(342b) Ash Aerosol and Deposition Formation during High Temperature Oxy-Combustion of Petroleum Coke

As a byproduct of petroleum refining process, petroleum coke is a potential fuel for power plants. However, the use of petroleum coke may be limited by its high sulfur contents, low volatiles and possible ash deposition issues. Currently available literature on petroleum coke combustion are largely about cofiring with coal in circulating fluidized beds. In order to assess the feasibility of switching to petroleum coke for steam generation, it’s essential to understand the behavior of ash aerosol and ash deposition of petroleum coke under a furnace that simulates time temperature histories of field units. Particulate matter with size smaller than 1 µm (PM₁) can only be partially removed by precipitators and the released PM₁becomes an issue for human health.

This paper is concerned with the formation of both the pet coke ash aerosol and the subsequent ash deposit on heat transfer surfaces. To this end a 100kw down-fired pilot scale oxy-fuel combustor (OFC) is used under two conditions: 1) air combustion (denoted as AIR); 2) oxy combustion with 70% O₂ in the inlet oxidant gas (denoted as OXY70). Experimental data consist of ash aerosol size distributions obtained through electric mobility (SMPS), light scattering (APS), and inertial impaction techniques (BLPI). The latter allows samples to be collected for subsequent size segregated elemental analysis. Ash deposition was collected using our own newly developed wall temperature controlled deposition probe, and compositions were analyzed by scanning electron microscopy (SEM). Results show that more submicron particles are generated in OXY70, which suggests more minerals get vaporized under high temperature conditions, which is not unexpected. The super-micron particle concentrations in the flue gas show little difference between these two cases, suggesting that ash fragmentation is not affected by flame temperature. However, the sodium content of the submicron particle is greatly diminished at the high OXY70 high flame temperature, because of increased rates of sodium scavenging by larger aluminosilicate particles. Although it is difficult to burn petroleum coke efficiently, high temperature oxy-combustion appears to be effective in lowering the loss on ignition (LOI) of the bulk ash aerosol compared to that from air combustion. Surprisingly, the deposits were found to contain carbonaceous particles very much larger than the pet coke particles in the fuel feed.