ECO demonstrates the attractions of multi-pollutant control

Despite long-standing environmental concerns, one-third of the world's electricity comes from coal-fired power plants. Nations with abundant coal supplies — such as China, India, the former Soviet states, and the USA — are looking at clean coal technologies to provide an answer to what has become a fundamental question in forging national energy policies: how can nations use this abundant fuel supply to satisfy the increasing demand for electricity and at the same time protect populations and natural resources from the environmental impact of burning fossil fuels?

One potentially important clean coal technology is a multi-pollutant control system for coal-fired boilers called Electro-Catalytic Oxidation (ECO). ECO, developed by Powerspan Corp, uses a combination of proven technologies to remove sulphur dioxide (SO2), nitrogen oxides (NOx), mercury (Hg), and fine particulate matter in one process. It is estimated that the capital cost is about half that of installing separate, single-pollutant control technologies.

With multi-pollutant environmental regulations now being debated in the US Congress, the time is right for commercialisation of integrated control technologies.

In pilot testing with a blend of bituminous and sub-bituminous coal, the ECO system has demonstrated greater than 95 per cent SO2 removal and 90 per cent NOx removal, based on 0.4 lb/MMBtu inlet NOx. Mercury reduction is greater than 80 per cent. Powerspan and FirstEnergy Corp, the US public utility holding company, are now installing a commercial ECO demonstration unit of approximately 50 MW at FirstEnergy's R.E. Burger power plant in Ohio.

The main components

The ECO emission control system can be installed on new or existing coal-fired power plants.

It is envisaged that each ECO system will be constructed as a separate, site-specific unit that can be connected to the power plant during a short outage.

The technology is designed to be retrofitted between a plant's existing electrostatic precipitator (ESP) or fabric filter and its stack. To connect the existing power plant to the ECO system, new connections are made on the existing flue gas outlet duct. Dampers are provided to direct gas flow to the ECO system and prevent untreated gas from entering the stack.

The ECO system consists of four components that integrate established technologies to effectively remove the primary air pollutants generated in coal-fired plants:

• ECO reactor - converts nitric oxide (NO) to nitrogen dioxide (NO2) and nitric acid (HNO3), and mercury (Hg) to mercuric oxide (HgO). The reactor also converts a small amount of SO2 (less than 20 per cent) into sulphur trioxide (SO3). The reactor design is similar to that used for industrial ozone production.

• Absorber vessel - removes SO and NO2

These pollutants are converted to useful ammonium nitrate (NH4NO3) and ammonium sulphate ((NH4)2SO4), along with nitrogen gas and water. The absorber is based on a scrubber design currently used for SO2 removal on several power plants.

• Wet ESP - captures aerosols and small particulates, including HgO, that pass through the absorber. The wet ESP has been widely used for aerosol collection in industrial process control for over 50 years.

• Byproduct recovery system - produces a commercial grade, granular fertiliser product that can be used directly in agriculture or blended with other fertiliser materials. Fertiliser granulation processes are well established commercially.

The ECO reactor

The core element of the system is the patented ECO reactor, a multi-tube assemblage fitted with centre electrodes, providing a plasma discharge for gas treatment. The reactor design is capable of real-time variation in pollutant removal as conditions in the flue gas duct change (eg, flow, temperature, and pollutant concentration). Remote control features allow system adjustment and optimisation.

As the coal combustion flue gas exits the pre-existing dry particulate collection device (ESP or fabric filter), the flue gas is routed to the ECO reactor. In the reactor the gas is exposed to a high voltage discharge, which generates high-energy electrons. The high-energy electrons initiate chemical reactions by colliding with water and oxygen molecules naturally occurring in the flue gas stream. These chemical reactions lead to the formation of oxygen and hydroxyl (OH) radicals that oxidise gaseous SO2, NO, and Hg pollutants to form different gases, aerosol mists and particulates, which are more easily collected:

• SO2 gas forms SO3 gas, which leads to formation of sulphuric acid aerosol mist;

• NO gas forms NO2 gas and nitric acid aerosol mist; and

• elemental mercury vapour forms mercuric oxide particulate.

Absorber vessel

The absorber vessel consists of two independent sections: a lower loop, which is primarily used to quench and saturate the flue gas; and an upper loop, which is considered the primary absorption and reduction zone.

Lower loop

After the flue gas passes through the reactor, it enters the absorber inlet, and then turns upwards into the lower loop spray zone. The gas passes through a set of recycle sprays, where it mixes with a liquid that cools and saturates the flue gas. The recycled liquid evaporates, concentrating the valuable ammonium sulphate and ammonium nitrate byproducts so they can be efficiently removed in the byproduct recovery system.

Upper loop

After the flue gas leaves the spray zone of the lower loop, it passes through a separator tray and into the upper loop absorption zone. From there, it passes through a series of dual flow trays and a single set of recycle spray nozzles. The upper loop recycle liquid is sprayed onto the trays. As flue gas flows up through the perforations in the trays, the flow of liquid down through the trays is restricted, causing a liquid froth level to collect on the trays. Sulphur dioxide and nitrogen dioxide transfer from the gaseous phase to the liquid phase and then react with ammonium hydroxide and ammonium sulphite in the recycle liquid to form ammonium bisulphite/bisulphate, ammonium sulphite/sulphate, ammonium nitrate, water and nitrogen gas.

After the recycle liquid drains through the trays, it is collected on the separator tray where it drains into an external tank. The liquid is pumped back to the recycle spray nozzles for further absorption/reduction reactions. Fresh ammonium hydroxide is added to the upper loop.

The treated flue gas continues upwards and into the mist elimination section, which removes entrained droplets from the gas before the flue gas enters the wet ESP.

Wet ESP

After the flue gas exits the absorber and mist eliminator, it continues into the wet ESP, which serves three primary objectives:

• Removes acid aerosols from the gas stream.

• Collects sub-micron particles such as fine ash and HgO that are carried in the flue gas and are too small to be collected by the absorber. And

• Collects and recycles excess ammonia and ammonia aerosols.

The design operates with a low-pressure drop, which minimises operating costs.

Wet ESPs have been used successfully in industrial applications for the collection of acid mists for over 50 years.

Byproduct recovery system

The fertiliser crystals are formed in the lower loop of the absorber, as the incoming flue gas evaporates water and the ammonia salts are concentrated. Effective control of the lower loop density is maintained by adding make-up liquor from the upper loop recycle tank and pumping the resulting liquid to the dewatering system.

After dewatering, the ammonium sulphate and nitrate crystals are passed through drying, granulation and screening steps in order to create a commercial grade, granulated fertiliser. As an alternative to on-site processing, crystalline fertiliser material could be used directly or provided to an existing fertiliser granulation plant.

Pilot tests

Pilot testing of the ECO technology is being conducted at FirstEnergy's R.E. Burger plant. The table below summarises the pilot test results:

The pilot test facility processes 1 500 to 3 000 scfm (standard cubic feet per minute) of flue gas, taken as a slipstream from one of the plant's 156 MW coal-fired units. This slipstream represents approximately one per cent of the plant's total flue gas flow or the equivalent of 1-2 MW.

The pilot test facility was designed, fabricated, and installed in 1998 under a $4.8 million contract with FirstEnergy.

The ECO equipment was retrofitted into a portable test ESP acquired from US-based Wheelabrator Air Pollution Control.

Currently, the pilot is showing removals of greater than 95 per cent SO2 and 90 per cent NOx, based on 0.4 lb/MMBtu inlet NOx.

Preliminary mercury tests show removal levels of over 80 per cent. This result closely matches independent testing conducted in March 2000 that showed mercury removals of 81 per cent and particulate reductions of 99.9 per cent.

The March 2000 testing was conducted with different scrubber chemistry from that being currently used, but the removal mechanisms are similar and performance is expected to be unchanged.

Mercury testing at the pilot is being funded under a $2.8 million co-operative agreement with the National Energy Technology Laboratory of the US Department of Energy.

Commercial demonstration

Powerspan has begun installation of the first commercial ECO demonstration, also at the R.E. Burger plant. The demonstration will treat 110 000 scfm of flue gas from the plant, equivalent to about 50 MW. The unit will burn Ohio coal with 2 to 4 per cent sulphur.

Objectives of this commercial demo project are: to demonstrate component performance and process reliability; to produce a commercially marketable fertiliser byproduct; and to demonstrate removal efficiency and reliability during extended runs.

The design specifications are as follows:

• SO2 emissions reduced from 4.65 pounds per million Btu (lb/MMBtu) to 0.04 lb/MMBtu;

• NOx emissions reduced from 0.4 lb/MMBtu to 0.04 lb/MMBtu;

• Hg emissions reduced from 10 micrograms per dry standard cubic meter (µg/dscm) to less than 1 µg/dscm; and

• Particulate emissions reduced from 0.03 lb/MMBtu to less than 0.004 lb/MMBtu.

Funding for the commercial demonstration is jointly provided by Powerspan, FirstEnergy, and the Ohio Coal Development Office within the Ohio Department of Development.

Wheelabrator Air Pollution Control, a leading engineering and construction firm in this field, has been retained to design and procure major equipment for the 50 MW demonstration. Powerspan is responsible for ECO process design, project management, and design and procurement for the reactor and its associated power supplies. Applied Chemical Technology is providing process development, design and equipment for production of a commercial grade fertiliser byproduct from the ECO effluent stream.

The major component in the 50 MW demonstration is the absorber vessel, which is based on a proven utility scrubber design. The wet ESP will be installed at the top of the scrubber tower, and is also based on a proven utility design. Using proven designs for major components in the 50 MW demonstration unit makes ECO readily scalable for future commercial installations.

The demonstration unit is expected to be operational in the second quarter of 2003.

Manufacturing and installation

The ECO system is designed to protect the power industry's sizeable investment in its existing infrastructure by minimising the amount of plant retrofit required for installation. The system is constructed as an independent unit and then connected to the plant, the idea being to limit the time, cost and disruption involved in the installation.

Since ECO technology provides multi-pollutant control with a single installation, it avoids the added construction and outage time required to perform multiple installations of conventional control equipment.

The ECO equipment also has a much smaller footprint than conventional control technologies, facilitating its installation on space-constrained sites that are typical of the existing coal-fired electricity generating fleet.

The system is readily adaptable to various types and sizes of coal-fired power plants, and Powerspan believes it is robust enough to achieve regulatory compliance when using coals of differing quality and operating under different conditions (eg, base loaded, load following, etc).

While ECO installations will be designed individually to suit a particular plant, the system components are largely standardised and in many cases modular. The reactor, power supplies, and wet ESP are of a modular design so that different-sized plants will require different numbers of the same module, not a unique design. This approach allows most of the ECO components and equipment to be manufactured using efficient and predictable processes.

Attractive alternative

Powerspan believes ECO technology will provide the owners of coal-fired generation with an attractive alternative to single-pollutant control systems. Based on commercial cost estimates, the ECO system can be installed as a competitive alternative to flue gas desulfurisation (FGD), with the added benefits of high NOx and Hg removals, along with the production of a valuable byproduct.

In situations where a generating plant initially requires only SO2 reductions, the ECO system can be designed and built without a reactor at a lower cost, and still provide 99 per cent SO2 reductions with a commercial byproduct stream. When required, the system could be easily retrofitted by adding the reactor and associated equipment for high removal efficiencies of NOx and Hg.

Coal is the most abundant fossil energy source and the least expensive of the world's fossil fuels with less price volatility than either natural gas or oil. Integrated technologies such as the ECO system offer a promise that coal can continue to play a major role in worldwide electricity generation.
Tables

ECO pilot test results