Technical paper. Biomass energy systems. Engineering a greener future

Transcription

1 Technical paper Biomass energy systems Engineering a greener future

2 Unique solutions... 2 Using biomass for energy production can give a significant help in building a greener and more sustainable future. A diverse array of technologies can exploit the energy content of biomass. WTE possess a significant experience in the direct combustion of MSW and is technical partner of choice for anyone interested in projects related to biomass direct combustion. The design of unique solutions In our approach, every furnace/boiler is carefully tailored to specific needs of our customers. Requirement analysis is focused on the characteristics of biomass used as combustible: energy content, moisture, quantity and chemical composition of ashes from combustion process, presence in flue gases of contaminant precursors. A major effort is made to design the system for maximum flexibility. In the real world - due to seasonal availability of biomass and other opportunities/constraints that can arise during the long lifecycle of these plants - a system will be fed with the combustibles actually available on the market. At this stage of the project the combustion diagram of the system is draft, It will be thoroughly analyzed and discussed with our customers, as it describes all the operating conditions of the system, varying fuel characteristics and throughput. Biomass energy is carbon neutral: CO 2 released during the combustion process equals the CO 2 absorbed by plants and trees during their growth. There is no net increase in the atmospheric CO 2 if the growth of new plants and trees fully replaces the supply of biomass consumed for energy. Energy crops Crop Productivity [tdb/ha] Energy content [GJ/tDB] Theoretical energy [GJ/ha] Moisture at harvest [%] Harvest season Arundo donax ,5-17, November-March Fibre Sorghum ,7-16, August-September Switchgrass , November-March Miscanthus ,6-17, November-March Poplar November-March Willow November-March Black locust November-March Productivity (expressed as tonnes of dry biomass per hectare per year), energy content and other relevant information for some energy crops. Value for theoretical energy in GJ/ha is only indicative, as calculated assuming moisture at harvest equal to 0%. A more realistic value should be half of the one indicated. It is commonly considered that hectares of land should give enough biomass to generate 1 Mwe. [Source: Candolo, Agronomica, 4/2006]

3 ...Designed to perform, built to last 3 System design We have developed and perfected over the years original approach and proprietary models Our methodology considers such variables as: Thermochemical equilibrium of combustion in furnace Relation between the quantity of heat exchanged through the furnace walls and the values assumed by parameters governing the combustion process (combustion air temperature, oxygen in flue gases, quantity of fumes, percentage of fumes to be recirculated, air temperature in post-combustion chamber) Model of thermal exchange in the boiler Computational Fluid Dynamics (CFD) to analyze flows within the boiler Process optimization Our systems are designed for flexibility and energetic efficiency. Design aims to: maximize the recovery of the combustible energy content optimize the parameters governing the combustion process insure the capability to self-sustain combustion even when processing low energy content materials insure a complete combustion, both in solid and gaseous state Reliability Maximum reliability is insured by system design and by the choice of machines and equipments. In our philosophy, there is no room for compromises when designing for the maximum reliability. Our outstanding results are obtained by: Selecting materials and construction process based on a detailed study of the system s operating conditions Applying even on small size plants - as long as this is economically feasible - the engineering practices typical of major power plants, such as the MSW incineration waste-to-energy facilities. Installing redundant machines (pumps and compressors). Operations and maintenance easiness Plant layout is carefully designed to insure easiness of maintenance and operations. Furnace and boiler are designed to easy the maintenance and the replacement of parts subject to wear. At the plant level, a major focus is given to: site access and internal circulation layout of the combustible receiving/storage/handling location of loading/unloading bays positioning of slag and ashes evacuation system location of Control Room and Power Centre Biomass power plant, 25 MWt/7 MWe This mid-sized system has been designed aiming at maximizing the production of electric energy. The system has can process materials with LHV as low as 1800 kcal/kg (7534 kj/kg). Temperature in combustion chamber is controlled by inflowing secondary air and recirculating flue gas. Bleeding of steam from turbine allows to feed a degaser and an air/steam exchanger, to preheat combustion air

4 Design driven by fuel characteristics 4 Furnace design Our firm specializes in design and manufacturing of grate-fired biomass combustion plants. Our calculation models allow determining the adiabatic grade of the furnace, depending on the energy content of the biomass used as fuel. Many intermediate solutions are possible between two extremes represented by the tube wall, non adiabatic furnace and the fully refractory lined, adiabatic one. Non adiabatic systems water tube walls suitable to process biomass with high energy content less sensitive to the problem of controlling temperature, as combustion chamber is cooled by the water, naturally circulating in the walls tubes low fumes specific production, best energetic performances insufficient flexibility to variations in combustible energy content. Adiabatic furnaces furnaces completely lined with refractory materials suitable to process materials with low energy content or high moisture cooling of combustion chamber by blowing air - this worsens energetic yield, increasing fumes generation. Our design solves this problem by recirculating exhaust fumes instead. This way the same high performances of a non adiabatic system can be attained. Understanding a system s combustion diagram To the left, the combustion diagram for a biomass energy system. Thermal capacity of the furnace depends on throughput and LHV of combustible. The light blue area shows the standard operating conditions. Systems can also operate in the blue area, corresponding to a full load situation. This diagram shows the fuel lines as function of the projected adiabatic degree of the furnace. Operating below the fuel line is possible only after firing auxiliary burners. Usually combustion diagram also shows the heating line (pre-heating of combustion air can be needed when processing low energy content materials)

5 5 Boiler design Our systems usually include horizontal tail end boilers. Vertical boilers can be realized as well, on customer request. Here are some of our boilers most remarkable characteristics: Radiation heat exchange in the high temperature pass by tube walls Control of thermal flow through tube walls of radiation channel, to avoid damages due to excessive temperatures Convection heat exchange by mean of tube bundles in the flue-gas path. Convective pass can be refractory-lined or realized with tubewalls. Natural circulation of water Dust removal by percussion (mechanic or pneumatic hammering) or vibration (rapping) Assessment of the best bundle tubes pitch ratio, to avoid obstructions generated by dust Optimization of flue gas speed Control of flue gas temperature after post-combustion, to avoid any corrosion Horizontal boilers for biomass energy systems Horizontal boilers have many advantages over vertical ones. Dust removed by rapping devices falls directly in the collection hoppers. No use of steam for cleaning operations is required. Bundles can be more easily extracted, for maintenance or replacement. In our design, bundles can be installed and removed avoiding any soldering operation. Due to these reasons, horizontal boilers - widely used in waste incineration plants - are best suitable when processing difficult materials, susceptible to give dust/fouling problems.

6 Maximize energy recovery 6 Energy recovery Our systems are designed to maximize the efficiency of energy recovery. The first design choice to be made concerns pressure and temperature of steam to be generated. Other options to be considered: Cogeneration (combined heat and power) or trigeneration (combined heats, power and refrigeration) systems can be designed. Regenerative thermal cycles can be designed with different complexity. Bleeding insures steam from the turbine to preheat water feeding the economizer(s) bundle(s), to preheat combustion air in one or more stages and to feed a degaser. A final economizer bundle may be installed downstream the flue gases treatment system Combustion air can be preheated also in suitable heat exchangers installed inside the boiler Biomass power plant, 5 MWt/1 MWe A small sized system, generating 1 MWe from biomass (wood chips and biomass from Short Rotation Forestry). This plant has can also supply thermal energy to a district heating system, at the cost of a slight reduction of the electric power generated.

7 Minimize environmental impact 7 An environment friendly technology Biomass energy is a green energy, and this is true also when considering gaseous and solid by-products of combustion. Air emissions and flue gas treatment Though depending on the characteristics of biomass used as combustible, the control of air pollution is seldom a significant issue. Our systems are designed to limit pollutant concentration in air emissions well below limits imposed by laws and regulations. Air pollution control systems (APC) include at least a filter (a fabric bag filter or an electro-static precipitator) to eliminate dust and particulate. When needed, a SNCR (selective non catalytic reduction) DeNOx is installed, to control nitrogen oxides. Solid residues Solid residues of the process are combustion slag and bottom ash, precipitated through the grate or in the slag pit, and fine ashes (flying ashes). Ashes can be returned to the locations where biomass was harvested, returning nutrients to the original soil. When this option is not feasible, ashes can be used as: fertilizer or in production of fertilizer building material or component of building products in cement kilns other niche applications can be possible Emission limits for CHP plants based on biomass combustion in IEA partner countries FEI [MWt] Austria W Finland W, S, P Finland BM / peat Belgium BM Denmark W, WW / S Denmark BM Sweden BM mg/nm 3 13% O2 mg/nm 3 13% O2 mg/nm 3 6% O2 mg/nm 3 11% O2 mg/nm 3 10% O2 mg/nm 3 6% O2 mg/nm 3 6% O2 Dust / CO / / 500 * > / 500 * NOx r.c.o > 10, < r.c.o SOx < / TOC > 0, PCDD/F > W = Wood WW = Waste Wood S = Straw P = Peat BM = Biomass FEI = fuel energy input r.c.o. = range covered otherwise *) = daily/hourly average

8 About WTE s.r.l. WTE is an engineering firm, established in 2000 by professional with a significant experience in the design waste-to-energy plants. We have provided our services to some of the best names in the Italian power and environmental industry. Our systems encompass all of our experience and know how in the demanding field of waste incineration with energy recovery where plant availability, efficiency and profitability, and control of environmental impact of operations are of the utmost importance. WTE can provide its customers with a wide range of services, from conceptual and feasibility study to the basic engineering, from detailed engineering to EPC services and can supply complete turnkey systems. Services Provided Conceptual and Feasibility studies Basic and Detailed Engineering Procurement, Construction and Project Management Design and supply of Turnkey systems Support to project financing Other consulting services WTE Waste to Energy Srl Via San Michele, Busto Arsizio (Varese) - ITALY Tel Fax sales@wastetoenergy.it Web: WTE Waste to Energy Srl C.F P. IVA REA Iscritta al registro delle imprese di VARESE al n Cap. soc. effettivamente versato