September 1, 1995, through August 31, 1996

Project Title: STABILIZATION OF SPENT SORBENTS FROM COAL-BASED POWER GENERATION PROCESSES

DOE Cooperative Agreement Number: DE-FC22-92PC92521(Year 4)

ICCI Project Number: 95-1/3.3A-2M

Principal Investigator: Javad Abbasian, Institute of Gas Technology

Other Investigators: James R. Wangerow, Institute of Gas Technology

Project Manager: Daniel D. Banerjee, ICCI

ABSTRACT

The overall objective of this study was to determine the effect of implementation of the new and more stringent EPA Protocol Test Method involving sulfide containing waste, on the suitability of the oxidized spent sorbents from gasification of high sulfur coals for disposal in landfills, and to determine the optimum operating conditions in a "final" hydrolysis stage for conversion of the residual calcium sulfide in these wastes to materials that are suitable for disposal in landfills. An additional objective was to study the effect of ash on the regeneration and ash-sorbent separation steps in the Spent Sorbent Regeneration Process (SSRP).

To achieve these objectives, a large set of oxidized samples of sulfided calcium-based sorbents (produced in earlier ICCI-funded programs) as well as oxidized samples of gasifier discharge (containing ash and spent sorbent) were tested according to the new EPA test protocol. Samples of the oxidized spent sorbents that did not pass the EPA procedure were reacted with water and carbon dioxide to convert the residual calcium sulfide to calcium carbonate.

The result of EPA strong acid tests conducted with the oxidized samples of sulfided calcium-based sorbents indicate that when the calcium sulfide content of these solid waste materials is below 4.5%, they will meet the EPA requirement for sulfur release, making them suitable for landfill disposal. When the calcium sulfide content of such solid waste materials are above 4.5%, they can be reacted with water and carbon dioxide to remove the residual sulfide to below 0.5%, making them suitable for landfill disposal. The presence of ash in the LASH (a mixture of limestone-derived and ash-derived) material did not appear to affect the sulfur leachability of such waste materials.

EXECUTIVE SUMMARY

Advanced or second-generation power generation systems such as Integrated Gasification Combined Cycle (IGCC) and two-stage Pressurized Fluidized-Bed Combustion (PFBC) plants can achieve higher thermal efficiencies and a lower cost of electricity than a conventional pulverized coal-fired (PC fired) plant equipped with flue gas desulfurization. Because these advanced processes can use high sulfur coals in an environmentally acceptable manner, they have the potential to expand the marketability of Illinois coals. Projected costs for IGCC and PFBC plants are substantially lower than comparable pulverized-coal (PC) combustion plants, especially if air emission regulations were to require much lower SO2 emissions.

These advanced coal conversion processes generally involve gasification of coal followed by combustion of the fuel gas in a gas turbine. In these processes, the sulfur compounds present in coal are converted to hydrogen sulfide when the coal is gasified. To comply with the New Source Performance Standards (NSPS) and lessen corrosion of the gas turbine, a high fraction of the sulfur must be removed from the fuel gas stream.

One approach for removing sulfur compounds from the fuel gas is the use of calcium-based sorbents, such as dolomite and limestone in the gasifier. In such processes, dolomite and limestone react with the sulfur compounds (mainly H₂S) to significantly reduce (i.e., more than 90%) the sulfur content of the fuel gas.

The solid wastes, produced from the reactions of calcium-based sorbents with hydrogen sulfide in the fuel gas, contain calcium sulfide which is unstable and has the tendency to decompose when it contacts moisture in the air, releasing hydrogen sulfide to the atmosphere. To dispose of the solid waste products in an environmentally acceptable manner, calcium sulfide can be converted either to stable calcium sulfate through reaction with oxygen, or calcium carbonate through reaction with water and carbon dioxide.

Stabilization of partially sulfided dolomite with oxygen has been studied by the principal investigator of this project in earlier ICCI-funded programs. The results of these studies indicate that limestone can only be partially sulfated (about 30%) while dolomite can be sulfated above 95%. The extent of sulfation depends not only on the sorbent type but also on the extent of sulfidation in the gasifier.

Surprisingly, essentially all the oxidized sorbents (especially those with high calcium sulfide content) produced in the earlier studies successfully passed the less stringent EPA test protocol, making them suitable for disposal. However, because the test procedure was believed to be inadequate, further studies were limited to dolomite where a high level of conversion of calcium sulfide could be achieved.

A new protocol for testing sulfide containing waste, which is much more stringent than the old method, has been proposed by EPA.

It was believed that because of stringent disposal requirements, singe-stage oxidative stabilization process involving limestone or even dolomite, would prove to be insufficient for production of acceptable material for disposal. Under such a scenario, a second stage would be necessary to remove the residual calcium sulfide from the sample. Earlier studies at IGT under sponsorship of ICCI have shown that over 99% of calcium sulfide in the spent sorbent can be removed through the reaction of calcium sulfide with water and carbon dioxide (hydrolysis). Such a process is believed to be suitable for conversion of the residual calcium sulfide to calcium carbonate.

The overall objective of this study was to determine the effect of implementation of the new and more stringent EPA Protocol Test Method involving sulfide containing waste, on the suitability of the oxidized spent sorbents from gasification of high sulfur coals, and to determine the optimum operating conditions in a "final" hydrolysis stage for conversion of the residual calcium sulfide in these wastes to materials that are suitable for disposal in landfills. An additional objective of this program was to study the effect of ash on the regeneration and ash-sorbent separation steps in the Spent Sorbent Regeneration Process (SSRP).

In this study, oxidized samples of the spent sorbents (produced in earlier ICCI-funded projects) covering a broad range of operating variables, including sorbent type, particle size, level of sulfidation, and level of sulfation, were evaluated in terms of their suitability for disposal under the new and more stringent EPA test procedure. The results of these tests were used to establish a guideline for selection of the relevant operating conditions in the one-step oxidative stabilization process.

Samples that did not pass the EPA test procedure were further reacted with water and carbon dioxide in the "hydrolysis" process to convert the residual calcium sulfide to calcium carbonate at the optimum operating conditions for this second stage determined in this project. The hydrolyzed sample were also tested under the new EPA procedure.

The result of EPA strong acid tests conducted with the oxidized samples of sulfided calcium-based sorbents indicate that when the calcium sulfide content of these solid waste materials is below 4.5%, they will meet the EPA requirement for sulfur release, making them suitable for landfill disposal.

When the calcium sulfide content of such solid waste materials are above 4.5%, they can be reacted with water and carbon dioxide in the temperature range of 60 to 120C at 5 bar to remove the residual sulfide to below 0.5%, making them suitable for landfill disposal.

The presence of ash in the LASH (a mixture of limestone-derived and ash-derived) material did not appear to affect either the sulfur leachability of such waste materials, or the removal of the residual sulfide through reaction with water and carbon dioxide. A modest separation of LASH material into sorbent-rich and ash-rich materials (suitable for recycle and disposal, respectively) in the context of SSRP process could be achieved with gravity-assisted separation technique. The sorbent-rich fraction had about 65% weight of the LASH material and contained 45% of the ash as well as 80% of the sorbent in the LASH. It should be noted that in order to achieve even such a modest level of separation, one has to deal with two separate streams in the context of SSRP process. While the sorbent-rich stream is hydrolyzed and recycled, the ash-rich stream should also be hydrolyzed separately to produce suitable material for landfill disposal.