Waste to Energy Conversion Routes

Waste-to-energy is the use of modern combustion and biological technologies to recover energy from urban wastes. There are three major waste to energy conversion routes – thermochemical, biochemical and physico-chemical. Thermochemical conversion, characterized by higher temperature and conversion rates, is best suited for lower moisture feedstock and is generally less selective for products. On the other hand, biochemical technologies are more suitable for wet wastes which are rich in organic matter.

Teesside-WTE-plant

Thermochemical Conversion

The three principal methods of thermochemical conversion are combustion in excess air, gasification in reduced air, and pyrolysis in the absence of air. The most common technique for producing both heat and electrical energy from household wastes is direct combustion.

Combined heat and power (CHP) or cogeneration systems, ranging from small-scale technology to large grid-connected facilities, provide significantly higher efficiencies than systems that only generate electricity.

WTE_Pathways

Combustion technology is the controlled combustion of waste with the recovery of heat to produce steam which in turn produces power through steam turbines. Pyrolysis and gasification represent refined thermal treatment methods as alternatives to incineration and are characterized by the transformation of the waste into product gas as energy carrier for later combustion in, for example, a boiler or a gas engine. Plasma gasification, which takes place at extremely high temperature, is also hogging limelight nowadays.

Biochemical Conversion

Biochemical processes, like anaerobic digestion, can also produce clean energy in the form of biogas which can be converted to power and heat using a gas engine. Anaerobic digestion is the natural biological process which stabilizes organic waste in the absence of air and transforms it into biofertilizer and biogas.

Anaerobic digestion is a reliable technology for the treatment of wet, organic waste.  Organic waste from various sources is biochemically degraded in highly controlled, oxygen-free conditions circumstances resulting in the production of biogas which can be used to produce both electricity and heat.

In addition, a variety of fuels can be produced from waste resources including liquid fuels, such as ethanol, methanol, biodiesel, Fischer-Tropsch diesel, and gaseous fuels, such as hydrogen and methane. The resource base for biofuel production is composed of a wide variety of forestry and agricultural resources, industrial processing residues, and municipal solid and urban wood residues. Globally, biofuels are most commonly used to power vehicles, heat homes, and for cooking.

Physico-chemical Conversion

The physico-chemical technology involves various processes to improve physical and chemical properties of solid waste. The combustible fraction of the waste is converted into high-energy fuel pellets which may be used in steam generation. The waste is first dried to bring down the high moisture levels. Sand, grit, and other incombustible matter are then mechanically separated before the waste is compacted and converted into fuel pellets or RDF.

Fuel pellets have several distinct advantages over coal and wood because it is cleaner, free from incombustibles, has lower ash and moisture contents, is of uniform size, cost-effective, and eco-friendly.

A Primer on Waste-to-Energy

Waste-to-Energy (also known as energy-from-waste) is the use of thermochemical and biochemical technologies to recover energy, usually in the form of electricity, steam and fuels, from urban wastes.These new technologies can reduce the volume of the original waste by 90%, depending upon composition and use of outputs.

Energy is the driving force for development in all countries of the world. The increasing clamor for energy and satisfying it with a combination of conventional and renewable resources is a big challenge. Accompanying energy problems in different parts of the world, another problem that is assuming critical proportions is that of urban waste accumulation.

The quantity of waste produced all over the world amounted to more than 12 billion tonnes in 2006, with estimates of up to 13 billion tonnes in 2011. The rapid increase in population coupled with changing lifestyle and consumption patterns is expected to result in an exponential increase in waste generation of upto 18 billion tonnes by year 2020.

Waste generation rates are affected by socio-economic development, degree of industrialization, and climate. Generally, the greater the economic prosperity and the higher percentage of urban population, the greater the amount of solid waste produced. Reduction in the volume and mass of solid waste is a crucial issue especially in the light of limited availability of final disposal sites in many parts of the world. Millions of tonnes of household wastes are generated each year with the vast majority disposed of in open fields or burnt wantonly.

The main categories of waste-to-energy technologies are physical technologies, which process waste to make it more useful as fuel; thermal technologies, which can yield heat, fuel oil, or syngas from both organic and inorganic wastes; and biological technologies, in which bacterial fermentation is used to digest organic wastes to yield fuel.

The three principal methods of thermochemical conversion are combustion in excess air, gasification in reduced air, and pyrolysis in the absence of air. The most common technique for producing both heat and electrical energy from wastes is direct combustion. Combined heat and power (CHP) or cogeneration systems, ranging from small-scale technology to large grid-connected facilities, provide significantly higher efficiencies than systems that only generate electricity.

Biochemical processes, like anaerobic digestion, can also produce clean energy in the form of biogas which can be converted to power and heat using a gas engine. In addition, wastes can also yield liquid fuels, such as cellulosic ethanol, which can be used to replace petroleum-based fuels. Cellulosic ethanol can be produced from grasses, wood chips and agricultural residues by biochemical route using heat, pressure, chemicals and enzymes to unlock the sugars in biomass wastes.

Waste-to-energy plants offer two important benefits of environmentally safe waste management and disposal, as well as the generation of clean electric power.  The growing use of waste-to-energy as a method to dispose of solid and liquid wastes and generate power has greatly reduced environmental impacts of municipal solid waste management, including emissions of greenhouse gases.

Biofuels from MSW – An Introduction

Nowadays, biofuels are in high demand for transportation, industrial heating and electricity generation. Different technologies are being tested for using MSW as feedstock for producing biofuels. This article will provide brief description of biochemical and thermochemical conversion routes for the production of biofuels from municipal solid wastes.

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Biochemical conversion

The waste is collected and milled, particles are shredded to reduce the size of 0.2-1.22 mm. MSW is pretreated to improve the accessibility of enzymes and make use of the enzymes in the bacteria for biological degradation on solid waste. The mixture of biomass is mixed with sulfuric acid and sodium hydroxide and autoclaved. After steam treatment, the mixture is filtered and washed with deionized water. The pre-treated mixture is then dried and drained overnight. The pre-treatment process improves the formation of sugars by enzymatic hydrolysis, avoids the loss of carbohydrate and avoids the formation of by-products inhibitory.

After pre-treatment (pre-hydrolysis), the mixture undergoes enzymatic hydrolysis for conversion of polysaccharides into monomer sugars, such as glucose and xylose. The common enzymes used for starch-based substrates are ?- and ?- amylase, pullulanase, isomylase and glucoamylase. Whereas for lignocellulose based substrates cellulases and ?- glucosidases.

Finally, the mixture is fermented; sugars are converted to ethanol by using microorganisms such as, bacteria, yeast or fungi. The cellulosic and starch hydrolysates ethanolic fermentation were fermented by M. indicus at 37 °C for 72 h. The fungus uses the hexoses and pentoses sugars with a high concentration of inhibitors (i.e. furfural, hydroxymethyl furfural, and acetic acid).

The composition of MSW feedstock effects the yield of the subsequent processes. A high composition of food and vegetable waste is more desirable, as these wastes are easily degradable and result in high yields compared to paper and cardboard.

Thermochemical conversion

Gasification process is carried out by treating carbon-based material with either oxygen or steam to produce a gaseous fuel which requires high temperature and pressure. It can be described as partial oxidation of the waste. At first waste is reduced in size and dried to reduce the amount of energy used in the gasifier.

Biomass_Gasification_Process

Layout of a Typical Biomass Gasification Plant

 

The carbonaceous material oxidizes (combines with oxygen) to produce syngas (carbon monoxide and hydrogen) along with carbon dioxide, methane, water vapor, char, slag, and trace gases (depending on the composition of the feedstock). The syngas is then cleaned to remove any sulfur or acid gases and trace metals (depending on the composition of the feedstock).

The main uses of syngas are direct burning on site to provide heat or energy (by using boilers, gas turbines or steam driven engines) and refined to liquid fuels such as gasoline or ethanol.

Syngas can then be converted into biofuels and chemicals via catalytic processes such as the Fischer-Tropsch process. The Fischer-Tropsch process is a series of catalytic chemical reactions that convert syngas into liquid hydrocarbons by applying heat and pressure. Hydrocracking, hydro-treating, and hydro-isomerization can also be part of the “upgrading” process to maximize quantities of different products.