coal selection criteria for industrial pfbc firing project 3.2 - CCSD

“Coal Selection Criteria for Industrial PFBC Firing”
Blockage of fuel feeding lines has been noted in Wakamatsu. This could be resolved by
improving the coal’s particle size distribution and equipment modifications (Sakanishi
1995). Such a problem was also reported in Osaki, where their fuel nozzle was clogged
several times by foreign material in the raw coal and coal lumps. As a countermeasure, a
reducer in front of the nozzle cut-off valve was installed (Matsumoto and Kawahara ).
Swelling coals are sticky and they could stick the surrounding bed particles together
forming agglomerates (Palit and Mandal 1995). Therefore, it was advised to use coals
with low crucible swelling number (CSN) or non-caking coals.
Bed agglomeration was also encountered in plants that used dry coal feed, such as
Tomatoh-Atsuma, instead of slurry feeding system. The temperature of the combustion
domain near the fuel nozzle outlet induced the agglomeration. A low ash fusion
temperature generated agglomeration. The Tomatoh-Atsuma plant selects coals based on
the iron content, coals with an iron content of 7% or more will have low ash melting
point (Kazuhiro 2002). Karita requires their coals to contain less than 7% Fe2O3 and to
have an ash fusion temperature higher than 1200 o C. If coals with low ash fusion
temperature are used, the bed temperature has to be kept below the ash fusion
temperature to prevent agglomeration (Palit and Mandal 1995).
Bed agglomeration is caused by amorphous clay mineral fragments and alkali species
adhering to sorbent and chars surfaces (Steenari, Lindqvist et al. 1998). Inside the
agglomerates, the chars are still burning, causing high temperature and reducing
conditions. Steenari et al. found that reducing conditions in the bed caused sintering
through reaction in the CaS-CaSO4 system and through eutectic melting of silicate-iron
mixtures (Steenari, Lindqvist et al. 1998). An increase in the coal’s clay content increased
the viscosity of the paste (Wright, Clark et al. 1991). Less agglomeration was found when
using dolomite instead of limestone as the sorbent. The reason was that dolomite contains
a higher quantity of MgO which raised the ash fusion temperature of the CaO-MgO-
Al2O3 (Marocco and Bauer 1993). Improved bed mixing and fluidization was observed
by using finer dolomite (