in a bio mass power plant
have you ever done the experiment where you disolve salt in water, then boil the water to only be left with salt? Same concept. The water used has all kinds of sediments in it. When the boiler heats up and makes steam to power the steam turbines, it leaves behind all of the sediment. Too much sediment build up could reduce the efficiency of the boiler or even cause it to rupture. By blowing it down, you remove the sediments from inside the boiler and basically flush them away.
The need for continuous blow down in any boiler is due to the fact that no steam system is 100% closed. There are always losses of steam from valve packing, flanges, turbine packing or seals, etc. Over a period of time the amount of water in the system would be depleted to the point that the boiler could not function. To counter this, water is added usually on a continuous basis. Since this water is never 100% pure there is, over time, a build up of chemicals in the the system. To deal with pH, oxygen in the system, etc, treatment chemicals are also added. To keep the chemistry in the right range, as water is added, some of the boiler water is removed as blowdown. You can do this on a batch basis but it is usually better for the system to do it on a continuous basis so the boiler water stays closer to the desired chemical balance.
Some boilers may have continuous blow down but others havw manual blow down only. Blow downs are done in the case of manual blowdowns at least once in an 8 hour shift. This is necessary in order to remove any water impurities such as calcium and magnesium carbonates. Although , water feed to boilers may be treated for solids, solids may always develop inside a boiler after a long period of steam production.
