I'd guess that this will never happen unless they figure out how to make a pond work.  Miles of plastic tubing would be a maintenance nightmare; any blockages would have to be cleared by hand.

A bioreactor might work for a furnace output, if you could find people willing to pay for the installation.

Pump cleaning plugs thru the pipes at regular intervalls. I would be more worried about finding a long lived transparent plastic that is not turned brittle or opaque from UV radiation.  Long lived tubing is important for good economy.
Won't work.  Phytofermentans did the math.  You're talking thousands of 20' plastic tubes all manifolded together in parallel.  To prevent blockages you would have to regularly backflush each tube.  The piping and valving alone for the gas, water, and cleaning would cost several times what the tubing would.