I bought a couple bag's last week to try as cat litter, well the cats didn't like it . So I dumped the unused ones in the P61 and wow I'm pretty impressed on the heat and low ash.
You're referring to the "pelletized" stuff, yes? That's crazy,'cos I was just at TSC and saw some, thought, "I wonder how those burn?" Now I know...
I bought several bags of the light-colored horse pellets from TSC and they burned well. I'd have no problem using them again.
I've burned a few bags over the years too. They are pine. They can be a bit of hit or miss. Folks say the bags with light colored pellets burn good but the dark colored ones maybe not so great. Normally they sell for $6/bag but frequently go on sale for $5/bag. Only thing is, technically they are not considered heating fuel so they charge tax on them too, at least they do around here.
I bought 5 bags of TSC pine horse bedding a couple of years ago after hearing people rave about them (that was during the last pellet shortage). I was so disgusted because they didn't put out any heat and were ashy as all get out (and I usually burn not-so-premium pellets, so that is saying something). Anyway, turns out the pellets I got were very dark whereas most others were getting the light pellets. I don't know how to tell if they are dark or light prior to getting them home since the bags are not see thru. guess you take your chances (or maybe different UPC's?).
Did you de-pelletize them before putting them in the cat box? I've used wood pellets that got wet and broke down a bit in the cat box and it works well. Course that cat used to be feral so he's not real picky about anything.
There was someone advertising wood pellets as horse bedding and cat litter during the summer. For cat litter he suggest wetting the pellets a little, so they start breaking down, then letting that dry before placing in the cat box. I would guess the result is more accepted by cats as close to cat litter. Some resources suggest a really slow method of transferring your cat to pelletized litter. As a note, only one of my two cats uses the pellets readily (the formerly feral cat). I have 3 cat boxes and 1 is of pellets and the other two litter. I should transfer all to pellets as they are much cheaper than clay litter, it clumps well and controls the odor too, but I'm too lazy to bother. the only reason I have one cat box that way is because I found two bags that had gotten wet and were basically sawdust so figured I would try them as litter.