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3-way Non-Selective Catalytic Reduction (NSCR) deNOx Catalyst

Three-way catalysts (TWC) used on stationary engines function in precisely the same way as those employed with such success over the past thirty years to control automotive emissions. A typical TWC will simultaneously convert over 98% of the NOx and CO and most of the unburned HC emissions according to the NSCR reactions below:

These reactions can only occur in this manner when the oxygen content of the exhaust is controlled to less than 1% vol. (typically about 0.5% vol.), accomplished by attaching an air/fuel controller (lambda sensor) to the engine to maintain the chemically correct (or stoichiometric) air/fuel ratio (AFR), such that all the fuel and oxygen in the mixture are consumed on combustion, and is typically referred to a rich-burn or stoichiometric operation.

NSCR as applied to IC engines typically comprises:

• Rich-burn, spark-ignited, natural gas-fuelled engine
• Three-way NSCR catalyst technology
• Air/fuel ratio (AFR) or lambda (λ) control system (to regulate fuel & air flow)

NSCR is unsuitable for lean-burn engine exhaust as there is insufficient CO and HC for the reduction of the NOx.

NSCR has been used routinely in the automotive industry since the mid-1970s to reduce vehicular CO, HC and NOx emissions. Application of NSCR to stationary gas engines for the control of NOx and CO first became commercially available in N America in the late 1980s. NSCR techniques have also been successfully employed in the control of NOx emissions from chemical manufacturing plants.

Typical non-automotive NSCR Applications include:

• Gas Gathering and Storage
• Gas Transmission
• Power Generation
• Combined Heat & Power (CHP) Cogeneration/Trigeneration
• Irrigation
• Inert Gas Production
• Non-Road Mobile Machinery (NRMM)

NSCR is a proven emission control catalyst technology with over a billion catalyst units equipped to automobiles since the mid-1970s and well over 2,000 stationary engine installations in service today.

+ NSCR for Rich-burn Stationary Reciprocating IC Engines
A rich-burn reciprocating IC engine (RICE) is defined as a spark-ignited, Otto-cycle (or two-stroke) engine operated with gaseous fuel with an exhaust stream oxygen concentration of < 4% vol. Whilst 1% oxygen content typically gives most efficient engine performance, a net oxygen content of less than 0.5% is required for the successful application of non-selective catalytic reduction (NSCR) emission control technology.

As with automotive sector, stationary NSCR involves the use of TWC technology to promote the reduction of NOx to nitrogen and water and simultaneous oxidation of CO and HC to carbon dioxide and water. NOx is reduced by the CO over the catalyst under slightly rich or stoichiometric conditions to produce CO2 and water with typical conversion efficiencies in the range 80-90% achievable together with corresponding decreases in HC and CO. NSCR-equipped rich burn engines operate at lambda values in the range 0.992 > λ > 0.988 to ensure net oxygen values < 0.5% vol. NSCR is generally not suitable for retrofit to stationary fuel-injected engines owing to AFR compatability issues but widely retrofitted to carburetted stationary rich-burn engines as an effective exhaust emissions aftertreatment technique.

In addition to catalysts and housings (converters), NSCR retrofits require installation of an oxygen sensor and feedback controller to maintain an appropriate AFR under variable load conditions.

Key operating parameters for effective stationary NSCR include:

• Slightly rich to stoichiometric air to fuel ratio (oxygen content range: 0.20-0.70%)
• Lambda setting on AFR controller of 0.97 to 0.99 (oxygen sensor range: 700-800 mV)
• Inlet NOx level range: 2000-4000 ppmv
• Inlet CO level range: 3000-6000 ppmv
• Inlet THC level range: 1000-2000 ppmv
• Inlet gas temperature range: 425- 650°C (800-1200°F)

Fuel & lube oil requirements:

• Sulphur levels should be < 200 ppmv (natural gas) or < 2000 ppm (#2 diesel).
• Lube oil consumption rate should be: < 0.0015 lb/bhphr.
• Lube oil components should be: < 0.09 wt%P, < 0.04 wt% Zn.
• Sulphated ash content should be: < 0.5 wt%.

AFR Control requirements:

• Off-the-shelf carburettor set to slightly lean of stoichiometric
• Supplemental fuel valve to enrich the mixture to required level
• Lambda sensor(s) for system feedback
• Pre- & post-catalyst thermocouples for temperature control
• A controller logic/display module

+ NSCR for Nitric Acid Plant Tail Gas Control

Johnson Matthey has four decades’ experience in the supply of non-selective catalytic reduction (NSCR) technology for control of oxides of nitrogen (NOx) - including nitrous oxide (N2O) - in reduced oxygen air streams such as those typically found in tail gas of nitric acid manufacturing plants.

The catalyst technology used is tried and tested and backed up by the highest manufacturing and quality control standards. The catalytic reaction is based on the reaction between NOx and a fuel to give nitrogen and water. The reducing fuel is generally chosen by site availability and price (typically natural gas, light naphtha or hydrogen).

The fuel gas is introduced into the NOx process upstream where it homogeneously mixes before entering the Honeycat® metal or ceramic honeycomb catalyst bed. The honeycomb substrate is impregnated with a special high temperature washcoat and various combinations of platinum group metals (pgm). The following reactions take place:

This reaction only takes place under reduced conditions when all the oxygen has reacted with the fuel. The maximum operating temperature for the reduction of the NOx using Honeycat® NSCR is 750°C. The advantages of the Honeycat® NSCR NOx abatement catalyst technology are:

• NOx – including N₂O - is reduced to harmless nitrogen and water vapour (in contrast to SCR technology which does not affect nitrous oxide)
• The catalyst has a long life and provides a very high NOX removal rate 95%+.

NOx Abatement from Nitric Acid Plants
In addition to NOx (NO, NO2) emissions, nitric acid production is also the largest source of nitrous oxide (N₂O) pollution within the chemical industry. Reductions in these emissions are urgently required as nitrous oxide plays a significant role in ozone layer depletion and is one of the primary greenhouse gases.

SCR (Selective Catalytic Reduction) has no effect on N₂O, 3-way NSCR (Non Selective Catalytic Reduction) catalyst technology is also routinely used to control tail gas emissions from nitric acid manufacturing plants.

The production of nitric acid by the oxidation of ammonia is an important industrial process throughout the world. Nitric acid is used in the manufacture of many products, notably fertilisers, explosives and nylon. While these products may be instrumental in improving the standard of life, nitric acid production can give rise to serious pollution problems. One way in which this problem can be alleviated is to use pgm catalyst technology to control the emissions of NOx from the vent stack.

Nitric Acid Production
The first part of the nitric acid process involves the reaction of ammonia and air under pressure over a rhodium-platinum gauze to give nitric oxide.

4NH₃ + 5O₂ => 4NO + 6H₂O

After the process gases have been cooled in a series of heat exchangers, boilers and steam super-heaters, they pass into an oxidising adsorption tower where, with air and water, they react to form nitrogen dioxide and subsequently nitric acid.

The residual nitric oxide is, in practise, re-oxidised to nitrogen dioxide for further conversion to nitric acid. There is an economic limit to the size of the adsorption tower that is practicable and the adsorption efficiency achieved is generally in the range 98.2 to 99.3%. It is the residual concentrations of nitrogen dioxide and nitric oxide (commonly referred to as NOx) that give rise to the pollution problem in the vent stack.

Image courtesy of AirProtekt Ltd

NOx Pollution
Nitrogen Oxide is a red-brown acidic gas with a pungent odour; nitric oxide is colourless and odourless but is oxides by the oxygen in air to nitrogen dioxide. The toxic effects of nitrogen dioxide are well documented. For example, human exposure to level of 300ppm may cause fatal bronchopneumonia. Long-term exposure to plant at levels below 1ppm can cause leaf damage and decreased fruit yield.

Example of NOx emissions prior to application of NSCR abatement technology
The effluent gas from a typical nitric acid plant of 350tons/day capacity has a volume of about 34,000 Nm3/hr (20,000 scfm) with a typical composition in the following range:

Expressed in weight terms, the total NOx emissions would over five tonnes/day.

NOx Removal
After the gases have left the absorption tower, they possess considerable energy, still being a the plant pressure, which may be up to eight atmospheres. This energy is recovered on passing through an expansion turbine; the rotational energy is normally used to drive the plant’s compressor. Several methods exist by which tail gas NOx concentration can be reduced to more acceptable levels.

Absorption
By increasing the size of the plant’s absorption tower it s efficiency may be increased. This is general a high capital outlay, particularly for an existing plant, and this outlay is not compensated by the small increase in nitric acid production that results.

Catalytic Combustion
The catalytic reduction of NOx to nitrogen gas takes place on a catalyst such as platinum or palladium on a ceramic honeycomb support or alumina pellets or nickel-chromium alloy ribbon.

The catalytic method for removing the nitrogen oxides is based on the reaction of NOx with a fuel to give colourless and harmless nitrogen and water. The reducing fuel is generally determined by availability and price: typical fuels are hydrogen (for example ammonia synthesis-loop purge gas or refinery off-gas), light naptha, methane (natural gas) or LPG (butane or propane).

The tail gas from the plant’s absorption tower is pre-heated in the heat exchangers, using the hot gases from the rhodium-platinum ammonia-oxidation gauze, to a temperature which is dependent on the design of the plant and the fuel used. Minimum inlet temperatures for good conversion efficiency and catalyst life are shown below for commonly used fuels with the Honeycat® catalyst specially developed by Johnson Matthey

The fuel gas is introduced into the gas stream and a homogeneous mixture of tail gas and fuel passed into a reactor containing a bed of catalyst as shown below:

Honeycat® catalyst consists of pgm deposited onto ceramic honeycomb. The bed is built-up from blocks of catalyst. The reaction taking place on the catalyst involves the reaction of the fuel with oxygen and nitrogen dioxide, reducing the latter to nitric oxide. These reactions take place first and are sometimes referred to as the decolourisation reactions and depend on the type of fuel used.

“Decolourisation” Reactions:

In the past, plants were been designed where sufficient fuel was added to raise the temperature of the tail gas and to decolourise the nitrogen oxides. The primary purpose of this operation is to increase the amount of power recovered at the expansion turbine by operation at a higher temperature. Decolourisation is a bonus, which, while it does not reduce the total acidity of the tail gas, does in fact improve the appearance of the plant and make it a less obvious polluters. However, no new plants are being built with decolourisation.

The second group of reactions does not commence until reducing conditions have been achieved by all the oxygen in the tail gas being reacted with the fuel. These are sometimes referred to as the abatement reactions and depend on the type of fuel used.

“Abatement” Reactions:

Provided that a sufficient quantity of fuel is added to react with the oxygen and the NOx, the gases leaving the catalyst reactor will contain substantially reduced concentrations of NOx and oxygen and will consist almost entirely of nitrogen gas and water vapour. Total abatement generally means NOx removal down to below 200ppm. The advantages of operating a fuel-rich system are lower light-off temperature and higher conversion efficiency when compared to operation under decolourising or oxidising conditions.

The reaction of the fuel gas with the oxygen in the tail gas is, of course, an exothermic reaction and the gas leaves the catalyst at an elevated temperature. This temperature rise is proportional to the oxygen content of the tail gas but varies according to the pressure and temperature of the reaction. As a general rule, however, the following temperature rise can be used for each one per cent of oxygen that is burnt with the named fuel:

This temperature rise puts a limitation on to the amount of oxygen that can be removed on a single stage. The maximum oxygen content can be calculated by subtracting the inlet temperature required with the chosen fuel from the maximum permissible operating temperature of 750°C and dividing by the temperature rise obtained if 1% oxygen were burnt with that fuel. For example, with hydrogen fuel and inlet temperature of 200°C the maximum temperature rise is 550°C and the maximum permissible oxygen concentration is 3.4%

Catalyst Form
The composition and form of the catalyst used for the above reactions has an important bearing on the operation, the degree of abatement achieved, the efficiency of fuel combustion, and the catalyst life.

The advantages of using ceramic honeycomb supports for pgm catalysts for various gas phase reactions, such as odour and NOx removal, and the treatment of diesel and gasoline engine exhausts may be summarised as compact process design, high activity, low pressure drop and good catalyst life. These advantages have led to honeycomb-based systems becoming widely accepted over pelleted or metal ribbon support systems for NOx abatement of tail gas streams from nitric acid plants.

The activity of the catalyst is particularly important when methane is used as the reducing fuel because the methane molecule has high stability and is difficult to oxidise. The Honeycat® range of catalyst is sued on platinum because Pt has high stability coupled with good activity and also under reducing conditions it does not allow carbon formation, which may occur with Pd-based systems. For methane combustion an improved version of the Honeycat® catalyst has been developed to give higher activity for methane oxidation while retaining the resistance to carbon formation.

Over the past four decades, many Honeycat® catalysts and systems have been installed in nitric acid plants in Europe and North America.

For details of Honeycat® NOx abatement installations, please click here.

For enquiries about Honeycat® tail gas NOx abatement catalyst technology, please click here.

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