The urgent need for sustainable propulsion solutions has accelerated the exploration of green bipropellant thrusters using high-test hydrogen peroxide (HTP) with kerosene. In this study, a transient, high-fidelity CFD model coupling droplet-phase dynamics, real-gas behavior, and finite-rate chemical kinetics was developed to simulate the ignition and combustion processes in a coaxial-injected HTP-kerosene thruster. Simulations investigated the impact of oxidizer purity (95 % and 98 %) and mixture ratio variations, targeting a vacuum thrust of 100 N. Results revealed that stoichiometric mixtures with 98 % HTP delivered the most favorable balance of thrust (63.22 N at sea level) and thermal loads, with combustion temperatures aligning within 1 % of CEA predictions. Fuel-rich mixtures exhibited significant inefficiencies, with up to 18 % unburnt kerosene detected at the nozzle exit. Wall temperatures peaked at 3271 K under adiabatic assumptions, exceeding material safety thresholds, highlighting the necessity of advanced thermal management strategies. Observations of flow separation, shock structures, and model-predicted oxygen backflow further reinforced the realism of the simulations. This study advances green propulsion by linking combustion dynamics with structural viability. It provides new insights into propellant formulation, thermal management, and injector optimization for future environmentally compliant engines.