Sulfur recirculation is an innovative technology that is able to reduce high temperature corrosion in superheaters. Alternatively, it can increase electricity generation at waste incineration installations, if superheater steam pressure and temperature are raised.

Sulfur recirculation technology is unique as no external sulfur is added and thus the amount of residual products does not rise. In the process, sulfur from a wet flue gas cleaning system is returned to the furnace. The recirculated sulfur raises the SO2 concentration in the furnace and reduces the Cl/S ratio in deposits and ashes, and the environment becomes less corrosive. Furthermore, the formation of dioxin is reduced, and the proportion of sulfates in the effluent water discharged from the wet flue gas cleaning is reduced substantially.

The process works in two stages. First sulfur dioxide is removed from the flue gases in the wet flue gas cleaning stage. The removed sulfur compounds are then sprayed into the boiler through nozzles with a surrounding carrier gas. In this way the level of sulfur in the water is raised. Thus each sulfur atom passes through the furnace several times and no external sulfur need be added.

We have demonstrated the process at the Renova waste incineration installation in Gothenburg. Measurements were taken in collaboration with Chalmers HTC and SP Technical Research Institute of Sweden with support from Waste Refinery and ProEnivro. The final report is available for download via the link to the right.

Dioxin samples, impactor measurements, deposit probe measurements, ash samples and 1,000-hour corrosion measurements were taken in full-scale trials with and without sulfur recirculation. With sulfur recirculation, corrosion rates in the superheaters for all materials evaluated (16Mo3, Sanicro 28 and Inconel 625) were reduced by more than 50 percent compared to the reference case. With sulfur recirculation, the nature of the corrosive attacks changed when compared to equivalent exposures in the reference case. The sulfur prevented the formation of metal chlorides at the metal/oxide layer and the formation of chromate.

The sulfur recirculation system provided sought-for SO 2 concentrations in the furnace except during a period with a very low sulfur content in the fuel. The sulfur concentration in the stacks was always at levels below applicable limits. Dewpoint measurements showed that sulfuric acid dosing did not give rise to increased SO3- levels in the furnace, which could otherwise lead to low temperature corrosion. Deposit rates were reduced by one third with sulfur recirculation.

Figure 1. Cross section of a corrosion probe in reference samples. A thick layer of oxide deposits is formed.
Figure 2. Cross section of a corrosion probe with sulfur recirculation in operation. A thin layer of corrosion products is formed.

FUELS WITH HIGH CHLORINE CONTENT CAUSE BOILER CORROSION

Compared to e.g. coal-fired boilers, waste-fueled boilers have low electrical generation efficiency. This is primarily because of the severe corrosive environment created. Deposits containing high levels of alkali chlorides are formed in boilers that use fuels with a high chlorine content. High concentrations of alkali chlorides increase high temperature corrosion. Elevated sulfur concentrations reduce boiler corrosion, thus allowing designs with higher steam parameters. Higher steam parameters means greater electrical generation efficiency. Today, leaks and the replacement of corroded superheaters cause availability problems and are major expenses for waste-fired energy plants.

Many studies have taken up the alternative of sulfur dosing in various forms. However, common to all of these methods is the need for large quantities of sulfur to be added, resulting in a considerable increase in residual products from the incinerator installation.