COMPARATIVE EXPLANATION OF
GASIFICATION AND INCINERATION

Introduction and Background

Biomass waste is a term for such material as agricultural and forest byproducts (rice hulls, cotton stover, sugar cane bagasse, wood bark and saw dust, etc.) and the sorted, non-toxic, burnable portion of municipal solid waste.  These waste streams represent both an asset, when used as energy, or an environmental liability, when improperly or overly disposed.  The energy potential represented by biomass waste is estimated at 2,740 quadrillion British Thermal Units (Btu’s), while the total annual world consumption of energy from all sources is estimated at 340 quadrillion Btu’s.  At present, only about seven percent of the world’s production of biomass is converted into energy.  Disposition of these biological products in capacity-limited landfills creates a myriad of adverse environmental impacts; atmospheric pollutants from landfill decay, ground and surface water contamination and uncontrolled bacteriological and algae growth.

This paper will focus on the beneficial use of a single biomass waste steam, the burnable, non-toxic portion of municipal solid waste or MSW. 

In reaction to the energy crisis of the mid-70's, several “trash to energy” facilities were constructed both in the United States and internationally.  Most of these facilities were designed with an energy conversion process called “mass burn” or “mass trash”.  Under this approach, all the material, which is received from the route trucks that picked up the MSW, is dumped into a reaction chamber.  The “mass trash”, including potentially recyclable materials, is rapidly combined with air, or incinerated, within a single reaction chamber of differing design. 

During the past several years, a global concern has been raised regarding the potentially harmful effects from the process of incineration. These warning flags of concern have been waved by such politically active groups as Green Peace, World Wild Life Association, the Audubon Society, etc. Perhaps, with a better understanding of the environmental pollutants, the chemical constituents, formation mechanism, methods for prevention of their formation and a basic understanding of oxidative chemistry will educate a more receptive audience for an improved methodology for the energy conversion of MSW.

The topic of energy conversion of biomass is somewhat complex and requires some basic understanding of chemistry and chemical interaction with the atmosphere, plant and animal life. Before we address the distinction between incineration and gasification, we will attempt to give a very brief explanation of fundamental oxidative chemistry, incineration, and environmental pollutants

Basic Oxidative Chemistry

Most people say that they do not understand chemistry, and that their only exposure was sitting through a fundamental course in high school.  And, although this may be your only exposure, you practice oxidative chemistry every day.

Oxidative chemistry refers to the combination of oxygen with a substance.  The air that we breathe contains oxygen.  When oxygen combines at low rates with metals, such as iron or copper, we call the reaction “rusting”.  When you breathe air, the oxygen combines rapidly with the food that you have eaten to produce heat and energy.  Oxygen laden air combined with gasoline releases energy in the form of heat and light, which cause the gases to rapidly expand, moving a piston, which propels your car.

All biomass waste is mostly composed of carbon, hydrogen and oxygen (proteins, sugars, cellulose, carbohydrates, etc.).  When these materials are combined with oxygen, the oxidative reaction produces carbon dioxide (CO2) and water (H2O), releasing energy in the form of heat and light.

In incineration processes, the primary objective is the destruction of the feed.  All of the oxidation or combustion takes place rapidly and, typically, in a single chamber.  Unfortunately, biomasses contain chemicals in addition to carbon, hydrogen and oxygen and air is comprised of both oxygen and nitrogen.  These additional elements produce side and competing reactions.  It is these competing reactions that produce environmental contaminants and because all of the reactions take place rapidly and in one chamber, the products of combustion are “fixed”.

Environmental Pollutants/ Air Contaminants

In an incineration reaction zone (that area in which the feed compounds are placed in intimate contact with oxygen to destroy them) several competing reactions will occur, as we stated previously.  Nitrogen enters the reactor with the oxygen-laden air.  At elevated temperatures, the nitrogen can combine with oxygen to produce NOx (oxides of nitrogen). Sulphur, chlorine and other trace chemicals enter the reaction with the biomass.  Sulphur will combine with oxygen to produce SOx (oxides of sulphur).  Chlorine and unburned hydrocarbons can produce dioxins and furans.   If MSW is unsorted, heavy metals, such as mercury (Hg) and lead (Pb) can be vaporized if exposed to the high temperature of the reactor.  Competing reactions can prohibit the complete oxidation of hydrocarbons and produce hydrocarbon gases in the exhaust (VOC’s or NMHC) and partially oxidized carbon or carbon monoxide (CO). 

All of the preceding (NOx, SOx, NMHC, CO, Hg, Pb, dioxins and furans) along with particulate matter  (PM) or soot are controlled by the US EPA and other nation’s environmental regulatory bodies with the intent of maintaining or, more preferentially, restoring our global atmosphere

The rationale behind the control of these particular chemical compounds is:

NOx and NMHC act as catalysts in the presence of sunlight to form ground level ozone, which is an oxygen molecule comprised of three atoms of oxygen instead of the diatomic or two-atom oxygen.  All air-breathing organisms, require the diatomic oxygen to run their own internal combustion engines, and cannot use ozone resulting in ozone warnings or Federal ozone control laws.  If you breathe an atmosphere containing no diatomic oxygen you will suffocate.

The general public may be confused between ground level ozone and stratospheric ozone.  The ozone molecule absorbs ultraviolet light, causing the chemical bonds to vibrate or resonant in the blue spectrum.  This is why the sky is blue.  When we speak of ozone depletion, we speak to the reduction in the amount of stratospheric ozone.  This reverse chemical reaction, from the tri-atomic ozone to the diatomic oxygen, occurs when ozone is exposed to chlorinated or fluorinated hydrocarbons (CFC’s).  This is why Freon was phased out as a refrigerant.

SOx, NOx and chlorine can combine with water (humidity) in the atmosphere to produce sulphuric acid, nitric acid and hydrochloric acid respectively.  Eventually, these acids will fall as acid rain, destroying vegetation and polluting water.

CO, as most are probably aware, is a poison.  Normal concentrations of carbon dioxide in air are non-reactive with body chemistry.  If you breathe these normal concentrations of CO2, they are harmlessly exhaled.  However, if you breathe CO, the body thinks that it is inhaling oxygen, the blood grabs onto it instead of oxygen.

PM, particulate matter makes for smoky cities, black buildings and lungs.

Dioxins and furans are toxic.  Probably the most widely known dioxin is Agent Orange, which, unfortunately, was used as an exfoliate by the US government during the Vietnam War.  Only after exposing both the US troops and citizens of Vietnam to Agent Orange did we become aware of the toxic nature of dioxins.

Lead and Mercury vapor are neurotoxins, blocking or killing the nervous system.

This is not a complete list of the pollutants controlled by environmental law, but represents the most common compounds of concern.  As we live in an energy intensive, industrial age, these pollutants can and are produced in amounts sufficient to negatively impact our global environment.

Conventional Mass Burn Incineration and Air Pollution Control

Traditional methods of municipal solid waste disposal have been in landfills or mass burn incinerators. In a mass burn incinerator, the un-segregated MSW is combined with oxygen to convert the burnable portion into energy.  As the MSW has not been segregated into a burnable and non-burnable portion, much of the potential energy available in the waste is spent heating the non-burnable portion.  Further, as the MSW is processed as received from the collection vehicles, the potential energy available from load to load varies dramatically, conceptually from bricks to oily rags.  To compensate for this wide fluctuation in potential energy, mass burn facilities add fossil fuel (diesel, natural gas, propane, etc.) in an attempt to control the temperature of the reaction chamber.

If you recall from the previous discussion, incineration is rapid combination of the biomass feed with oxygen within a single chamber.  Since in mass burn incineration the MSW is unsorted, the number of reactions competing with the energy conversion reaction is increased.  Further, there are present in the unsorted waste a number of potential environmental pollutants; mercury from mercury batteries, lead from lead acid batteries, hazardous industrial waste, and energy consuming non-burnable material. 

The products of combustion that exit the incinerator and were fixed with the incinerator, with its constantly changing reactions and oxygen requirements, are also constantly changing in the amount and content of the atmospheric pollutants.  To be in compliance with environmental restrictions, several post-incineration air pollution control (APC) devices are employed.

Acid gas scrubbers, either dry or wet, to remove the SOx and chlorides from the vent steam, making an acidic water in wet scrubbers that must be subsequently treated to remove the acid, or gypsum in dry scrubbing,

Cyclonic separators, to remove large particles from the exhaust,

Bag houses or electrostatic precipitators to remove small particulate matter from the vent, collecting the soot as dust,

Afterburners or, as in your automobile, catalytic converters, to burn (oxidize) any residual hydrocarbons.

Activated carbon injection to remove heavy metals (mercury, lead, chrome, etc.), dioxins and furans.

Catalytic reduction of NOx with ammonia injection equipment.

The cost of the APC equipment designed to comply with industrial exhaust restrictions in the US can exceed the cost of the energy conversion equipment, and our (US) laws are not as restrictive as some European laws. 

The deficiencies in the ability to control the oxidation and feed quality of mass burn MSW, the limitation of incinerator to a rapid oxidation, and the high cost of multiple air pollution control devices, we believe are the wrong underlying assumptions of conventional trash to energy facilities. 

Our approach is to correct these deficiencies through the application of current technologies; materials recovery facilities and biomass gasification.

The Proposed Solution

Materials Segregation

The energy conversion process begins with the separation and sorting of the bulk MSW in a Materials Recovery Facility (MRF).  Within the operations of the MRF, recyclables, nonflammable or potentially hazardous materials are segregated from the waste stream.  The recyclable materials, plastics, paper, aluminum, glass, copper, etc., are returned to centers to be recycled.  The remaining combustible, or burnable, material is referred to as Post Recycle Municipal Biomass.  The segregation technology was developed within the last ten years to increase recycling of valuable wastes and produce a renewable, efficient and environmentally friendly alternate fuel.

Gasification

Gasification is a technology that has been widely used in commercial applications for more than 50 years in the production of fuels and chemicals. Current trends in the chemical manufacturing and petroleum refinery industries indicate that use of gasification facilities to produce synthesis gas (“syngas”) will continue to increase. Attractive features of the technology include: 1) the ability to produce a consistent, high-quality syngas product that can be used for energy production; and 2) the ability to accommodate a wide variety of gaseous, liquid, and solid feedstocks. Conventional fuels such as coal and oil, as well as low- or negative-value materials and wastes such as petroleum coke, heavy refinery residuals, secondary oil-bearing refinery materials, municipal sewage sludge, hydrocarbon contaminated soils, and chlorinated hydrocarbon byproducts have all been used successfully in gasification operations.  Gasification of these materials has many potential benefits when compared with conventional options such combustion or disposal by incineration.

In a position paper entitled Powering Ahead: A New Standard for Clean Energy and Stable Prices in California, the Union of Concerned Scientists supports the deployment of “high-value technologies” for power generation including biomass gasification.  Recently, the U.S. Environmental Protection Agency (EPA) announced that the Agency is considering an exclusion from the Resource Conservation and Recovery Act (RCRA) for listed secondary oil-bearing refinery materials when processed in a gasification system. 

Following the sorting, sizing and drying of the PRMB, this engineered alternate fuel is fed into a gasification process.  Unlike mass burn technology, the production of potentially harmful pollutants is significantly minimized in the gasification process as that total burning or oxidation of the RDF does not occur in the gasifier.  Within the gasifier, process conditions are maintained to produce a combustible synthesis gas or syngas.  With lower operating temperatures, most of the potentially harmful air pollutants remain in the discharged solid ash. 

The hot combustible synthesis gas or “syngas” evolved in the gasification process is then oxidized in a series of staged for the proactive control of NOx.  In the final oxidative step, the syngas is oxidized in a boiler where the energy is recovered as steam.  High pressure, superheated steam is directed to a condensing turbine that ultimately drives an electrical generator.  Unlike incinerator technology, the boiler is a standard gas fired boiler, and neither constructed of exotic alloys nor exorbitantly expensive.

Environmental Benefits

This process approach is not mass burn incineration in which the MSW is incinerated in the same, un-segregated physical form as collected and delivered.  Mass burn technologies are energy inefficient, and as the feed is constantly changing, the products of combustion are constantly changing, requiring elaborate and expensive air pollution control equipment.

Remnant ash from the gasifier may be used in the manufacture of cement or disposed in an engineered landfill will be biologically inert and disease free, eliminating the anaerobic production of odoriferous landfill gas and leachate.

In this process design, aqueous process discharges are eliminated.

The evolved syngas is remarkably constant in chemical composition and potential energy, reducing the complexity of control of the oxidation reaction in the boiler.

Utilizing PRMB from municipal solid waste as a primary fuel will eliminate thousands of tons per year of carbon dioxide that would have been released from the burning of fossil fuel in the creation of an equal amount of energy. 

Economic Benefits

Recyclable materials are recovered from the MSW, producing additional operating income and the return valuable resources into the economy.

This beneficial use of renewable, sustainable domestic waste will annually displace million of gallons of imported oil.

The monies that would have been spent with foreign countries to import oil will be spent domestically in salaries, taxes and funds for other domestic goods and services.

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