Keeping it cozy, while thinking about the environment

Everything now a days is about protecting the environment, especially when you have utilities in your own home that could conflict with environmental health.
A thing very popular in the cozyness of your own home, is fireplaces and woodburning stoves. The sinner on this subject is the smoke that the stoves releases, and it is difficult to find wood that respect the environment.

In Denmark wood pellets and the so called briketter is among the most popular products, but what are the right thing to use?

Obviously, the product with the least CO2 emissions is preferable. But isn't the market lacking a more sustainable product?
That is exactly what I find as the most promising entrepreneurship, if we could find a product that respects both environment and can have a long lifetime of use.

Elon Musk go nuts please