Flue gas treatment by electron beam technology

Various methods have been developed over the years for SO2 and NOx removal from coal firing exhausts; the conventional technologies deal with SO2 by wet scrubbing and NOx by catalytic reduction, but neither method can be adapted to remove all these gases.

The concept of treatment of flue gas by electron beam technology was first put forward over twenty years ago. Since then research on the process, carried out mainly at pilot plants in Japan, the USA, Germany and Poland, has established a practicable process. Recent findings, based on experiments performed on pilot plants at Kawêczyn power station and the Chubu power station, Nagoya, have led to developments which have brought the process to maturity just at the dawn of the 21st century. Now the process is being carried out on a fully industrial scale at Pomorzany power station by the Dolna Odra EPS group. Other developments, primarily on oil firing plants have been reported from Japan and after the Nagoya pilot plant experiments in 1995, an industrial coal burning plant was built in China while another oil fired demonstration plant, this time of 200 MWe, has recently started up by Chubu Electric Co, in Nagoya.

There are remarkable differences in the technological and design solutions applied in these installations. Developments achieved at EPS Kawêczyn pilot plant and at the INCT (Institute of Nuclear Chemistry and Technology) laboratory unit were the basis for the project realised at EPS Pomorzany.

The current situation is that two industrial plants are in operation – one in Chengdu, China, the other in Szczecin, Poland. Two further industrial plants are under construction, in Japan and China.

The technology has demonstrated the following advantages;

• simultaneous SO2 and NOx removal,

• usable by product formation

• dry operation

• that it meets the stringent requirements internationally for the removal of SO2 and NOx.

Pilot plant

In 1991 a pilot plant with a capacity of 20 000 Nm3/h was constructed under an IAEA/Poland technical cooperation project and installed on a bypass of the main flue gas stream from a WP-120 type boiler at Kawêczyn power station. Two ELV-3a accelerators rated at 50 kW, 700 keV, were installed in series on a reaction vessel. For the first time on an industrial pilot installation, cascade double gas irradiation was applied.

The flue gases are cooled in a spray cooler working with “dry-bottom” conditions and then irradiated in two steps in the reaction vessel, a cylinder 7 m long and 1.6 m in diameter. The double titanium window system is employed. An air curtain was installed to separate the secondary window from corrosive flue gas. Gaseous ammonia is injected by nozzles upstream of the flue gas irradiation stage. Aerosol particles formed in the process are small, with a diameter less than 1 micron, and “sticky”. Three types of filters were tested – a bag filter, a gravel bed filter, and electrostatic precipitator. The electrostatic precipitator, constructed by ELWO SA at its Pszczyna factory has performed well throughout the testing process.

The pilot plant is equipped with a modern monitoring and control system and manual sampling methods were also used for to verify results.

Emissions performance

Results showed that NOx removal efficiency depends to a large extent on the dose and on the inlet NOx concentration while SO2 is mostly removed by the thermal reaction.

High enthalpy water and steam were used for flue gas conditioning. The humidity of flue gases can be increased to 15 per cent by volume and the gas inlet temperature reduced to 55 - 65°C. Under these conditions an SO2 removal efficiency as high as 98 per cent can be achieved. Owing to the thermal reaction very low dosing is required to obtain a high removal efficiency of SO2. When moderate NOx removal efficiency is required the economical competitiveness of the electron beam process with conventional technologies increases further; and it has been demonstrated that the higher the SO2 content the more economical the electron beam process becomes, especially when taken together with the costs of NOx removal.

By-product

The by-product obtained during electron beam purification of flue gases – ammonium sulphate – is of a quality that can be used as commercial fertiliser. Another possibility is to use the by-product as a component of NPK fertiliser. A test performed in Poland has shown that a blend obtained using the by-product as an ingredient meets the standards established for this kind of fertiliser. Horticultural tests have been performed for the pure by-product and the by-product with different levels of fly-ash content. Results have been very positive. The heavy metals content is much lower than the values allowable for commercial fertilisers.

Scaling up

The most important problem to be solved in scaling up the process was construction of the irradiation vessel. In the Kawêczyn pilot plant longitudinal, double irradiation of the flue gases had been applied. It had been discovered that further optimisation can be achieved by adequate dose distribution between irradiation stages.

Industrial project

The experience gathered during laboratory and pilot plant tests was encouraging enough to go to the next stage – the preparation of a full scale industrial plant design (see schematic on page 53). Consequently, starting in 1999, the process was installed at the Pomorzany power station near the port of Szczecin in northern Poland for the simultaneous removal of SO2 and NOx from flue gases emitted by Benson boilers. The installation cost $20 million, with 40 per cent of that sum coming from Japan, The Republic of Korea and the IAEA. The plant was built by Energobudowa SA.

After detailed analysis it was decided to design and construct the electron-beam installation to treat a maximum 300 000 Nm3/h flow of flue gas. The high concentration of NOx and relatively low content of SO2 in flue gas emitted from Benson boilers established the specific conditions for flue gas treatment. The parameters of the electron beam process were adjusted to achieve efficiency of removal of NOx at 60-80 per cent, together with 70-90 per cent for the SO2. The conceptual system arrangement is shown in the schematic on page 53.

Solutions developed at the EPS Kawêczyn pilot plant have been applied at the EPS Pomorzany industrial plant. Two parallel irradiation vessels with longitudinal gas irradiation have been installed. This solution has been found to give the best electron beam energy transfer efficiency. Other solutions developed at the pilot plant and applied in the industrial plant were the air curtain which separates the secondary window from corrosive flue gases, and the injection of high enthalpy water at the humidification stage in order to reduce the size of the humidification tower.

Testing procedure

The optimisation tests for the installation were started in January 2001. In the first quarter of 2001, four weeks of continuous operation of the installation were observed and recorded, the operational conditions being set to achieve an outlet temperature above 85°C. After mixing with untreated gases the stack temperature was greater than 110°C.