coal-fired boiler design

5/25/2006

1

Advanced Environmental Systems, Inc. 2001

Coal-Fired Boiler Design

By Bob Brown, P.E.Bibb and Associates, Inc.

5/25/2006

2


EarthWind& Fire

Using Coal and Air to Produce Flames, or…Boilers:

Boiler design can be a rather bland topic, so I started thinking of ways to spice up the presentation.

In boilers, you supply coal and air to produce flames to extract the energy.

Coal, air and flames can be interpreted as Earth Wind and Fire, my ambassadors of boiler design.

5/25/2006

3


Earth (aka Coal)Huge emerging coal-fired energy markets

– India and China – rapid increases in demand– China is world’s largest producer/consumer of coal

Coal found on every continentAbundant domestic supply

– Current production ~1 billion tons/year– 275 billion-ton reserves = 200-year supply– More energy in U.S. coal than total world oil supply

The United States has the more recoverable coal reserves than any other country in the world.

The PRB region of Wyoming/Montana puts out a unit train of coal every 10 minutes.

5/25/2006

4


U.S. Coal Reserves

Up until the 1980s, the vast majority of the coal came from the Appalachia region, mostly West Virginia, Kentucky, and Pennsylvania.The coal from that region is high in heating value (12,500-14,000 Btu/lb) and mostly high in sulfur.

The Illinois basin coals are lower grade bituminous (10,500 to 12,000 Btu/lb) high sulfur, often 3%+.

Powder River Basin Coals now make up more than half the coal production in the country. These coals are sub-bituminous (8,300 – 9,500 Btu/lb) and low sulfur <0.5%.

5/25/2006

5


Coal Mining

Eastern mines are mostly deep shaft mines. In addition to being dangerous, shaft mining is also expensive.

Western mines are surface mines. Surface mines move the overburden of dirt and rock (20-40 feet deep) to expose the coal seam (60-100 feet thick). The coal is blasted with explosives, and then scooped up by giant shovels and loaded into large earth-mover trucks. The trucks run down a haul road to a train loading facility.

Surface mining is a much more cost effective mining technique. PRB mines are able to sell coal for $7/ton, while shaft mines are often $35/ton

After a section is mined, the land is reclaimed by replacing the overburden and replanting with native vegetation. More than 2 million acres of land have been reclaimed.

Surface mines are also used in Appalachia (mostly Kentucky). The practice there is much more controversial because mountain tops are removed to expose the coal, with the fill going into the adjacent valleys. This causes a more drastic change to the landscape and often affects streams.

5/25/2006

6


It’s cheap– Delivered price is often under $1.50/MBtu vs.

natural gas that is currently at:

$6.00/MBtuXXXX$10.00/MBtuXXXX$12.00/MBtu

Why Coal?

XXXX$8.00/MBtu$?????

The abundance of U.S. coal, in conjunction with cost-effective mining and transportation, makes coal a very cheap source of energy.

After the cheap gas days of the 1990s ($2.50 to $3.00/MBtu), gas began a rapid price increase. From 2002 to 2006, gas increased at about 30%/year to its current rates between $7.00 and $8.00/MBtu.

So the relatively high capital cost of a coal plant can be justified when fuel costs are factored in.

5/25/2006

7


Why Coal? (continued)

Gas prices remained flat during the 1990s, but then began rapidly increasing.

5/25/2006

8



The U.S. Energy Information Agency predicts the future price of coal and gas. Coal is expected to closely follow general inflation. Gas prices are expected to rise fairly slowly for the next 10 years, followed by steeper prices increases.

5/25/2006

9



Levelized costs are a common way to assess the long term power production costs of different technologies. The values shown represent fairly typical costs associated with circulating fluidized bed (CFB) and combined cycle (CC) plants.

5/25/2006

10



This shows graphically how the lower fuel price of coal more than offsets the higher capital cost, maintenance costs and lower efficiency.

5/25/2006

11


What Exactly is Coal?Rank (Anthracite, Bituminous, Sub-bituminous, Lignite, Peat)

Proximate Analysis(Btu, Moisture, Ash, Volatiles, Fixed Carbon)

Ultimate Analysis (Carbon, Hydrogen, Sulfur, Nitrogen, Ash, Moisture, Oxygen)

Ash Analysis (SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, K2O, P2O5, TiO2)

Ash Fusion Temperatures(Slagging Predictors)

Ranking coals gives a general idea of coal properties.

The proximate analysis is performed frequently to monitor the coal quality (sulfur is often added) Moisture is very important – not only does it “water down” the Btu content, but it also requires about 1000 Btu/lb to vaporize in the boiler. Each 1% increase in moisture translates to a heat rate increase of about 20 Btu/kWh and a cost of $40-50k per year to our 250 MW plant

The ultimate analysis gives the chemical analysis of the coal, which is required for combustion calculations. Since these analyses are performed much less frequently for a coal, I recommend normalizing the properties to conform to the Btu, ash, and moisture from the proximate for that mine.

Ash analyses give us the composition of the burned ash. High silica indicates high erosion; sodium and potassium are often considered to correlate with fouling. Some published indices correlate iron or base/acid ratios with slagging. Calcium and magnesium can actually assist in sulfur removal.

What you can’t tell is what was the contaminate in the coal, such as clays, quartz, pyrites or calcite.

5/25/2006

12


Slagging and Fouling Impacts on Design

Higher slagging and/or fouling coals cause the need for larger, more expensive pulverized coal boilers.

5/25/2006

13


Combustion Calculations

Calculations (excess air)Unburned carbonAdiabatic flame temperatureUse of sorbent for SO2 removalProducts (and byproducts) of combustion

Important engineering calculations associated with the boiler include:•Air requirements for fan sizing•Limestone use rates (CFB)•Ash composition (possible sales)•Ash disposal rates (Bottom ash and fly ash)

5/25/2006

14


Boiler Terminology

This shows an overview of the boiler equipment to understand the nomenclature used.

5/25/2006

15


Boiler Processes

Radiant heat transferConvective heat transfer to water/steamConvective heat transfer to incoming airCombustion

As I stated earlier, the function of the boiler is to move heat into the steam. Before I talk about the boiler as a whole, I’d like to briefly go over the processes that take place within the boiler.

Radiant heat transfer is really not a significant factor in HRSGs, but it is very significant in utility-sized boilers. Radiation is a function of T4, so it is extremely temperature-related. Other factors affecting radiant heating are the emissivity of the heat source and the absorptivity of the heat receptor. Coal properties affect both of these.

Convective heat transfer is a function of temperature differential and heat transfer coefficient. The heat transfer coefficient depends on velocity and fluid properties.

Inside the boiler, convection is used to transfer heat into water/steam as it flows through tubes crossing the gas flow path. After the boiler, the hot gas is used to heat air, either in the same way (tubular air heaters) or by heating metal that is used in turn to heat the air (regenerative air heaters).

Of course, all of this heat has to come from somewhere, which leads us to the earth, wind and fire of the boiler. Combustion of solid fuels, unlike natural gas, is a rather involved process.

5/25/2006

16


Combustion

Coal is a heterogeneous material consisting of a composite of: • Hydrocarbons (referred to as volatiles)• Non-volatile combustible material (referred to as char; this is mostly carbon in solid form)• Ash, which doesn’t really play much of a part in combustion, but it is very important to the design of the boiler• Water

To burn the coal, there must be sufficient oxygen, sufficient mixing of the fuel and oxygen, sufficient temperature for ignition, and sufficient time to complete the process.

For combustion to take place, the coal particle must heat up, and it can’t really do that until the moisture is vaporized and removed.As the particle continues to heat up, the volatile hydrocarbons are released as a gas, and they burn very readily.Finally, the char burns. This process can take several seconds, and some of it never does burn and just becomes part of the ash.

5/25/2006

17


Coal Feeders

Coal is sent by conveyor to silos near the boiler. Below each silo is a feeder, which is basically a short conveyor belt with load sensors on the middle idlers that weigh the coal on the belt. Fuel flow is controlled using the feeders by adjusting the speed of the belt.

5/25/2006

18


Pulverizers(mills)

In a PC unit, the coal goes from the feeder down into a pulverizer (also called a mill). The coal feeds in through the center of the mill onto a circular trough. Large, heavy wheels (called rollers) roll around the trough and crush the coal. Hot primary air comes into the mill and flows in an upward direction, entraining the smaller coal particles. At the top is a classifier, which is used to send larger pieces back down for more grinding. The finely crushed coal (70% passing through a 200 mesh screen) that has also been dried by the hot air leaves with the air and is divided into several burner lines.

5/25/2006

19


Wind

This shows an elevation view of the flow path of air to a PC boiler. Air is delivered in two parts: primary air and secondary air. Primary air flow is roughly the same as the full-load coal flow on a mass basis. Secondary air flow is much higher (approximately 8 times more). The total air supplied is regulated to maintain the target oxygen content in the flue gas, which indicates the amount of excess air.

Part of the primary air is heated, and part is not. The tempering air (unheated) flow is controlled to maintain the target pulverizer outlet temperature. Too cold can lead to problems with insufficient drying, and too hot can lead to mill fires.

All of the secondary air is heated and sent to the burners via the windbox on the side of the boiler.

5/25/2006

20


Wind

This shows the physical layout of the air supply systems.

5/25/2006

21


FANS Primary Air, Forced Draft, Induced Draft

Typical power plant centrifugal fan.

5/25/2006

22


FANS Primary Air, Forced Draft, Induced Draft (cont.)

Axial fans are often used for forced draft (secondary air) and induced draft duty. The pitch of the blades can be hydraulically controlled to vary the flow without wasting energy in dampers, much like using a variable speed motor.

5/25/2006

23


Fire

After all that complicated process, we finally get to burn the fuel……come on baby, light my FIRE!

This shows a schematic of a burner. The flame shape is very important for proper combustion, flame detection and NOx formation. The secondary air distribution in the burner is controlled by manual adjusters at the burner front.

5/25/2006

24


Mass and Heat Flows

This shows a heat/mass balance on a boiler using our 250 MW design discussed earlier.

First, we need 120 tph of PRB coal, 4 tph of limestone, and nearly 2,000,000 pph of air.

As a result of the combustion process, we have a little more than 2,000,000 lb/hr of hot flue gas and nearly 10 tph of ash.

On the fluid side, we have 1.6 million pph of feedwater that is boiled and superheated.

Then we have a slightly lower amount of cold reheat steam that must be brought up to temperature. Anyone know where the “lost” steam went? (extraction to FWHs)

As far as a heat balance goes, the vast majority of the heat goes into producing the main steam, a much lower amount to reheat, and about 15% of which is lost.Most of the losses are heat leaving out of the stack or used up to vaporize water in the fuel; some other losses include general heat losses from the boiler surface and carbon that never burns.

5/25/2006

25


Water/Steam Flow Path

Natural circulation (shown here) uses the density difference between the water in the downcomers and the water/steam mixture in the waterwalls to drive the flow.

Some boilers use forced circulation, where there are boiler water circulating pumps located at the bottom of the downcomers. This is done to allow circulation before the fire is even started and to allow smaller, higher pressure drop waterwall tubes.

Subcritical boilers such as the one in the drawing have a drum to separate water from steam. Supercritical water/steam have no such distinction; therefore, a drum would be of no use. The flow can still be mostly as shown, or the walls can be spiral wound.

5/25/2006

26


Tube Banks

Typical layout of a horizontal economizer. Tubes could be about 1” diameter with 1” gaps between circuits. Tube bank spacing increases for the sections closer to the furnace to reduce the chance of plugging and because the volumetric flow rate is higher where the gas is hotter.

5/25/2006

27


InteriorWaterwalls

Exterior

The waterwalls are tubes that are welded to spacers between the tubes to form a gas-tight wall. The water flows up through the walls as it turns to steam.

5/25/2006

28


Boiler Design:Traditional Pulverized Coal

This is the traditional PC unit I have been showing. Coal is ground up and mixed with air. Heat is released at the burners. Notice that below the burners there is not much going on – just a water-cooled hopper that is sealed in a water trough. The ash drops out and is taken away by drag chain or a water sluice system.

Water is boiled in the waterwalls (mostly by radiant heat transfer), and then the flue gas goes through the backpass to give up its heat to the various tube banks (much like an HRSG).

Then (unlike an HRSG) the flue gas gives up the remainder of its available heat to the incoming air in the air heater.

5/25/2006

29


Air Heaters

The air heater is a key part of a fossil plant design. The air heater captures much of the heat leaving the boiler and sends it back to the furnace.

Air heaters can be tubular (shell and tube) or regenerative as shown here.

Flue gas goes from ~730° F to ~220° F and heats the air from ambient to ~680° F, some of the air leaks over into the gas stream. An important parameter to monitor is AH cold-end average temp (the average of the to-gas streams on the cold side) This must stay above a certain number depending on the sulfur in the coal, ~150° F for low sulfur PRB.

5/25/2006

30


Boiler Design:Bubbling Fluidized Bed

Fluidized bed boilers don’t use pulverizers and burners. The fuel is thrown in and allowed to burn more slowly at the bottom of the boiler in a bed formed by the coal, ash, limestone, and usually some sand as well. The fluidized part of the name comes from the fact that the bed is constantly stirred by air forced up from the bottom. This also makes the bed less dense and causes it to act like a fluid.

5/25/2006

31


Fluidized Bed

This is a diagram of the different parts of the fluidized bed. You can see a photograph of some bubble caps connected to the air ducts.

5/25/2006

32


Boiler Design:Circulating Fluidized Bed

In a circulating fluidized bed (CFB), the air velocity is increased from 6 fps to maybe 15 fps. Smaller particles become entrained and flow with the flue gas up and out of the furnace. The hot ash is captured and returned to the furnace.

The schematic on the right shows a 2-stage variation used by B&W

5/25/2006

33


CFB Plant Schematic

This diagram shows the components of a CFB power plant and the primary boiler-related processes:•Fuel and limestone are fed into the boiler•Air from the PA and FD fans is heated and delivered to the boiler•Some of the ash is removed from the bed, cooled and disposed of•The remainder of the ash travels with the flue gas through the boiler backpass and air heater before being collected by the baghouse•A portion of this ash is hydrated and re-circulated into the flue gas stream to increase SO2 removal (called a polishing scrubber)•Finally, the ID fans send the flue gas out the stack

5/25/2006

34


Why CFB boilers?• Emissions

Lower NOX emissions (<100 vs 800)allows for cheaper SNCR, not SCR

In-bed SO2 removal up to 95%simpler/cheaper flue gas scrubbing

• Eliminate slagging and reduce fouling(less soot blowing)

Because they’re cool

• Fuel flexibilityAny grade coal, waste coal, pet coke, even wood, agricultural waste, tire chips, etc.WITHOUT significant boiler modifications

MONEY: Potential savings on capital, maintenance and aux power

CFB furnaces operate around 1600° F vs. 2500° F for PC boilers. This lower temperature greatly reduces the amount of NOx formed during combustion. Another big advantage of this particular operating temperature is SO2 removal. To remove SO2, limestone is added to the furnace. The heat breaks down the CaCO3into CaO and CO2, then the CaO reacts with SO2 to form CaSO4. The CFB temperature is hot enough to calcine the limestone into lime and cool enough to allow the sulfation of the lime.Even though current emissions limits demand post-combustion air quality control, much cheaper technologies can be used due to the lower boiler emissions. SNCR instead of SCR and FDA (or polishing scrubber) instead of a dry lime scrubber or wet limestone scrubber.The lower temperatures in a CFB eliminate slagging and fouling, which also eliminates the need to buy and maintain sootblowers.Pulverized coal units are very fuel-specific in their design. The only way to get fuel into the system is through the pulverizers. As we have seen with the natural gas market, predictions of future fuel prices are often not accurate. Fuel flexibility allows the owner to pursue low-cost fuels as the market changes and to go after short-term opportunity fuels that can pop up. You could possibly even charge a disposal fee for wood-waste if the alternative is land-filling.As always, the bottom line answer is…. money. It is possible for a CFB plant to be cheaper to build and operate, mostly due to eliminating air quality control equipment.

5/25/2006

35


CFB Design(Typical)

Most manufacturers (Alstom, Foster Wheeler and Kvaerner) use large cyclones that separate the furnace from the backpass

The ash flow from the cyclones goes through a J-valve (or loop seal). The J-valve is a goose-neck trap (like under your kitchen sink) that prevents hot furnace gases from short ciruiting the upper furnace and flowing up through the cyclones. But, because ash is a solid, it would tend to just pack itself in and plug the flow. So air is used to fluidize the ash and keep it flowing.

The ash is very hot (1200°-1500° F). One way to regulate the bed temperature is to take a portion of the hot ash and cool it in a FBHX located outside of the fluid bed.

5/25/2006

36


Bed Ash Coolers

The bed ash is at 1600° F, so it must be cooled before disposal.

There are several types of coolers for this job.

Some use water-cooled surface, some just use water-cooled screw conveyors and others use air (called stripper coolers).

5/25/2006

37


CFB Design(Alstom Modular)

Alstom recently came here and gave a presentation on their CFB boilers, including their modular design for sizes from 10 to 70 MW.

The modular design uses furnace boxes that are completely refractory-lined (no waterwalls) and bottom-supported.

The squat height of the furnace allows the cyclones to be mounted on the roof and return the ash directly back to the bed (no J-Valve or FBHX).

The backpass (also bottom-supported) does have waterwalls. Below the backpass is a 3-pass tubular air heater.

This shows a diagram of the furnace – notice the small amount of heat transfer surface in the bed zone of the furnace.

Alstom claims this design can be built faster and cheaper than a traditional waterwall CFB in the size range offered.

5/25/2006

38


Coal-Fired Boiler Design

By Bob Brown, P.E.Bibb and Associates, Inc.

coal-fired boiler design

Documents

coal production

recoverable coal reserves

unit train of coal

btulb high sulfur

earth wind

illinois basin coals

appalachia region

ambassadors of boiler