The burning and biodegradation of wastes as a source of energy is also significant.
1. Water- Based Biomass:
Aquatic plants do not require irrigation or appreciable rainfall, the temperatures fluctuate less than land temperatures and light absorption is high—giving rise to high photosynthetic efficiencies.
Algae, seaweed and other aquatic lower plants can be intensively grown in certain areas of the sea or in inland lakes or ponds. This would eliminate the competition for land with conventional agriculture. On average, the oceans are low in plant nutrients.
Surface waters have low natural productivity but sometimes act as nutrient repositories due to runoff from the land. Deep ocean waters are rich in nutrients. If such deep water can be pumped to the surface a biomass crop such as giant kelp (i.e., large brown seaweed) could be grown.
A typical algae farm might contain a hectare of land, excavated to a depth of 18 inches (45 cm), flooded and algae plants introduced. The pond would be covered with plastic, injected with carbon dioxide and maintained at a temperature of 100°F (38°C).
2. Energy from Wastes:
All human and industrial processes produce waste. Wastes that can form sources of biofuels include domestic refuse, industrial wastes, agricultural wastes, forestry residues, sewage and industrial effluents.
It is highly desirable that the recovery of energy from waste should form part of an integrated approach to waste management, designed to maximise waste recycling and reclamation. Using wastes as a source of fuel can be highly cost effective, especially if the alternative disposal cost is discounted.
Wastes do have certain disadvantages when used as fuel sources. They may be difficult to handle and process and generally they have low energy density. Some wastes may be contaminated with non-fuel materials. It may be necessary to transport the waste from its source to a conversion site.
3. Use of Solid Wastes:
In a highly developed society like Great Britain or the USA, solid waste is produced at the rate of about one tonne per person per year (2000). Most domestic waste is solid in form. The constituent proportions typical of UK municipal waste are shown in Table 24.12.
Each year the UK produces 28 million tonnes of solid domestic wastes plus a similar amount of industrial wastes. Solid wastes are usually disposed of either by burning or by burial in landfill sites. In the UK a “landfill tax” of £7/ton has been introduced which affects both the costs and usage of landfill sites.
For combustion in a modern waste incinerator the refuse can be first sorted to separate out materials such as glass for recycling. The remainder is shredded to convert the burnable component into RDF (Refuse Derived Fuel) pellets.
The RDF pellets burn like wood but produce less heat than coal and leave more ash. Modern municipal waste incinerators produce no smoke and reduce the. bulk of the waste to about one-eighth. Some wastes, such as synthetic polymers (e.g. plastics produced from petrochemicals), produce toxic gases on combustion and these have to be safely contained within the incineration process.
General industrial waste consists mainly of paper, cardboard, wood, and plastics. It contains a lower moisture and lower ash content than municipal waste, making combustion easier to control and is less contaminated with non-fuel ingredients.
Special forms of industrial waste such as batteries, motor tyres, poultry litter, and hospital wastes require special forms of treatment and are all useful biofuels for combustion systems. Refuse incineration in municipal plants for energy recovery is a relatively new but growing technology in the UK, as illustrated in Table 24.13.
Most of the domestic waste produced in the UK is not burned but is buried in about 5,000 landfill sties. Decay of the waste produces a methane-rich gas called landfill gas. At the end of 1990 there were 33 landfill gas schemes in operation in the UK, including 18 MWe of electricity generation. By 1995 there were 50 gas-producing sites in operation and an estimated further 400 sites that could be used.
The largest current site in the UK produces 3,500 m3 of methane per hour to make steam for use in paper production.
A diagrammatic representation of a landfill gas system is given in Fig. 24.20:
The venting of landfill gases poses some environmental problems. Within the site vicinity there may be objectionable odours. Uncontrolled discharges from landfill using (say) power station cooling water, which is presently dumped into rivers. As the algae is harvested the nutrient-rich water used for growth is returned to the pond and re-seeded to recycle the operation.
The aquatic weed known as water hyacinth has been studied as a tropical water source of biogases, particularly by the US National Aeronautics and Space Administration (NASA). On a dry weight base, one kilogram of water hyacinth can produce 0.4 m3 of biogas with a calorific value of 22 MJ/m3. Aquatic weeds are a hazard in some waterways and have to be harvested, of necessity. The biofuel value is then a useful by-product.
However, there are a number of advantages and disadvantages of use of alternative fuels sources.
The details are given in Table 24.14:
Finally different types of power plant operation and their energy generation systems are summarised in Fig. 24.21.