Yes, there are many fanless Mini PCs available in the market that are an excellent choice to set up as a firewall. Their compact size and silent operation make them perfect for both home offices and small offices (SOHO). However, before purchasing any Mini PC to use as a firewall, make sure it fulfils the requirements of the firewall software you wish to install. I have written a separate post related to pfSense hardware and OPNsense hardware. These posts contain the list of Mini PCs that are compatible with the two popular FreeBSD-based firewall solutions – pfSense and OPNsense.