Abstract
In order to reveal the migration of trace elements from coal to gasification residues, the modes of occurrence of potentially-hazardous trace elements (Be, V, Cr, Co, Ni, Cu, Zn, As, Se, Mo, Cd, Sb, Ba, Hg, Tl, Pb, and U) were determined by a five steps sequential chemical extraction procedure. Samples were collected from a coal-to-methanol plant (GE water-slurry coal gasification, formerly Texaco) and a coal-to-olefins plant (Gaskombimat Schwarze Pumpe pulverized coal gasification, GSP) in the Ningdong Energy and Chemical Industry Base, China. Concentrations of As and Se were determined using atomic fluorescence spectrometry (AFS). The content of Hg was determined using a DMA-80 mercury analyzer. Other trace elements (Be, Cr, Co, Ni, Cu, Zn, Mo, Cd, Sb, Ba, Tl, Pb, and U) were analyzed using inductively-coupled plasma mass spectrometry (ICP-MS). XRD and SEM-EDX were employed to determine the minerals or other inorganic phases in samples. The modes of occurrence of trace elements in feed coals can influence their behavior, including their volatility during coal gasification and, ultimately, the element’s mode of occurrence in the gasification residues. Knowing an element’s mode of occurrence in the feed coal may aid in anticipating which components the elements are likely to combine with during liquid slag cooling. Based on the relative enrichment of trace elements in the residues, elements Be, V, Cu, Mo, Ba, and Hg showed volatility during the GE and GSP gasification processes; As and Se showed volatilization-condensation behavior during the GE and GSP gasification processes; Cr, Ni, Zn, Cd, Sb, Tl, Pb, and U showed volatility during the GE gasification process; Zn, Cd, Sb, Pb, and Tl in the GSP samples, as well as Co in the GE samples, showed volatilization-condensation behavior; and Cr, Co, Ni, and U showed less volatility during the GSP gasification process. In the gasification residues, quartz, calcite, and Al–Si glass were the main inorganic phases, carbonates and iron and manganese oxides (likely recrystallized calcite) were the main hosts of most trace elements in the residues, including Be, V, Cr, Co, Ni, Zn, As, Cd, Ba, Hg, Tl, Pb, and U. Copper, Zn, Se, Cd, and Sb tended to stay in the Al–Si glass. Molybdenum is likely precipitated with the sulfides in the residues.
Highlights
Utilizing coal cleanly and effectively will be increasingly important as countries try to reduce environmental impacts caused by coal use [1]
Trace elements migrate into the coarse gasification residue and fine residue unevenly in gasifiers due to the properties of the feed coal and the gasification conditions [4,5,6]
Seven samples consisting of two feed coals and five corresponding residues were collected from two commercial-scale gasifiers (GE water-slurry slagging entrained flow gasifier and GSP slagging pulverized entrained flow gasifier) from the Ningdong Energy and Chemical Industry Base located in Yinchuan, Ningxia
Summary
Utilizing coal cleanly and effectively will be increasingly important as countries try to reduce environmental impacts caused by coal use [1]. Coal gasification is a less polluting form of coal use and aims for “zero emissions” [2]. The “zero emissions” means that trace elements in coal are not released to air directly during coal gasification due to the closed gasification system. Organic matter and minerals will be destroyed by high-temperature gasification (1250–1450 ◦ C). Some of the trace elements (such as Be, As, Se, and Hg) associated with the organic matter or in the sulfides tend to vaporize [5] and condense on the finer particles which have relatively high surface areas [6,8,9,10,11]
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