I want to design alien plants in which wood and cell walls are not made of the lignin/cellulose mixture wood and plant cell walls on Earth are made from, but I don't actually know what materials would be suitable. On it's own, I admit, this isn't a very helpful thing to say, as what counts as "suitable" could be almost infinitely variable, so I have a list of criteria for my definition of suitability:
Its strongest form must have a ratio of compressive strength to weight at least 75% that of average Earth wood
It should be sufficiently rigid that tall trees made of it would remain upright
It should be composed entirely of carbon, hydrogen, oxygen, and nitrogen, with no other elements as part of its core chemical structure
It should not be so reactive that it would be significantly more unstable than flammable wood in an oxygen-rich atmosphere
It should be possible to biodegrade but not so easy to digest that it cannot form fossil fuels; If it cannot biodegrade at all, it is not viable for use by living things, but I would also like the planet these plants to inhabit to experience an industrial revolution at some point, which requires at least coal, so it must be plausible for it to take long enough for biodegradation of it to evolve that something like what happened in the Earth Carboniferous era could take place whereby it could trap enough carbon before biodegradation evolved that large amounts of carbon-rich material derived from it could be fossilized
With all that out of the way, what materials meet the above criteria and, therefore, could comprise the wood and plant cell walls of an alien world, and what interesting uniquenesses do these have?