As a novel portable and robust broadband coherent light source, mid-infrared (MIR) Kerr microresonator-based frequency combs (microcombs) have prospective applications in the precision spectroscopy of molecules and biochemical sensing. The mature integrated silicon photonics platform is well suited for the MIR microcombs study because silicon has both large linear and nonlinear refractive index, but the transparency window of the platform is limited by the cladding material. Here, we numerically demonstrate the generation of a broadband MIR comb in a silicon microring resonator, harnessing the large-cross-section air-cladding waveguide to alleviate the absorption loss. The effects of higher order nonlinearities are also investigated, which show that the effect of five-photon absorption around the pump wavelength (4.78 mu m) is negligible while an octave-spanning (3.5-8 mu m) Raman-Kerr comb line can be obtained with the assistance of Raman effect and a quite pure Kerr frequency soliton comb can also be achieved at large detuning. The proposed structure can be compatible with the CMOS technology, thus can be a very promising solution to the MIR integrated photonics.