Introduction

Waste disposal was until recently an easily overlooked problem for several reasons:

Historically, a large percentage of waste was disposed of at the site. This was true of everything from household trash burning and septic fields to large incinerators operated by industrial facilities and hospitals. For instance at one time it was estimated that of the approximately 300 million tons per year of industrial waste generated in the U.S., more than 50 percent was disposed of on site. Increased regulation of waste incineration and changes in lifestyles have resulted in a tendency for waste to be centrally accumulated and processed.

The amount of waste generated per capita has increased. One example is that in Sweden the amount of household waste increased almost 25% in a ten year period. Since waste volume generally increases with commercialization the volume of waste on a global scale is expected to increase exponentially.

As both regulation and scientific knowledge advances, waste treatment procedures are adjusted. A good example of this is the disposal of sludge from municipal wastewater facilities. This material which was at one time routinely spread on crops has been determined in some cases to be inappropriate for that use. (A recent survey indicates that 8.4 million dry tons of municipal sewage sludge are generated annually. Twenty percent is incinerated, 25.4 percent is land-applied, 6.6 percent is disposed of in the ocean, and 46.4 percent is landfilled.)

There are a variety of methods used to handle our waste including recycling, land-filling (with and without gas collection), processing to create biofuels  and incineration.

Most people would agree that the solution to the global waste problem is not disposal, but reduction of waste. However, that and government regulations and incentives affecting waste treatment philosophies are beyond the scope of this report. Globally, incineration is a common waste disposal procedure. Many existing facilities (except in Europe and the U.S.) have ESP equipment for emission remediation. As regulations change fabric filtration is expected to be used in those applications.

Landfill Alternate

Waste to Energy

In almost every case, regulations governing waste disposal and processing are primary drivers of the WTE market. For instance, in the 1970's the Public Utility Regulatory Policies Act which mandated that utilities must pay alternate energy producers their avoided cost of energy allowed the industry to obtain a higher price for their energy than they might have obtained otherwise. This led to an increases in the production of energy from alternative sources. In the early 1990's when Clean Air Act restrictions were enacted the number of waste incineration facilities in the  U.S. was near 1,100. Within a decade more than 90% of the facilities were no longer operating. Although in most cases environmental laws are becoming more restrictive, green incentives and other government subsidies and regulations sometimes affect in the marketplace. There are many waste stream sources including municipal solid waste (MSW), industrial waste, agricultural waste and waste generated by municipal waste water treatment facilities. In addition, existing landfills are being "mined" for valuable waste which might include both materials that can be recycled and materials which might be used as fuel. Methane gas from decomposing products in landfills is used to generate heat or power. Waste tires are a large individual waste stream with more than 1billion tires being disposed of globally each year (approximately 300 million in the US.)

Government subsidies or incentives are often used to encourage the development of processes perceived to be  beneficial to the local or even global welfare. In the case of WTE the increase in legislation and regulation controlling landfills and the related shortage of landfill options has worked in concert with incentives to the WTE industry to focus investment in the WTE industry. For instance, in the U.S., operators of the Lee County WTE facility have been able to sell Renewable Fuel Credits (RFC.) The recent yearly income of approximately $1M has allowed them to process waste competitively with local landfills.

Cap and Trade Regulations Will Impact Both Landfill and WTE Operators

Landfill Cap and Trade

A “cap and trade” system is premised on the fact that the emission of green house gases (GHG) is almost unavoidable (or at least economically imperative) for some industries. At the onset of any program maximum levels (caps) of GHG emission are established for emitters. Those entities that produce excess greenhouse gases and those that destroy or sequester them can “trade” “credits.” The assumption is that some entities can reduce emissions or destroy GHGs more cheaply than others and a free market would encourage resources to flow to the most advantageous GHG reduction technologies or strategies, indirectly financing renewable strategies.

The US House of Representatives passed a “cap and trade” bill in 2009. Although the fate of US cap and trade is unknown the EU and others are already using this type of system and it is expected that the US will eventually adopt some sort of framework for trading carbon credits.

Both the science and the economics underlying this type of legislation is hotly debated. Proponents and critics alike say the carbon credit market could be lucrative. Some analysts predict that the market could be worth as much as $700 billion by 2013.

Voluntary carbon credit trading markets, such as the Chicago Climate Exchange, already exist. The Chicago Climate Exchange trades Carbon Financial Instruments (CFIs.) Since its inception, CFIs have traded for as much as $7 (although the price in early 2010 has generally been in the range of 10 to 15 cents.

California has plans to begin a cap and trade system in 2012. Some members of the Western Climate Initiative, a consortium of Western states including California and Canadian provinces, have shown an interest in developing a regional cap and trade frame-work. (There are indications that the possibility of establishing this sort of potentially expensive, binding agreement has led to fractures of the alliance.)

The degradation of organic waste produces a significant amount of methane. Both landfills and WTE processes destroy GHGs and as such could generate carbon credits. This could create financial incentives for these industries.

For instance, flaring off methane produces water and carbon dioxide. Although CO2 is also a GHG methane absorbs more of the sun's infrared radiation than carbon dioxide. Flaring methane destroys it (ie reduces a greenhouse gas emission) and potentially generates carbon credits that could be sold or traded.

(One CFI equals 100 metric tons of carbon dioxide equivalent. The EPA defines carbon dioxide equivalent as a measurement unit used to compare the emissions from various greenhouse gases based upon their global warming potential.)

 

Regulations Relating to Emissions Will Impact WTE

The increasing cost of complying with environmental legislation has led to an acceleration of the trend to use our trash to generate electricity, provide heat and reduce the production of methane gas which would be created were the trash installed in landfills. Facilities which produce heat and electricity are referred to as Combined Heat and Power (CHP.) Producing heat and electricity from municipal waste has the advantage of providing a fuel source close to the power or heat consumer thus reducing both transportation and transmission costs.

Utilizing MSW as a fuel has several disadvantages. The waste is generally bulky and heterogeneous. Combustion facilities can be either "mass burn" or utilize refuse derived fuel. Combustion facilities can operate more efficiently if they use refuse derived fuel (RFD) which is the material which is produced after waste is processed to recover recyclables (ie metals, glass and cans) and then shredded or pellatized. Some statistics indicate that over one-fifth of US municipal waste incinerators use this type of fuel. The heterogeneous nature of the fuel makes it more difficult to optimize the combustion process. More expensive and advanced air pollution control equipment is required to assure that environmental restrictions are met.

HCl emissions are considered to be a key issue in waste incineration applications. The injection of sodium sorbents has proven to be an effective control technology. As with any injected material, the residence time is crucial for removal. Injection upstream of a baghouse allows the filter cake to serve as a secondary reaction point (the baghouse serves as a reaction vessel) marketedly improving removal efficiency.

Studies by Vogg and Stieglitz (1986) showed that particulate carbon can react with oxygen and inorganic chlorines with copper (II) as a catalyst to form organochlorine compounds including dioxins and furans, and concluded that this de novo synthesis is the primary source of dioxins/furans produced in municipal waste incineration.

Solid Waste Combustors

Types of Incinerators

Commercially available incinerators fall into 3 categories: rotary kilns, grates and fluidized beds. Rotary kilns and grates can accommodate a wider range of waste input but operate at a higher temperature. Fluidized be incinerators require a uniform sized waste (typically RFD) but have better heat recovery and are easier to control when the caloric value of the waste input varies.

A Typical WTE Incineration Facility

 

1. 2. 3. 4. 5. 6.

Tipping Floor RefuseHolding Pit Grapple Feed Chute Feed Chute Stoker Grate Combustion Air Fan

7. 8. |9. 10. 11. 12.

Ash Discharger Combustion Chamber Radiant Zone (furnace) Convection Zone Superheater Economizer

13. 14. 15. 16. 17.

Dry Gas Scrubber Baghouse Fly Ash Handling System Induced Draft Air Fan Stack

Waste Disposal Using Gasification Processes

Gasification is a process that converts carbonaceous materials, such as coal, petroleum, biofuel, or biomass, into carbon monoxide and hydrogen by reacting the raw material, such as house waste, or compost at high temperatures with a controlled amount of oxygen and/or steam. The resulting gas mixture is called synthesis gas or syngas. The syngas is composed of  hydrogen, carbon monoxide and some methane. The syngas is then used as a fuel for the production of heat and/or steam for electricity. In other words, gasification is a method for extracting energy from many different types of organic materials.

As early as the 17th century street lighting and cooking gas were produced in this manner. Currently gasification is used on industrial scales to generate electricity from coal or petroleum (ie. fossil fuels.) The fact that almost any type of organic material can be used as the raw material for gasification makes it a candidate for waste disposal. 

The advantage of gasification is that using the syngas is potentially more efficient than direct combustion of the original fuel because it can be combusted at higher temperatures. Syngas may be burned directly in internal combustion engines, used to produce methanol and hydrogen, or converted into synthetic fuel. In addition, the high-temperature combustion refines out corrosive ash elements such as chloride and potassium, allowing clean gas production from otherwise problematic fuels.gasification itself and subsequent processing neither directly emits nor traps greenhouse gasses such as carbon dioxide.

Combustion of syngas or derived fuels emits the exact same amount of carbon dioxide as would have been emitted from direct combustion of the initial fuel. Biomass gasification and combustion could theroetically play a role in a renewable energy economy, because biomass production could remove the same amount of CO2 from the atmosphere as is emitted from gasification and combustion. While other biofuel technologies such as biogas and biodiesel are carbon neutral, gasification in principle may run on a wider variety of input materials and produce a wider variety of output fuels.

Power consumption in the gasification and syngas conversion processes may be significant, and may indirectly cause CO2 emissions. Gasification relies on chemical processes at elevated temperatures >700°C. Typically the elevated temperatures are produced using electrically powered plasma heating elements.Several of the gasification processes currently available use more energy than is produces (therefore using them to destroy hazardous waste is logical; using them for pwer production does not make economic sense.)

Gasification uses the principles of combustion, but the conditions are controlled so that they remain sub-stoichiometric, that is to say one part of the reaction is at a level less than required for the perfect reaction. In the case of gasification, the oxygen levels are kept purposefully low so that the combustion reaction only gets to the volatilization stage where combustible gases are produced.

The low level of oxygen ensures that these gases cannot combust themselves to complete the combustion reaction where all energy in the waste materials is then spent. The syngas generated can then be tapped off and used as a fuel.

A $225M facility planned by Alliance Federated Energy of Milwaukee could be the first commercial waste disposal facility using  gasification technology in the U.S.. The 25-megawatt project would go online in 2013, and would deploy a plasma gasification technology developed by Westinghouse Plasma Corp.

A planned facility in the UK will use gasification

Pyrolysis

Like gasification, pyrolysis is a thermal reaction with restricted air or Oxygen. Gasification, however, is exothermic and requires a higher operating temperature. Pyrolysis (with no added O or air) is endothermic;  lower temperatures avoid the production of dioxins. Both processes are currently used for hazardous waste.

Refuse Derived Fuel

Most facilties use Refuse Derived Fuel (RDF.) RDF is typically made by compacting combustible products such as papers and plastic into "balls" approximately 3/4 inch in diameter. A typical waste recycling facility produces 55% by weight RDF (ie the other 45% of waste is either recycled or inappropriate for RDF use.)
    Approx 4,800 kcalories per 1kg (2.2 pounds) of waste.
    Approx 1kg of waste  is equivalent to 500 liter (132 gallons) of petroleum.
    Approx 1 ton of RDF to produce 1 MW power.

Hospital Waste Incineration

Hospital Waste Incineration is a specialized field due to infectious and hazardous components of the waste.
Hospital Incineration

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