A few days ago we had a guest post on glycerol, a waste product from the biodiesel production process. There’s lots of it these days as the biodiesel industry grows, and it turns out it has some useful applications, including generating electricity. There are lots of different waste products that can be used to create energy or heat, including household waste. Sometimes, like glycerol, there are specific by-products from particular industries that present unique opportunities. Here are two more:
Sugar is an important global commodity, and many African countries grow sugar cane on a large scale. To produce sugar from the cane, it is crushed to extract the juice. When it’s in season, you can buy fresh sugarcane as a snack and if you’ve ever tried it, you’ll know how this goes – you chew out the sweetness, and spit out the fibrous leftovers. The same thing happens in the factories, where roughly a tonne of waste is produced for every three tonnes of sugar cane processed. The waste fibre is called bagasse, and it has various uses. It can be pressed into plywood, mixed into animal feed, or used to make paper. It can also be used to generate electricity.
In Mauritius, half of the country’s electricity needs come from the sugar factories. The bagasse is fired in co-generation power stations, displacing coal during the harvest season. It produces electricity and steam, the latter being piped back through to the factories for further sugar processing. This is efficient and deals with waste that would otherwise be a problem for the island nation. It’s also helping to keep the sugar industry profitable at a time of falling global prices. The government has been instrumental in nurturing the industry and pushing it to greater efficiency, creating tax breaks for bagasse use and encouraging higher value exports.
Several other African countries use bagasse for electricity already, and the photo is from a plant in India. Others are investing in it as part of the transition to renewable energy. In Brazil, the world’s leading sugar exporter, they are also experimenting with using bagasse to produce ‘second generation’ ethanol. The first plant to create biofuel from bagasse opened this summer.
Closer to home, I read about another innovative form of energy from waste this week. Celtic Renewables are a Scottish company that has pioneered a method to make biofuels out of the waste from the whisky industry. As you will know if you’ve taken a distillery tour, one of the early stages of the whisky process is to steep the malted barley in warm water in a mash tun. When the water is siphoned off to continue the process, a remaining mash of barley husks is left behind. This is called draff and it’s usually used as animal feed.
Celtic Renewables have developed a way to ferment this waste, using bacteria to convert it into ethanol, butanol and acetone. At the end of the process, the mash is dried for animal feed, so nothing goes to waste.
These are good examples of locally appropriate technologies, making good use of local resources. These are technologies that don’t make sense outside of their context, as they’ve developed in response to the specific industries nearby. By using local waste materials, they reduce the need for imported fuels such as coal. This improves local energy resilience as well as reducing carbon emissions.
The Earthbound Report 
The easiest way of using waste material is to burn it in a steam generator. The steam can be used in a reciprocating steam engine, either for stationary power or in boats and railway locomotives.
Converting it to a liquid fuel gains no energy but merely enables it to be used in an internal combustion engine, which is of course more convenient in many situations. However, internal combustion is less difficult to control and gives rise to NOx and particulates, so if the aim is for clean exhaust, external combustion is preferable.