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From Science & Research

From Science & Research Disposal of Bio-Polymers via Energy Recovery Article contributed by Christian Laußmann, Umweltreferendar Land Nordrhein-Westfalen Bezirksregierung Münster, Germany Hans-Josef Endres FH Hannover, Germany Ulrich Giese, Dt. Inst. f. Kautschuktechn. e. V. Hannover, Germany Ann-Sophie Kitzler Achilles Papierveredelung, Celle, Germany Calorific values of bio-polymers [MJ/kg] Polyethylene (PE) Polypropylene (PP) Polystyrene (PS) Polyamide (PA) Polycarbonate (PA) Polyethyleneterephthalate (PET) Polyvinylchloride (PVC) Polytetrafluoroethylene (PTFE) Bio-polyethylene Polycaprolactone (PCL) blend Bio-polyester Polyvinylalcohol (PVAL) Polyhydroxyalkanoate Polyester-PLA blend Starch blend Polylactide (PLA) Cellulose derivative / blend PP + 30% by wt. of wood flour Fuel oil Coal Wood Paper 0 5 10 15 20 25 30 35 40 45 50 Fig 1. Measured calorific values of bio-polymers compared with those of conventional plastics and petrochemical fuels [5] References [1] General literature on the calorific value of gasoline and fuel oil [2] Troitzsch, J.: The combustion behaviour of plastics: basis, legislation, test procedures; Carl Hanser Verlag, Munich, Vienna 1982. [3] Kaminsky, W.; Rössler, H.; Sinn, H.: in KGK – Kautschuk Gummi Kunststoffe magazine 44 (1991), pp. 846 [4] Endres, H.-J.; Hausmann, K.; Helmke, P.: Research into the influence of various adhesion agents and their content on PP/Wood compounds in: KGK – Kautschuk Gummi Kunststoffe magazine 7/8 (2006), pp. 399-404. [5] Endres, H.-J.; Siebert-Raths, A.: Technische Biopolymere, Carl Hanser Verlag, Munich 2009 To achieve a maximum degree of sustainability for biopolymers, even in the method of their disposal, there is increasing discussion on the subject of cascade benefits and CO 2 reduction costs in connection with so-called ‘end of life options’ by adopting appropriate disposal options. The advantages of the ‘incineration’ option as a method of disposal, as opposed to simple waste disposal, is that additional energy recovery benefits are achieved which in view of the overwhelming bio-based component of bio-polymers represents a largely CO 2 neutral method of energy production. Alongside this contribution to climate protection the incineration of bio-polymer waste also contributes to resource conservation in that petrochemical based sources of energy (e.g. heating oil and gasoline) can be substituted [1]. From the point of view of environmental protection one also needs to consider the question of the composition of the combustion gases emitted by bio-polymers when considering their incineration and energy potential. Incineration of polymers In general, incineration (or burning) refers to the reaction of a substance in the presence of oxygen that is submitted to increasing temperature. It is a catalytic, exothermic reaction whose progress is maintained by the free radicals and heat radiation that it emits [2]. Pyrolysis, on the other hand, is an irreversible chemical breakdown resulting from increased temperature without the presence of oxygen and with no oxidation process [2, 3]. The significant factors that affect the composition of the incineration gases are (i) the way in which energy is produced, (ii) the amount of oxygen available (ventilation) and (iii) the physical properties or chemical composition of the incinerated materials. Experiments To carry out comparative experiments, in addition to various bio-polymers, two conventional thermoplastics, polypropylene (PP) and a natural fibre reinforced polymer (WPC - wood plastic composite) with a high PP content (70%) and the coupling agent maleic acid anhydride, were selected [4]. The conventional polymers served as a reference against which one could evaluate the performance of the bio-polymers. For the experiments biopolymers from the following groups were selected: • Various bio-polyesters • Polyvinyl alcohol • Polycaprolactone • Polylactide • Starch polymers • Cellulose polymers • Bio-polyethylene • Various polymer blends 42 bioplastics MAGAZINE [01/10] Vol. 5

From Science & Research Results a) Calorific values In figure 1 the calorific values of the substances tested are presented and compared with some conventional plastics, a wood-filled plastic and various fuels. The comparison of the calorific values shows that the biopolymers tested are without exception suitable for thermal recovery because their calorific values are at least as high as that of wood and comparable to conventional polymers. Furthermore, the calorific values of a certain few biopolymers can compete with the values obtained from coal or fuel oil. The values of the various bio-polymers are almost always the same as the conventional plastics, i.e. they are a factor of the fundamental composition of the polymer, with the presence of oxidisable components (in the case of the materials tested these were carbon and hydrogen) in relation to the non-oxidisable components (in the case of the materials tested these were water, and in particular oxygen or nitrogen) being of major significance. Even the conventional plastics polyamides and PET, have lower calorific values than polypropylene and polyethylene, because of the heteroatoms nitrogen and oxygen. b) Emissions When investigating the combustion emissions it was seen that these were mainly influenced by the chemical composition of the bio-polymers and the combustion temperature. At the lower combustion temperature (400°C) the gases in many cases exhibit, as expected, structural compositions similar to those from the incinerated polymers. The composition of the combustion gas hence consists, in a large part, of the relevant monomers, oligomers and chain breaks which are partially oxidised to form aldehydes and ketones. And so the bio-polymers emit the corresponding carbonic acid esters, caprolactone in the case of polycaprolactone and dilactide and lactide oligomers in the case of polylactides. With increasing temperature of combustion the increased atomization of the fuel fragments the structural relationship between polymers and the associated combustion product is reduced. A general view of the influence exerted by the combustion temperature on the bio-polymer tested, and on PP as a classic petrochemical olefin, is given in table 1. Combustion gas Structural relationship to the original polymer Key factor in the type of combustion emission Product spectrum Completeness of combustion (Eco)toxological hazard of the substances Combustion temperature 400°C Often present Fundamental elements and polymer structure Diversified several substance groups Combustion temperature 800°C Hardly ever present Almost exclusively fundamental elements Very little diversification aromatic compounds dominant Lower Higher more CO 2 , CO and H 2 O than the break-up product Less frequent incidence More significant incidence Table 1: Comparative impact of temperature on the character of the combustion gases Amongst the combustion gases from almost all of the polymers tested certain substances classified as (eco)toxologically critical were found, with the aromatics benzene, toluene and naphthaline being the most common. The formation of these substances is observed principally at the 800°C combustion temperature, but also, to a reduced extent, at 400°C. In this connection it is important to note that the formation of these critical substances is not limited to the purely hydrocarbon based plastics such as PP, but that the substances were detected in the combustion gases of almost all of the tested polymers, i.e. also in those containing oxygen. At the higher combustion temperatures it can be seen that the dependency of composition of a polymer‘s combustion gases of the elementary structure of the polymer is reduced, and that the origin of the raw materials is of no significance. Furthermore, the fact that a renewable source for the raw materials is of no significance in determining the nature of the combustion gas, is seen in the example of bio-PE where the same products were identified in the combustion gas as those seen in the combustion gas of conventional PE. The composition of the combustion gases is therefore not determined by the raw material basis but above all by the elementary composition of the polymer. Summary The evaluation of the test results showed that the biopolymer materials tested for their calorific value are without exception suitable for thermal energy recovery. As with other materials and fuels, the calorific value and the composition of the combustion gas of a bio-polymer is in principal determined only by the elementary composition of the material and any additives. With regard to the composition of the combustion gas, even with bio-polymers a few (eco)toxicologically critical substances were identified. The fact that a substance is biodegradable does not necessarily mean that when such a substance is burned there will be no emission of (eco)toxicologically critical substances. But in this context it should should however be pointed out that these types of decomposition products also occur during thermal energy recovery of conventional plastics and even natural materials such as wood. In addition the general recognition that the higher combustion temperature of 800°C is not favourable from an ecotoxicological point of view in terms of the combustion gases produced, has been confirmed. When burning biopolymers there is no higher potential for the emission of hazardous substances than when burning conventional domestic and trade waste. Biobased polymers do however have an additional decisive advantage: burning bio-polymers is a largely CO 2 neutral source of energy creation thanks to their basis of overwhelmingly renewable raw materials, and hence the burning of bio-polymers represents a logical and sustainable waste disposal system with an additional energy cascade benefit. bioplastics MAGAZINE [01/10] Vol. 5 43

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