Of Scrubbers, Bag Houses and Flue Gas Cleanup

The problem that Governments face is that, while they can take political and idealistic stances about issues, when the chips start to fall they are in charge of governing. Which means that, regardless of idealism, they have to recognize the reality of (among other things) providing energy to their nation. Thus, while we see a considerable public posture by the British Government over Climate Change, they are moving ahead with plans for increased coal mining activity. In the short term the industry is to be awarded permission to mine an additional 15 million tons, and production is rising by 15% from surface mines (opencast) as imports also go up. (In the UK surface mining now accounts for more coal than does underground production). Some 54 new mines have been approved in the past four years. It is a recognition that idealism or not, power must be provided.

Carbon capture and sequestration is still perhaps ten years away from large scale application, but the flue gases produced from the combustion of the coal are already being cleaned of some of their content gases, and I thought that I would talk today about scrubber technology and what is currently done to flue gases before they are released.

While I will describe how a modern power plant works in another post, suffice it to say for now that coal is used to heat water to steam, and that steam then turns turbines that spin generators, that produce electricity. After the coal is burned, the combustion gases are released to the atmosphere and are referred to as flue gases. But, because of the chemicals and particles that are left in these gases, they are first cleaned or “scrubbed” to remove some of the noxious ones. The most infamous of these is sulfur dioxide (SO2) which, if released into the air can combine with water vapor in the air to form sulphuric acid. The vast majority of the sulfur dioxide in the air comes from electricity generation and at historic levels, the acid that was produced and precipitated as a rain which could not only cause damage to surfaces it fell on, but also posed a health hazard. Hydrogen chloride might also be formed, and some of the nitrous oxides that are emitted also combine with water to form nitric acid, and commonly the combination is referred to as acid rain. To prevent it, it is easier to remove the acid forming gases at the power stations before they are released.

U.S. Sulfur dioxide emission levels in 2002 (EPA)

Formation of acid rain (EPA)

So how do we capture a gas that is coming out of a tall chimney? Simple answer is at the bottom, before it goes up the chimney. In a more complex answer the gas is fed into a chamber where either hydrated lime is sprayed as a fine particulate into the flue gas (known as dry injection) or the limestone/lime is sprayed through nozzles in a slurry form (the wet scrubbing). Efficiency of removal of the sulfur dioxide is in the 95 to 99% range. Wet scrubbing is the most widely practiced of the two with limestone being the preferred absorbent over lime. (Mainly because of cost).

Simplified scrubber schematic (Source Penn State)

The result is a wet mixture of calcium sulphate and calcium sulphite (the heat of the flue gas is supposed to dry the particles in some designs). In the process the reaction also releases carbon dioxide.

SO2 + CaCO3 = CaSO3 + CO2

Or, where the remaining air in the flue gas is supplemented

SO2 + CaCO3 + 1/2O2 + 2H2O = CaSO4.2H2O + CO2

With the gypsum being a saleable product into either wallboard manufacture, cement making or agricultural products. The solid must be removed from any residual liquid (often using hydrocyclones) and then is ready for processing. The process can also pick up some of the mercury in the flue gas, and in that case the circuit may be modified to strip that from the waste water.

Waste water treatment at the Merrimack Power Plant (Siemens)

In an alternative version of scrubber technology, the Chinese have used salt water, rather than lime or limestone, as the scrubbing agent. In the schematic of the process the dust collector is the bag house and the Absorbers are the scrubbers:

Schematic of the cleaning circuit at the Mawan Power Plant

Mawan Seawater Scrubber Power Plant Layout

But at the same time the power plant will also capture the fine particles of ash left over from the combustion and some of the lime particles may get picked up in the cleaned flue gas and they need to be captured before the gas is released to the atmosphere.

The fine residue from the furnace is called fly ash, and because of its small but regular size and some of its chemical properties it has some commercial value (when mixed into cement it can significantly improve it, for example). Thus up to 45% of the fly ash that is collected can be used in this and other ways, and the EPA is hoping that up to 50% will find a use in the next couple of years, since that which has no use must be land-filled.

Because the fly ash is small in size (5 – 100 micron ) and hot (about 140 deg C) as it comes out of the boiler it is trapped as the gas passes through fine filter bags of a special weave that collect this residue, and at intervals the filters are shaken, loosening the ash, which drops into hoppers and can be collected and removed. This operation takes place in what is known as a bag house. On rare occasions bag houses can catch fire, if all procedures aren’t followed properly. The filter weave must, obviously be smaller than the particles if it is to trap them.

Typical baghouse construction

Rules for flue gas emissions are being tightened, and as a result emission levels continue to fall

Sulfur dioxide emissions were down 24 percent compared to the first half of 2008, much more than would be expected due to the recession and lower electricity demand, the power industry data provider said in its quarterly review of energy trends.

"The industry is clearly going through a dress rehearsal for the implementation of the Clean Air Interstate Rule (CAIR) in 2010, and judging by allowance prices as well as the fundamental data, it is a stellar performance," Genscape said.

The change in rules requires scrubbers in plants where there was no need before, since the rule caps emissions of SO2

But the decline in SO2 is largely because of the new rules coming in 2010 and an allowance scheme that favors early implementation, the power data provider said.

"Most of the decline in sulfur emissions is not due to the recession or even to the switch from high-sulfur coal to lower sulfur grades and to gas," Genscape said, noting many plants have installed equipment to remove SO2 from emissions.

"It makes sense to start cutting emissions early if the equipment is in place since pre-CAIR vintage allowances will retain their full face value of a ton of SO2, while from 2010 onward, each permit will be worth only half a ton," Genscape said.

Well this is fairly brief review of what a scrubber and baghouse are and what they do when attached to a Power Station. As with all Tech Talks I have had to simplify and condense things, perhaps a little more than is clear. In which case please ether comment if you know more, or ask if something is not clear.