Advantages and possibilities of solid recovered fuel cocombustion in the European energy sector.

ABSTRACT

The 1999/31 Elemental Carbon Directive sets strict rules on the disposal of untreated municipal solid waste in the European Union countries and forces a reduction of the biodegradable quantities disposed off to landfills up to 35% of the amount produced in 1995 in the coming decade. More environmentally friendly waste management options shall be promoted under the framework of the Community Waste Strategy ([96] 399 Final). In this context, the production and thermal use of solid recovered fuels (SRFs), derived from nonhazardous bioresidues and mixed- and mono-waste streams, could be a key element in a future waste management system. Within the scope of the European Demonstration Project, RECOFUEL, SRF cocombustion was demonstrated in two largescale lignite-fired coal boilers at RWE power station in Weisweiler, Germany. As a consequence of the high biogenic share of the cocombusted material, this approach can be considered beneficial following European Directive 2001/77/EC on electricity from renewable energy sources (directive). During the experimental campaign, the share of SRF in the overall thermal input was adjusted to approximately 2%, resulting into a feeding rate of approximately 25 t/hr. The measurement campaign included boiler measurements in different locations, fuel and ash sampling, and its characterization. The corrosion rates were monitored by dedicated corrosion probes. The overall results showed no significant influence of SRF cocombustion on boiler operation, emissions behavior, and residues quality for the thermal shares applied. Also, no effect of the increased chlorine concentration of the recovered fuel was observed in the flue gas path after the desulfurization unit.

INTRODUCTION

In the present work, the role of solid recovered fuels (SRFs) as substitute fuel in coal-fired utility boilers is examined. SRF is proven as an advantageous substitute fuel because of its low production cost and the significant thermal value (14-16 MJ/kg, raw). (1) The main SRF consumers have been cement and lime industries up to now. However, the advantages and the potential of this substitute fuel are gradually appreciated by energy utilities too, and the co-utilization of SRF in coal-fired power plants is continuously increasing. In this framework, the need on the fuel standardization becomes clear. Mandated by the European Union Commission, the standardization and classification work of SRFs by Committee for Standardization Technical Committee (CEN TC) 343 has started in 2002.

SRFs

Mixed SRFs mainly consist of biogenic components (40-80 weight percent [wt %]) like paper, cardboard, textiles, and wood. A further significant fraction consists of mixed plastics, such as polyethylene, polypropylene, or polystyrene, in the form of foils or (hard) plastic pieces. They derive from nonhazardous mixed waste streams, such as municipal solid waste (MSW), commercial, or bulky waste, but also from certain mono-waste streams. More specifically, the input materials suitable for SRF production in accordance with the Bundesgutegemeinschaft Sekundarbrennstoffe und Altholzrecyling e.V. are defined as the following five main groups referenced in the waste catalog and the Commission Decision 2000/532/EC: (1) group 1: wood, paper, cardboard, and cardboard boxes; (2) group 2: textiles and fibers; (3) group 3: plastics and rubber; (4) group 4: other materials (e.g., waste ink, used absorbents, and spend activated carbon); and (5) group 5: high calorific fractions (HCFs) from nonhazardous mixed collected wastes.

The HCF is usually sorted out from the mixed streams by positive or negative sorting methods and mixed with defined production specific waste streams to achieve the required quality for the final product. Process steps contain size reduction, screening, mechanical sorting, ferrous metals or nonferrous metals separation, biological drying, and so forth. The two main approaches on the SRF production contain either only mechanical processing steps to separate the HCF and to remove unwanted components, for example, polyvinyl chloride, or mechanical-biological treatment where a biological drying step is integrated in the process. SRF is usually produced in the form of bales, fluff, or soft or hard pellets according to the market demand. It is mainly used in the cement, lime, and steel industries as a coal substitute and in the Scandinavian countries as fuel for district heating. Major countries producing SRFs in the European Union are Austria, Germany, Italy, the Netherlands, and Scandinavian countries. Currently approximately 5 million t of SRFs with a biomass fraction of 40-80 wt % are produced and used in Europe.

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Current Situation on the European Market of SRFs

The European waste market and, accordingly, the market of recovered fuels go through a transition period. It is expected, that the necessary changes in the countries' waste treatment policies imposed by the directive 2000/76/EC will lead to a gradual decrease of the waste quantities disposed of to landfills in all of the European Union countries. (2) The need for alternative waste treatment options becomes gradually visible. In Germany, the prohibition of landfilling of MSW from June 2005 led to serious waste treatment capacity problems (Figure 1). The demand on incineration and coincineration capacities became clear. According to dedicated Prognos studies, this demand will continue also in the near future (Figure 2). Both incineration and coincineration facilities are needed as two supplementary parts of a future waste treatment policy. Therefore, no obvious concurrence between these two concepts is expected.

SRF cocombustion in existing utility boilers may play a key role at this point. Partial substitution of coal by SRFs in large-scale power plants can effectively assist in covering the capacity limitations to a certain extent and will, furthermore, result in savings of valuable fossil fuel sources and reduction of carbon dioxide (C[O.sub.2]) emissions (~1 mg of C[O.sub.2]/Mg SRF), (34) SRF co-utilization in existing thermal plants usually requires low additional investments, and in this way reduced electricity generation costs are expected (<0.05 [Euro]/kWh) compared with the generation costs from other renewable energy sources, such as wind energy or photovoltaics. This reduction of the electricity generation costs from renewable energy sources is also a key issue in the European energy policy.

[FIGURE 2 OMITTED]

As the number of SRF production plants grows in the European area, there is an increasing demand on efficient quality control mechanisms in the waste treatment processes. Although in the previous decades the input material for SRF was mainly product-specific waste streams, the development of the sorting and separating technologies at the present time enabled the use of mixed-waste streams, which are more difficult to handle and control. The need for quality assurance and fuel standardization was recognized by the SRF producers and users. An increased acceptance of SRFs by transparent quality management and reliable SRF qualities was and is a matter of prime importance. National regulations were developed like the Regulation of the German Institute for Quality Assurance and Certification (RAL-GZ 724), the Finnish regulation (SFS 5875), and the according Italian regulation. On the European level, the standardization activities related to SRFs are combined and coordinated in the CEN TC 343 (5,6) and the related national mirror committees.

EXPERIMENTAL WORK

Description of the Weisweiler Power Plant

The full-scale cocombustion trials took place in the RWE Power's site at Weisweiler, which is located 50 km west of Cologne in the Rheinish brown coal area. The Weisweiler site consists of six units with total installed capacity of some 2060 [MW.sub.el] (megawatts of electric power). The tests took place in the Units G and H, with a nominal capacity of 600 [MW.sub.el] each, which already use paper sludge for cocombustion (Figure 3). The scope of the trials was the examination of the SRF cocombustion and its effects on the plant operation and the residue quality. The primary activities can be summarized as follows (7): (1) characterization of boiler combustion behavior (profile measurements at furnace exit and evaluation of the operational data monitored); (2) flue gas measurements in front of the air preheater (APH) and stack emission measurements; (3) mill measurements and periodic mill inspections; (4) fuel (lignite, SRF, paper sludge, and ready fuel) and ash (fly ash and wet bottom ash) sampling; and (5) corrosion monitoring.

During the tests, the SRF feeding rate per boiler was 12.5 Mg/hr, corresponding with 2% thermal share, whereas it was doubled to 4% for specific time periods. The total feedstock consumption for both units during the cofiring tests was 4200 Mg of SRF, 13,300 Mg of paper sludge, and 345,000 Mg of lignite. A special unloading station has been built at the Weisweiler site for the cofiring of substitute fuels like paper sludge. It consists of a twin 180-[m.sup.3] bunker and a single 90-[m.sup.3] one together with two intermediate storage areas summing 4000 t or less. The feeding capacity of the three screw feeders amounts to 125 [m.sup.3]/hr each, resulting to a feeding capacity of 62.5 mg/hr for paper sludge or 25 t/hr for SRFs. The existing infrastructure could be successfully applied during the SRF cofiring tests.

Fuel Characterization