MODELING AND SIMULATION OF SMALL-SCALE BIOGAS DIGESTER BASED ON KITCHEN WASTE

Anaerobic digestion (AD) is a collection of biological processes where the organic material is converted by microorganisms to produce a mixture of mainly methane and carbon dioxide (biogas) in the absence of oxygen. Methane is a very powerful greenhouse gas. The combustion of methane releases energy, which can be used to generate heat and electricity. AD proves to be a beneficial technology in various spheres. Biogas technology has the potential to meet the energy requirements in many places, it can be designed to meet the electrical and/or heat demand in rural areas. On the other hand, kitchen waste can be used to produce biogas due to its high biodegradability which can reduce the dependency on fossil fuels. This paper presents a proposed design, modeling and simulation of small-scale biogas digester based on kitchen waste. The biological processes of the AD are mathematically modeled to give a complete representation of the physic-chemical reactions depending on several aspects such as microbial activity, substrate degradation, and temperature. A small-scale family size kitchen waste digester is designed to utilize the kitchen waste of an average Egyptian family and provides the required cooking heat of the house. The model is then simulated in Matlab/Simulink environment. The proposed model is simulated under different conditions to investigate the impacts of digester temperature, feed type, and reaction time on biogas production. The simulation results identify the best parameters for the operation of the proposed model. The study explains that the suitable size for a biogas digester based on the kitchen waste of an average Egyptian family is 0.06 m3, with a diameter of 0.4 m and a height of 0.5 m. The results show that there is a regular increase in methane production at 30 ºC for about 18 days before it becomes constant, and best volume of methane equals to 0.05369 m3 /day.
Keywords: Organic wastes; Anaerobic digester; Kitchen waste, Biogas; Simulink modeling.
Nomenclature
b          Retention time factor,
BVS    Biodegradable Volatile Solids
F_feed     Influent or feed flow (m³/d),
F_meth    Methane gas flow (m³ CH₄/d),
k₁         Yield factor obtained from experimental data,
k₂         Yield factor obtained from experimental data,
k₃         Yield factor corresponding to the growth rate of methane,
k₅         A factor correlated to the methane flow and obtained from experimental data,
K_d        Specific death rate of acidogens (d⁻¹),
K_dc       Specific death rate of a methanogens (d⁻¹),
K_s        A constant represents Monod half-velocity for acidogens (Kg BVS/ m³),
K_sc       A constant represents Monod half-velocity for methanogens (Kg BVS/ m³),
Sbvs    Concentration of BVS in the AD digester (Kg BVS/ m³),
Sbvs_in Concentration of BVS in the feed substrate (Kg BVS/ m³),
Svfa     Concentration of total VFA in the AD digester (Kg VFA/ m³),
Svfa_in   Concentration of total VFA in the feed substrate (Kg VFA/ m³),
T_reac     Digester temperature (°C),
V         Effective digester volume (m³),
VFA    Volatile Fatty Acids
X_acid     Concentration of acidogens (Kg organism/ m³),
X_meth    Concentration of methanogens (Kg organism/ m³),
μ          Growth rate of acidogens (d⁻¹),
μ_c         Growth rate of methanogens (d⁻¹),
μ_m(T_reac)          Maximum growth rate for acidogens (d⁻¹),
μ_mc(T_reac)         Maximum growth rate for methanogens (d⁻¹).