Long dryer vent

[Mark Forest]

The utility room in one of my rentals is in the center of the house without access to an exterior wall. The dryer vents to the crawl space into PVC pipe. The run is about 15 ft. with a few 90 degree turns. When the dryer is on very little of the exhaust makes it to the outside vent.

I have heard that venting to the roof could cause dangerous lint build up. I would really appreciate some ideas.