By Yougen Kong, Solvay Chemicals, and Terry Lindsay, Ulman Associates
The Mirant Potomac River Power Plant is in Alexandria, Va., close to Washington, D.C. The plant has five coal-fired boilers built between the late 1940s and the mid-1950s. The nominal design capacity of each unit is around 100 MW.
Due to the plant’s unique location, it is critical to control air pollutants such as sulfur dioxide (SO2) and particulate matter. The plant has been injecting trona (Na2CO3.NaHCO3.2H2O) upstream of the hot electrostatic precipitators as a sorbent to control SO2 emission, but plant operators wanted to be able to remove more SO2 at a reasonable cost. Milled sodium bicarbonate has been known to be more reactive than trona in mitigating SO2, so in late 2007 a trial with sodium bicarbonate was conducted with the existing trona handling system to assess its performance.
Unit 4 of the Potomac River plant was used for this trial. Its operating parameters at the time of these tests were:
- Unit load: 36 ~ 108 MW
- Coal heating value: 12,927 ~ 13,179 Btu/lb.
- Sulfur content of coal: 1.08 ~ 1.37 lb./MMBtu
- Stack SO2 emission rate: Less than 0.4 lb./MMBtu
The trona is transported directly from Solvay Chemicals’ mine in Green River, Wyo., via 100-ton railcars. The unloading station pneumatically transfers trona directly from the railcars to five storage silos, one for each boiler. Each silo has several dry air injection points for aeration and to prevent bridging inside the silo.
The raw sodium bicarbonate has a bulk density of 68 lbs./ft3 and is too coarse to be injected directly. An air-classifying hammer mill was used to grind sodium bicarbonate to d50 = 12 µm and d90= 30 µm. The milled sodium bicarbonate was pneumatically transported into a trailer that had four conic hoppers. Dry aeration air was injected at each hopper and a pulsed vibrator was used at each hopper to help the sorbent flow.
To see what differences sodium bicarbonate could make, the existing trona system was operated at various unit loads and trona federates. Then, milled sodium bicarbonate was injected from the storage trailer under the same conditions. Results showed that sodium bicarbonate was more effective than trona in removing SO2. (See Figure 1.)
Click here to enlarge image The normalized stoichiometric ratio (NSR) is calculated with the following formula:
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At the theoretical usage rate of NSR = 1, 2.41 pounds of trona react with one pound of SO2 while 2.63 pounds of sodium bicarbonate are needed to react with the same amount. As required removal rates increase trona becomes less efficient—as does sodium bicarbonate—but trona loses efficiency much faster so the practical use rates of trona are higher than sodium bicarbonate.
The operating cost depends on how much sorbent is used and is figured by calculating the weight ratio between trona and sodium bicarbonate for the same level of SO2 removal.
At 36 MW, the ratio is 1.8, so 1.8 times more trona than sodium bicarbonate is needed to remove 75 percent of SO2. At 104 MW, this ratio increased to 2.1, so sodium bicarbonate is more effective at full load for this unit.
The sodium bicarbonate used is a milled product and trona is unmilled. The weight ratios between trona and sodium bicarbonate are expected to be lower if the trona was milled to a particle size similar to milled sodium bicarbonate.
A five-hour sodium bicarbonate injection test was conducted on Unit 2 in late November 2007. An inspection of the hot and cold precipitators a week later yielded no discernible signs of deposits left by the sodium bicarbonate injection. Gas temperatures measured at the inlet to the hot precipitators during the November test were in the 610 F to 620 F range, which is above the temperatures where sticky sulfate deposits were expected to form. However, deposit coupons inserted in the gas ductwork to the hot precipitators during each test yielded little sign of a sticky deposition on either Unit 2 or Unit 4.
The flue gas concentrations of hydrochloric acid (HCl) and hydrofluoric acid (HF) were also analyzed from the samples taken during the sodium bicarbonate and trona injections. They were effectively removed by both sodium bicarbonate and trona.
The fly ash from using sodium bicarbonate was analyzed. The sodium content was lower than trona, which should help ash storage characteristics.
A test was conducted on Unit 4 to determine the effects on SO2 mitigation with unmilled sodium bicarbonate as a sorbent. Results showed that raw sodium bicarbonate needs to be milled before injection.
In conclusion, tests show that milled sodium bicarbonate is more effective in removing SO2 than unmilled trona. Sodium bicarbonate was able to remove around 80 percent of SO2 at full load with NSR = 1.0, while unmilled trona needed NSR = 2.5 to remove the same amount of SO2. The savings in the amount of sorbent needed also means less fly ash generated and like trona, sodium bicarbonate was shown to be very effective in removing both HCl and HF. Lower sodium levels in sodium bicarbonate fly ash also result in improved ash storage characteristics.
Authors: Yougen Kong is technical development manager, SOLVAir Products, Solvay Chemicals. He has a Ph.D in chemical engineering and is a licensed professional engineer. Terry Lindsay is senior engineer at Ulman Associates. He has an MS in chemical engineering.
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