10" class 150 butterfly valve is in a service that operates at 1300 °F, has 14% oxygen, 7% SO2, and balance N2 in the process stream.  Valve steam leaks are a real problem.  Conventional graphite packing oxidizes and disappears above 900 °F, which we have proved.
Unless you can keep air away from the graphite I don't think you have a choice but to extend the bonnet and/or add heat fins.
The only other thing I can think of is asbestos, but there aren't very many places left that still allow it to be used.They have done extensive type research and testing on valves and valve packing at high temperature.  They offer a triple offset butterfly valve that is used extensively in the power industry and Petrochemical.  Use their engineering resources and expertise to help solve your problem.

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