Furthermore, this wasn't ANFO, just ammonium nitrate. So, no fuel--only oxidizer. Some analysis has suggested that the efficiency of a detonation here would only be .15. Furthermore, probably a low-order detonation (only part of the material detonating). Even actual ANFO typically needs an intermediate booster explosive to achieve a good high-order detonation. For example, instead of just having a detonator on its own a blaster might add a PETN booster charge to make sure the ANFO all detonates. Without that you typically see pellets of undetonated ANFO scattering around the explosion. Hell, even with a booster you still tend to see ANFO pellets lying around the blast area.
All that is to say, this was probably closer to 200 tons of TNT, very roughly speaking.
Ammonium ion is the fuel when pure ammonium nitrate explodes.
Adding fuel oil increases the power for a couple reasons - 1) fuel oil adds additional fuel 2) ammonium nitrate absorbs water from the atmosphere and makes it much less sensitive. The fuel oil “waterproofs” the ammonium nitrate.
According to Wikipedia the Texas City disaster had just over 2000 tonnes of ammonium nitrate explode (as opposed to 2700 in Beirut) which they claim to be “one of history's largest non-nuclear explosions”.
Was there some other factor that caused the Texas City to have a bigger blast?
If I'm reading it right, the Texas City disaster had 2300 tons on the Grandcamp, and 961 tons on a nearby ship, the High Flyer. That second ship exploded later in the day. There was also a chemical plant that was caught in the blast and a warehouse with fertilizer.
All that is to say, this was probably closer to 200 tons of TNT, very roughly speaking.