Côte d'Ivoire is the second palm oil producer country in Africa, with
an annual production of 276 000 tons (Jacquemard, 1995).
Fifteen tons of mode of palm can give, in average conditions, more than 3.5
tons of oil (Ribier and Rouziere, 1995; Departement
de l'agriculture, 1991). Thus, residues resulting from the transformation
are about 906 857 tons for this country. This waste can be used for energy production
apart from what they can be used as feeding stuffs or elements of compost. Various
technologies exist which make it possible to transform this waste into energy.
These technologies are usually gathered in first and second generation (FAO,
2008; Clashausen, 2007).
Wastes resulting from transformation of fruits of the palm tree in oil (seeds)
are used as fuels (Fuel 1) in boilers in order to produce electricity for the
oil mill. The nuts (almond) obtained after the production of the palm oil were
sold in the state. Given the increase of oil price and concerns related to global
warming. The production of energy from biomass is in full revival both in developed
and in developing countries (Uwe, 2007; Jurgen,
2007). In Côte dIvoire for example, operators of sector of oilseeds
started to transform nuts which they sold rough, to extract oil. The added-value
of such operation is in a ratio of 4.6 (nuts initially sold to 100 euros/ton
whereas the oil of almond is sold to 461.54 euros/ton). However, this transformation
involves residues made up of hull and oil cakes, by-products which can be used
as fuels in the boilers of the oil mills. These residues made it possible to
implement Fuel 2 and Fuel 3. These fuels are composed for Fuel 2 exclusively
by oil cakes (by-product of the transformation of the almond) and Fuel 3 is
a combination of Fuel 1 and 2.
The object of this study is to evaluate the energy contribution of two new fuels and compare them to Fuel.
MATERIALS AND METHODS
In Côte d'Ivoire, the oil mills are usually installed in isolated agro-industrial
areas (Tounon, 1996). To achieve their objectives, they
need to produce energy, usually steam. This study was conducted from March 2006
to July 2009.
The produced vapour is partly used for production of electrical energy necessary
to machinery of the factory and this starting from turbo alternators groups.
The other part is used for specific operations such as palm bunch, fruits mixing,
palm oil clarification and the drying of fibre, nuts, almond, hulls used as
fuel in the boilers (Graille et al., 1981).
Production of vapour and electricity in the oil mill Boiler: A boiler with a water pipe type BABCOCK SFMV 1157 ensure the production of superheated steam at 20-22 bars; 255-275°C. A set of equipment (water balloons, lateral collector screen, rear collector screen, super heater, fans of primary and secondary air, fan of pulling, fireplace, chimney sweep, power supply fuel) contribute to its optimum operation. In general bad combustion (excess of air) and thermal losses of the boiler are mainly due to the malfunctioning of the chimney sweeps and equipment to remove the ashes.
Turboalternators: The production of electricity of the oil mill is ensured by two turboalternator groups. They are equipped with steam turbines with a single wheel (single stage) and a back pressure. This back-pressure is at the origin of the racking of superheated steam at the exhaust. The vapour obtained at the exhaust is cooled until 2.5-3 bars ≈130°C to be used in the machining process (sterilization, mixing, clarification and then drying of solid waste).
Each turbine is coupled to a mechanical reducer which reduces the speed of its shaft (6000-7000 rpm). This allows the alternator shaft to turn at a speed of 1500 rpm.
Thus alternators convert mechanical energy into electrical energy distributed by the power station of oil mill. At start-up, in the event of call of significant loads, two generators CUMMINS provide additional electrical energy.
CHARACTERIZATION OF THE FUELS (SOLID WASTE, OIL CAKE)
Characteristic of fuel (solid waste) used in boilers Components: Solid
waste resulting from the treatment of palm bunches is the main fuel used in
boilers of the oil mill. It is a mixture of fibres, hulls and fragment, representing
on average 24% of the weight of the fresh bunch of palm trees (12% of fibres
resulting from some pulp, 7% of hulls and 5% of fragment of palm-kernel resulting
from palm nut) as shown in Fig. 1.
The components of this fuel are hydrogen, carbon, oxygen, sulphur and ash. Table 1 shows the mass composition of each component of the fuel used.
Mass composition: Three samples taken from the fuel are used in interval of 8 h.
Having weighed and sorted out them manually (separation components) we found that the fuel is composed by an average of 94.65% of fibres, 3.54% of hulls and 1.81% of fragment.
The drying of these three samples for 12 h at a temperature of 105°C provides a moisture (% H2O) average of 30.9% after a double weighing (before and after steaming).
||Mass composition of the bunch and granulate of palm tree
||Mass composition of each component of fuel
||Summary of the characteristics of three (3) samples of fuel
|*The superior calorific power
Calorific value: The energetic potential of fuel is given by its lower
caloric power. It depends on water content (% H2O) and hydrogen (%
H2) in the fuel. The Lower Caloric Power (PCI) is determined by using
Lucien Delaplace relation (Delaplace and Philippe, 1972):
where, PCIcombj represents the lower caloric power of fuel of constituent j expressed in kcal kg-1, % combj represents the mass of constituent j in the fuel mixture.
The tests performed on fuel samples taken at intervals of 8 h a day of machining have shown the performance table as shown in Table 2.
From the relation Eq. 1 and previous sampling results, the
average lower calorific value of the fuel mixture is determined by relation
Eq. 3. It is 2646 kcal kg-1 compared to those of
ethanol and of the gas oil which are respectively 6096 and 10191 kcal kg-1
(Guibet, 1997). The difference between the first value
and the two others is certainly due to the water content.
Impacts of the residues of combustion of the mixture used in the boilers of the oil mill: The formation of slag, derived mainly from the fusion of ashes resulting from combustion of fuel mixture (fibres, hulls, debris) plugs the grids and spray tubes of the boiler.
This formation is even so accentuated that the operation of the chimney sweepers and system of cleaning of ashes is defective.
The photographs of boiler furnace, one month after the general maintenance
during the peak period of the factory show the real impact of slag on boiler
equipment (Fig. 2, 3).
Characteristic of the oil cake: The oil cake is a solid residue obtained after oil extraction from almonds of palm fruit. With an oil content of about 9%, it is dry and gritty (Fig. 4).
The agronomic studies show that oil cake constitutes, after cereals, the second
class of animal feeds (IE - Groupe Economie du Betail, 2008).
The analyses carried out in one of the laboratories of an oil mill of Côte
d'Ivoire (Iboké) make it possible to affect to them a humidity (%H2O)
of 5%, a lower calorific value (PCI) of 4000 kcal kg-1 and a content
of oil (% oil) of 9.66%. The humidity of oil cake was determined by double weighing,
||Slag formed on the tubes
||Slag formed on the refractory bricks sprays
||Photograph of a sample of oil cake
The Lower Calorific Value (PCI) of oil cake was determined from Lucien Delaplace formula (1), superior calorific values and the hydrogen content of 4200 kcal kg-1 and 6.6%, respectively provided by the documentation of the management of the council and industrial Development (DCDI) of oil mill.
As for the content of oil, by the extraction process solid-liquid (oil cake-hexane), it was noticed that 30.523 g of oil cake provide 2.948 g oil of palm-kernel, or an oil content of 9.66%. Using the oil cake as fuel, this residual oil contributes among others to slag formation. The knowledge of the characteristic content of oil is a necessary data for the conduct of boilers.
RESULTS AND DISCUSSION
Comparative study of the energy contribution of the fuels (solid waste, oil cake, mixture): In this part, we present the results of the energy balance of the use of three types of fuel:
||Quantities of fuel for boilers
||Fuel 1 is essentially consisted of solid waste only
||Fuel 2 is only oil cake
||Fuel 3 is the mixture of oil cake and solid waste
This study allows to determine:
||The total quantity of superheated steam produced (depending
on the efficiency of boilers)
||The quantity of electrical energy which can be produced (depending
on consumption of turbo alternators)
The energy balance of fuel depends on efficiency of boilers and the specific consumption of turbo alternators. Thus the lower calorific values only, determined in the previous part, are not sufficient to compare the energy inputs.
Fuels availability: The analyses of monthly production of palm bunches of agro industrial units available in Côte d'Ivoire permit to determine the quantities of Fuels 1, 2 and 3.
Total quantity of Fuel 1 produced by a single unit out of ten is equivalent to 18% (by mass) of the total regime treated.
The rate of extraction of palm oil is 43% (in mass), the total quantity of Fuel 2 is equivalent to 57% (in mass) of the total quantity of palm produced. Palm-kernels are available from four units.
As for Fuel 3, it results from the sum of the total quantities of Fuel 1 and Fuel 2.
Investigations on the production units of a year are recorded in the results of Table 3 and shown on Fig. 5.
Amount of steam to be produced
Boiler efficiency of oil mills: The boilers used in oil mills type BABCOCK SFMV 1157. Their performance can be calculated according to the Eq. 4.
||Monthly quantities of fuel
||The mass flow of steam (tons h-1)
||The mass enthalpy variation between mass superheated steam and water supply
||The mass flow of fuel (tons h-1)
||The average lower calorific value of fuel (kcal kg-1)
These parameters are evaluated with daily data of machining during a month of high production (month of May.
Mass flow of steam produced (qvap): It is calculated by Eq. 5:
||15492 kwh as mean energy delivered by the turbo alternators
||24 kg kwh-1 as specific consumption of turbo alternators
||24 h as operating time
A mass flow rate of 15.492 t h-1 of steam produced by the three boilers of the agro industrial unit.
Variation of enthalpy mass (ΔH): Water supply of boilers, levied
at 70°C is superheated to 22 bars/265°C; its variation of enthalpy is
630 kcal kg-1 (Goodall, 1981).
Mass flow of fuel used (qcomb): It takes into account total mass in tons of treated fresh regime (Mr), the mass ratio (% Comb) of fuel elements of treated regimes and the operating time (tp) in hours presses:
The 635.86 tons of fresh regimes containing 18% of fuel are treated daily for 20 h. This provides a mass flow of fuel of 5.72 t h-1.
The efficiency of boilers used in agro industrial units is estimated at 64.5%.
Mass flow of steam to produce: The quantity of superheated steam produced
by ton of fuel (QTfuel) is function of (Delaplace
and Philippe, 1972):
||The mass enthalpy variation between superheated steam (Hvap)
and water (Hwater) supply of boilers (ΔH)
||The efficiency of boilers (ηboiler)
||Lower calorific value of fuel considered
It is calculated by the Eq. 7:
Water supply is taken at 70°C, its enthalpy is 70 kcal kg-1.
The Fuel 3 is a mixture, its calorific lower value is given by the formula
of the balanced calorific value (1). It depends on the mass proportions of Fuel
1 and Fuel 2 from the analysis of Table 3.
100%Fuel 3 = 67% Fuel 1 + 33% Fuel 2
Thus, the lower calorific value of Fuel 3 is estimated at 3092.82 kcal kg-1.
The quantities of superheated steam produced by ton of fuel are respectively:
||Monthly quantities superheated steam which can be produced
by tons of each fuel
||Quantities of superheated steam which can be produced in the
boilers of the oil mill
Figure 6 which results from Table 4 shows the monthly quantities of superheated steam likely to be produced per ton of fuel.
Quantities of electricity which can be produced
Specific consumption of turbo alternators of the oil mill: The yield (data manufacturer) of turbine and the reducer is 0.97 and that of the alternator is 0.96. Thus, the turbo alternator has a yield of 0.90.
The states of steam in admission are 20 bars and 260°C and exhaust 3 bars
and 150°C. The variation of enthalpy (ΔH) is 40 kcal kg-1
The energy unit (Eu) provided by the turbo alternators is the kilowatt-hour (860 kcal). Their specific consumption (C) calculated from the Eq. 8, gives a value of 24 kg kwh-1.
Quantity of electrical energy to produce by the oil mill: The low value of the efficiency of boilers is mainly due to the dysfunctions previously stated.
||Estimated quantification of electrical energy produced by
turbo alternators of the oil mill of Iboké (2006)
The total quantities of electricity that could be produced by Fuel 1, 2 and 3 in the waste boilers of palm oil mills of Iboké in Côte dIvoire are listed in Table 5.
The results in Table 5 show that the production of electricity by the oil cake (Fuel 2) only is not a viable option. Indeed Fig. 5 shows that the electrical energy produced by the oil cakes (Fuel 1) is lower than that of Fuel 2. In addition, the current use of Fuel 1 requires the addition of generators to cover the needs of the Agro Industrial Unit (IAU).
About the Fuel 3, it can produce an electrical power of 3672432 kWh. This production exceeds the needs of the agro industrial unit, which was 3611142 kWh in 2006.
Experimental furnace: Early we showed the energy contribution of Fuel 1, 2 and 3. In order to supplement this study, combustion tests were carried out in an experimental furnace at the oil mill. These tests aim to examine the conduct of the burning of different fuels studied to compare with previous results.
To this end, an experimental furnace was developed by modifying an oven for cooking. Figure 7 shows the different views of the cooking oven before the changes.
This cooking oven presented:
||Cooking area (second stage) with walls
||Fireplace linked to a chimney (Fig. 7)
Changes made (Fig. 8) on the kitchen oven are:
||Replacement of the plate separating the cooking area of the
fireplace by a grid. This grid allows removing the ash and also allows supplying
the oven with primary air
||Installation of a chimney communicating with the cooking area
by its higher wall
||Installation of pipelines (equipped with shutters), one of
which leads to a nozzle located in the cooking area (secondary air) and
the other to the original location of the chimney (primary air)
||Diagram of front and profile views of the cooking oven
||Shows changes made on the oven cooking. 1: Chimney, 2: Insulator,
3: Secondary base of air, 4: Grid of health, 5: Refractory bricks and 6:
Dorr of the hearth
Combustion tests: The experimental furnace made it possible to test the combustion of studied fuels (Fuel 1, 2 and 3). The tests are carried out in the same conditions (5 kg of each fuel, primary and secondary air flows are identical). Combustion temperature was measured with infrared thermometer. The colour of the exhaust gas of the experimental furnace was observed. The results of these tests are shown in the Table 6.
Comparing temperatures of smoke is like comparing those of the fireplace because the combustion gas route is very short. The data in Table 6 show that the fireplace of Fuel 1 reached the highest temperature (309°C).
It burned with a suitable quantity of air because there are no unburnt residues and smoke is clear grey. The sample of this fuel was consumed faster than the others and generated the smallest quantity of ash.
||Temperatures of combustion of the various fuel samples
The operating conditions of furnace did not make it possible to keep the oil cake (Fuel 2) in suspension during the combustion. This one was consumed in heap (black smoke and unburnt residues). By putting it in suspension using a metal bar (the furnace door opened), the oil cake burns very quickly by causing a sharper flame.
The bad conditions of its combustion probably justify the low temperatures of smoke recorded and the long time for its total combustion.
As Fuel 3 (3.35 kg of Fuel 1 plus 1.65 kg of Fuel 2), it is noted that its combustion requires more air. Despite of the bad conditions of its combustion, the smoke temperatures are higher than those of the oil cake. In addition, the quantity of ash is quite low.
This study has highlighted the energy potentialities of oil cake. The analysis and tests allowed giving their contribution as fuel for boilers waste. Countries such as Côte d'Ivoire, which have oil mills of palm have a great potential in biomass. The recovery of the oil cakes to produce energy could contribute to reduce the use of the petroleum products. It could also help to reduce production costs of agro industrial units.
Simultaneously, this recovery would help reduce the negative effects and degrading environment which the activities of the oil mills generate.
We thank Prof B. K. YAO our research supervisor for his help. We also thank Dr M. K. Kouadio, teacher of English for specific purposes who has helped us to write this paper in English.