Byproduct Sulfur: Environmental Hazard and Wasted Resource

Figure 1: Sulfur cycle on geological time scales (Rappold & Lackner 2009).

Millions of tons of waste sulfur accumulate near oil and gas operations around the world. Yellow waste piles and hydrogen sulfide-enriched underground reservoirs store a lot of carbon-free energy that’s currently not utilized. At the same time, the storage of these reduced sulfur compounds presents a number of environmental and human health hazards that could be eliminated by converting petroleum-derived sulfur to benign sulfate salts.

The energy potential of sulfur is substantial. Mole for mole, H2S has as much chemical (redox) potential as the most energetic hydrocarbons. Currently extracted fossil fuels contain on average 1 – 2% sulfur compounds. Since roughly 85% of the world’s total primary energy supply derives from fossil fuels, the total chemical energy extracted in the form of sulfur compounds probably exceeds the equivalent of 1% of the world’s energy consumption.

Figure 2: Sulfur management by sequestration in soluble salts (Rappold & Lackner 2008).

Most of that chemical energy is utilized today in sulfuric acid manufacture and related applications. Yet an ever growing share of fossil sulfur has no market and as a consequence piles up as hazardous waste. The world’s current sulfur surplus is on the order of 10 million tons S per year, which corresponds to 10 GW thermal energy.

Our research focused on making the best of waste sulfur by extracting its chemical energy and sequestering sulfur waste products in benign salts that are chemically similar to sea salt. The salts can be disposed of by dilution in seawater or injection as brines into saline aquifers.

We provided detailed arguments for sulfur management in Rappold TA, Lackner KS, Large scale disposal of waste sulfur: From sulfide fuels to sulfate sequestration, Energy (2009), doi:10.1016/, in press.

Figure 3: Ocean outfall system for sulfate brine (Rappold & Lackner 2009).


In this research project, we focused on enhancing catalytic conversion of SO2 to dilute sulfuric acid with activated carbon catalysts. Catalytic SO2 oxidation is a necessary step in the sulfur management process chain. Understanding the fundamental processes at work in the pore space of carbon particles might lead to reduced costs and improved process efficiencies in SO2 capture and sequestration. The resulting sulfuric acid would be concentrated enough for reasonably fast neutralization of the acid with alkaline silicate minerals.
Klaus Lackner, Ewing-Worzel Professor of Geophysics, (PI)
Tim Rappold, PhD 2011, Earth and Environmental Engineering