I think the wool flannel you are referring to is what is often seen in men's suits and topcoats, not the fuzzy cotton we think of for bedding and sleepwear.
Can you sacrifice a small piece of it, say a 12" square? I would serge or zigzag all 4 sides of the square to keep it from raveling. Trace it off on a piece of paper because the stitching and handling will alter the size and shape, then wash the square in a lingerie bag in a washer load with something like sheets, etc. Run it through the dryer on cotton setting or whatever you use most. If it shrinks and there is still enough for the blanket/quilt you are planning, go for it. I have only one item of clothing that requires dry cleaning, the rest goes into the washer & dryer. I like to shop for old wool blankets, mend any moth holes I find, then run through a hot water wash and dryer on cotton or permanent press. Yes, they shrink, some a lot. I will cut up one of the blankets to add large patches to sides and top or bottom, making sure all the edges are serged, then the seams top-stitched with a decorative stitch. Most of the blankets are in shades of dusky rose or muted green. Some of the left over scraps make beautiful leaves, etc. for applique on the blanket/quilt or anything else. Somewhere in one of my notebooks is the tutorial for replacing the frayed and ragged satin binding on blankets with extra wide binding of purchased satin yardage or quilter's cottons.