This paper is the second of two describing the use of shredded waste gypsum board (SWGB) as an additive during combustion of biomass. The focus of this paper is to determine whether SWGB can be used as a fuel additive providing CaO and SO2/SO3 for mitigation of ash-related operational problems during combustion of biomass and waste derived fuels in grate fired fixed bed applications. The former study in this series was performed in a fluidized bed and thus allow for comparison of results. Gypsum may decompose at elevated temperatures and forms solid CaO and gaseous SO2/SO3 which have been shown to reduce problems with slagging on the fixed bed and alkali chloride deposit formation. Three different biomasses, spruce bark (SB), reed canary grass (RG), and wheat straw (WS), were combusted with and without addition of SWGB in a residential pellet burner (20 kWth). Waste derived fuel with and without the addition of SWGB was combusted in a large scale grate-fired boiler (25 MWth). The amount of added SWGB varied between 1 and 4 wt %. Ash, slag, and particulate matter (PM) were sampled and subsequently analyzed with scanning electron microscopy/ energy dispersive spectroscopy and X-ray diffraction. Decomposition of CaSO4 originating from SWGB was observed as elevated SO2 emissions in both the large scale and small scale facilities and significantly higher than was observed in the fluidized bed study. Slag formation was significantly reduced due to formation of calcium-silicates in small scale application, but no conclusive observations regarding calcium reactivity could be made in the large scale application. In the small scale study the formation of K2SO4 was favored over KCl in PM, while in the large scale study K3Na(SO4)2 and K2Zn2(SO4)3 increased. It is concluded that SWGB can be used as a source of CaO and SO2/SO3 to mitigate slag formation on the grate and chloride-induced high temperature corrosion and that fixed bed applications are likely more suitable than bubbling fluidized beds when using SWGB as an additive.