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Biomass Based Electricity Production: TANWAT Case Study, Tanzania

Prepared by TaTEDO Tanzania, for SPARKNET, April 2004

Background

The Company

TANWAT was founded in 1949, when the Commonwealth Development Corporation (CDC) took responsibility for a forest development project set up two years earlier by the Forest, Land Timber and Railways Company, located in the Southern Highlands of Tanzania with 15,000 hectares of private forest business.

Production of tannin from wattle was the major source of revenue for the business until late 1960's when a decline in demand for leather goods, brought about by availability of cheaper synthetic alternatives. This resulted in an initiative aimed at exploring new global forestry product opportunities, reducing reliance on tannin products and creating prospects for increased revenue on a long-term basis. The Tanganyika Wattle Company is a fully owned subsidiary of CDC capital fund.

The Fuel Source

The forest estate comprises 8000 hectares of wattle trees, 4000 hectares of pine and 1000 hectares of eucalyptus. The wattle bark is rich in tannin. The bark in the field and transported to the factory for processing and manufacture of wattle extract. The wood, which is effectively a waste product, is transported to the power station for use as fuelwood for boilers. At the current wattle extract production levels, wood volumes in excess of 60,000 tonnes are available per annum.

Once the tannin has been boiled out, the wattle's bark is a waste product and upon pre-conditioning by drying, the (waste) bark provides a further fuel source for the boilers. At current production levels, 10,000 tonnes of spent bark is available per annum.

The eucalyptus in the forest is available in a range of species, some of which are not suitable for conversion into poles. Planting of these species has been discontinued but there is a residual 60,000 tonnes available in the forest

The sawmill produces 3,000 tonnes sawn timber per annum at a recovery rate of 40% from the pine trees. As such, 4500 tonnes of pine waste is produced per annum, comprising off-cuts and sawdust.

The Power Station And Associated Infrastructure.

The Tanzania's first commercial wood-fired power plant was commissioned in mid 1995 with an installed capacity of 2.5MW. The plant provides power to the Njombe/TANESCO mini grid. The plant is composed of fuel handling and processing facilities that include a hydraulic feeder or logger, a drum chipper with a capacity of 70m³ loose chips per hour, chip belt conveyor and two silos, each capable of storing 17 tons of chips.

The furnace or combustion unit burns wattle tree, pine and eucalyptus chips. The fuels are normally burnt separately. However, the dominant fuel is the wattle tree chips, which have a heat value of about 17 Kcal per kg at about 40% moisture content. The combustion unit operates at a temperature range of 1100-12000C thereby generating enough heat energy from the fuel that forms a "mat" of about 100 mm thick during combustion. The superheated steam enters the turbine that rotates at a speed of 5345 RPM. This speed is reduced by a gearbox mechanism to 1500-RPM shaft power that is coupled to a generator.

The generated high voltage is at 3.3 kV. The turbine exhaust steam is at 1 bar and is directed to a condenser all through to a cooling tower and back to the boiler thereby increasing the overall plant efficiency. Below are the plant's technical details;

Plant

Year erected 1995
Manufacturer Mech Mar, Malaysia
Consultant W.S. Atkins U.K

Boilers (2 Units)

Type Water tube
Related Output 15 tons per hour
Temperature 3330C.
Pressure 30 bar
Wood consumption 60-80 tons per day
Chipper plant Capacity 70 m3 loose chips per hour

Steam turbine

Make Dresser Rands (USA)
Type Condensing
RPM 5345/1500

Generators

Make AVK Germany
Rated Output 2.5MW
Rated voltage 3300 Volts
Frequency 50 Hertz
Net power 3125 KVA/547 Amps

Power Distribution

TANESCO 40%
Tea factory 30%
Wattle and Timber 10%
Power station 20%

Steam Distribution

Turbine 62.5%
Wattle process 25%
Dearator 9.6%
Ejector 0.5%
Loss due to transmission 1.8%

The TANWAT plant, which is a typical condensing turbine flow has similar features like the Saohill plant (Figure 1).

Fig. 1: Steam flow diagram of TANWAT

TANWAT Power Plant Performance

Apart from generating power for its own use, the plant sells the surplus power to the national utility TANESCO on power purchase arrangement (PPA). During operation, the plant consumes some 110-150 tons of woody biomass of average moisture content of 35-40% and generating power at 3.3 kV (3300 Volts) at the voltage panel or switchyard.

The major components of the switchyard are the busbars, high voltage circuit breaker and a step-up transformer to 33 kV. The 33 kV is the output voltage of the Power Island or mini grid. The switchyard has step down transformers 3.3 kV/415V for the wattle factory and domestic power supply. The power for both the wattle factory and for domestic customers goes through a 600kVA transformer that minimises the power losses. There is also a step-up transformer 3.3 kV/11 kV for the tea factory, which is, located 15 km away. A 2000 kVA transformer is also installed in order to stabilise the power for the tea factory and irrigation for activities.

The 33KV-transmission line used at Njombe Power Island is the most common primary voltage used in distribution systems in Tanzania. The common medium level distribution voltage is 11 kV. The low level distribution voltage is a 400 volts and the final connection to consumers is at 220-240 Volts.

As per the TANWAT-TANESCO PPA, the utility has statutory obligation of maintaining the quality of power supplies. For example variations in voltages can cause serious faulty operations and sometimes damage equipment. It is the responsibility of the utility to ensure good quality power supplies.

Load shedding, which is the deliberate removal pre-determined customer demand from a power system in an effort to maintain the integrity of the system and minimise customer outages, is monitored by TANESCO. This responsibility is to switch over to EWURA (Energy Water Utility Regulatory Authority) when TANESCO is unbundled in 2005.

Technical Performance

Several assumptions are made in order to compute the technical performance, such as water and fuel consumption as well as financial performance. Fuel consumption is given in m3, tons or kg per unit time and is calculated as the energy consumption in the boiler divided by the energy content of the fuel on use.

The energy consumption in the boiler has direct relationship with the boiler efficiency and the output. In order not to underestimate the fuel supply quantities, the boiler efficiency assumed is 20%. And the boiler output is 2500KW (2.5MW)

With regard to the water quantity, normally in a condensing steam cycle, about one third of the boiler output is converted into electricity and the two thirds is lost in the condenser. For each degree of temperature rise, one kg of cooling water absorbs 4.2KJ of energy -(heat capacity). The cooling water flow is therefore calculated as heat transfer in the condenser (KJ/S) divided by the product of heat capacity and cooling water temperature rise.

Financial Performance

The cost of other variable parameters has been assumed as percentages of the cost of water. Spare part costs are assumed to be percentage of capital costs (Table 1.0).

Table 1.0 TANWAT costs

No.	Item	US$
1.	Capital Costs:	4,875,000.00
2.	Fixed O&M Costs: Administration Spare Parts Other, @30%	52,133.33 114,666.67 50,000.00
3.	Variable O&M Costs: Fuel (biomass) Handling Lubricant Water	475,089.66 37,152.00 81,028.15
4.	Total Costs	1,340,139.04
5.	Fuel Cost ($/kWh)	0.0247
6.	Variable O & M ($/kWh)	0.0062
7.	Total Variable ($/kWh)	0.0309
8.	Estimated Power Demand -kWh	18,240,000.00
9.	Power Costs US$(Own Gen. @80% Cu)	1,616,398.16
10.	Power Costs US$(Import-TANESCO)	1,641,600.00
11.	Estimated Surplus Energy-kWh	960,000.00
12.	Expected Revenue- Surplus Energy (US$)	76,800.00
13.	Total income	1,693,198.16
14.	Benefit/Cost-Ration	1.26

Source: TAWNAT/Ngeleja, 2002

On the basis of the costs in Table 1.0, the unit energy costs are calculated (Table 2.0)

Table 2.0: Energy cost analysis for 2.5MW of power generated at TANWAT

Item	US DOLLAR
Fixed costs: Capital cost ($ x 10) Total capital $/kW Shadow price ($ x 10) Interest during construction (IDC) Cost recovery factor (CRF) Interim Replacement Fixed (0& M) &.kW-yr Fixed (0 & M) $/kW-yr Total Outage Rate Outage Adjustment Fixed Cost ($/kW-yr)	5.0 2000.0 4.90 1.0 0.1339 0.00335 274.76 86.74 0.12 412.33
Variable costs: Fuel cost $/kW Variable O & M- $/kWh Total variable	0.0340 0.0077 0.0417
Unit cost $/kWh Plant Factor 60% Plant Factor 65% Plant Factor 70% Plant Factor 75% Plant Factor 80% Plant Factor 90%	0.1201 0.1141 0.1089 0.1044 0.1005 0.0940

Source: TANWAT/Ngeleja 2002

From the above costs, a quick analysis indicates that the unit energy cost decreases when the plant capacity utilization increases. This means TANWAT plant utilization ought to be above 75% in order to compete with the grid power. In this situation, Demand Side Management (DSM) is necessary. DSM is a managerial tool where by a utility can influence the consumer to alter the consumption pattern so as to suit supply capacity and energy limitation of the utility provided the customers are also rewarded (Katyega, 2001). However, power generation at TANWAT is not core business and so TANWAT may have no incentives for carrying out DSM.

TANWAT Power Supply

A review of electricity supply to Njombe in early 1990s indicates that the minigrid capacity was 2.75 MW. By then the minigrid comprised of 3 diesel generators (gensets) with a total load capacity of 1.92MW. The Uwemba Mini Hydro that forms part of the minigrid has a gross capacity of 0.83 MW.

The diesel power from Njombe catered for the base load whereas Uwemba catered for the peak load. The use of diesel power for base load was found to be expensive. In early 2002, the diesel fuel price range from Tshs 600-610 per litre (equivalent to US$ 0.66 per litre in 2002), making the diesel power generation very expensive.

The establishment of TANWAT in mid 1995 and the subsequent sale of surplus power to the minigrid through a PPA changed the power generation pattern in Njombe. TANWAT power supply is reported to have influenced the removal of one diesel genset to another power needy destination. On the other hand, TANESCO has a policy to reduce the use of diesel/thermal plants because they are expensive to operate especially on far distant localities such as Njombe where the diesel fuel is costly.

The combination of the above factors make TANWAT plant highly dependent on power from the Njombe minigrid. For example during dry season from August to November, TANWAT power sales to the grid increases to the order of 400,000 -500,000 kWh. This is because the Uwemba minihydro is stopped because of shortage of water for power production. Furthermore the remaining two diesel gensets of 1.28 MW do not meet the energy demand. Coincidentally during this dry season the wattle factory is stopped and therefore the power for this consumer is redirected to the minigrid.

TANWAT And The Njombe District Electrification

Njombe district had an estimated population of about 462,000 people in 2002 on an area of 10668 km3. Njombe district has agro forestry potential.

TANWAT power supply share to the district is reported to be about 50% during the last three years (TANESCO -Iringa Regional Office -2002). This remarkable share is reflected in the increase in rural electrification. Accordingly, during the last four years, three power transmission lines have been constructed. These transmission lines are; the 25km long Njombe -Kidugala-11kV, the 25km long Muhaji-Halali-11kV, the 16km long Halali- Wanging'ombe. -11kV.The cost of this power transmission with a total length of 66 km is estimated to be Tshs 0.875 billion which implies Tshs 13.26 million per km or US $ 13, 668 per km.

The current cost of constructing an 11kV distribution line is about Tshs 15 million per km i.e. about US$15500 (TANESCO Iringa -2002). This capital investment is high but affordable by local private sector investors, who could invest in local rural electrification.

In a related development, TANWAT is in the process of acquiring the Lupembe tea estates and factory. At present, the power at Lupembe tea estates is on diesel generated. This future TANWAT arrangement to extend power line to Lupembe is to be completed by 2005/6. The plans involve connecting the Lupembe factory with a 33 kV primary transmission line that is to be owned by TANWAT from the TANWAT plant. This new line will also connect the Kidengembwe-Malembe Mission. At present this mission gets its power from its own minihydro plant of 250 kW. The mission is engaged in piggery and poultry keeping and therefore is a potential energy customer.

TANWAT cogen system has therefore a significant role to play in the 5 year Njombe electrification plan (Table 3 and 4).

Table 3.0: Njombe electrification plan (Domestic and Commercial -2001-2007)

Tariff /Category	Year	Forecast Units (kWh)	kWh Increase	Annual Increase %	Estimated Customer	Remark
I-Domestic Sub total II-Commercial Sub total Total	2002 2003 2004 2005 2006 2007 2002 2003 2004 2005 2006 2007	8188788 9007666 9908413 10899275 11989202 13188122 968352 1065182 1171705 1288875 1417762 1559538	818900 990662 1198926 3,008,688 96835 117170 141776 355781 3,364,469	9 10 9 Average 9.3 9 9 9	1969 2382 2882 7232	-Improved rate of rural electrification. At present Tanzania rural electrification rate is about 1% - Increase in the TANESCO generation capacity is necessary i.e. double the current capacity of 211MW.

Source: Tanesco -Iringa 2002

Table 4.0 Njombe Industrial Electrification Plan 2002-2007

Tariff/category	Year	Forecast kVA	KVA increase	% annual increase	Remarks
III Industrial	2002 2003 2004 2005 2006 2007	1788 1966 2162 2378 2615 5491	178 216 2876	10 10 109	During the year 2006/7 The Njombe - Luponde and Njombe Lupembe transmission lines at 33kV will increase irrigation at tea estates
Totals		16400	3270

Source: Tanesco -Iringa 2002

Njombe electrification plans are an integral part of the new national energy policy that aims at ensuring availability of reliable and affordable energy supplies in a sustainable manner for both rural and urban. The major challenge is to increase the per capita electricity consumption. Tanzania's has per capita electricity consumption sector of about 53 kWh per annum. (Marandu-2002)

References

TANWAT report, 2002
Tanesco -Njombe internal reports reports, 2001
J. Ngeleja 2002, Biomass Cogeneration in Tazania study
M. Katyega,2001, Biomass electricity generation in Tanzania study

Pagename: :TANWATBiomassElectricityTanzania