Civil Engineering Research

Dr. Howell H. Heck, III

Dr. Heck's research activities are focused in two distinct areas, solid and hazardous waste management, and water quality in lakes and reservoirs. In the area of waste management, he has investigated the composition, energy content, disposal rates, recycling rates, anaerobic degradation and composting of solid waste. He has also directed numerous assessments on contamination of underground storage tank facilities.

By Howell H. Heck and Marwan E. Jubran

The early experiences with MSW pyrolysis processes in the United States were disappointing. Development of the technology can be envisioned as an endless number of technical and mechanical problems and short- and long-term shutdowns. In most cases, redesign or modification of the system solved the problem, but other problems continued to appear. Even during periods when the plants were operating, there were problems in developing markets for the products produced. All of these factors worked together to increase processing costs to the point that other management options were chosen. Unlike pyrolysis, gasification has continued to be developed. Most of the work has occurred outside of the US. Pilot-scale gasification has been proven viable in other countries for processing MSW. Commercial facilities are being built but have not proven themselves as a viable alternative to incineration

The economic viability of any thermo processing technique within the US will continue to be affected by three major factors. First, the lack of control over how MSW will be disposed of within any region (commonly referred to as flow control); second, the availability of cheap landfill space; and third, the low price that power companies will pay for electricity. Other factors are also important, such as the recycling policy, MSW/refuse-derived fuel (RDF) availability, heat and power prices, energy demand, fossil-fuel prices, pyrolysis product markets, air-emissions requirements, and disposal costs for different types of residues. Outside the US, stringent landfill regulations, high disposal costs, lack of space for siting new landfills, and a concern for ash toxicity have forced the solid waste managers to search for alternatives to incineration and land filling.

MSW pyrolysis and gasification have been investigated in the US since the 1960s, and laboratory research continues to this day. A similar process is actively being developed in Europe. Numerous MSW pyrolysis pilot plant facilities were constructed in the US and a few commercial-scale facilities were built. The last commercial facility closed in the early 1980s. During the 1980s and throughout the 1990s, pilot- and commercial-scale facilities were constructed in Europe and Japan, and they are still being built today.

Regulatory and economic differences between the US and other countries will be considered to explain why these thermo processing techniques are being developed abroad but not in the US and if future development within the US is likely.

Pyrolysis

Pyrolysis and thermolysis, commonly referred to as destructive distillation, are defined as an irreversible chemical change brought about by the action of heat in an oxygen-deficient, inert-gas environment. An oxygen-free atmosphere in the pyrolytic chamber is hard to achieve; therefore, an oxidation zone may exist. Oxygen bound in the refuse and from the air reacts with solid carbonized fuel, producing carbon monoxide. The principal reactions that take place in the oxidative zone follow. Plus and minus signs indicate the release and demand, respectively, of heat energy during the process.

C + O₂                     Þ            CO₂ + 406 [MJ/kmol]

Also, hydrogen in the feedstock material reacts with oxygen present in the system, producing steam.

H₂ + ½O₂                Þ            H₂O + 242 [MJ/kmol]

In the reduction zone, a number of high-temperature chemical reactions take place in the absence of oxygen. Reduction reactions show that heat is required during the reduction process, causing a drop in the gas temperature. The principal reactions that take place in reduction are listed below:

Boudouard reaction: C + CO₂                            Þ            2CO - 172.6 [MJ/kmol]

Water-gas reaction: C + H₂O                             Þ            CO + H₂ -131.4 [MJ/kmol]

Water-shift reaction: CO₂ + H₂                           Þ            CO + H₂O + 41.2 [MJ/kmol]

Methane-production reaction: C + 2H₂                Þ            CH₄ + 74.6 [MJ/kmol]

The pyrolysis process requires temperatures ranging from 752°F to 1,652°F. Pyrolysis systems use a source of heat to drive the endothermic pyrolysis reactions in the complete absence of oxygen. The only difference between pyrolysis and thermolysis is that the former employs a direct heat source within the reactor (retort), while the latter employs an indirect external source of heat to the reactor (retort). MSW can be reduced in weight by 90% to solid char, municipal plastic refuse by 80%, and industrial wastes by 65% (Sanner et al., 1970). Solid biomass and wastes are difficult and costly to manage, while liquids, char, and gas from the pyrolysis process have advantages in transport, storage, combustion, retrofitting, and flexibility for production and marketing. In transport, bulk density is important; oil and slurry mixtures have a clear advantage over feed materials. Storage and handling might be important because of seasonal variations in production and demand. Crude biomass, such as wood chips and straw, will deteriorate during storage as a result of the biological degradation process. Char, however, is very stable.

Generally, liquid and gas products are easier to handle in a combustion process. Coal-fired furnaces can easily accept char as a partial fuel replacement as long as the volatile content is compatible with the furnace design. Existing oil-fired burners may be fueled directly with liquid or gaseous pyrolysis products without major reconstruction of the unit, which might not be attractive in unreliable fuel markets.

Most organic substances are thermally unstable; under pyrolysis conditions they can be split through a combination of thermal cracking and condensation reactions into gaseous, liquid, and solid fractions. The pyrolysis of organic material causes the volatile fraction to distill, forming combustible liquids and vapors. The vapors are composed primarily of methane (CH₄), hydrogen (H₂), carbon monoxide (CO), carbon dioxide (CO₂), water (H₂O), and more complex hydrocarbons, such as ethane (C₂H₆), propane (C₃H₈), oils, and tars. The exact components in percent composition of these gases formed by pyrolysis of either MSW or any other feed stream cannot accurately be predicted in a real system. The complex multi component fractions could be converted to more stable gases, such as ethylene (C₂H₄), through a continuing pyrolysis action and are the result of complex kinetic reactions from variations in time and temperature. No matter how pyrolytic reactors are built, there always exists a low-temperature zone within the pyrolysis reactor generating condensable hydrocarbons.

Gasification

The gasification processes involve partial combustion of a carbonaceous biomass in the reactor to generate a combustible-fuel gas rich in carbon monoxide, hydrogen, and some saturated hydrocarbons, primarily methane. Gasification is achieved in the presence of heat and a limited supply (less than stoichiometric) of oxygen (O₂). When air (as opposed to pure oxygen) is used as the gasifying agent, nitrogen (N₂) becomes a part of the product gas. Gasification is a high-temperature (1,650°F-2,200°F) process that is optimized to produce a fuel gas with a minimum of liquids and solids. The high temperature is partially achieved by heating the gasifying agent before it enters the gasification chamber. The char produced is actually a vitrified ash with a low carbon content. During the gasification process, five principal reactions occur as follows, where the heat to sustain the process is derived from the exothermic reactions.

Feedstock Properties. Important feedstock properties include energy content, water content, ash content and composition, particle size, bulk density, volatile-matter content, and reactivity. Material with high-energy content is always better for pyrolysis. Wood and agricultural wastes have heating values in the range of 4,500 Btu/lb. to 10,000 Btu/lb. (10 MJ/kg to 22 MJ/kg), whereas fossil fuels possess higher heating values as shown in Figure 1. The moisture content of biomass fuel depends on the type of refuse and its origin and treatment prior to the pyrolysis process. Moisture content below 10% by weight is desirable (Bridgwater and Grassi, 1991). Material with a high moisture content is difficult to ignite, reduces the thermal efficiency of the process, and results in a low product yield and poor gas and liquid quality. Moisture content can be controlled through drying at low temperatures between 212°F and 230°F (Bridgwater and Grassi, 1991), whereas high temperatures may cause changes in the crystallinity of cellulose, lignin degradation, and volatile losses that can affect the yield and composition of the products from thermal treatment. Particle size and distribution of the material affect the pressure drop across the reactor and the power that must be applied to draw the gas out of the reactor. Excessively large particles have a low reactivity, can cause startup problems, and produce a poor gas quality. Bulk density varies significantly with the material, moisture content, and particle size. Bulk density affects the volume occupied by stored material and has considerable impact on gas quality because it influences the refuse velocity, its residence time in the firebox, and the gas flow rate. In general, no slagging occurs with refuse having ash content below 5% (Bridgwater and Grassi, 1991). Reactivity determines the rate of many reactions within the reactor and can be improved by using catalysts (e.g., potassium, sodium, and zinc).

Feedstock Processing. Typical pretreatment processing steps that control the physical characteristics of the processed raw material are liberation and screening to remove contaminants, shredding to reduce particle size, magnetic separation to remove conductors, classifying to refine, drying to increase the calorific value, and pelletizing to obtain homogeneity. Each of these operations may vary in sequence and has a capital and operating cost associated with it. Generally, all pelletizing systems require feed moisture-content levels below 15%-preferably below 10%-to achieve a stable, dense product that prevents temperature fluctuations and reduces a pressure drop inside the reactor (Ferrero et al., 1989).

Reactor Types. Reactors that have been widely used in pyrolysis include vertical shaft (e.g., fixed-bed updraft, downdraft, twin fire, and cross draft), rotary kiln, fluidized and dual fluidized, and multiple hearth.

Vertical shaft reactors are simple and have a relatively low cost, but they are sensitive to the mechanical characteristics of the refuse. They require a uniform, homogenous fuel, such as densified RDF. They have a potential of achieving low air-pollution emissions with simplified air-pollution devices.

Although several types of reactors have been used in the pyrolytic conversion of MSW, approximately 70% of the cumulative, worldwide MSW pyrolysis systems utilized vertical shaft reactors between 1969 and 1988. Fixed-bed designs accounted for 50% of vertical shaft reactors, followed by dual fluidized-bed systems and entrained beds (Gupta and Shepherd, 1992).

Gas Thermoprocessing produces primary vapors first, the characteristics of which are most influenced by the heating rate. These primary vapors then degrade to secondary tars and gases, the proportions and characteristics of which are a function of temperature and time. The product gas from pyrolysis or gasification is usually a fuel with a medium to low heating value. The product gas contains carbon monoxide, which varies from 15% to 30% by volume. Carbon monoxide possesses an octane number of 106, but its ignition speed is slow. Carbon monoxide is toxic in nature and requires proper handling. Hydrogen is also a part of the product gas and varies from 10% to 20% by volume. Hydrogen possesses an octane number of 60-66, which increases the gas ignitability. Methane constitutes 2%-4% by volume and adds to the heating value. Noncombustible gases, such as carbon dioxide and nitrogen, constitute a large part of the product gas. Carbon dioxide varies from 5% to 15%, and nitrogen  from 45% to 60% by volume. Higher percentages of carbon dioxide indicate incomplete reduction of the waste material. Water vapor is also a part of the product gas, and its quantity varies from 6% to 8% by volume. Water vapor occurs because of the moisture content of air trapped in the refuse, the injection of steam, or the moisture content of the feed material.

The gas product is contaminated with particulate matter and condensable vapors. Particulate-matter removal is required unless the gas is utilized in the pyrolysis reactor or some other gas combustion chamber as a fuel source. Preliminary calculations indicate that about 2 M Btu might be required to pyrolyze 1 ton of municipal refuse at 1,652°F. With approximately 8 M Btu available from the gas produced per ton of refuse, more than enough gas is available to sustain the gas requirements of the process (Sanner et al., 1970). The highest energy efficiency occurs when the gas is used hot, because the sensible heat from the pyrolysis process represents a significant amount of the product-gas energy content; therefore, it should have markets that are nearby. Markets for the gas can be established for use in boilers, furnaces, kilns, or dryers. If the gas needs to be compressed to be transported by pipeline, the condensable vapors must be separated from the gas product by cooling, and an enormous loss of energy occurs in the form of sensible heat.

Liquids The pyrolysis liquids consist mainly of tar, light oil, and liquor. The tar contains from 16% to 25% olefins, 62% to 80% aromatics, and 3% to 14.5% paraffins and naphthenes, and the remainder is organic compounds that have been identified as acids, bases, ketones, and aldehydes containing from one to eight carbon atoms. The major components of light oil are benzene and toluene. The pyrolysis liquor is generally 90%-97% water (Sanner et al., 1970).

A high level of char and ash carryover can be assimilated in the liquid product, where particulate sizes and proportions can influence the product quality. Care must be taken in storage, handling, combustion, and upgrading of pyrolysis liquids because of the water and high oxygen content. Polymerization of the liquid product can be caused by temperatures around 212°F. Polymerization adversely affects physical properties such as viscosity, phase separation, and deposition of a bitumenlike substance. Heating the liquid to reduce its viscosity for pumping needs to be considered cautiously. Exposure of the pyrolysis liquid product to air during storage or transportation may cause deterioration of its chemical and physical properties. The water content in the liquid product is important and has several effects. It reduces the heating value, affects the pH, reduces the viscosity, and influences both physical and chemical stability. A high water content in the liquid-product stream will affect the refining and upgrading process and increase the wastewater generated during these process steps. The generated wastewater is highly contaminated by dissolved and suspended organics with typical chemical oxygen demands of 150,000 mg/lit. The wastewater will need to be treated and disposed of, thus increasing the operating cost. Pyrolysis liquids require additional processing to be directly assimilated into a conventional fuel-marketing infrastructure. This would allow uses such as synthetic oil for boilers and power stations, synthetic oil as refinery feedstock, and synthetic oil for gas turbines and modified diesel engines. Pyrolysis liquids can be mixed, without upgrading, into bulk fuels by fuel blenders and sold in conventional markets. Conventional markets are preferred because they allow multiple customers.

Solid Residue The solid residue is a lightweight, flaky char that can be readily sieved to remove extraneous materials such as bottle caps, tin-can lids, and aluminum. Processed municipal refuse containing mainly plastics yielded a char with the highest fixed carbon (56.7%) and a heating value of 17.7 M Btu/ton. Industrial refuse produced a char with the lowest fixed carbon (9.7%-17.0%) and heating values ranging from 3.3 M Btu/ton to 5.8 M Btu/ton (Sanner et al., 1970). The metals content of the char are similar to ash produced by an RDF incinerator, but the metal concentrations are less because of large amounts of unburned carbon. The metals leachability is unknown. The char from pyrolysis is very stable and will not deteriorate, although it can absorb moisture. Some uses of the solid residue are as a fuel, a landfill cover, an oil conditioner, and a filter medium. Tests have shown that the residue from municipal waste can be readily briquetted with a starch binder. These briquettes ignite easily and burn under normal atmospheric conditions. A solid residue with high char content may allow its use as a fuel in boilers and furnaces, or it can be utilized as an external heating source in the pyrolysis process. Char-based slurries such as charcoal-heavy fuel-oil mixture, charcoal-domestic fuel-oil mixture, charcoal-water slurry, and charcoal-mineral coal-water slurry may permit markets for boilers and power stations. Unacceptably high viscosity will limit the amount of char that can be introduced into oils or water. The residue is adequate for filtering sewage sludge and could be used as a filter medium for removing organic substances from the wastewater produced during upgrading (Sanner et al., 1970). The organic compounds present in the liquor can be absorbed on the char for disposal by combustion. Activating the carbon in the solid residue will create a market in adsorption packing.

Application of pyrolysis to MSW in the US is limited to a few companies. The pyrolysis processes that were demonstrated during the 1970s and early 1980s include the Union Carbide process, the Andco-Torrax process, the Monsanto Landgard process, the Occidental Petroleum Liquefaction process, and the Waste Distillation Technology process. All US attempts to apply the pyrolysis technology to MSW failed to achieve acceptable technical or economic performance, and the facilities have been shut down.Worldwide activities in MSW pyrolysis diminished in the late 1980s and the 1990s, and the interest in pyrolysis technology has been replaced by gasification. There may have been only six commercial-scale facilities operating worldwide as of 1998. These include the 400-tpd Andco-Torrax system in Creteil, France; the 450-tpd Nippon Steel system in Ibaraki, Japan; the 200-tpd TPS fluidized system in Greve, Italy; the 20-tpd Pyroflam system in Budapest, Hungary; the 100-tpd Thermoselect system in Verbania, Italy; and the 120-tpd Kiener-Siemens system in Fürth, Germany. A summary of previous and recent facilities that have been constructed worldwide, some of which are currently operating, is presented in Table 1. A brief discussion of each process follows.

Table 1. Previous and Recent Pyrolysis Facilities Worldwide

Process	Plant Size	Location	Duration of Operation
Danish Destrugas	5 tpd	Kolding, Denmark	1967
Danish Destrugas	5 tpd	Kalundsborg, Denmark	1970
Andco-Torrax Systems	75 tpd	Orchard Park, NY	1971-1977
Monsanto Enviro-Chem Systems	35 tpd	St. Louis, MO	1972
Occidental Petroleum Company	4 tpd	La Verne, CA	1972
Monsanto Enviro-Chem Systems	1,000 tpd	Baltimore, MD	1973-1979
Union Carbide	180 tpd	South Charleston, WV	1974-1978
Occidental Petroleum Company	200 tpd	El Cajon, CA	1975-1979
Danish Destrugas	5 tpd	Japan	1976
Danish Destrugas	1 tpd	Berlin, Germany	1978
Andco-Torrax System	400 tpd	Creteil, France	1979-1998+
Nippon Steel	450 tpd	Ibaraki, Japan	1980-1998+
Union Carbide System	150 tpd	Chichibu City, Japan	1981-1988
Andco-Torrax Systems	100 tpd	Orlando, FL	1982-1983
Waste Distillation Technology	50 tpd	Elmwood Park, NJ	1982-1984
TPD Tsukishima Kikai System	450 tpd	Funabashi City, Japan	1983-1990
Kiener-Siemens	72 tpd	Goldshöfe, Germany	1984-1987
Kiener-Siemens	5 tpd	Ulm-Wibinsen, Germany	1988-1996
Voest Alpine	72 tpd	Linz, Austria	1991
TPS Fluidized System	200 tpd	Greve, Italy	1993-1998+
Kiener-Siemens	24 tpd	Yokohama, Japan	1994-1996
Nippon Steel	300 tpd	Ibaraki, Japan	1996+
Pyroflam System	20 tpd	Budapest, Hungary	1996-1998+
Nippon Steel	120 tpd	Iryu-kumiai, Japan	1997*
Nippon Steel	130 tpd	Kagawatobu-kumiai, Japan	1997*
Kiener-Siemens	120 tpd	Fürth, Germany	1998+
Nippon Steel	180 tpd	Iizuka, Japan	1998*
Thermoselect	100 tpd	Verbania, Italy	1992+
Nippon Steel	150 tpd	Ibaraki, Japan	1999*