Local Fluidised Bed Combustion Steam Power Plants Fuelled by Pre- Screening Material (PSM)

Transcription

1 Local Fluidised Bed Combustion Steam Power Plants Fuelled by Pre- Screening Material (PSM) Dieter Steinbrecht, University of Rostock, Chair of Environmental Technology Abdallah Nassour, University of Rostock, Chair of Waste Management Volker Spiegelberg, ES+S GmbH Rostock, Germany Summary One of the principles for future waste policy in Germany is the conversion of waste management into a material flow, energy and resources management. The objective is the use of all combustible materials for the replacement of primary fuels and thus a contribution to saving resources and to climate protection. The review covers the technical, economic and environmental conditions for local utilization of fuels derived from waste in in SFBC plant energy systems with firing capacity of 6 MW. Pre- and fine screening materials, the by-products from the RDF production, are actually disposed of. This is connected with known negative actions on the environment. For an energetic utilisation of pre-screening material are additional screening steps necessary to remove stones, glass and metals. The cleaned materials contain enough energy to run a Fluidised Bubbling bed Combustion process economically. With exception of their ash content SFBC combustion tests had the result that their fuel properties are similar to common RDF. Pre- and fine screening materials should be energetically used, in future. Introduction Waste fuels can be generated by treatment of municipal waste, bulky, construction, trade and industrial waste and by sorting of recyclables (paper, cardboard, wood, textiles, plastics). Here happens a distinction between RDF and pre-screening material / Table 1 /. Table 1 Sources of fuel from waste RDF fuel Pre-screening material origin Calorific fraction from mechanical or mechanical-biological waste treatment plants (MA, MBA, MBS) Calorific sorting residues from the sorting of municipal, construction and commercial waste Pre-screening material (0-40 mm) from the sort of municipal, construction and commercial waste

2 The pre-screening material is a potential fuel with high share of impurities (mainly stones, glass, metals, but also limestone rubble). It accrues in the first technological process step of sorting materials. After removal of coarse contaminants is the prescreening a fuel with low calorific value. Depending on the disposal options and sorting techniques are produced the prescreening in different sizes (0-10 mm, 0-30 mm, 0-40 mm and 0-50 mm) for disposal or further processing. The calorific value is in the range of about 2-12 MJ/kg (own investigations [ 1 ]). This is a very wide range. For the operation of thermal treatment facilities, especially facilities of SFBC, this material flow is a sufficient potential energy. Table 2 shows as an example the analysis values of a commercial waste sorting plant in Mecklenburg-Western Pomerania. Analyses show that limits for landfilling can not be hold. Table 2 Analysis of the pre-screening and sorting remains of a commercial waste sorting Pre-screening 0-30 mm Sorting residues mm DepV TOC Masse % TS 12 17,2 18 ph 7,2 7,2 5,5 13,0 TOC in Eluat mg/l GB 21 L Gas/kg TM 33,1 34,9 20 Calorific Value kj/kg The Chair of Environmental Technology of the University of Rostock deals with Stationary Fluidised Bubbling bed Combustion (SFBC) processes for non-regular fuels. This paper is the result of a long-lasting successful cooperation with the Chair of Waste Management of the University of Rostock. The ES+S company stands for + Project developing, planning and realisation of SFBC plants + Training of plant personnel + Plants supervision and plants operation What differentiates our SFBC plants from other SFBC plants? The Chair of Environmental Technology had developed and experimentally confirmed a so called Rostock SFBC operational philosophy. In comparison to other SFBC plants this principle is characterized by several advantages:

3 + flameless combustion, combustion reactions only run within the fluidised bed. + pre-selectable combustion temperature level + pre-selectable combustion oxygen level (depending on LCV and heat in- / decoupling) + low flue gas emission values realized by fluidised bed process-internal measures The Rostock SFBC operational philosophy enables to burn difficult fuels, e.g. Pre- Screening Material (PSM) ore Fine Screening Material (FSM). Present situation RDF as SFBC fuel has already been introduced in Germany. Several examples exist which demonstrate the excellent RDF properties for its utilisation as SFBC fuel in power stations. Meanwhile, RDF got defined properties nearly like a normal fuel. But there is hardly free RDF material (up to this moment not used for new energy plants) available in the market. During the RDF preparation process for SFBC fuel purposes are generated new wastes Pre - Screening Material PSM ( > 30 mm, LCV = 2 14 MJ/kg ) and Fine Screening Material (FSM) ( 0 10 mm, LCV = 2 14 MJ/kg ) characterized by a relatively wide range of LCV. Independent on its real LCV the PSM / FSM material is mainly disposed of in spite of its interesting part of combustible components. In the past (e.g. in 2009) there arose several landfill problems with illegal disposal of these PSM and FSM materials in Germany. Presently these materials are not yet used for energy production purposes. The PSM / FSM masses to be disposed of are in an interesting order for an alternative energetic application, in future. As well Pre-Screening Material (PSM) as Fine Screening Material (FSM) are available in greater masses at RDF preparation plants. The idea PSM / FSM material should be investigated and, if possible, introduced as an alternative SFBC fuel. In whole Germany there are great masses available. Only in the German State of Mecklenburg-Western Pomerania this available mass is ca tons PSM per annum. This is interesting as well from technical as well from economic and ecologic aspects. A special SFBC plant design must take into account the un-usual PSM / FSM fuel properties. As possible, the avoided landfill receipt charge fees should give an economic input to the SFBC steam power plant. That means the reception of PSM / FSM as SFBC fuel realizes an economic advantage. The following information is based on the results of experimental investigations with a 250 kw SFBC pilot plant at the University of Rostock.

4 Tests at the 250 kw SFBC pilot plant at Rostock University Fig. 1 Raw PSM input > 30 mm, contaminated by stones, glass, metal Fig. 2 Cleaned PSM input, nearly free from stones, glass, metal Fig kw-sfbc pilot plant at Rostock University Fig. 4 Crushed PSM before combustion Tests with shreddered PSM in a 250 kw SFBC at the University pilot plant have demonstrated its good applicability at low flue gas emission levels, table 3. During the tests the combustion oxygen concentration at rather constant SFBC bed temperature had been changed, fig. 5. This was possible by variable heat decoupling from the fluidised bed by an air-cooled in-bed high temperature heat

5 exchanger (HT-EX). This procedure gives information about CO and NOx emissions of this new fuel and about the possibilities of influencing them. The tests demonstrate the great influence of air excess on the NOx emissions. Table 3: Pre-screening material test, CO 2,max calculated from measured O 2 & CO 2 Fig. 5 SFBC test: Plot of relevant information belonging to the variation of combustion oxygen concentration Fig. 6 gives the information of a technical optimum for SFBC flue gas oxygen concentrations fulfilling German 17 th BImSchV conditions to minimize the NOx emissions without any additional additive reaction. It is possible to reduce NOx emissions without a significant increase of CO emissions. This situation can be achieved at flue gas oxygen concentration of 6 Vol%.

6 NO x Combustion tests: O 2 6 Vol% CO Fig. 6 CO and NOx emissions in dependency oxygen concentration of flue gas during the SFBC combustion of PSM. No additive is used. Further important information could be demonstrated, fig.7: Pre- screening material looks rather heterogeneous (fig. 3) but its combustion behaviour is like a homogeneous fuel, fig.6. All the calculated CO 2,max concentrations (= fuel characteristics) have nearly the same value, Table 1. This comes from a nearly constant composition of the combustible parts (h/c-ratio). In comparison to RDF the PSM and FSM seem to contain a little bit greater share of particles with wood origin. Combustion tests: PSM can be used like a homogeneous fuel Fig. 7 Measured data pairs of CO 2 and O 2 and calculated CO 2,max

7 These test results (fig. 5 and 6) have also been confirmed when PSM and Fine Screening Material (FSM) with origin from different RDF production plants in northern Germany was used. Table 4: High calorific values (HCV) for different samples from PSM material [ 1 ] Indication Dry mass, in % Ash, in % HCV Heating value HCV Heating value of solid mass of solid mass MJ/kg Solid Mass MJ/kg Dry Mass sample 1 92,7% 86,6% 13,61 14,68 sample 2 86,3% 18,8% 16,82 19,48 sample 3 89,3% 19,3% 15,52 17,38 sample ,8% 22,0% 19,26 19,69 sample 4 81,7% 70,7% 5,70 6,98 sample ,7% 70,4% 5,28 6,47 sample ,7% 28,7% 15,60 17,58 sample 5 88,5% 80,6% 2,74 3,09 sample 6 87,0% 76,1% sample 7 92,2% 12,7% 18,90 20,50 sample 8 89,5% 83,7% For further calculation purposes a medium LCV = 8 MJ/kg had been assumed. Investigations on the combustion properties of the Pre-Screening material 1 had the following result: The original dry mass content during random tests of PSM is rather constant at high level. But the ash content in the dry mass changes in a wide spread range between 12.7 and 86.6 %. Connected with this the higher heating value (HCV) of the PSM varies in the range between practically zero to nearly 19 MJ/kg, see table 4. This important information has a great influence on the design of an industrial SFBC plant. Its design must be able to react on the changes in HCV (or LCV) as well on the changing ash mass flow. What will happen with the share of non-combustibles? The possibly high share of inertia in the fuel leads to an increase of the bed mass in the SFBC. Connected with it are + An increase of the bed pressure drop (this leads to an increase of fluidisation blower energy consumption) and + A possible reduction of the plant power (caused by reduction of the combustion air mass flow). This situation must be avoided and makes a management system for bed material necessary. A part of the bed material is continuously removed from the bottom of the bed. Its energy content must be used. The part of removed bed material is sieved.

8 Fine particles (e.g. ash) as well as large particles (e.g. stones) are to be removed and disposed of. 6 MW Stationary Fluidised Bubbling bed Combustion steam power plant operated with Pre-screening material Based on the R & D experience at the University of Rostock and at the ES+S company is proposed a local 6 MW SFBC plant fuelled by + substitute fuel (RDF) + or by pre- screening material (PSM) + or by fine screening material (FSM). Fig. 8 presents the calculated chart of the combustion performance for this plant. It can be seen clearly that the theoretical LCV limit is LCV = 4 MJ/kg. But because of the high share of inertia in this case is plant full load not possible. So follows for such a plant a necessary second (fuel) management system to realize a minimum LCV = 6 MJ/kg. Fig. 8 Chart of 6 MW SFB combustion plant performance Fig.9 presents the conceptual design of the 6 MW SFBC steam boiler plant. The main components are: Fuel receiving station, fuel transport system, SFBC and steam boiler, feed water preparation plant, steam turbine & generator, air-cooled steam condenser, flue gas cleaning plant, stack This conceptual plant has been investigated by the help of economic models.

9 Fuel Receiving Station Fuel transport Service rooms Daily storage SFBC Steam boiler Steam Turbine Flue gas Stack silo feed water preparation Generator cleaning Fig.9 Conceptual design of the 6 MW SFBC steam boiler plant, Design: companies ES+S, Birkhahn & Nolte Economic chances of a 6 MW SFBC plant Fig. 10 Development of the difference between revenue and expense in dependency of annual plant full load and fuel revenue

10 Model investigations are made on possible economic effects of the planned 6 MW SFBC, see fig 10. Following assumptions are made: Investment 7.5 Mio Heat sale 40 / MWh, electricity sale 40 / MWh, variable fuel revenue The most important influence is the possible number of annual plant full-load hours. This number must be as high as possible. Conclusions An integrated system to ensure the environmental and economic waste management is the subject of article. The waste material flows should be prepared, utilized and disposed of according to the physical and chemical properties. The low and medium caloric fractions from the trade and industrial waste sorting and mechanical-biological residual waste treatment plants are potential streams for decentral local thermal plants after stationary fluidized bed technology. The sources of the material are the pre-screening (0-40 mm) from trade waste and sieving of the organic fraction the mechanical-biological waste treatment. Based on the positive results of the presented investigations a 6 MW SFBC steam power plant fuelled with pre-screening material (PSM) and / or fine screening material (FSM) should give positive economic and environmental effects. The deposition of PSM and FSM materials can be avoided in future. The technical solution is innovative. A long-term test of such an industrial 6 MW SFBC plant should be advised. Reference: [ 1 ] Ministerium für Wirtschaft, Arbeit und Tourismus Mecklenburg-Vorpommern, Germany, Study, made by the University of Rostock, Chair of Waste Management, December 2009, Title Dezentrale Verwertung von Ersatzbrennstoffen und heizwertreichen Sortierresten in Mecklenburg-Vorpommern