for   carbon C d = C a d ( 100 100 − M a d ) for   Nitrogen N d = N a d ( 100 100 − M a d ) for   Hydrogen H d = H a d ( 100 100 − M a d )

where: d = the dry basis;

ad = as determined basis;

M_ad = the moisture content of the general analysis sample when analyzed. During the calibration process six points for every component are taken and the sample was run in duplicate finally the average values to be taken are as in Table 4.

Proximate analysis helps to assess the percentage of volatile matter, fixed carbon, moisture and ash contents. This analysis is very important to study the combustion phenomenon of the coffee husk. Therefore, to know the proximate analysis and calorific value of the coffee husk different laboratory equipment is used in Geological Survey of Ethiopian. The experiment is conducted by international standard test method using three main apparatus, these are CARBOLITE furnace, Carbolite oven and Adiabatic Calorimeter. 200 GM of coffee husk sample which came from the study area is taken from this analysis (Fan, Soccol, Pandey, & Soccol, 2003) (Figure 11).

The experiment started from the sample preparation room. In this room, giving a code for the sample was the primary task and Y-M-001 was the sample code number. Next, removal of unwanted moisture from the sample is done in a Retsch model RS 200 (Fiori & Florio, 2010) . Then grinding of the sample using an electrical milling machine is done. Sieving the crushed sample with the help of 0.60 mesh sieve and packing the prepared sample with plastic sheet was the last work in the sample preparation room. After this the sample is sent to the hydrocarbon laboratory where the analysis of the prepared sample is conducted (Figure 12).

Proximate analysis indicates the behavior of the husk when it is heated. Therefore the following tasks are conducted in the hydrocarbon laboratory.

1) When 1 gram sample of husk is subjected to a temperature of about 110˚C in the oven for a period of 1 hour, the loss in weight of the sample gives the moisture content of the husk.

2) When 1 gram sample of husk is placed in covered crucible and heated to 950˚C inside Carbolite Furnace and maintained at that temperature for about 6 minutes. There is a loss in weight due to the eliminating of moisture and volatile matter. So present of volatile matter is calculated here.

3) When 1 gram sample of husk is placed on uncovering crucible and heated to 750˚C until the husk is completely burned, a constant weight reached, which indicates that there is only ash remaining in the crucible. Complete combustion of husk is determined by repeated weighing of the sample.

4) The amount of fixed carbon is determined as follows, using subtraction of percentage by mass.

The calorific value (heat of combustion) of a sample may be broadly defined as the number of heat units liberated by a unit mass of a sample when burned with oxygen in an enclosure of constant volume (Wilen, Stahlberg, Sipila, & Ahokas, 1987; Ogada, 1995; Nussbaumer & Hustad, 1997) . In this research to determine the calorific value of the coffee husk, the oxygen bomb adiabatic calorimeter is used, which is the standard instrument for measuring calorific values of solid and liquid combustible samples in Geological Survey of Ethiopian shown in Figure 13.

The amount of heat obtained from the sample was then determined by multiplying the observed temperature rise by a previously determined energy equivalent of the calorimeter. Then, by dividing this value by the weight of the sample, we obtain the calorific value (heat of combustion) of the sample on a unit weight basis. Corrections must be applied to adjust these values for any heat transfer occurring in the calorimeter, as well as for any side reactions which are unique to the bomb combustion process in Table 5 which were obtained (Kreith, 2000) .

Formulas used for Post processing

Theenergyequivalent ( W ) = ( WightofStandardmaterial ∗ Heatofcombustionofstandardmaterial ) ( Temperatureriseproducedinthetest ) Thegrossheatofcombustion ( Hg ) = ( temperaturerise ∗ energyequivalent ) weightofthesample

From the results of experiments, it can be concluded that Coffee husk is characterized by low moisture contents. The low moisture content is an important factor in the combustion of coffee husks, since the problems associated with the combustion of high moisture content of fuels are avoided. One such problem is the effect of moisture on the heating values of fuels. It is seen that husk has a

higher heating value when we compare it with other agricultural residues (Menendez, Dominguez, Fernandez, & Pis, 2007) . The ash contents of the husk are also low and are within acceptable range. This gives a great advantage during furnace design. Because for the combustion of agricultural residues with high ash contents, such as rice husks, consideration must be given to incorporate an efficient ash removal equipment from the flue gas to eliminate or reduce particulate pollution. Due to the low contents of sulfur, SO₂ emission problems would not be expected during coffee husks combustion (Wilson, John, Mhilu, Yang, & Blasiak, 2010) . The high volatile matter, content indicates easy ignition of fuel, although it has a significant effect on the combustion mechanisms and consequently on the design and operation of the combustion systems for this fuel. The coffee husk is lighter and smaller than wood chips and coals burned. Thus, there may be a tendency of the particles to be carried out of the furnace with the flue gas. Furthermore, without proper design of the furnace, part of the combustible volatile gases may leave the furnace unbound. The bulk density of the coffee husks is low, raising the costs of transportation and storage. Furthermore, coffee briquettes are fragile and not suitable for long distance transportation. Coffee husks are therefore more suitable for firing within the production region.

Combustion chamber of boiler is designed to use the chemical energy in the coffee husk to raise the energy content of fluid in the steam generators (Boiler), so that it can be used for power and heating applications. During the combustion process, oxygen reacts with carbon, hydrogen, and other elements in the husk to produce a flame and hot combustion gases (Flue gas). As these gases are drawn through the boiler it cools as heat is transferred to the working fluid. Eventually the gases flow through a stack and into the atmosphere. As long as husks and air are both available to continue the combustion process, heat will be generated. The combustion chamber is the most important part of the boiler. It is constructed of bricks that causes to minimize heat dissipation to the surrounding. It also has no involvement to do any kind of work. In addition, the kinetic and potential energies of the fluid streams are usually negligible. Then only total energies of the incoming streams and the outgoing mixture remained same for analysis. The conservation of mass and energy principle requires that these two equal each other that is shown in Figure 14 which was obtained from (Nag, 2008) .

Considering the theoretical combustion reaction for the organic component of the husk, such as carbon, hydrogen and sulfur (Raja, Srivastava, & Dwivedi, 2006) for mass of air required. The combustion process is assumed as the ideal case (stoichiometry). So, nitrogen is not considered to react with oxygen during combustion reaction. It limits the intimacy between the fuel molecules and O₂. When the fuel composition is known, the carbon balance method is quite accurate for mass balance calculation (Werthera, Saengera, Hartgea, Ogadab, & Siagib, 2001) . In this work, combustion takes place with excess air and when free carbon is not present in the products. The products of combustion are mainly gaseous. When a sample is taken for analysis it is usually cooled down to a temperature which is below the saturation temperature of the steam present. The steam content is therefore not included in the analysis, which is then quoted as the analysis of the dry product. Since the products are gaseous, it is usual to quote the analysis by volume. An analysis, which includes the steam in the exhaust is called a wet analysis (Coskun, Oktay, & Ilten, 2009) . To obtain the maximum temperature attained in the furnace, the analysis of heat balance is necessary (Harry, 1998) .

A steam generator generates steam at the desired rate, at the desired pressure and temperature by burning the coffee husk in its furnace. It is a complex integration of com-buster, economizer, evaporator and super heater. Steam generator is a closed vessel in which the heat produced by the combustion of fuel is transferred to water for its conversion into steam at the desired temperature and pressure. While selecting a boiler the factors should be considered: the working pressure and quality of steam required, which is whether wet or dry or superheated, Steam generation rate, Floor area available, Accessibility for repair and inspection, Comparative initial cost, Erection facilities, The probable load available, The fuel and water available, Operating and maintenance cost and etc. Heat exchangers (Steam generators) are devices where two moving fluid streams exchange heat without mixing. A heat exchangers typically involve no work interactions (w = 0) and negligible kinetic and potential energy changes for each fluid streams (Frank, Peter de, Sarah, & Jeremy, 2007) . Depending upon whether the pressure of steam is below or above the critical pressure (221.2 bar), steam generators can be either subcritical or supercritical units. The subcritical steam generators are water tube drum type and they usually operate at between 130 and 180 bar steam pressure. Which produce superheated steam at about 540˚C - 560˚C (Rajput, 2008) . But here, for this specific case a subcritical steam generator which operates at 10 bar is used, for this, satisfies the main goal of this paper that is developing a CHP steam plant with minimum cost and maximum efficiency. The increase in steam pressure is limited by the consideration of mechanical stresses and the ensuing higher cost of equipment. And the increase in steam temperature is limited by the properties of the construction materials of boiler and turbine. Increasing the steam temperature above 540˚C, increases the metallurgical quality of the nozzle and steam turbine blades by 30%. Very high pressures at the maximum temperature are undesirable because they create material strength problems (Aljundi, 2009) . Heat transfer to water in the steam generators takes place in the three different regimes (Kenya Planters Cooperation Union (KPCU), 1996) . Water is first heated sensibly in the economizer till it becomes saturated liquid. In the evaporator there is phase change from by absorbing the latent heat of vaporization at 10 bar the saturated vapor is farther heated at constant pressure in the superheated (Eastop & McConkey, 1993) .

An economizer is a feed water heater which raises the temperature of feed water to about the saturated liquid temperature at the boiler pressure using the flue gases discharged from the boiler. The justifiable cost for an economizer depends on the total gain in efficiency (Bbergman, Lavine, Incropera, & Dewitt, 2011) . In turn, this depends on the gas temperature out of the boiler and feed water temperature to the boiler. Economizer tubes are commonly 45 - 70 mm in diameter. And also the coils are installed at 45 - 50 mm spacing between them as obtained from Table 6 (Treybal, 1981) .

As shown in Figure 15, first the air is impelled by a blower and passed through a duct and it is heated by a set of heat exchanger and then it entered the dryer in countercurrent flow with the coffee. The air outlet is opposite to the coffee inlet. The solid coffee is fed by a container and is removed on the side opposite to the coffee inlet. The dryer cylinder is 9.46 m long with a diameter of 1.54 m. Rotary dryer is a class of dryer commonly used in industry to dry particulate solids. It is made of a long cylindrical shell that is rotated. The shell is usually slightly inclined to the horizontal to induce solids flow from one end of the dryer to the other. Indirect heat rotary dryers, a hot gas flowing through the dryer provides the heat required for vaporization of the water. To promote gas-solid contact, most direct heat dryers have flights placed parallel along the length of the shell, which lifts solids and make them rain across the dryer section. The transport of solids through the drum takes place by the action of the solids cascading from the flights, each cascade comprising the cycle of lifting on a flight and falling through the air stream as shown in Figure 16 which was obtained from Rigassa, Sundaraa, and Seboka (2011) .

Dryer and drying process selection for a specific operation is a complex problem and many factors have to be taken into account. Though, the overall selection and design of a drying system for a particular material is dictated by the desire to achieve a favorable combination of a product quality and process economics. In general with respect to the rate and total drying time in dryer performance is dependent on the factors such as air characteristics, product characteristics, equipment characteristics etc. As a consequence of dryer specialization the selection of the type of dryer appropriate to the specific product to be dried becomes a critical step in the specification and design of the processing plant. The choice of the wrong type of dryer can lead to inefficient operation, reduced product quality, and loss of profit. In the wet solid drying process three sub processes take place. Therefore the dryer is divided into three zones and the section wise calculation of temperature and humidity of the stream can be obtained by material and energy balance as demonstrated in Figure 17 mentioned by Chattopadhyay (2006) .

The basis of calculation is 1 hour operation and the initial and final moisture values of the solid (Cherry, Parchment, and Washed coffee) are taken from literature data (Dry coffee processing handbook) which tabulate data conducted in

the laboratory of Panama research (Geankoplis, 1993) . In Sheka Zone there are two types of dry coffee processing machineries (local and foreign) with a working capacity of 700 - 1000 and 1200 - 1500 kg/h respectively (Ginzburg & Savina, 1982; Kinch, 1967; Walas, 1990) . Since the dryer is the supplier for the processing machineries so, as much as possible, it must be work with the same working capacity of the pulping machine. Therefore, to make the design safe the minimum working capacity 700 kg/h is taken for this work.

The cogeneration thermal power plant analysis is based on a simple Rankinecycle, steam generated from saturated liquid water (feed water) is used as the working fluid. The super saturated steam flows through the turbine, where its internal energy is converted into mechanical work to run an electricity generating system. And the steam comes out from the turbine supplies heat to the dry process. Not all the energy from steam can be utilized for running the generating system because of losses due to friction, viscosity, bend on blades, heat losses from boilers i.e. hot flue gas losses, radiation losses and blow down losses etc. (Hewitt & Barbosa, 2008; Kreith, Manglik, & Bohn, 2003) . Rankine cycle is the theoretical cycle constructed from four main processes on which the generation steam power plant works. The main processes in the Rankine cycle are:

Process 1-2: Reversible adiabatic expansion in the turbine.

Process 2-3 (2’-3’): Constant pressure transfer of heat in the condenser (Drying processes).

Process 3-4: Reversible adiabatic pumping processes in the feed pump.

Process 4-1: Constant pressure transfer of heat in the boiler.

Then, I am going to analyze energy of the cogeneration plant considering 1 kg of fluid and applying steady flow energy equation (SFEE) to Steam generators (boiler), Turbine, Condenser and Pump (Figure 18).

The power plant itself consumes a part of the electricity produced. This is due to the various auxiliary equipment required like feed water pumps, circulation pumps and air/flue gas blowers. In forced circulation boilers the share of electricity consumed by the circulation pump is about 0.5% of the electricity produced by the plant. Normally the internal power consumption is about 5% of the electricity produced by the power plant. In this context, since the power used is electrical (and taken from the grid), the internal power consumption share is reduced from the final boiler efficiency in boiler calculations (Rajput, 1990; Saidur, Ahamed, & Masjuki, 2010; Duffie & Beckman, 1991; Glikin, 1978) .