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 LoTOx^TM 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.

LoTOx^TM Process Description

The LoTOx^TM 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. LoTOx^TM 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 LoTOx^TM 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 LoTOx^TM 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 LoTOx^TM 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 LoTOx^TM System.

The LoTOx^TM 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 + O₃à 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 LoTOx^TM 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, LoTO^TM technology uses ozone to selectively oxidize NOx.

Reduction Using NH₃ / Urea Oxidation Using O3/O2

(SCR, SNCR) (LoTOx^TM)

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

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

	SNCR	SCR	LoTOx^TM
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 O₃Consumption 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 LoTOx^TM System’s key advantages over technologies based on reduction is that the LoTOx^TM 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 LoTOx^TM system are shown in Table 3.

Table 3 - NOx Emissions Control Technologies from Various Sources –
LoTOx^TMAllows Multi-Pollutant Emissions Control

Emissions / Source Characteristics	SNCR	SCR	LoTOx^TM
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 LoTOx^TM 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 LoTOx^TM 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 LoTOx^TM 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 LoTOx^TM 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 LoTOx^TM System will reduce NOx emissions by up to 70 tons per year, depending on production schedule. The LoTOx^TM 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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ICAT Funded Projects