Petroleum Coke Burns Hotter - Burns Grates | Firebars quicker!
Some cheaper (imported coal) is made from or include Petroleum Coke "Petcoke" A derivative of fuel oil, these fuels / coals are not recommended and burn in excess of the designed temperature for stove parts. Ceramic Glass is tested up to a temperature of 760c Degrees.
More information can be found:
http://en.wikipedia.org/wiki/Petroleum_coke
Specifically of Interest on the Wikipedia Page
Fuel grade coke
Fuel Grade Coke is classified as either sponge coke or shot coke morphology. While oil refiners have been producing coke for well over 100 years, the mechanisms that cause sponge coke or shot coke to form are not well understood and cannot be accurately predicted. In general, lower temperatures and higher pressures promote sponge coke formation. Additionally, the amount of heptane insolubles present, and fraction of light components in the coker feed contribute.
While its high heat and low ash content make it a decent fuel for power generation in coal fired boilers, petroleum coke is high in sulfur and low in volatile content, and this poses environmental (and technical) problems with its combustion. To meet current North American emissions standards, some form of sulfur capture is required, a common choice of sulfur recovering unit for burning petroleum coke is the SNOX Flue gas desulphurisation technology,[3] which is based on the well-known WSA Process. Fluidized bed combustion is commonly used to burn petroleum coke. Gasification is increasingly used with this feedstock (often using gasifiers placed in the refineries themselves).