Particle size inside the pellets

Very important   indicator is  also the particle size inside the pellets. According I2 specification  2,5 mm size particles should not exceed 90%   and particles over 3,15 mm should not be more than 98%.This limitation should avoid retaining of the bigger particles in the mills of the power plant which can slow down the speed of the passing the mills and may  create fire in the hammer  mills of the power plant.

Particle size in the pellets is regulated with EN 15149-2 and EN 16126. According this standard approx. 200 g  of pellets are put for soak  in 600 ml distilled water where they will stay for  abt 12 hours for the pellets to be dissolved in the water. Pellets particles dissolved in the water  are exposed to a temperature of 105 ºC until they are dry. The size of the sieves to be used may be as following:   0,25mm- 0,5 mm - 1,0mm – 1,4mm – 2,0 mm -3,15 mm, which means that  the fractional distribution of the particle size can be done after the evaluation of the results of the testAccording International Specification there is no limitation for small particles inside the pellets. It is on the contrary mentioned that 50% of the particles should be below 1mm size But there is quite a lot of discussion around the amount of very small particles in the pellets.  

Minimal Explosive Concentration for very thin wood dust ( pine flour)  can be about 38g per m3 or just 1,08 g per cubic feet. This is 30 mg per m3.Not so much! Actually the problem seems to be serious as some articles say that in a case  the average  size of  particles  in the wood dust is  below 420 mcm ( 420 :1000= 0,42 mm) it can be dangerous because of the possible explosion.Some end users are especially worried about small particles. One big   UK  utility  has  included  into their specs the demand of  particle size less than  420 mcm (0,42mm) to be  not more than 50% of all particles  and the number of  particles  with the  size less than 85mcm (0,085 mm) to be  not more than 5%.