What if the mountains of plastic wastes that blight our landscapes and beaches spewing poisons from incinerators and landfills could be transformed overnight into combustible gas and diesel oil. Dr. Mae-Wan Ho
As the price of oil and gas soar, alternative energy sources are rapidly becoming cost-effective by comparison. One attractive option that has emerged is diesel oil from waste plastics.
The first report of turning plastic wastes into oil came in 2001 from the People’s Daily, China’s English language newspaper . An oil refinery in Hunan province had succeeded in processing 30 000 tonnes of plastic wastes into 20 000 tonnes of gasoline and diesel oil that satisfied the provincial standards. Wang Xu, who built the refinery in 1999, started experimenting with waste plastic processing in the 1980s, and later teamed up with Hunan University doctoral tutor Zeng Guangming who gave him scientific advice on decomposing plastic wastes. This may be one reason why China has been importing enormous amounts of plastic wastes (“Redemption from the plastic wasteland”, this series).
Although no details were given on the technology used, it is most likely based on thermal depolymerization, a process for breaking down organic wastes under heat and pressure into light crude oil, which has been studied in the West since the 1970s . It mimics the natural geological processes thought to be involved in producing fossil fuels. Under high pressure and heat, long chain polymers of carbon, hydrogen and oxygen decompose into short-chain petroleum hydrocarbons in a matter of hours. Until quite recently, however, the artificial process was far from energy-efficient, as more energy had to be put in than was produced, and the product, a crude oil, was also full of impurities.
In the 1980s, Illinois microbiologist Paul Baskis in the United States modified the process to produce a lighter, cleaner oil, but failed to convince investors until 1996, when a company called Changing World Technologies began development with Baskis to make the process commercially viable .
Changing World Technologies opened a demonstration plant in Philadelphia, Pennsylvania, and in 2001, the first full-scale plant was built in Carthage, Missouri, and the company applied to patent a “thermal conversion process” (TCP) for converting organic wastes, such as pig manure, into oil and other products.
The first pilot TCP plant was built to treat turkey offal, which it succeeded in converting to diesel fuel, along with fertilizer and absorbent carbon. The full-scale plant, located in Carthage, Missouri, can process up to 191 tonnes of turkey wastes a day .
(The TCP plant looks like a small refinery operation, and it is likely that, given the earlier report from China, existing oil refineries could easily be converted into TCP plants.)
Processing is divided into two main stages. In the first stage, the turkey waste is pulped into a slurry and heated at a pressure of 40 bar (1 bar ~ 1 atmosphere) to 200-300C. The solids are separated and the liquid is ‘flashed’ to a lower pressure to separate the oil from water. The oil is then heated in a second stage reactor to a higher temperature around 500C to ‘crack’ it into light hydrocarbon oil, leaving a solid product. Recovered from the first stage are solid minerals and a liquid concentrate rich in nitrogen and other nutrients. From the second stage, fuel gas, carbon, and diesel oil are recovered. The fuel gas produced is a mixture of methane, carbon monoxide, carbon dioxide and low molecular weight hydrocarbons. The oil contains predominantly straight chain hydrocarbons with a chain length between 15 and 20.
One advantage of TCP is that it claims to break down the prion proteins associated with mad cow disease - which survives normal boiling or autoclaving - and is therefore suitable for treating slaughterhouse wastes, disinfecting the wastes at the same time that biodiesel, gas fuels and fertilizers are produced. However, no evidence was presented for this claim.
The process also appears to be energy efficient and environmentally friendly in a lifecycle audit . The audit did not include energy and carbon emission costs involved in building the TCP plant, however.
From an input of 191 tonnes of wet turkey offal per day (50 percent moisture) plus 3.0 tonnes of sulphuric acid and 91.2MJ of grid electricity, the output are 2506 GJ of diesel oil, 274GJ fuel gas, 30.6 tonnes liquid nitrogen fertilizer, 7.5 tonnes mineral fertilizer, 6.1 tonnes of carbon and 79.9 m3 waste water.
There are no discharges to the atmosphere from the plant during the processing. The only gaseous product is the medium to high heat-content fuel-gas (heating values between 9 and 19 MJ/m3) used for heat in processing, or as fuel for a boiler or turbine. Emissions from the turbine have been independently verified to be in compliance with the Clean Air Act. The oil product is typically a light hydrocarbon similar to diesel fuel that could be used as heating oil or converted into higher value products.
There are two types of fertilizers/soil amendments produced by the TCP: Res minerals and Res liquid concentrate . Both are produced from food and agricultural wastes such as turkey offal, feathers, bones, pig manure, used cooking oil and slaughterhouse waste. Res minerals consist of N, P K, and Ca, representing nearly 30 percent of the total fertilizer, the rest is made up of organic material such as carbohydrates, amino acids, fatty acids and moisture (40 percent).
The Res liquid concentrate is a mixture rich in nitrogen that also contains phosphorus, potassium, sulphur and trace minerals, similar to a fish emulsion, and is rich in amino acids and derivatives.
The products leave the unit at about 100C after heat recovery. With full heat recovery, the overall energy efficiency could be above 85 percent based on the heating value of the products and the dry weight of the feedstock. In other words, it generates 467 percent more energy than it takes to produce it; except that this figure leaves out energy needed to construct the TCP plant.
For comparison, biofuel from maize crops, according to the latest study, generates at best only 35 percent more energy than it takes to produce , and has the added disadvantage that growing crops for biofuels uses up valuable agricultural land that could produce food.
Each tonne wet weight of turkey wastes processed was estimated to save more than a tonne of carbon dioxide equivalents in green house gas emissions, largely on account of the savings due to substituting for diesel.
There were various setbacks experienced by the Carthage plant . The plant was shut down for a period due to reported noxious smell, though it could not be confirmed to have come from the plant. In addition, the oil produced by the plant did not qualify as a biofuel for tax purposes, and so the plant did not qualify for the $42 per barrel of No. 2 oil in tax credits. But the definitions have since been changed to allow explicitly for diesel generated from thermal depolymerization process, taking effect at the end of 2005.
Despite the setbacks, the process appears cost effective. In January 2005, the Carthage plant was producing refined No. 2 oil (used for diesel and gasoline) for about $80/barrel, compared to the on-highway prices for diesel in the US at $101/barrel or $2.40/gallon (8 August 2005), which is likely to continue to rise.
Changing World Technologies aims to tackle plastic wastes next . A major source of plastic wastes comes from some 15 million cars that are de-registered in the US each year, 95 percent shredded to recover metals, leaving a ‘shredder residue’ of fluff 25 percent by weight that’s buried in landfills. Most of this two million tonnes of shredder residue is plastic: polyurethane foams and rubber.
In a pilot bench experiment, the company demonstrated the conversion of two different mixtures of shredder residue (see Table 1) into a variety of products including light hydrocarbon oil. The two mixtures of shredder residues differed both in their composition of solids and in the amount of moisture. They were first screened to remove a small portion of the inorganic material (mainly iron oxides and small pieces of glass and rocks) that would not fit in the bench-scale reactor.
Table 1. Average composition of shredder residue (SR) #1 and #2
|Content (weight %)||SR #1||SR #2|
Rubber & Elastomeres
The bench reactors were capable of operating at temperatures above 900C and pressures above 138 atmospheres.
In SR #1 the polymers that make up the rubber and plastics were mostly transformed into the hydrocarbon oil, which is similar to diesel, with a small fraction going to the 2nd stage gas. The first stage gas was primarily carbon dioxide. But the second stage was rich in hydrocarbons and would support combustion. It had a heating value of approximately 80 percent that of natural gas. The yield of hydrocarbon oil was 41.9 percent of the total input mass or 65 percent of the initial weight of solids. Less than 20 percent of the solids went into the carbon matrix.
For SR #2, which contained more water, oil yield was 29.8 percent of overall mass, or 52.7 percent of the initial solid matter. The carbon matrix yield was 32 percent of the solids.
The two samples of fuel gas were quite similar, as were the diesel oil obtained.
Shredder residue is often contaminated with toxic chemicals such as PCB (polychlorinated biphenyls) and heavy metals. The samples treated were spiked with water containing PCB. The PCB was largely degraded in the process. The heavy metals arsenic, barium cadmium, chromium, copper, lead, mercury, silver and zinc were detected, mainly concentrated in the carbon matrix. Selenium was not detected. Small amounts of zinc were detected in the oil from SR #1, and zinc, chromium and lead in oil from SR #2. Practically no bromine (from polybrominated flame retardant) were found in oils, but ended up in the water and the carbon matrix.
TCP and similar processes look promising both for extending the use of oil by recycling mixed plastics wastes and for turning food and agricultural wastes into renewable fuel and saving on greenhouse gas emissions, though certain questions remains unanswered.
Article first published 29/11/05
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