Get talking: energy from waste

Materials World magazine
4 Jan 2017

Allan Childs proposes we shift focus from new nuclear to energy from waste.

There has been a considerable amount of reportage regarding the advantages and disadvantages of nuclear generated electricity. Very little consideration seems to have been given to some of the alternatives, probably because they are still in rather early stages of development. A quick look at recent history could contribute to more reliable solutions to the problem.

Records show that a couple of centuries ago the majority of people were either farm labourers or fishermen who lived a hand-to-mouth existence in squalid cottages and worked outdoors in all weather. In the 18th Century, industrialisation took over and revolutionised the western world. People then lived in slums and worked a 60-hour week in mines and factories. Illiteracy and infant mortality were rife. Since then, things have improved considerably and, although agriculture and fishing are still major employers, industrialisation has become the prime mover.   

Major developments have occurred in the construction, transport and engineering fields, medical science has conquered many fatal diseases, food security has greatly improved, as has the provision of housing, electricity, clean water and waste disposal systems. All these developments have contributed to a considerable increase in life expectancy. There are now more people producing children who live longer than ever before. 

Around 12,000 years ago, the world population is estimated to have been 1 million. By 1800 it had reached 1 billion, by 1900 it was 1.6 billion, by 2000 it had soared to 6.1 billion and in 2011 the 7 billion mark was met. The World Health Organisation (WHO) has forecast that by 2050 the world’s population will be 9 billion. This is the root cause of global warming. The WHO has also observed that as the population grows it migrates to built up areas. All these people will need to be fed, housed, employed and transported. They will produce waste and sewage, which the local authorities will be required to collect and dispose of in an acceptable manner. 

Records both in South Africa and the UK indicate that the population on average produce about one tonne of household waste per person per year. About 30% of this is biodegradable. It is normally disposed of in a landfill site where the degradable portion gradually decomposes. During this process, living organisms eat the material and excrete a 50/50 mixture of carbon dioxide and methane. Methane is 22 times more effective as a greenhouse gas (GHG) than the carbon dioxide standard. Hence one tonne of landfill gas (LFG) is equivalent to 11.5 tonnes of GHG. Methane has a low toxicity rating but is a severe fire hazard and mixtures with 5–15% in air are explosive. One of the alternative methods of generating electricity is using LFG to drive a spark ignition engine connected to a dynamo. The problem with the process is that current collection methods only retrieve about 70% of the available gas and it takes the living organisms about 25–30 years to complete the decomposition process.   

Under normal landfill site conditions, one tonne of degradable material produces 70Nm3 of LFG. A typical methane-driven generator uses 620Nm3 to generate 1Mw of electricity. Therefore, in round numbers, 1Mw can be generated for every nine tonnes of degradable waste or 30 tonnes of general waste. With a population of 50 million, it could be possible to generate 1.5 million megawatts of electricity per year. One of the attractions of the process is that the gas can be stored as a compressed gas in periods of low power demand and used when demand increases.  

Humans have domesticated plants and animals for around 14,000 years, using selective breeding or artificial selection. The process of selective breeding, in which organisms with desired genes, and thus desired traits, are used to breed the next generation was a precursor to genetic modification. Various advancements in genetics allowed humans to directly alter the DNA of organisms. Progress was relatively slow at first. In 1982, the first human antibodies were produced in plants and the first genetically modified crop, an antibiotic-resistant tobacco plant, was produced. China was the first country to commercialise transgenic plants, introducing a virus-resistant tobacco in 1992. Since then, progress has spread to a fairly wide range of applications. Genetically modified organisms are now used in biological and medical research, production of pharmaceuticals, experimental medicine and agriculture.

The use of genetic engineering to increase the yield from certain liquid fuel producing processes was introduced some years ago, including the production of ethanol from biodegradable waste. A number of technologists have suggested that genetic engineering be used to speed up the landfill site decomposing process, eventually turning a 20-year process into a 24-hour process. There are reports from several sources of work being undertaken that could contribute to this target being achieved. There is still a considerable amount of work required to perfect the process, but interim scaled up application during development could be attractive. It needs a special organisation to undertake the necessary coordination, funding etc, but it seems most unlikely to cost as much as a new nuclear power station. 

Sewage is another waste material that slowly produces methane as it decomposes. It is also collected and treated by the local municipalities – some already collect the methane and use it to generate electricity. Animal dung is also used as a source of electricity but it too suffers from a slow rate of conversion.   

As the population grows and migrates to built-up areas, there will be an increasing quantity of household waste and sewage to be collected and processed, municipalities charge for this service that could render these everlasting potential raw materials free. 

The elimination of methane from the waste and sewage decomposition cycle would contribute significantly to the reduction of greenhouse gas production, thus reducing global warming.  

The proposed new South African nuclear power station has been put back to 2037 and is forecast to generate 1.359Mw. That is slightly less than the waste potential, without a contribution from sewage.  

Allan Childs AIMMM is a chemist by profession and a business technologist by trade. His professional life was spent in the chemical industry producing emulsion polymers, mainly in senior positions. In 1973, he was transferred to South Africa and later set up as a private consultant, sourcing technology, writing business plans and feasibility studies and implementing projects. In 2003, he became involved in a landfill site and environmental matters, particularly global warming. He has written a number of reports on the use of waste materials to produce fuel and electricity and reduce greenhouse gas emmissions.