Speciation of Chromium in Feed Coals and Ash Byproducts from Canadian Power Plants burning Subbituminous and Bituminous Coals

The chromium species in the feed coals and ash byproducts from seven Canadian coal-fired power plants that were burning local subbituminous or bituminous coals with sulfur contents in the range of 0.30−3.5 wt % have been examined using Cr X-ray absorption near-edge spectroscopy (XANES). Chromium in the Canadian feed coals is always found as Cr3+ but generally has a dual occurrence, as Cr3+ is distributed to varying degrees between the clay mineral illite (Cr3+/illite) and a poorly crystallized chromium oxyhydroxide (CrOOH) phase associated with the organic fraction. In two subbituminous feed coals from Alberta, chromium is present largely as Cr3+/illite, whereas in two other such coals, it is present predominantly as CrOOH. Chromium in a low-sulfur (0.50 wt %) bituminous feed coal from Alberta is found mostly as Cr3+/illite, whereas for feed coals from Nova Scotia with high sulfur contents (2.60−3.56 wt %), chromium is distributed between both Cr3+/illite and CrOOH. Very little chromium was found in the limestone used in a fluidized-bed combustor. The chromium species in most bottom ash samples from all seven combustion units is predominantly, if not entirely (>95%), Cr3+ associated with aluminosilicate phases. Chromium speciation for subbituminous electrostatic precipitator (ESP) fly ash is mostly Cr3+ (>95%), but in some cases, it is slightly less (>80%) and varies by sampling location at the plant. Chromium in fly ash from the combustion of bituminous feed coals is predominantly (>95%) Cr3+. A unique species of chromium found in one feed coal and an unrelated fly ash is metallic chromium (Cr0), similar to that in stainless steel. The occurrence of this form of chromium in these materials indicates contamination from machinery, such as the coal milling machine or possibly wearing down of stainless steel parts by the coal or ash. The observation of this unexpected contamination demonstrates the power and usefulness of X-ray absorption fine-structure (XAFS) spectroscopy for speciation determination.