The answer is: Municipal solid waste (“MSW”) and its conversion into energy! There are few, if any, energy technologies that can claim to be both renewable and sustainable but also reliable and financially attractive. Why is this so?
To understand this, it’s necessary to understand what MSW is and is not. According to the U.S. EPA, MSW includes non-hazardous solid waste from residential, multifamily, commercial, and institutional (eg., schools, government offices) sources, see following chart. This definition excludes many materials that are frequently disposed with MSW in landfills, such as combustion ash, water and wastewater treatment residuals, and construction and demolition debris.
If the MSW is sorted, so all the recyclables are removed and the construction debris is eliminated, then the residual is considered renewable for tax credits and other renewable energy credits.
MSW can also be classified as sustainable if it allows landfills to remain open due to lower volumes of waste to be handled. In this case sustainable is societal, economic, and environmental if the inherent value of the total resource is used appropriately.
Historically, “waste management practices have evolved over time in response to two important factors: first, from a desire to protect public health, followed much later by a desire to protect the environment. Garbage has been an issue since at least 10,000 BC when humans began moving away from their nomadic habits to establish primitive societies. The increased population densities resulted in more waste being concentrated in a smaller area. In approximately 500 BC, Athenians developed one of the first municipal disposal sites in the Western world and required citizens to dump their waste at least one mile from the city limits. Around AD 200, the Romans established an early form of garbage collection service where teams of two men would walk along the streets collecting garbage and tossing it into a wagon.” Today, the problems remain basically the same, though more extensive, complex and potentially hazardous. Technology is now available to transform the problems to an environmentally and socially beneficial opportunity.
The value chain of MWM consists of three main stages that give rise to three distinct markets. The first one regards collection services, whose counterparts are waste producers and operators. The second is the market for the handling and disposal of waste, whose counterparts are operators of collection services and owners of disposal sites. The third is recovery/recycling, whose counterparts are again collection operators and final users of waste-derived materials. Within each market further additional secondary transactions take place, that identifies additional side markets (e.g., contracting out of specific activities from waste collection operators to specialized companies).
“Some cities, primarily in the northeastern and mid-Atlantic U.S., burned part of their municipal solid wastes. Hemmed in by major population centers, landfill space was at a premium, so burning wastes to reduce their volume and weight made sense. Combustion reduces the volume of material by about 90 percent and its weight by 75 percent. The heat generated by burning wastes has other uses, as well, as it can be used directly for heating, to produce steam or to generate electricity. Other than solving the “landfill gap,” municipal waste combustion facilities made little of no economic returns.”
Then in 1970, Congress passed the Clean Air Act, which authorized the end of open burning at U.S. landfills. City incinerators were required to install pollution controls or cease operation, and a number of the worst polluters were closed down. Losing incinerators forced cities to consider alternate methods of waste disposal.
Combined with rising concerns for sustainability and renewable energy, the prohibition on incineration has spawned renewed interest in converting waste-to-energy. The force behind this movement is gasification and pyrolysis. Like incineration, pyrolysis and gasification are thermal processes that use high temperatures to break down waste. The main difference is that they use less oxygen than traditional mass-burn incineration. The waste is broken down to create gas, solid and liquid residues. The gases can then be combusted in a secondary process. The pyrolysis process thermally degrades waste in the absence of air (and oxygen). Gasification is a process in which materials are exposed to some oxygen, but not enough to allow combustion to occur. Temperatures are usually above 750 degrees C. In some systems the pyrolysis phase is followed by a second gasification stage, in order that more of the energy carrying gases are liberated from the waste.
The main product of gasification and pyrolysis is syngas, which is composed mainly of carbon monoxide and hydrogen (85 per cent), with smaller quantities of carbon dioxide, nitrogen, methane and various other hydrocarbon gases. Syngas has a calorific value, so it can be used as a fuel to generate electricity or steam or as a basic chemical feedstock in the petrochemical and refining industries. The calorific value of this syngas will depend upon the composition of the input waste to the gasifier.
Most gasification and pyrolysis processes have four stages:
“1. Preparation of the waste feedstock: The feedstock may be in the form of a refuse derived fuel, produced by a Mechanical Biological Treatment plant or an autoclave. Alternatively, the plant may take mixed waste and process it first through some sort of materials recycling facility, to remove some recyclables and materials that have no calorific value (e.g. grit)
2. Heating the waste in a low-oxygen atmosphere to produce a gas, oils and char (ash)
3. ‘Scrubbing’ (cleaning) the gas to remove some of the particulates, hydrocarbons and soluble matter
4. Using the scrubbed gas to generate electricity and, in some cases, heat (through combined heat and power – CHP). There are different ways of generating the electricity from the scrubbed gas – steam turbine, gas engine and maybe some time in the future, hydrogen fuel cells. The scrubbed syngas can also be fed into a Fisher-Tropsch (FT) reactor to produce liquid hydrocarbons such as diesel fuel.”
Source: Green Car Congress
As previously noted, incinerators emit nitrogen oxides, sulfur dioxide, particulate matter, carbon monoxide, carbon dioxide, acid gases, lead, cadmium and mercury, and organic compounds, such as dioxins and furans, into the atmosphere. However, gasification plants’ air emissions also include nitrogen oxides, sulfur dioxide, particulate matter, carbon monoxide, carbon dioxide, methane, hydrogen chloride, hydrogen fluoride, ammonia, heavy metals mercury and cadmium, dioxins and furans. Gasification facilities share the same environmental problems associated with mass burn incinerators. Pyrolysis, using no oxygen, reduces the volume of product gases by a factor of 5 to 20. Furthermore, the pollutants are concentrated in a coke-like residue matrix for easy disposal.
Current improvements in feedstock selectivity, process control, equipment design and modularization, and more efficient catalysts for the Fisher-Tropsch synthesis has significantly lowered the capital cost, enhanced the environmental friendliness and boosted the performance of waste to energy conversion by pyrolysis.
In closing, the solid waste problem is part of live – ancient and modern. The need to control the problems has led to the development of waste management, which has become a service of public and general interest, a basis for civilization, environment control and health protection. The demanding environmental standards have brought to the forefront the need to prevent, recycle and recover prior to disposal. This has increased the valuation of once financially unsuccessful methods to reclaim the stored energy inherent in MSW. Pyrolyis in combination with power producing systems or hydrocarbon synthesis reactors is that game changing process. It is not only sustainable but renewable too. Landfills can breathe a sigh of relief and incineration can now rest in peace.
By. Dr. Barry Stevens
Dr. Barry Stevens has over 25 years of proven international experience building technology-driven enterprises and bringing superior products and services to market ahead of the competition. He is the founder of TBD America Inc., a technology business development group. In this role, he is responsible for monetizing technologies and leading globally-competitive companies to higher levels of revenue, earnings, and growth. Please visit TBD's website at http://www.tbdamericainc.com and his blog at http://barryonenergy.wordpress.com