Electricity cogeneration

The sugar industry is the second largest electricity producer in Cuba. Its share in electricity generation decreased to 5% of the total electricity in 2003, although in 1970 it cogenerated 18% of the total electricity in the country (Fig. 4.1) [4.5].

In 1959, the installed capacity of cogeneration in the existing 159 sugar mills was 317 MW, which generated 369 GW·h. In 1990, the installed capacity

0 200 400 600 800 1000 1200 1400 1600

1970 1973 1976 1979 1982 1985 1988 1991 1994 1997 2000 2003

GWh·

FIG. 4.1. Cogeneration in the sugar industry. Source: Refs [4.1–4.3].

increased to 726 MW and generation reached 1449 GW·h. In 2003, capacity decreased to 662.3 MW and 720 GW·h was generated [4.5]. Nevertheless, important changes have occurred in terms of efficiency. Electricity cogen-eration per tonne of crushed sugarcane grew from approximately 17 kW·h/t in 1989 to 22.4 kW·h/t in 1997 and to 31 kW·h/t in 2003 [4.5].

This increase in efficiency has been achieved mainly as a result of the increase in the energy efficiency of the main equipment and facilities. This increase was realized by using solutions designed domestically, by installing 42 backpressure turbogenerators that were locally available and by keeping in operation only the most efficient sugar mills. As part of the restructuring of the sugar industry, 15 additional turbogenerators with a total capacity of 35 MW were relocated and brought into operation [4.8].

In spite of the lack of recovery in the sugar industry, in the past four years it has been possible to standardize¹⁷ the electricity distribution systems in 62 small settlements situated near sugar mills comprising more than 50 000 families. This has had an important social impact by improving the quality of life of those families and represents a reduction in energy use estimated to range between 30 and 40% of the electricity supplied to these communities.

Although the sugar industry produces an amount of renewable biomass equal to 5.2 million toe annually, use of this biomass in electricity generation in the country’s thermal power plants only amounts to 0.4 million toe. This fact is related, above all, to the suboptimal use of the installed capacity and the technological obsolescence of the industrial sector’s energy base. Different variants for better use of this potential have been identified. These variants are classified or grouped according to the terms and financial resources required for their implementation [4.9]:

— Optimal use of installed capacity. The quantity of electricity cogenerated in a sugar mill can be maximized by operating the milling plant at its full capacity and ensuring the stable operation of the electric plant. These factors are determined by the regular supply of sugarcane to the sugar mill and the amount of time lost due to shutdowns.

— Increase in the efficiency of electricity cogeneration using low pressure boilers. The efficiency of the thermal cycle of electricity cogeneration is

17 The standardization process transferred the ownership of distribution networks (which provide electricity to small settlements near sugar mills) to the Electric Union. It eliminates the illegal connections and electricity theft, and increases the quality of the service. Now, all the benefits and obligations of all electricity consumers are applied.

determined by the characteristics of the generator and steam turbine. The modernization or replacement of steam generators with the purpose of achieving a thermal efficiency of 80% and reaching steam pressures of 40 kg/cm² would make possible the generation of 27.5 kW·h/t of crushed sugarcane. If the backpressure turbines were also replaced with extraction condensation turbines, it would be possible to attain 33.5 kW·h/t of crushed sugarcane.

— Use of thermoelectric plants in the sugar industry. A significant increase in electricity cogeneration in the sugar industry has been achieved with the introduction of high pressure steam generators (80 kg/cm²), which require the use of a generation scheme similar to those used in thermal power plants. In this case, because of the high total investment required, to be economically viable it is necessary to cogenerate electricity throughout the year. In Cuba, the most appropriate fuel is SAR, as these residues are gathered annually in quantities amounting to 1 million toe. The main technological problem to be solved is the compression and preservation of this material for long periods of time. Under current conditions, it is possible to produce 120 kW·h/t of milled sugarcane during the harvest and 300 kW·h/t of sugarcane residues outside the harvest season. Studies show the possibility of installing 1675 MW in Cuba’s sugar mills and 41 units of 25 MW and 11 units of 50 MW in thermal power plants, with the possibility of generating up to 9648 GW·h/a.

— Use of gas turbine cycles. Thermal cycles for electricity generation with gas turbines have reached a commercial level with the use of natural gas or liquid fuels in thermal power plants. The high thermal efficiency of these technologies (over 40%) makes their investment costs per installed kW(e) the lowest of the technologies that are commercialized in the world at present. The use of sugarcane biomass as a fuel in these cycles has been delayed because the biomass gasifiers required for these appli-cations are still in the research and development stage and work is being done to improve gas cleaning systems. In addition, aeroderivative turbines have not been evaluated under the working conditions of these thermodynamic cycles.

At present, financial support is being negotiated for the three plants Melanio Hernández, Héctor Molina and Jesús Rabí. These power plants are designed to operate throughout the year and to supply electricity to the sugar industry, to the sugarcane by-products plants and to the national grid. The first project, for the Melanio Hernández mill, will use regular bagasse during the sugarcane cutting time. Bagasse from a different variety of sugarcane (known as the ‘energy sugarcane’) and wood residues or forestry wastes will be used for

the rest of the year, depending on their availability and price at each location.

In the second project, for the Héctor Molina sugar mill, the use of bagasse during the harvest season and crude oil for the rest of the year is planned. Work is expected to be carried out until 2010 on these three biomass power plants, which will have a total installed capacity of 66 MW.

In addition, cogeneration projects using biomass integrated gasification combined cycle are being analysed. This technology uses an external gasifier for the production of combustible gases from bagasse, which are burned in a modified gas turbine. The combustion gases are circulated through a heat recovery boiler for steam generation; a portion of the gases is used for drying the bagasse. The efficiency in the conversion of biomass to electricity can reach 37%. This type of technology is considered to be at an initial commercial stage at the international level, as there are some facilities of this kind in Brazil.