Trace element analysis of gasification plant samples by i.c.p.–m.s.: validation by comparison of results from two laboratories

In the absence of either standard methods for trace element analysis of coal-derived chars and ash or standard reference materials for coal gasification samples, a comparative exercise has been undertaken to validate analyses of chars and fines recovered from a coal gasification pilot plant. Several sample digestion methods, used for converting solid samples to aqueous solutions for trace element analysis by i.c.p.–m.s. and i.c.p.–a.e.s., have been compared: peroxide fusion (sodium peroxide+sodium carbonate) and microwave digestion by AEA Technology and wet-ashing (open acid digestion) and microwave digestion by Imperial College (IC). The aim was to analyse samples from a pilot plant scale coal gasification rig. Digestions were carried out using a range of sample sizes, at times as small as 10 mg and analyses of small quantities of these samples were compared with results on the same samples from a specialist contract laboratory (AEA Technology). Elements studied were As, B, Ba, Be, Cd, Co, Cr, Cu, Ga, Hg, Mn, Mo, Ni, Pb, Sb, Se, Sn, V and Zn. For the majority of the elements studied, elemental concentrations (1–150 wppm range) were generally in agreement between the two laboratories within 50%. Limitations of the various digestion methods are discussed. All the digestion techniques appeared to lose mercury and therefore a Leco atomic absorption spectrophotometer based method was used to determine Hg. Output solid stream distributions of trace elements from the gasifier indicate that all of the Hg in the fuels was released to the fuel gas. The primary cyclone fines stream contained the major part of the trace elements from the fuels. The causes of apparent losses of As, Mo, Pb, Sn, Zn and Se are discussed.