Energy and Products from Waste

A quick overview

A metric ton of USA Municipal Solid Waste (MSW) can yield up to 145 liters of ultra clean Diesel fuel or up to 165 liters of ultra clean gasoline.

If the choice is to generate electric energy, then a metric ton of USA MSW can generate up to 1,500 kiloWatt-hours through the Integrated Gasification and Combined Cycle (IGCC) method of power generation.

In addition, roughly 70 kilograms of metal can be recovered and 250 kilograms of vitrified construction material can be produced from a metric ton of USA MSW.

The value of the products from a converted metric ton of MSW is up to $200 in the USA. The cost of conversion can be up to $160 per ton.  It is profitable to convert MSW to energy and products.  It also leaves no residue to be placed in a landfill.

Taking MSW to a landfill is not only is a monetary loss but also harms the environment.  There seems to be little justification for this practice.

Although we use MSW as a specific example of the possibilities, the same argument applies to waste in general but the energy and product yield will vary with the chemical and physical makeup of the waste in question.

For example, used automobile tires are a good feedstock for conversion - they can yield 450 to 500 liters of ultra clean Diesel fuel per metric ton.

The following diagram illustrates the products that can be derived from used automobile tires.

Some Details:

MSW conversion and synthesis to F-T products such as Diesel fuel or gasoline along with heat recovery can make beneficial and commercial use of 70% or more of the energy value of MSW as shown in the example below. 

 

For generations, crude oil and natural gas have been major sources of energy.  By extracting these from the earth and  burning them,  we add carbon dioxide to our atmosphere.  The carbon dioxide released by combustion influences climate change.  The rate of release of carbon dioxide from this source is increasing exponentially.

Natural gas and fuels made from crude oil can be replaced by equivalents derived from material of biological origin.  This is a vital key to the reduction of greenhouse gas.  By converting organic material such as biomass, municipal solid waste and all other organic matter originating from the natural life cycle we eliminate the increase of carbon dioxide which would otherwise occur by burning fossil fuels.

Carbon dioxide from burning biological matter is re-absorbed by the biosphere to produce more biomass by the photosynthetic  process.  There is no net increase in greenhouse gas in this scenario.  Many view this as nature's way of using carbon dioxide and water to capture and store the energy of the sun.

How Biomass is Converted to Synthetic Fuel

By deconstructing the biomass organic molecules and re-arranging them we can produce ultra clean equivalents (substitutes) for the fuel types in wide use.

Referring to the diagram below, when organic material is heated in the absence of oxygen it breaks down to make a gas - this is known as destructive distillation which has a long history of exploitation by mankind.  If a bit of oxygen is added to the pyrolytic process , the carbon and hydrogen in the organic matter react to generate heat and become partially oxidized.  This starts by the formation of hydrogen, carbon monoxide, carbon dioxide and water vapor and as the availability of oxygen increases the carbon and hydrogen continue to oxidize until it is all converted to carbon dioxide and water.  Adding enough or more than enough oxygen to the fuel to oxidize all of the hydrogen and carbon is called combustion and results in a hot gas which is useful for the generation of steam and using the steam to generate electric energy

Thermal Oxidation Spectrum

Traditionally, three types of thermal decomposition of organic matter are recognized, these are as follow:

Pyrolysis which is the heating of the waste in the absence or very low levels of air or oxygen to cause thermal decomposition of the organic fraction. In this process there is a significant residue of carbon and long chain hydrocarbons.

Gasification which is the thermal decomposition of the organic fraction using sufficient oxygen or air along with steam so as to render all hydrocarbon species into a combination of hydrogen, carbon monoxide and carbon dioxide (and other gaseous compounds depending on what other elements are in the hydrocarbons) the ratio of which varies in accordance with stochiometry and process parameters.

Combustion which is the thermal decomposition of the organic fraction in excess oxygen or air resulting in the oxidation of the hydrocarbon species into water, carbon dioxide and other gases depending on what other elements are present in the hydrocarbons

The following diagram illustrates the evolution of gases as the oxygen concentration is increased.  This is for coal, but the same principles apply for all organic material.  The basic form of the diagram remains the same but the molar ratios will vary somewhat.

Why Gasification?

In the gasification process, the aim is to produce a maximum amount of carbon monoxide and hydrogen and a minimum amount of water and carbon dioxide.  The carbon monoxide and hydrogen are then cleaned and passed through a synthesis stage where they react to form the basic organic molecule building block CH2 .  This building block molecule then combines in various ways to form chains and branches resulting in a mixture of various organic  molecules corresponding to the desired product such as ultra clean Diesel fuel, gasoline, aviation fuel or kerosene.

The production of liquid or gaseous fuels from biomass is a narrow example of a wider range of possibilities that exist.  The diagram below is a simplified compilation of the possibilities.

As can be seen in reading the diagram from left to right, there is waste and biomass going into the process and no waste exiting the process.  This is in tune with the ecology of our world and a desirable departure from the primitive practice of simply burying waste.

One of the more interesting products is hydrogen which is regarded by many to be the fuel of the future.