Slag and ash chemistry after high-calcium lignite combustion in a pulverized coal-fired power plant

More than 73% of the electrical power requirements of Greece are generated in lignite-fired
power plants. Greece is the thirteenth largest coal and the fifth largest lignite producer in the
world. The lack of domestic high-rank coals makes necessary to use low quality lignite for
power generation in Greece. These lignites are characterized by a high water and ash content
and a low calorific value. The low quality of such lignites generates important technical and
environmental problems during combustion. Slagging and fouling are common inside the
power units and affect with particular severity the power plants performance. Slagging
deposits take place in the high temperature radiant sections of the boiler, and are usually
associated with some degree of melting of the ash. Fouling deposits are produced in the
lower temperature convective sections of the boiler, and are generally related to condensation
on the low temperature tube surfaces. Problems in boilers associated with ash deposits
include modification of the heat transfer in different sections of the furnace, physical distortion
of metal pieces due to the weight of the deposit, clogging of burners, and corrosion and
erosion of metal walls. Feed lignite always carries more than 20% of inorganic matter, and its
mineralogy and chemistry are originally related to geological factors. Five representative
samples were collected from the inner surfaces of Unit 1 of the Agios Dimitrios Power Plant,
Northern Greece, reflecting the main types of ash deposits occurring in the combustion
facility. The chemistry of these high-calcium ash deposits has been investigated. Moreover, a
fly ash and a bottom ash samples were taken and analyzed in order to investigate their
possible impact on the environment after land-filling. All samples were digested by using 2.5
ml HNO3, 5ml HF and 2.5 ml HClO4 (1:2:1). Sixty element concentrations were determined in
all samples by inductively coupled plasma–mass spectrometry (ICP–MS) and inductively
coupled plasma – optical emission spectrometry (ICP-OES). Calcium is the most abundant
element in all samples due to the dominance of calcium phases. The chemical composition of
the bottom ash, fly ash, slag and fouling deposits, is mainly influenced by the chemical
composition of the feed lignite and the co-excavated sterile materials, which are marly
limestones containing, on average, 93% of calcite.