Flue gas desulfurization: SNOX process with unique catalytic converter for NOx and SO2

In order to meet stringent environmental regulations limiting SO2 emissions enacted in many countries, SO2 removal from flue gases is an important requirement for plants use fossil fuels. Conventional flue gas desulfurization (FGD) methods are troubled by high operating costs, massive limestone consumption, and production of contaminated gypsum that must be deposited.

SNOX technology

It was funded by the U.S. Department of Energy (DOE) under the Clean Coal Technology (CCT) program

One of the major thrusts of the CCT program is to develop and demonstrate technology options for reducing the emissions of acid rain precursors that result from utility and industrial combustion of coal.

In brief, the SNOX process involves catalytic reduction of NOx in the presence of ammonia (NH3), followed by catalytic oxidation of SO2 to SO3. The exit gas from the SO3 converter passes through a novel glass-tube condenser in which the SO3 is hydrated to H2SO4 vapor and then condensed to a concentrated liquid sulfuric acid (H2SO4). Before entering the SNOX system, most of the fly ash is removed from the flue gas, leaving the boiler in a high-efficiency, fabric filter baghouse to minimize the cleaning frequency of the catalysts in the two downstream conversion processes. The SNOX process removes virtually all of the remaining fine particulates by capture on the catalyst or in the condensation of H2SO4.

Credit: By Anne Mette S?rensen?

Process

In the first step, the particulates are significantly reduced in a conventional pulse-jet baghouse fitted with high-efficiency bags. The flue gas is heated to SCR reaction temperature 730 degs F in gas/gas heat exchanger is then sent to the SCR [selective catalytic reactor] unit for NOx removal. In the SCR reactor, nitrogen oxides are selectively reduced with ammonia to elemental nitrogen and water vapor.

The reduction follows the equation

?NO + NH3 + 0.25 O2 = N2 + 1.5 H2O + 410 kJ/mole (175100 Btu/lb mole) In this equation, NO is taken to represent NOx. The small amount of NO2 present in the flue gas is similarly reduced.

The gas leaving the SCR reactor, containing residual ammonia and minimal residual fine particulates, is reheated with natural gas, oil, or steam to reach the optimum SO2 converter inlet temperature 780 deg F In the converter, filled with Haldor Topsoe VK-WSA catalyst, SO2 is oxidized to SO3 without any reagents or additives, over 95% of the entering SO2 is oxidized via the following equation.

The oxidation follows the following equation

SO2 + 0.5 O2 = SO3 + 98 kJ/mole (42600 Btu/lb mole)

This is then followed by hydration of SO3 to H2SO4 in the vapor state and then condensation to produce a high-quality, commercial-grade acid.

In addition, unburned hydrocarbons in the flue gas and ammonia slip from the SCR reactor are completely oxidized. This allows high NOx removal with high SCR space velocities, without the risk of downstream ammonium salt scaling.

SO2 hydration to H2SO4

Flue gas leaving the hot side of the gas/gas heat exchanger is further cooled to about 210 deg F in the WSA condenser. During cooling, SO3 and water react exothermically to form H2SO4, which condenses and is collected as concentrated acid. The acid formation reaction is shown in the following equation

The hydration of SO3 follows the following equation

SO3 + H2O = H2SO4 (vapor) + 100 kJ/mole (44300 Btu/lb mole)

The condensation of H2SO4 vapor follows following equation

?The condensation of acid vapor is described by following

H2SO4 (vapor) = H2SO4 (liquid) + 69 kJ/mole (29500 Btu/lb mole) (4)

The proprietary WSA condenser is in principle an air-cooled multi-tube falling film heat exchanger. The tubes are made of borosilicate glass, and other acid-wetted parts are either constructed of or lined with acid-proof materials such as acid brick and fluoropolymers. Cooling air leaving the condenser can be integrated with the boiler as preheated combustion air to significantly offset flue gas cleaning system operating costs.

Credit: Google