Mercury
Mercury can be present in both coal and natural gas, although the quantities vary widely from source to source. Mercury presents a potential hazard both for the integrity of the plant and as a toxic emission for the environment. Whether gasifying coal or partially oxidizing natural gas, mercury from the feed will appear at least in part in the synthesis gas, and so for these feeds it is necessary to address this feed contaminant.
Wilhelm describes a number of different mechanisms by which mercury degrades engineering materials (Wilhelm 1990). In particular, he mentions liquid metal embrittlement of high-strength steels. He also describes the formation of the highly explosive compound mercury nitride in the presence of ammonia.
Mercury is gaining increasing recognition as an important atmospheric pollutant from coal-fired power stations. The U. S. Department of Energy has reported that for conventional coal-fired power stations, there is “currently no single technology” available that can control mercury from all power plant flue gas emissions. The DoE has a major test and development program for processes to control mercury emission in flue gas (U. S. Department of Energy 2002).
The situation for gasification technologies is different. Proven and economic methods for mercury removal are available and have been practiced for many years. Mercury can be adsorbed onto sulfur-impregnated carbon, which can achieve an effluent concentration of less than 0.1 (ig/m3 (Wilhelm 1990).
A prominent example of mercury removal in a coal gasification environment is provided by the Eastman Chemical operation in Kingsport, Tennessee. A sulfur — impregnated activated carbon bed was installed upstream of the acid-gas removal unit from the plant’s inception in 1983 to protect downstream chemical processes from contamination and has operated successfully for nearly 20 years (Trapp 2002). Mercury capture is estimated to be between 90% and 95%. This experience was used as the basis for a cost-comparison study performed for the U. S. DoE showing that mercury removal from an IGCC plant could be as little as one-tenth of the cost of removal from a conventional PC power plant (Rutkowski, Klett, and Maxwell 2002).
Koss and Meyer (2002) report also on mercury removal from an existing coal gasification plant, in which metallic mercury removal is integrated into a Rectisol desulfurization unit operating at -57°C. Total mercury slip through the unit was measured at l-2ppbv.
In the case of a natural gas feed, Marsch (1990) has reported on the explosion of an ammonia separator after 10 years of operation that was attributed to the presence of mercury. The natural gas feed to the primary reformer of this 1000 t/d ammonia plant contained on average 150-180 ^tg/m3 mercury, amounting to an annual intake of 60-72 kg per year. Significant quantities of mercury passed through primary and secondary reformer (essentially a catalytic partial oxidation process), as well as a CO shift and acid-gas removal system, to enter the ammonia synthesis unit, where it caused the damage described. In evaluating the conclusions from this accident, Marsch recommends removal of any mercury in the feed to the lowest possible level. This message applies not only to ammonia manufacture but equally to any other application involving the partial oxidation of natural gas.
One of the problems associated with coal gasification is that in coal many of the elements of the periodic table can be found in minor concentrations. An element of emerging concern is arsenic, which may be present in concentrations in the order of 1-10 ppmw in coal (see Table 4-7). Toxic elements are of no concern when they end up bound in the slag or in stable chemical compounds. The problem with arsenic is that under reducing conditions it forms the volatile compound AsH3. Arsenic is a known poison for ammonia catalysts, but recorded instances of this occurring in commercial plants have not been found.
Raw gas shift catalyst is reported as “removing arsenic very selectively,” though arsenic deposits on the catalyst were low compared with those of nickel and carbon (BASF undated).