Closest substitute for alder wood chips when smoking

I'm confused. You don't want fake smoke but you don't want to use wood? Are you asking for a synthetic substance you can burn that will not be worse for the environment than burning wood?

First of all- burning wood is not bad for the environment- that carbon has not been sequestered and so it would be released into the atmosphere anyway when the wood decayed.

Secondly- mesquite is a cancerous weed here in Texas and I encourage everyone to smoke with it to restore the damaged ecosystem.

Thirdly- You can't fake the taste of smoking with wood. That's the point. Even liquid smoke is actually made by distilling real smoke.

(I originally posted a comment and decided to modify it into an answer.)

If you're putting your wood chips directly on a fire, it's good to soak them (and that means real soaking--a few days--not an hour or two). This prevents the wood chips from actually catching fire, which can cause off flavors from combustion chemicals settling back on your food. Tastes kinda like a brand new telephone pole smells on a hot day. Tar/Creosote, and not a nice flavor.

But on a gas grill, if you've got your chips in a pan or wrapped in a fairly tight foil pouch, you needn't worry so much about the chips actually catching on fire. As long as they just smoulder, you're good to go.