‘Lectricity from lignite

Should you be one of those who receives a lump of coal in your stocking this Christmas, don’t despair and think you’ve been bad: Santa may just be urging you to give thanks for a remarkable resource which, among other things, may well be powering the lights on your Christmas tree.

Coal is fossil plant matter: ancient plants buried for millions of years beneath the earth and compressed and heated until they’re mostly pure carbon.

Saskatchewan has a lot of the stuff, and down in the southeast it’s particularly close to the surface—right on the surface in some places, as native people kindly pointed out to John Palliser when he traveled through the Souris Valley on his famous 1857 expedition.

Mining of Saskatchewan coal begin in the 1870s, with the first commercial mine opening around 1890. Strip mining didn’t begin until 1927 (before that the coal mines were underground) but by the mid 1950s it had replaced underground mining altogether.

It used to be that much of Saskatchewan coal was exported. Now, though, 90 percent of it is used in the province—and mostly it’s used to produce electricity.

The Boundary Dam power station, which opened in 1959, is the largest lignite-fired power generating station in Canada. Coal is divided into four major classes, based primarily on the proportions of combustible matter, water and ash. Lignite contains the least amount of energy per kilogram, and the most water, of any the classes of coal, so it’s often been neglected as a power source.

Not in Saskatchewan, though. It’s hard to ignore something you have so much of. But then again, for power production it takes a lot.

Boundary Dam, with a total generating capacity of 872 megawatts from six generating units, produces almost a third of the province’s electrical requirements all by itself. To do so, it burns about 5 million tonnes of coal a year—15,000 tonnes on an average day at full load.

The coal is mined just a few kilometers away, by large draglines with buckets capable of holding almost 70 cubic metres of material at a time. It’s dumped into one of many giant coal-hauler trucks, each of which can hold about 145 tonnes, which continuously haul coal to the plant’s stockpile. From the stockpile, coal is constantly being pushed into the coal-crushing facility. There are three different stages of crushing. The primary crushers have to break up chunks that can be the size of a car. The secondary crushers break everything down to about 2.5 centimetres in diameter. At that stage the coal is dropped into the coal bunkers near the boilers.

From there, the coal drops continuously into the coal pulverizers, five or six per boiler, which crush it to the consistency of fine flour. Large primary air fans constantly blow air (heated to 370 degrees C. so the moisture in the coal is instantly evaporated) through the pulverizers, which forces the coal dust through pipes into the boiler furnaces. The furnaces sustain a constant fireball at around 1,370 degrees C.

Surrounding the furnace are hundreds of tubes five to 7.5 centimetres in diameter, filled with water ,which quickly turns to steam. The steam is heated further, until it emerges, superheated to around 540 degrees C.and at about 2,000 psi, onto the blades of the turbines, spinning them at 3,600 rpm. The turbines turn the generators, and the generators produce electricity.

Coal is not perfectly combustible. One tenth of it becomes ash: or, to put it another way, each day’s burning of 15,000 tonnes of coal produces 1,500 tonnes of ash.

There are two types. A heavy bottom ash that is collected at the bottom of the furnaces and slurried out to large ash lagoons; about ten percent of it is sold to the cement industry for mixing with cement. A light fly ash is carried in the gas train through the furnaces, but 99.8 percent of it is collected by electrostatic precipitators at their outlets—which is why, in the summer time, you can’t see anything coming out of the Boundary Dam smokestacks.

In the winter, of course, you can see big white plumes as the water vapour from the wet coal hits the cold atmosphere.

They look, in fact, rather like Santa Claus’s beard.

See? There’s a natural connection between Santa and coal—and a lump of the latter in your stocking should therefore be a reason to rejoice and reflect, rather than weep and wail.

Though personally, I admit, I’d rather find the key to a Ferrari.

Permanent link to this article: https://edwardwillett.com/2005/12/lectricity-from-lignite/

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