While previous studies experimentally demonstrated that loop reactor (LR) can be sustained with a lean feed (using ethylene combustion) and have analyzed the single-reaction adiabatic case, this work analyzes the effects of heat loss and of reactor size to determine the leanest stream (expressed in terms of adiabatic temperature rise T-lim) that will sustain the operation. For an adiabatic infinitely long reactor T(lim)0 while for a finite reactor T-lim scales as (1+Pe/4)(-1) where Pe=Luc(pf)/k, and heat loss increases this limit by (/Pe)(1/2). Thus, a good design of a LR will aim to decrease conductivity (k) and radial heat-transfer coefficient () while increasing throughput (u) and reactor length. This article is also the first experimental demonstration of auto-thermal operation in a LR for catalytic abatement of low-concentration of methane, showing the leanest stream to be of 8000 ppm vs. 33,000 ppm in a once-through reactor. Experimental combustion results of methane and of ethylene are compared with model predictions. (c) 2016 American Institute of Chemical Engineers AIChE J, 63: 2030-2042, 2017