Some silicon-rich mineral additives are known to be capable of capturing volatile elements evolved during combustion and, thus, may be used to mitigate ash-related problems (e.g., particulate matter emissions, fouling/slagging, and corrosion). Considering that silicon is also rich in some biomasses (e.g., rice husk), this work aims to explore how co-firing various silicon-rich biomasses with coal would affect fine ash formation and slagging deposition. Combustion experiments of individual biomasses, a low-rank coal, and their blends were conducted on a drop-tube furnace in simulated air and at 1300 degrees C. The fine ash was collected by a Dekati low-pressure impactor, and the high-temperature slags were collected by uncooled mullite tubes. The particle size distributions of fine ash were obtained. The morphology and composition of fly ash and the slags were carefully characterized by a scanning electron microscope equipped with an energy-dispersive spectrometer. The results indicate that the particle size distributions of particulate matter whose aerodynamic diameter is less than 10 mu m (PM10) from combustion of all of the fuels are similarly bimodal. The ash-based yields of particulate matter whose aerodynamic diameter is less than 1 mu m (PM1) and particulate matter whose aerodynamic diameter is between 1 and 10 mu m (PM1-10) from individual fuels decrease in the order: coal >> corn stalk > bagasse > rice husk. Interactions between biomass and coal promote the release of alkali chlorides while inhibit the vaporization of Ca, Mg, and Fe from the coal during co-combustion. For the rice husk and bagasse with a relatively low content of K, the emission of PM1 during co-combustion is suppressed. However, for the corn stalk with a high content of K, the emission of PM, from co-combustion is promoted evidently. The rice husk ash shows a weak tendency of slagging, while the corn stalk shows a serious tendency of slagging. The slagging tendency of bagasse ash is between that of rice husk and corn stalk ashes. During co-combustion of silicon-rich biomass and coal, the interactions between Si and Mg/Ca are favored, while those between Si and Na/K are suppressed. These result in more Mg and Ca in the coarse particles. Therefore, the ash slagging tendency in co-combustion is generally reduced.