Sure, there are at least two ways, but you won't get it fuel-efficient enough for commercial service. You could do research flights into the volcanic ash. One

Sure, there are at least two ways, but you won't get it fuel-efficient enough for commercial service. You could do research flights into the volcanic ash. One choice is a low-compression jet engine that doesn't burn hot. Temperatures need to stay lower than the melting point of volcanic ash. Add a durable coating (maybe: sapphire, diamond, tungsten carbide...) to resist abrasion and you're all set. Another choice is to use an engine that is part turboprop and part rocket. The turbine portion is fed entirely from internal stores, for example liquid oxygen and liquid methane. Keep them balanced for maximum power, or dilute/unbalance them to avoid the need for exotic materials. Ash never enters the turbine. To get back a little efficiency, you put a prop on the front instead of just having a rocket. You'll want to also put a sapphire coating on the windshield. Treating the wing leading edges could also be a good idea; maybe for that you could consider titanium-aluminum-nitride.