  We use a three-specie fluid model of electric discharge in air to simulate
streamer evolution from the avalanche-to-streamer transition to the collision
of opposite-polarity streamers. We estimate the upper limit on the production
of thermal runaway electrons, which is dominant during the second of these
processes. More thermal runaways are produced if the ionization due to natural
background and photoionization is reduced, due to possibility of creation of
higher electric fields at streamer tips. The test-particle simulation shows,
however, that these thermal runaway electrons have insufficient energies to
become relativistic runaways. The simulations are done in constant uniform
background fields of $\mathit{E}_{0}$ = 4 and 6 MV/m. A simulation was also
performed in $\mathit{E}_{0}$ = 2 MV/m after formation of streamers in 4-MV/m
field, in order to approximate the average background field created by
$\sim$1-MV voltage over a $\sim$1-m electrode gap used in laboratory spark
experiments. We conclude that the used fluid model is insufficient to explain
X-ray observations during such experiments. We discuss the possible role of
mechanisms which were not included in this or previous modeling but may play
the deciding role in the electron acceleration and X-ray production during a
streamer collision.
