Particle aggregation by magnetic seeding was proposed for removing the coal-fired PM (10). To better understand the particle aggregation properties by magnetic seeding, experiments on the fly ash particles in the size range of 0.023-9.314 mu m were conducted in a uniform magnetic field by seeding magnetic particles of Fe3O4 and gamma-Fe2O3. The fly ash particles were produced from combustion of bituminous coal originated in Dongshen, China. A dedicated fluidized bed aerosol generator was developed to disperse particles to generate aerosol at a constant rate. The fly ash particles mixed with magnetic seed particles, underwent agglomeration during passing through the magnetic field. The variation in particles number concentration induced by aggregation was measured in real time by an electrical low pressure impactor. Characteristics of particles aggregation by seeding the two kinds of magnetic particles were examined. Experimental results show that particle removal efficiencies can be increased by increasing the magnetic flux density, the mass ratio of magnetic seed particles/fly ash and the particle residence time in the magnetic field. The removal efficiencies by seeding Fe3O4 are higher than those by seeding gamma-Fe2O3 under the same conditions. When particles are saturatedly magnetized, further increasing the magnetic flux density no longer has effect on particle aggregation. Both the single-sized and total removal efficiencies of fly ash particles for seeding Fe3O4 are higher than those for seeding gamma-Fe2O3- Mid-sized particles removal efficiencies are higher than those of the bigger ones or the smaller ones. Particle number median diameter decreases during aggregation, and the decrement for seeding Fe3O4 is lower than that for seeding gamma-Fe2O3. Numerical simulation results indicated that particle removal efficiencies increase remarkably with the increase in total particle mass concentrations. The total removal efficiencies reach 84% and 62% for seeding Fe3O4 and gamma-Fe2O3 respectively, and the particle number median diameter decrease from original 0.151 to 0.098 and 0.085 mu m when total particle mass concentration is 40 g m(-3). (C) 2007 Elsevier B.V All rights reserved.