**4. Current conversion technologies of bio wastes to energy**

Conversion of biowaste aims to recover energy, remove residues, and increase material value [3]. Effective conversion technologies should address financial requirements, effective recovery of energy, waste removal, and environmental protection. The development, improvement, and implementation of conversion technologies have been made due to surge in wastes that pollute environment and need for energy sustainability. The need for land has emerged as another challenge. For example, Waste to Energy (WTE) plant that can treat 30 million tons of MSW for 30 years only will use 10 hectare compared to landfill that needs 30 hectare [34].

A number of conversion technologies have been developed and applied in many parts of the world. Advanced waste to energy conversion has led the increase of energy mix from biowastes. The technologies range from direct energy conversion to more upgraded fuel. The technologies have advanced in a way that they produce energy efficiently and meet the requirements for public health while reducing air pollution and obligatory number of dumping locations [3]. These methods include direct combustion/incineration, carbonization, pyrolysis, gasification, anaerobic digestion, and oil pressing for biodiesel production. Some conversion leads to production of feedstock for chemical production. Each method has the advantages and disadvantages and thus limitation in use in African countries. The common and simple technologies in biowaste conversion have been anaerobic digestion and incineration.

### **4.1 Direct combustion/incineration**

This is the oldest and simple, and well utilized conversion technology [34]. It deals with direct heating of fuel in presence of oxygen. It takes place at high temperature (850-1200°C) and release energy in form of heat [3]. It has been used in many biowaste plants ranging from bagasse, MSW to saw dust. Apart from energy recovery, the bottom and fly ashes can be used in other applications such as construction. The challenges of this technology are that feedstock should be of low moisture content and thus limiting many of African MSW which have high moisture content to above 40% [35]. It also may lead to pollution if poorly controlled, since it operates at high temperatures and thus releasing pollutant gases and dust. Its advantages and appropriateness in African biowastes are that it is simple, mature, and low investment cost especially if heat is the final conversion. It is also appropriate in dry biowastes such as forest residue since no pretreatment costs [3] and thus utilizing wood and crop residues produced in Africa.

### **4.2 Carbonization**

It is the process of heating biowaste at low temperature in the absence of oxygen [36]. This process upgrades biowastes to produce usable charcoal. It is mature, cheap, and simple technology that has been used on other biomass for years and years [37, 38]. The technology works efficiently for large size and low moisture contents.

Most of biowastes such as forest and crop residues have been converted into charcoal by this process. Its advantages include the cheapness, simplicity, and ability to handle variety of feedstock including lignocellulosic biomass. This method is appropriate for African biowastes although for fine materials such as sawdust and a wet material such as MSW becomes inappropriate. It is applicable in every part of Africa.
