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.

P49. Pick Points

Sometimes there are situations that don’t know when to quit. So it is with the Ukraine:Russia dispute over gas. Today Prime Minister Putin “harshly accused” Ukraine of threatening the passage of Russian gas west. (He also appears to be ignoring the “President guy”) and said he would cut off gas this Saturday. This whole situation is now starting to threaten Russian credibility. But the demagoguery seems to have worked and Ukraine is coughing up the ready.

Naftogaz on Thursday transferred the final $50 million installment of a $360 million payment for gas consumed in February, the Ukrainian company's spokesman Valentyn Zemlyansky said. Gazprom confirmed that Naftogaz had paid in full for February supplies.

Part of the problem now appears to be a political struggle in Ukraine

Tymoshenko and her allies deny that Naftogaz diverted Russian gas. They claim Yushchenko initiated the search in order to get his hands on the company's profits and hinder the company's dealings with Russia by confiscating vital documents.

The message has spread beyond just government and industry also is now learning about using renewable energy, and conservation.

And speaking of revisiting crises, the Pakistan Chief Minister would like to reassure the country

Qaim Ali Shah said, “A meeting will soon be convened to resolve power outage crisis in which KESC administration, representatives from trade community and local citizens will be invited.”

The Canadians are trying to help but supplies are short, and still the Pakistan Senate is urging that prices be lowered. Imports of fuel oil are up 87% over this time last year. Sadly, and a little along the lines of my Tech Talk on Sunday, some 14 miners were killed and 19 injured in a gas explosion in a coal mine in Pakistan on Thursday.

Regulation of the coal industry has become the intent du jour, the European Commission has now set in place emission rules that will likely, through constriction of sulfur dioxide emissions , threaten to close the deep mines of Britain. The threats of increased regulation have just caused cancellation of another coal-fired power plant, this one a 649 MW plant in Iowa. Wisconsin also recently cancelled expansion of the Nelson Dewey facility. The utility planning the facility is now looking into wind power but, from a statistical point of view

Power producers have canceled more than 90 coal-fired projects totaling more than 55,000 MW of capacity for various reasons, over the past few years, according to data from the Sierra Club. There are still about 70 coal plants under construction or in development in the United States.

I believe I heard someone comment recently that all the wind and power installed in the U.S. to date would be the equivalent of one medium coal-fired power plant, just to put this in perspective.

Regardless, the pressure is on coal operators to find solutions but what might get lost in the shuffle is that, whether in Europe or the United States, if we shot the old, before the new can carry the load, life could go downhill really quickly. To counter that the IEA suggests that the US should consider new gas and diesel taxes, and open up offshore areas for drilling.


More stories can be found at The Energy Bulletin and Drumbeat at The Oil Drum.

7. The limited Energy Growth Options of the Future

We’re now getting closer to the start of the new Administration, with President-elect Obama having travelled to Washington, to begin his residence there. And, with his arrival, one can start to look forward to the change that his Administration is going to bring, in many of the Departments that form the Federal Government.

Obviously the one that most concerns this site is going to be that of the Department of Energy, although rulings from the EPA on the future of coal power plant emissions, and a number of issues that relate more to the Departments of the Interior and to the Department of Agriculture will also impact the supplies of energy that we will need more critically as the Obama years develop. Given the very clear message that the nominees to head these various divisions have already articulated in regard to the need to move away from technologies that are being blamed for climate change, one might, at first think that their path forward was clear, but I suspect that a harsh dose of reality is going to temper some of the steps that lead into that path.

Consider first of all the options that have prevailed until now, in the basic forms of energy that we use, and how those must change in the future. It was over at The Gristmill just over a week ago, that Sean Casten gave this table on recent power construction:

Our total U.S. electric grid has a peak capacity of just over 1,000 GW. (That's 1 billion kilowatts or, if you prefer, enough to power 10 billion hundred-watt light bulbs.)
Of that total, here's what we've installed just since 1995:
~200 MW of solar PV

~10,000 MW of wind

~45,000 MW of combined heat & power

~200,000 MW of natural gas (about half of which was combined cycle, which runs at almost 2x the fuel efficiency of the U.S. grid)

He then goes on to note that in the same time interval there has been no construction of coal or nuclear power plants.

In the same time interval the amount of ethanol produced from corn in the United States has grown to meet mandated demand, with the stated goal of having this at 15 billion gallons/yr by 2022, by which time ethanol from cellulosic sources is mandated to reach at least 16 billion gallons for a total of 36 billion gallons of renewable fuel a year (that’s 2.3 mbd roughly). By September 2008, the industry was producing 0.64 mbd against a demand of 0.69 mbd. In November the largest ethanol producer, VeraSun, filed for bankruptcy protection.

Cellulosic ethanol, as I noted last week has yet to yield any significant production plant performance.

So here you are with a clean slate, and where would one then start developing future supplies. One could presume that the initial thought would be “more of the same,” particularly given the lack of other alternative options in the immediate future.

That would imply, given the hostility toward coal that is evidenced in at least some of the remarks attributed to Dr. Chu about this being his worst nightmare, that the focus remains on solar power, wind and natural gas.

And that is where the rub comes in, for there are a number of issues that will start to come to the fore as the current technologies move closer to larger-scale adoption.

Consider, for example, solar power, Robert Rapier recently invited Tom Standing to comment on a couple of these, and he looked at An Arizona Plant and a review of plans in France. His conclusions, that a 280 MW plant in Arizona would cover about 6 sq. miles (at 50% collector density) and produce around 1.8 billion kWh per year. This is about 2.3% of the growth in U.S. electrical demand per year. Thus even if the plants could be made sufficiently cost-effective that they can compete with the cheaper electricity from coal and nuclear (in delivered cost/kWh) they are unlikely to be a major player within the next few years. And that perhaps reflects the percentage of the total build of the last decade.

Moving on to wind turbine development, this is very much a question of where to put them to best advantage, and how then to distribute the power.

Montana, for example, has lots of wind potential, but as Ben Arnoldy in the CSM points out there are two problems left unaddressed. The first is the installation of transmission lines to carry the power to where it is going to be used, and the second is the provision of standby power for when the winds don’t blow. And here one comes back to that comment about coal again, because the logical back-up might be a coal powered plant, given all the coal in Montana. However, the resistance of the new Administration might be such, that the development of coal bed methane might be a better alternative, although there is some opposition to the idea.

But that brings us back to natural gas, which is where, in the above table, some 80% of the power growth has been. And this is a real concern, since the presumption is, in much the same way as has been presumed in Europe, that there will be enough natural gas going into the future, not only to supply the existing plants, but also to supply the growth needed in the future. And that is where there has to be concern.

As I pointed out in an earlier post back in my Oil Drum days the current oversupply of natural gas in this country is based on increasing amounts of production from wells in the shales of the country. These wells are expensive to create and relatively short in life. Betting the whole of our energy future on them seems to be highly rash.

Which brings us back to the apparently unacceptable options of coal and nuclear power plants. It will be interesting (since the utility companies seem to think that they are the best option, given that they are planning on installing more than 100 of them) to see how this plays out over the next four years.