Low Temperature Oxidation System

This page updated November 18, 2005.

BOC Gases

Low Temperature Oxidation System

CARB Grant Number ICAT 99-2


The statements and conclusions in this Report are those of the grantee and not necessarily those of the California Air Resources Board. The mention of commercial products, their source, or their use in connection with material reported herein is not to be construed as actual or implied endorsement of such products.

Overview
BOC Gases has demonstrated it’s patented LoTOxTM technology for low temperature NOx removal at a reverbertory furnace used for lead smelting, operated by Quemetco, Inc., City of Industry, California, a subsidiary of RSR Corporation. The demonstration was funded in part by an Innovative Clean Air Technologies grant from the California Air Resources Board. The demonstration was very successful, accomplishing > 90 percent NOx removal under varying reverb furnace operating conditions, and the installation of a full scale system for control of NOx emissions from the Quemetco, Inc., process is expected to be completed by the end of 2001.
LoTOxTM Process Description
The LoTOxTM System is a NOx removal system that injects ozone into the flue gas stream to oxidize insoluble NOx to soluble oxidized compounds. Ozone is produced on site and on demand by passing oxygen through an ozone generator. LoTOxTM is a low temperature system; therefore, it does not require heat input to maintain operational efficiency or to prevent the "slip" of treatment chemicals, such as ammonia, as is common with SCR and SNCR systems.
Ozone is produced in response to the amount of NOx present in the flue gas generated by the process. The low operating temperature allows stable and consistent control regardless of variation in flow, load or NOx content. There are no adverse effects of acid gases or particles on the LoTOxTM System, and some particles may even enhance the reaction by producing sites for nucleation of moisture and by catalyzing the oxidation reaction.
Ozone rapidly reacts with insoluble NO and NO2 molecules to form soluble N2O5. The species N2O5 is highly soluble and will rapidly react with moisture in the gas stream to form nitric acid. The conversion of NOx into the aqueous phase in the scrubber is rapid and irreversible, allowing nearly complete removal of NOx. The nitric acid, along with unreacted N2O5 and nitrous acid formed by reaction of NO2 with water, can be easily scrubbed out of the gas stream in a wet scrubber with water or neutralized with a caustic solution.
The rapid reaction rate of ozone with NOx makes ozone highly selective for treatment of NOx in the presence of other compounds such as CO and SOx, resulting in a high ozone utilization efficiency for NOx removal with no wasteful consumption of ozone by CO and SOx.
The LoTOxTM System can be installed as a stand-alone design or as a polishing system in compliment to combustion modifications or other post-combustion technologies for NOx removal. Because the system is modular, it requires minimal downtime to install and can be made to fit into limited space, with auxiliary components located elsewhere on-site. The economic and environmental benefits of heat recovery are achieved readily because of the LoTOxTM technology’s low optimum operating temperature. Since the system efficiently removes more than 90 percent of the NOx downstream of the combustion process, the operator can tune combustion components for stable burner operating conditions, allowing more efficient combustion and lower CO emissions. This reduces fuel consumption and maintenance costs and extends boiler / burner life.

Figure 1 is an overall process diagram of the LoTOxTM System.


The LoTOxTM technology is based on the high solubility of higher-order nitrogen oxides in water. NO and NO2 are relatively insoluble in water as compared to other oxides as shown in Table 1, where typical constituents in the combustion flue exhaust are listed.

Table 1 - Relative Solubility of Various Gases in Water at 70ºF and 1 Atmosphere

Gas

Relative Solubility

Nitrogen, N2

1.0

Oxygen, O2

4.4

Nitric Oxide, NO

6.3

Nitrogen Dioxide, NO2

126

Carbon Dioxide, CO2

174

Sulfur Dioxide, SO2

11,700

However, once the NOx is oxidized to higher orders of nitrogen oxides, their solubility and reactivity with water increase significantly. The NOx oxidation step proceeds according to the following reactions:

NO + O3 à NO2 + O2k298 = 1.8 x 10-14 (cm3/molecule*second)

NO2 + O3 à NO3 + O2k298 = 3.2 x 10-17 (cm3/molecule*second)

NO2 + NO3 à N2 + O5k298 = 2.0 x 10-12 (cm3/molecule*second)

These reactions are much faster than the ozone oxidation of CO to CO2 [k298 ~ 1.1 x 10-21 (cm3/molecule*second)] and of SO2 to SO3 [k298 = 2.2 x 10-22 (cm3/molecule*second)]. Thus ozone is highly selective for the oxidation of NOx relative to other combustion flue gas components. Nitrogen pentoxide (N2O5) formed during the oxidation process reacts vigorously with moisture in the gas stream to form nitric acid (HNO3).
Comparison of Post-Combustion NOx Abatement Technologies
Although there are other competing NOx technologies, SNCR and SCR remain the most commonly used post combustion NOx abatement technologies. The LoTOxTM technology works quite differently from SCR and SNCR in the principal for NOx abatement; i.e., while SNCR and SCR reduce NOx using ammonia and urea, LoTOTM technology uses ozone to selectively oxidize NOx.

Reduction Using NH3 / Urea Oxidation Using O3/O2

(SCR, SNCR) (LoTOxTM)


The technical and operating features of SNCR and SCR are compared to the LoTOxTM technology in Table 2.

Table 2 - Comparison of Common Post Combustion NOx Abatement
Technologies for NOx Emissions Control

 

SNCR

SCR

LoTOxTM

Mode of Treatment

Reduction

Reduction

Oxidation

Active Chemical

NH3 / Urea

NH3 / Urea

Ozone

Gas Temperature
Required °F

1650 - 2000

500 - 900

150 – 250

Pressure Required,
psig

0+

0++

0+

Placement in
Exhaust System

Near Combustion
Chamber

Between Air Pre-Heater
and Economizer

Tail End

Catalyst Bed

No

Yes

No

Gas Phase
Reaction Duct

Yes

No

Yes

Scrubber

Optional

Optional

Yes

NOx Reduction
Achieved

40 - 70 Percent

60 - 95 Percent

90 - 98 Percent

Slip of Active Agent

NH3 – Yes

NH3- Yes

Ozone - No

CO Emissions After
Treatment

May Increase

May Increase

No Effect

SOx Emissions After
Treatment

Little Effect – Maybe
H2S

Little Effect - Maybe
H2S

No Effect or
Significantly Reduced

Gas Temperature
Outside Operating
Range - Overshoot

More NOx Emissions
Through NH3
Oxidation

More NOx Emissions
Through NH3 Oxidation,
Ammonia Slip

Increased O3
Consumption for
the Time Period
of Overshoot

Gas Temperature
Outside Operating
Range - Undershoot

More NOx Emissions
Through Reduced
Reduction by NH3

More NOx Emissions
Through Reduced
Reduction by NH3,
Ammonia Slip

No Effect


One of LoTOxTM System’s key advantages over technologies based on reduction is that the LoTOxTM System can conveniently handle multi-pollutants. Since SOx and particulate emissions commonly accompany NOx emission, integrated scrubber systems can be implemented to remove NOx, SOx and particles without significant additional costs. These features of the LoTOxTM system are shown in Table 3.

Table 3 - NOx Emissions Control Technologies from Various Sources –
LoTOx
TM Allows Multi-Pollutant Emissions Control

Emissions / Source Characteristics

SNCR

SCR

LoTOxTM

Utility Boilers      
Industrial Furnaces      
FCC / Refining with NOx Only      
Electric Arcs for Metals      
Gas-Fired Glass Furnaces      
Low Temperature Sources      
Pickling / HNO3 Production      
Battery Processing / Acid Recycling      
Coal-Fired Utility Boilers with Mercury / NOx Emissions      
Emissions with VOC’s and NOx      
Emissions with Highly Variable NOx      
Description of Testing
The testing at Quemetco, Inc., focused on measuring incoming NOx and flow variations, varied ozone dosage, residence time and temperature data, and the impact of these variations on treatment efficiency. The experimentation was conducted in manual operation mode to effect these parametric changes, varying specific parametric settings and recording the results as part of the data acquisition system. The residence time and treatment effectiveness test series provided valuable information for characterization of the process gas stream. Information was tracked for ozone usage in relation to percentage of NOx removal, residual O3 and temperature effects.

Range of Gas Test Conditions

Gas Flow Range

150 to 300 scfm

Temperature Inlet

150 to 250 ºF

Inlet NOx

0 - 500 ppmvd
(Parts / Million by Volume, Dry Basis)

Inlet SOx

0 - 5000 ppmvd

Inlet O2

0 - 25 Volume Percent

Inlet CO

0 - 100 ppmvd

Demonstration Test Results
There are three primary process variables which impact the NOx control capabilities of LoTOxTM process: Temperature of the flue gas (T), Residence Time of the flue gas in the reactor duct (RT) and Amount of Ozone (O3 Factors) injected into the reactor duct. Based on the results of the LoTOxTM demonstration testing, the most efficient control parameters for a full scale system on the Quemetco, Inc., furnace are a temperature of 175°F or less, a residence time of four seconds, and an ozone injection factor of 80 to 90. The O3 Factor represents the O3 to NOx ratio at the inlet to the reaction duct. The presented data uses O3 Factor for the sake of clarity and to provide meaningful data while maintaining confidentiality of proprietary intellectual information.
Graph 1 depicts the performance conditions of the LoTOxTM process for the design conditions of RT = 4 seconds, T = 175°F and O3 Factor = 70 to 90. Under these design conditions the NOx removal achieved ranges from 80 percent at an O3 Factor of 70 to 95 percent at an O3 Factor of 90.

Graph 1


Summary
From the demonstration test data, it is clear that the LoTOxTM process is extremely effective and efficient at removing NOx from the Quemetco, Inc., furnace flue gas. NOx removal efficiencies ranging from 80 percent to 95 percent were demonstrated as achievable and controllable. When installed to treat the entire furnace exhaust (at 100 ppm NOx) the LoTOxTM System will reduce NOx emissions by up to 70 tons per year, depending on production schedule. The LoTOxTM System also allows the ability to tailor the NOx emissions to the source allowables, providing market-sensitive NOx control. By injecting more or less ozone, the degree of NOx removal can be adjusted to optimize the economic value of creating saleable NOx credits. Full scale systems include automated control systems which automatically adjust the ozone injection rates in order to maintain a desired outlet NOx level, thereby enabling the system to automatically respond to variations in the process conditions. The testing also demonstrated that residual ozone levels can be maintained at undetectable levels despite variations in the process conditions, due to the effectiveness of sulfite, which is formed as a by-product of Quemetco’s SOx removal system, as an ozone scavenger.

Funding Source

Funding Amount


ICAT

$199,790

Grantee

$555,540


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