Colben Energy Pte Ltd

- Shape and size (e.g., dust, pellets, shavings, bales).
- Density and compaction (which affects the calorific value and volatile content per unit volume. This in   turn affects the combustion characteristics, product handling and storage space required)
- Water content (depends on type of biomass, time of harvest and type of storage. It affects not only the   calorific value but also the combustion temperature, exhaust and ash characteristics, plus the time   and type of storage).
- Ash content and fusion characteristics (depends on type of biomass and degree of contamination. It   affects the calorific value and its acceptance in certain types of burner).

Dry waste can include food crops such as rape and sugar-producing crops. From these, liquid fuels can be produced to replace diesel or gasoline in automotive engines. The term also includes energy crops, grown specifically as fuel. Short rotation coppice (SRC) such as willow and poplar has been identified as showing potential for energy supply from farm land no longer required for food production. Eucalyptus and alder also produce reasonable yields. The grasses miscanthus and spartina appear to have the ability to produce high yields of biomass.

Design of the utilisation plant has to be co-ordinated, not only with the characteristics of the biomass used, but also with its preparation. In the case of combustion, this affects the type of burner used, e.g. underfeed or chain-grate stoker, pulverised or fluidised bed combustion. A homogeneous fuel will make automation easier. It will reduce the need for maintenance and manual intervention. Some of the essential characteristics which need to be determined are:

At present, plants using direct combustion for steam generation are the only fully developed biomass systems available for the heat and electricity generation markets. For generation and for Combined Heat and Power (CHP) they operate in combination with turbines or motors (screw or reciprocating types) Complete systems, with normal commercial guarantees, are available from competing vendors (see attachment “chiklitt”).

In such a process the biomass is simply burnt to recover energy in the form of heat. Electricity can then be generated, using a conventional steam cycle. Numerous boiler designs are available, many of which have been developed specifically for the use of biomass fuels. A plant for the combustion of wood chippings, straw or “dry” energy crops has the same basic design as that for coal combustion. More refractory lining is added to allow the relatively wet fuels to dry on the grate, with extra secondary air to ensure complete combustion of volatiles. Such plant can handle fuels with a moisture content as high as 60% by weight. They are available in a wide range of sizes, from a few hundred kilowatts to many megawatts. The emissions from these plants is low, with only a vapour plume from the stack following start-up. At the larger sizes, fluidised bed combustion is possible.

The higher the combustion temperature, the more efficient the process and the more energy available for electricity generation. At high temperatures however, fusion of the ash could cause deposition problems in boilers. In all combustion plant types, cereal straw presents problems as the ash has a low melting point. This may lead to boiler fouling.

For large power stations, biomass may be burnt together with coal. Most biomass fuelled plants will be small in size, due to the limitation imposed by having to transport the fuel over long distances. Consequently, the overall thermal efficiency will be low – 20-25% in the output range of 1 – 30 MW (elect). Where the plant size is less than 1 MW (elect) the efficiency falls to a value making such small scale electricity generation uneconomic in almost all situations. The exceptions are where the biomass fuel is obtained free, or where the price of fossil fuel (gas, oil, coal) is high.

There is significant development activity in this area, with numerous pilot and demonstration projects in operation (see attachment “biogas”). Green agricultural crops and residues, as well as animal slurries and sewage sludge can be subjected to anaerobic digestion. This is the bacteriological fermentation of organic material in the absence of oxygen. This can take place in fabricated tanks. The activity of these “anaerobic” bacteria leads to the formation of a methane-rich biogas, comprising about 60% methane, with a calorific value of about 25 Mjoules/ sq.m. After filtering and drying, the gas can be used as a fuel, either for boilers, gas turbines or conventional spark ignition gas engines. Most new applications use spark ignition engines for the driving of alternators.

- Gasification is the thermal oxidation of an organic feedstock, with too little oxygen to enable complete combustion. This results in the production of a gas mixture, containing carbon monoxide, hydrogen and methane. All of these have a fuel value and can be used to fuel internal combustion engines. At the scales appropriate to biomass, overall thermal efficiency is increased beyond that for conventional combustion, from the low 20% range to over 40%. This process has a longer history than the other two below.

Where biomass is used to fuel generator prime movers, be they internal combustion engines, steam or gas turbines, the possibility exists of utilising the waste heat from the prime mover exhaust. Such Combined Heat and Power (CHP) schemes offer an interesting alternative where heat is required in addition to electricity. Where air conditioning is required, this heat can be utilised as the energy input to an absorption chiller. The tendency at the lower power range (<3 mw) is for motors to be more economic as a prime mover than turbines.

The economics of the project will be affected by the purchase price of a kilowatt-hour, and hence the reduction in the cost of power purchased from the public supply. Whether the public supply will allow parallel operation is of great significance economically, in particular what price they will pay for kilowatt-hours exported.

Power systems have traditionally been designed to take power “down” to the consumer. Local, or “embedded” generation acts in the opposite sense. There is therefore a tendency to raise the local voltage. This has to be taken into account when calculating the effect of local generation on other users and on the distribution system as a whole. Fault level and system capacity have to be considered and a protection and control scheme agreed with the public supply authority.