Production of Energy from Biomass Residues 

(Nuts and Fruit Processing Waste) 

 

1. Aim of the Project 

 
The aim of the project was to construct a combined heat and power demonstration plant 
fuelled by agro-food industry residues, in particular: hazelnut shells, olive kernels, fruit 
stones, pine cone seed wastes, other fruit processing wastes and wood chips. 
 

 

 

 

Photo 1: General view of the plant 

 

2. Introduction 

 
The company Energonut srl in Naples has built in the industrial area of Pozzilli this 
cogeneration plant. The plant construction was completed by the end of 1996 and is operating 
since early 1997. The fuel material is biomass derived from agro-forestry and agro-industrial 
activities, such as olive husk, pine cones, hazelnut shells, fruit stones, wood residue etc., 
gathered inside an area in a range of about 300 km from the power station. The typology of 
the various fuels, their physical characteristics (calorific value, moisture and ash content etc.) 
and the seasonal properties of the products required the development of a specific and 
thorough strategy for purchase and use, specially as every single product needs an individual 
storage care and mixing conditions. This influences considerably the management of the 
feedstock supply to the boiler in order to obtain the best fuel mix while at the same time 
trying to meet very diverging fuel specifications.  
 
Italian laws n.9 and 10 of 1991 govern the production and sale of electricity from renewable 
resources (like biomass) from private operations.  The ENERGONUT plant fully complies 
with the requirements of the above laws and will therefore benefit by the price per kWh for 

the electric energy produced by it and sold to ENEL. The electric energy price actually paid 
covers the production costs of the plant after the significant modifications introduced as 
suggested by experience. 
 

3. Technical description 

 
The project included the design, engineering, construction and commissioning of the 
complete power station, consisting of the biomass storage yard and handling facilities; the 
grate steam boiler and flue gas treatment; the turbo-generator and steam condenser, the 
transformers, switchboards and power connection system to the national electrical network 
(ENEL);  the condenser water cooling system; the ashes recovery and shipping unit; ancillary 
plants; and the waste heat supply for third party's utilisation (greenhouses, etc.).  
 
The steam boiler is equipped with a moving grate specially designed for proper regulation of 
the combustion rate and a design value for unburned carbon of less than 0.5 %. The ashes 
(about 200-300 kg/h depending on the fuel mix) are conveyed to an ash silo for storage of 30 
m

3

. The silo is periodically emptied into trucks and the ash is sent to an authorised landfill 

nearby. ENERGONUT is investigating the possibility to sell the ash as a soil conditioner due 
to its high potassium content, compost component or cement additive. 
 
Natural gas is used as auxiliary fuel for start up and transient phases. 

 

4. Performance of the Plant 

 
One of the main objectives was to demonstrate the multi-fuel operation of the boiler since this 
will give great operational flexibility for this and future plants. Table 1 gives a typical blend 
of the biomass fuel based on the various types of residues available. The net calorific value, 
on dry basis, of the fuel fed to the boiler is in the range of 19 - 21 MJ/kg. Photo 2 shows the 
biomass storage area. 
 

 

 

Photo 2: The biomass storage silos 

Table 1: Typical blend of the biomass  

 

Type of biomass residue 

Weight % 

Olive oil residues 

10-20 

Almond, walnut shells & apricot stones 

10-20 

Pine seeds shells and cones 

10-30 

Other wood residues 

50-60 

 
The steam data are summarised in table 2. The electricity production, 70,000 MWh/y, is 
totally sold to ENEL. The waste heat is available for the customers in the form of warm water 
at 40 °C. The plant has an annual consumption of 80,000 t/y of biomass fuels corresponding 
to 36,000 Toe saving. The net rated power output is 9.8 MWe. Figure 3 shows a general view 
of the plant. 
 

Table 2: Steam data and steam turbine performance 

 

                          Steam data 

 

                        Steam turbine  

 

Steam output 

43 t/h Steam generation 

43 t/h

Steam temperature 

505 °C Exhaust steam rate to condenser 

38 t/h

Steam pressure 

67 Bar a Nominal output power 

11,000 kW

Primary fuel consumption 

8-10 t/h Total plant efficiency 

28-29 %

Boiler efficiency 

94.3 % Net power delivery 

9,800 MW 

 
 

 

Photo 3 : General view of the installation 

During the commissioning of the plant several tests were made with various feedstock 
mixtures in terms of their quality and of the moisture content. A significant data basis has 
been established on operating performance with several feedstocks, their characteristics,  the 
transport properties as well as their combustion efficiency. This data basis has been used to 
improve and optimise the operational status of the plant and to introduce specific and 
necessary experience based modifications.  
 
It has been necessary to make equipment modifications and operational adjustments for the 
fuel preparation and feeding system, the combustion system and the cooling towers. Before 
these modifications the plant was operating at significantly reduced availability of about 50 
%. After the modifications were completed the availability was improved to 90 % of the rated 
value. The operators plan to reach the 100 % operational capacity by optimisation in several 
areas of the plant with emphasis on the feedstock handling, preparation and feeding. 
Considering the complicated nature of the feedstocks, these problems are understandable. 
 
This concept can be disseminated in several countries with similar biomass and agricultural 
residues especially in the Mediterranean area such as Spain, Portugal, France and Greece. 
 

5. Economic Performance 

 
As the plant has reached its full operational capacity only since 1999, it is not possible to have 
sufficient data concerning its economic performance. However, the total amortisation of the 
investment is estimated to be completed over the first 8 years of the operation of the plant 
during which ENEL will buy the exported electricity at subsidized prices. 
 
In the same period possible additional income may be gained from the sale of waste heat and 
the high potassium content ashes which will significantly improve the economic viability of 
the plant. 
 
The useful life time of the plant is estimated to 25 years. 
 

6. Project Identifiers 

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Project: BM/496/93 
Owner: ENERGONUT 

Srl 

  Via 

Privata 

D. 

Giustino 

3/A 

80125 Napoli 
Italy 
Tel: +39-0865-911028 
Fax: + 39-0865-911777 

Contractors:  SITICEM Spa of Pisa &  

BABCOCK ENTERPRISE, Group CNIM-Paris. 

Technology: BABCOCK ENTERPRISE 
Total Cost:  21,340,000 ECU 
EC Support:    2,118,135 ECU