A global overview of emerging and innovative thermal drying technologies shows that they are driven by the need for enhanced dried product quality, higher thermal efficiency, improved safety, and reduced environmental degradation. This includes many technologies that are already commercialized or show the potential for imminent industrial exploitation. In the near future, it will be essential to ensure net or near net zero emissions of greenhouse gases, which means any carbon emissions created are balanced by taking the same amount of carbon out of the atmosphere.
Biomass has historically been widely used in many drying applications. In recent years, it has been converted into biogas or liquid biofuels. Further, biofuels are being derived from non-edible vegetable oils, so as not to interfere with the food chain and concerns over food insecurity. Alcohols can be made from sugar or sugar-containing materials, starch or starch-containing materials, and also lignocellulosic biomass. Biodiesel can be obtained from various edible and non-edible oils, seeds of various crops and also from algae, fungi and lignocellulosic biomass. Dimethyl ether, which is a synthetic future fuel, can be obtained from lignocellulosic biomass and can be blended with diesel for use in vehicles. Such renewable fuels are a good option to replace fossil fuels as the source of thermal energy for industrial drying.
Renewable fuels such as hydrogen, compressed air and synthetic methanol have a large potential to provide sustainable energy for drying. Methane is a powerful greenhouse gas, but it is a good fuel, which can be obtained both from renewable and nonrenewable sources. Renewable methane can be obtained from a number of routes: (i) anaerobic decay/biological or chemical processes (ii) thermal gasification of organic materials and (iii) landfills generally from municipal solid wastes.
The dry mass of biological materials cycling in the biosphere is about 250 × 109 ton/year, incorporating about 100 × 109 ton/year of carbon. The associated energy bound in photosynthesis is 0.634 × 1011 kW. Of this, about 0.5% by weight of biomass is used as crops for human food. Biomass provides about 13% of mankind’s energy consumption, including much for domestic use in developing countries, but also significant amounts in developed economies. Carbon released from the burning of biomass is neutralized via photosynthesis, which generates energy from biomass and is ‘carbon neutral.’
Approximately one out of every nine people in the world is undernourished. Worldwide, approximately 1.3 billion tons of food meant for human consumption is lost for diverse reasons on a yearly basis. This is equivalent to 32.5% of the total production. To address this challenge, several technologies and methods have been developed. Although there are numerous mechanical, electrical and natural convection methods of drying, for small-scale farmers in the developing world, the use of passive solar dryers is a viable option, which is already being used, but not on a scale needed to ensure food security by preservation of freshly harvested produce.
Sustainable energy refers to a renewable type of energy, which is utilized with high efficiency. Drying applications include producing heat through the use of solar collectors and photovoltaics. Solar, wind, hydroenergy, geothermal, biomass-derived energy and renewable fuels are sustainable energy sources. Wind energy, small-scale hydroelectric power plants, tidal energy, wind turbines, and underwater turbines using river or sea currents can also be used to supply electricity to large-scale dryers. The heat content of underground soil can be used for the production of geothermal energy, available in many regions of the world, but has not been considered at present for drying.
Carbon capture and Sequestration (CCS) results in greenhouse gas reduction, improved energy efficiency and lower impact on climate change. Emissions from power plants, which burn fossil fuels, would go into the atmosphere if not captured and sequestered. CCS involves capturing the emissions and storing them deep underground in geological structures. CCS may be biological, geological or technological. Technological developments are required for cost reduction. The main methods of reduction in the atmospheric concentration of carbon include sequestration in biomass such as plants and trees by large-scale planting of trees and forestation, putting biomass in soil, sequestration by algae in oceans, its capture and sequestration deep underground or in the oceans and use of renewable fuels. Renewable energy technologies provide an excellent opportunity for mitigation of greenhouse gas emissions and reducing global warming, thus making the drying operations more energy efficient and environment friendly. This is critically important for industrial-scale drying of numerous commodities such as grains, pulp and paper, wood, ceramics, and minerals since drying is highly energy-intensive and conventional use of fossil fuels can lead to extremely large amounts of greenhouse gas emissions, which are detrimental to climate. It has been estimated that national energy consumption belonging to industrial drying operations in developing countries ranges from 12 to 20%. This clearly presents an attractive situation for the use of renewable fuels; their eventual use would help reduce greenhouse gases and mitigate the effects of climate change. Owing to space limitations, details of the various ideas mentioned above cannot be presented here.
Department of Mechanical Engineering, Indian Institute of Technology Kanpur, India
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