An OFA (over-fire air) program and matched measures supporting efficient-combustion are proposed for the FW (Foster Wheeler)-type down-fired boiler, where the OFA is supplied from the arches of the boiler. A gas/solid cold test was conducted at a scale of 1:15, using a two-dimensional laser phase Doppler analyzer to measure parameters in the model furnace, such as the two-phase gas/solid flow velocity, fluctuation velocity, particle size, particle volume flux and particle number concentration. The effects of the OFA ratio on the gas/solid flow characteristics in the furnace were investigated. With an increase in the OFA ratio, the jet depth of the fuel-rich flow decreased. For an OFA ratio of 25%, the jet depth of the fuel-rich flow was much less than that for other OFA ratios, and the residence time of the gas/solid flow in the lower furnace was shorter. An increase in the OFA ratio can increase the penetration depth and allow fuller mixing with the upward airflow, while an OFA ratio that is too great will hinder the upward flow back to the burner nozzle region. An OFA ratio of 20% is considered appropriate for the proposed OFA technical scheme. (C) 2014 Elsevier Ltd. All rights reserved.