Escalating sludge generation and enforced environmental regulations dictates advanced thermal treatment methods for sustainable energy recovery and zero carbon. This study develops a comprehensive Euler-Euler multiphase model for sludge incineration in a bubbling fluidized bed (BFB) integrated into a circulating fluidized bed (CFB). The modular reactor (0.6 m x 14 m) comprises five sequential thermal zones: drying, devolatilization, volatile combustion, char oxidation, and burnout. Governing equations are derived for gas and solid phases using the interpenetrating continuum approach in the CFB riser, incorporating conservation of mass, momentum, species, and energy for each phase. Euler-Euler formulation with Kinetic Theory of Granular Flow (KTGF), volumetric reactions, species transport, and interphase heat exchange is applied. Gas phase is modeled as Newtonian, while sludge follows non-Newtonian granular behavior via kinetic theory, improving particle drying and fast fluidization. Convective, conductive, and radiative heat transfer are incorporated. Moisture evaporation and multi-step pyrolysis kinetics for volatile release are included using semi-empirical and literature models (EDM-FRC). For preliminary assessment, simplified zone-wise finite difference simulations are implemented in Python to evaluate axial temperature, pressure drop, and species concentration. Final transient 3D CFD simulation is conducted in Ansys Fluent using PISO and finite volume method, with detailed species transport and heat transfer with UDF based boundary conditions. Results capture low-temperature drying, sharp combustion rise, and nearly linear pressure drop along the freeboard. Model predictions of temperature distribution, heat-flux, conversion rates, and flow dynamics are validated against pilot-scale experimental data. The model pro-vides realistic insights for sludge to energy incineration.