
+ 4HNOSO4 -> 2SO4 + 5N2 + 5H2O
The elimination of NOx
using hydrazine is affected by several factors:
Acid Strength –
reaction rate is higher in 93% H2SO4 than 98% H2SO4
Acid Temperature
– reaction rate increases with increasing temperature
Excess Hydrazine
– the reaction rate is roughly proportional to % excess hydrazine
Sulphur Dioxide –
the presence of SO2 reduces the reaction rate
Other chemical reducing
agents can be used but none are as effective as the hydrazine compounds. The addition of any treatment chemical to the
product acid should be done carefully to avoid excess unreacted chemical which may in
itself be a contaminant. As well, chemicals
such as hydrazine are toxic and possible carcinogenics.
Hydrazine is added to
the mist eliminator drainings to destroy the NOx and then return the treated acid to the
drying acid system. The nitrosylsulphuric
acid is collected in a holding tank and is them fed to a NOx reaction column at a constant
rate. Water is added to dilute the acid to
93% H2SO4 and hydrazine is added.
Dilution of the acid to 93% H2SO4 and the subsequent
increase in temperature both contribute to an increase in the rate of reaction.
The treated acid is
returned to the drying acid in system and gas from the NOx reactor column are vented up
the stack. Excess hydrazine is added to the
treated acid where it will react with the NOx in the drying acid circulating system to
produce an overall product acid within specification.
Hydrogen peroxide is added to the product acid stream to eliminate any
excess hydrazine.
Lurgi have develop a NOx
removal process based on the lead chamber process. The
reactions involved are:
2HNOSO4
+ 2H2O <-> 2H2SO4 + 2HNO2
3SO2
+ 2H2O <-> 3H2SO3
2HNO2
+ 3H2SO3 <-> 3H2SO4 + N2 + H2O
The process is
characterized by its simplicity, low capital and operating cost. No expensive chemical reagents are required and
there are no effluents.
The process is being
used at the Lurgi designed and built metallurgical sulphuric acid plant operated by
Western Mining Fertilizer at Mount Isa, Australia.
Case
Study - INCO
In October 1991, the new
acid plant at INCO Copper Cliff, Ontario, Canada began operation. The acid quality produced from the plant generally
met specification, however, the presence of NOx was of concern.
In 1992, a program was
initiated to identify the source, concentrations and distribution of NOx within the acid
plant. The operation of the furnaces with
oxy-natural gas burners produced a gas containing typically 20 ppm NOx . With increased throughput and the use of coke
instead of natural, NOx levels were reduced to the 0 to 10 ppm range. The NOx levels would spike to approximately 400
ppm whenever combustion gases were vented to the acid plant but they were for short
durations and somewhat controllable.
In August 1992, the
drains from the candle mist eliminators were piped together and directed outside of the
tower for disposal, to ensure product acid specification could be met with respect to NOx.
In May 1993, the copper
reactor was started resulting in NOx levels in the off-gas of 300 to 400 ppm. Operating the conventional oxy-fuel burners at 50
to 70% stoichiometry was able to reduce NOx levels to approximately 100 ppm. This resulted in high NOx levels in the product
acid despite the fact that the candle drainings were being collected and separated from
the circulating acid.
It was hypothesized that
some of the mist collected by the candles was draining to the outside of the candle
instead of the inside where it is collected. To
test this theory, a collecting bath was installed on one candle in order to collect any
mist draining to the outside of the candle. The
results of the testwork showed that a significant portion of mist was draining to the
outside of the candle rather than the inside and this occurred most often when the plant
was operating at less than design flows.
The solution was to
equip the bottom of each candle with a collecting bath.
The bath was deep enough to allow acid to accumulate to a sufficient depth
that the acid would begin to drain through the candle to the inside where it would be
collected through the drain system. The
modifications to the candles proved to be effective in preventing mist from draining back
into the circulating acid and contaminating the product acid. The interpass system collects about 40 kg/h of 4
to 6% NO3 and the final system about 30 kg/h of 2% NO3.
References
Humphris, M.J., Liu, J., Javor, F., Gas Cleaning and Acid
Plant Operations at the INCO Copper Cliff Smelter, Nickel-Colbalt '97.
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