TECHNOLOGY-ANALYSIS: Biomass combustion -

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1 TECHNOLOGY-ANALYSIS: Biomass combustion - Overview of Key Technologies, Benchmarking and Potentials (Summary) This report has been prepared within the project»smart Energy Network of Excellence, Nr. 5403«, Interreg IV Program Italy Austria Project co-funded by the European Union, European fund for regional development. Authors: Daniele Dell'Antonia. Università degli Studi di Udine

2 Summary 1. Introduction Background Thermo-chemical conversion Combustion Gasification Pyrolysis Combustion technologies Type of applications Small-scale appliances for space heating District Heating Networks Power generation and co-generation Pyrolysis technologies Gasification technologies Utilisation of Product gas Co-firing Combined heat and power (CHP) Gas cleaning

3 1. INTRODUCTION 1.1. BACKGROUND Biomass combustion such as burning fuelwood to provide heat, power, or combined heat and power (CHP) is a link in the energy chain from producing renewable biomass resources to providing sustainable services in the form of heat (or refrigeration), shaft power and electricity. Biomass combustion supplies about 11% of the world s total primary energy. There are several broad categories of combustion applications: heat for daily living use (stoves); community applications including district heating; industrial use for both process heat and electricity production, combined heat and power (CHP) uses in the pulp and paper, forest industries, and sugarcane processing; finally the production of electricity. Each of these applications will use different biomass materials according to the local availability of such fuels. Nevertheless the majority of biomass used is a lignocellulosic fiber (wood, straws, stalks, nuts shells, etc.) The most common application of biomass energy in developing countries is its use as a source of heat for cooking, sometimes called traditional biomass use. Industrial use of biomass combustion takes place in a combustor or furnace with the heat being used to in a manufacturing process, or to raise steam in a boiler which can expand through a steam turbine to generate shaft power in the so called Rankine cycle. Other prime movers that utilize the heat of combustion to provide power include gas turbines, Stirling engines and some direct conversion devices. The produced shaft power or electricity can be used directly to drive a mill or other machine. In combined heat and power (CHP) applications the most common variant is when the electricity is generated first using high grade heat from the combustor and the lower temperature heat is taken from the exhaust of the electricity cycle. Properly applied biomass combustion, unlike that of fossil fuels is climate neutral with respect to greenhouse gas production. In the case of biomass the combustion process releases carbon dioxide, which was removed from the atmosphere by photosynthesis, and thus maintains an equilibrium level of carbon dioxide, unlike the combustion of fossil fuels which increase the level of carbon dioxide by releasing the carbon that was long ago stored in the earth. Carbon dioxide is the leading greenhouse gas and the use of biomass is as a result neutral with respect to global warming potential. Combustion is the process by which more than 90% of the world s primary energy supply is realized in order to provide heat and energy services such as materials processing including food preparation; space heating, ventilation and cooling; electricity, and transportation. The non-thermal sources of energy in the primary energy mix are hydro and nuclear power, as well as renewable sources. Only about 11% of the fuels used are biomass resources, and the wide range of fuels used in combustion include: coals ranging from anthracite (an almost pure form of carbon) to lignite or brown coal; peat; natural gas; crude and refined oils including liquid petroleum gas (LPG), gasoline, diesel and kerosene. Coal, oil, gas and biomass, at shares respectively 23.3, 35.7, 20.3, and 11.2 %, are the recent major primary energy sources for the world. Combustion is a process in which the fuel is burnt with the oxygen from the air to release the stored chemical energy as heat in burners, boilers, internal combustion engines and turbines. The scale of combustion devices encompasses a few kw of thermal input such as a single gas ring for cooking, to huge coal fired combustion boilers with inputs of 3-5 GW in a single unit serving the electrical needs of almost 1 to 2 million households. There are probably over 1 billion small biomass cook stoves in use rated at less than 5 kw. In the case of biomass the combustion process releases carbon dioxide, which was removed from the atmosphere by photosynthesis, and thus maintains an equilibrium level of carbon dioxide, unlike the combustion of fossil fuels which increase the level of carbon dioxide by releasing the carbon that was long ago stored in the earth. Carbon dioxide is the leading greenhouse gas and the use of biomass is as a result neutral with respect to global warming potential. There are however, a number of environmental costs of biomass combustion, which require innovation and significant 3

4 investment for their mitigation. These costs include both direct human health impacts as well as environmental damage to the earth s productive ecosystems. The damage caused can be very local in the form of health impacts from indoor air pollution produced by cook stoves in household energy provision, or at a scale of several kilometers as in the case of carbon monoxide and much of the particulate material released in combustion. Regional impacts (at the 1000 to 3000 km path length) include acid rain and ground level ozone for which the precursors are sulfur dioxide (SO 2 ), nitrogen oxides (NO X ), and non-methane hydrocarbons (NMHC), which are sometimes described as volatile organic compounds (VOC). While the production of excess ozone at ground level is hazardous, it is perverse that the reduction of stratospheric ozone by chemical reactions with the products of combustion diffusing into the stratosphere is also hazardous as it allows ultraviolet light to reach the ground. On a global scale the release of greenhouse gases (mainly non-renewable carbon dioxide), methane, nitrous oxide and halogenated materials increases the likelihood of global climate change. As harvested from the field or forest, the biomass materials are not purely a mix of carbon, hydrogen and oxygen (CH&O), but also contain the elements N, P, K, S, and Cl as well as many trace elements that in the living plant are essential to maintaining metabolism, respiration and growth. These additional chemical elements present challenges to combustion engineering technology in the form of fouling, deposition, slagging and corrosion of the internal burner structure and heat transfer surfaces. The emission of metals and other elements to the air and soil may also have environmental impacts. However, depending on the quality of the combustion process and the investment in emissions controls, the use of biomass fuels can be as clean as natural gas utilization or even dirtier than coal THERMO-CHEMICAL CONVERSION Three main processes are used for the thermo-chemical conversion of biomass, together with two lesser-used options.the main processes, the intermediate energy carriers and the final energy products resulting from thermo-chemical conversion are illustrated in the flow- chart shown in Figura 1. Figura 1 - Main processes, intermediate energy carriers and final energy products from the thermochemical conversion of biomass. 4

5 Combustion The burning of biomass in air, i.e.combustion, is used over a wide range of outputs to convert the chemical energy stored in biomass into heat, mechanical power, or electricity using various items of process equipment (e.g. stoves, furnaces, boilers, steam turbines, turbo-generators, etc). Combustion of biomass produces hot gases at temperatures around 800 1,000 C. It is possible to burn any type of biomass but in practice combustion is feasible only for biomass with a moisture content<50%, unless the biomass is pre-dried. High moisture content biomass is better suited to biological conversion processes. The scale of combustion plant ranges from very small scale (e.g. for domestic heating) up to large-scale industrial plants in the range 100 3,000 MW. Cocombustion of biomass in coal-fired power plants is an especially attractive option because of the high conversion efficiency of these plants. Net bioenergy conversion efficiencies for biomass combustion power plants range from 20% to 40%. The higher efficiencies are obtained with systems over 100 MW (e) or when the biomass is co-combusted in coal-fired power plants. Combustion per se is not considered any further in this study, as it does not produce a fuel suitable for use in either an s.i.g.e. or a gas turbine. One heat engine cycle, the Stirling cycle, uses combustion to provide shaft power directly but the development of the cycle is presently limited to small power outputs. Figura 2 Map and potential of biomass plant in Italy and in Friuli Venezia Giulia. 5

6 Gasification Gasification is a process by which either a solid or liquid carbonaceous material, containing mostly chemically bound carbon, hydrogen, oxygen, and a variety of inorganic and organic constituents, is reacted with air, oxygen, and/or steam. The reactions provide sufficient exothermic energy to produce a primary gaseous product containing mostly CO, H 2, CO 2, H 2 O (g), and light hydrocarbons laced with volatile and condensable organic and inorganic compounds. Most of the inorganic constituents in the feedstock are chemically altered and either discharged as bottom ash or entrained with the raw product gas as fly-ash. Unless the raw gas is combusted immediately, it is cooled, filtered, and scrubbed with water or a process-derived liquid to remove condensables and any carry-over particles. Alternatively, the raw gas can undergo either medium-temperature (350 to 400 C) or high-temperature (up to gasifier exit temperatures) gas cleaning to provide a fuel gas that can be used in a variety of energy conversion devices, including internal combustion engines, gas turbines, and fuel cells. Biomass when gasified with steam and/or oxygen will produce synthesis gas, rich in CO and H 2, which in turn can be catalytically converted to produce high-value fuels and chemicals Pyrolysis Pyrolysis is the conversion of biomass to liquid (termed bio-oil or bio-crude), solid and gaseous fractions, by heating the biomass in the absence of air to around 500 C. Pyrolysis can be used to produce predominantly bio-oil if flash pyrolysis is used, enabling the conversion of biomass to biocrude with an efficiency of up to 80%. The bio-oil can be used in engines and turbines and its use as a feedstock for refineries is also being considered. Problems with the conversion process and subsequent use of the oil, such as its poor thermal stability and its corrosivity, still need to be overcome. Upgrading bio-oils by lowering the oxygen content and removing alkalis by means of hydrogenation and catalytic cracking of the oil may be required for certain applications. 6

7 2. COMBUSTION TECHNOLOGIES 2.1. TYPE OF APPLICATIONS The selection and design of any biomass combustion system are determined mainly by the characteristics of the fuel to be used, existing environmental legislation, the costs and performance of the equipment available, as well as the energy and capacity needed (heat, electricity). Due to economy of scale effects concerning the fuel feeding system, the combustion technology, and the flue gas cleaning system, usually large-scale systems use low-quality fuels, while high-quality fuels are typically used for small-scale systems. Therefore, large-scale biomass combustion technologies are often similar to waste combustion systems, but when clean biomass fuels are utilised, the flue gas cleaning technologies are less complex and therefore cheaper. Improvements are continuously being made in fuel preparation, combustion and flue gas cleaning technologies. This leads to significant improvements in efficiencies, and reductions in emissions and costs, as well as improved fuel flexibility and plant availability, and opens new opportunities for biomass combustion applications under conditions that were too expensive or inadequate before. For any biomass combustion application, emission reduction and efficiency improvement are major goals SMALL-SCALE APPLIANCES FOR SPACE HEATING Wood fires have been used as a local heat source for thousands of years, progressing from open pit to semi-open pit (a fireplace) to enclosed pit (a stove). The interest in using wood for heating purposes is increasing. Besides heating, wood-burning appliances are also used for cooking, for producing a pleasant atmosphere, and for interior decoration. Domestic wood-burning appliances include fireplaces, fireplace inserts, heat storing stoves, pellet stoves and burners, central heating furnaces and boilers for wood logs and wood chips, and different kinds of automatic wood chip and pellet appliances. Over-fire boilers are commonly used to burn logs and are relatively inexpensive. In such systems, a fuel batch is placed on a grate and the whole batch burns at the same time. The stove or boiler is normally equipped with a primary air inlet under the grate and a secondary air inlet above the fuel batch, into the gas combustion zone. Wood is fed from above and ashes are removed from a door below the grate. These boilers work on the principle of natural draught and, as the fuel bed is cooled by fresh fuel, the initial CO emissions can be relatively high. Very strict emission limits in some countries have made it necessary to introduce downdraught burners. Here, unburned wood gases released by wood placed on a ceramic grate are forced by a fan to flow downward through holes in the grate. Air is introduced below the grate in the secondary combustion chamber, where the gases flow along ceramic tunnels, and final combustion takes place at high temperatures. By using lambda control probes to measure and control flue gas oxygen concentration, staged air combustion, and even fuzzy-logic control, very low emissions are achieved. Naturally, down-draught boilers are much more expensive than conventional boilers. A recent innovation in space heating is automatic pellet combustion.the excellent handling properties of pellets mean that the fuel is gaining popularity rapidly in Sweden, Denmark, and Austria. In other countries, the interest in pellet burners is starting to increase. Pellet burners are of special interest since they can replace an oil burner in an existing oil-fired boiler. If the burner-boiler combination is well designed, efficiencies over 90% can be achieved at nominal thermal output. At part load, and varying load, the efficiency decreases but for the best burners efficiencies over 86% have been obtained. 7

8 2.3. DISTRICT HEATING NETWORKS District energy is a means for delivering heat and/or cooling to multiple buildings from a central source. The countries that have developed this technology tend to have been those in which heating demands are of most concern, so that district heating is prevalent. However, even Northerncities have significant cooling demands, so that district cooling is a growing market. The properly operating district heating systems can provide possible improvement of energy efficiency, reduce emissions, improve energy security and competitiveness and creating of new jobs. A district heating network includes the infrastructure for centralised heat production and distribution to the consumers for providing space heating and hot tap water in a wider area (Figura 4). The district heating system can be considered energy efficient and cost-effective only at optimal operation conditions and minimum heat loss. Figura 3 - Domestic wood-burning appliances: a) fireplaces of Palazzetti in Porcia, Italy; b) pellet stoves of MCZ in Fontanafredda, Italy. Figura 4 - The principles of district heating and the hydraulic interface unit. 8

9 There are more than 5,000 district heating systems in Europe, currently supplying more than 9% of total European heat demand. District heating systems are mainly used in the northern European countries, such as Sweden and Finland. As regards to Latvia, Lithuania and Estonia, the percentage of district-heated households is around 60-75%. Due to the fact that the connection to a singlefamily house is rather expensive, district heating is a less attractive solution for the countryside. Large district heating networks supply heat usually in big cities, since the level of heat consumption in large areas is high. Furthermore, new buildings are generally more energy efficient than existing buildings. This trend will continue in the future due to the strengthening of the energy efficiency requirements that buildings will have to comply with. An immediate effect of the strengthening in the energy efficiency standards for buildings is the reduction in the amount of energy that would be required to heat future buildings. The developed district heating systems promote cogeneration development. When cogeneration plant supplies heat to district heating system, its capacity is defined by maximum heat load of this system. In some cases thermal storage unit is attached to cogeneration plant for efficient operation of district heating system. Cogeneration plant with district heating provides an alternative energy production and delivery mechanism that is less resource intensive, more efficient and provides greater energy security than many popular alternatives. Figura 5 Cogeneration (1,5 MWe) and district heating plant (87,000 m) in Dobbiaco, Italy. 9

10 2.4. POWER GENERATION AND CO-GENERATION Cogeneration is defined as the sequential generation of two different forms of useful energy from a single primary energy source, typically mechanical energy and thermal energy. Mechanical energy may be used either to drive an alternator for producing electricity, or rotating equipment such as motor, compressor, pump or fan for delivering various services. Thermal energy can be used either for direct process applications or for indirectly producing steam, hot water, hot air for dryer or chilled water for process cooling. Various technologies are available for the electricity production based on biomass combustion; the steam turbine process, the steam piston engine process, the steam screw-type engine process, the ORC process, and the Stirling engine process. These technologies are applicable for the following power ranges: 1. up to 100 kwel: the only applicable technology for small-scale CHP plants is the Stirling engine process which is at the demonstration phase at present; 2. from 200 2,000 kwel: suitable technologies for medium-scale CHP plants are steam engines, steam turbines and especially the ORC process. These technologies are already available on the market; 3. larger than 2,000 kwel: the steam turbine process is the most relevant technology for large-scale CHP plants. 3. PYROLYSIS TECHNOLOGIES The liquid bio-oil product from fast pyrolysis has the considerable advantage of being storable and transportable as well as the potential to supply a number of valuable chemicals. In this respect it offers a unique advantage and should be considered complementary to the other thermal conversion processes. Biomass fast pyrolysis technologies for liquid fuel production have been successfully demonstrated at small-scale, and several large pilot plants or demonstration projects (up to 200 ton/day biomass feed design capacity) are in operation or at an advanced stage of construction. Earlier economic studies suggested that bio-oil is relatively expensive compared to fossil based energy (Bridgwater et al. 2001): however, in light of recent dramatic increases in petroleum prices, bio-oil may be cost competitive for direct utilization and potentially is competitive in the transportation fuel market after catalytic processing for deoxygenation. Commercialization of bio-oil production continues to face economic and other non-technical barriers when trying to penetrate the energy markets. As with biomass gasification, fast pyrolysis will only be able to penetrate energy markets if completely integrated into a biomass system. Thus the innovation in practically all demonstration projects under implementation lies not only in the technical aspects of the various processes but also in the integration of pyrolysis technologies in existing or newly developed systems where it can be demonstrated that the overall system offers better prospects for economic development. Niche markets are likely to be the most attractive in the short term, such as utilization of bio-oil in a marine environment where bio-oil is much less damaging than fuel oil in case of accidents. The overall challenges include small-scale efficient and cost effective power generation (electricity and mechanical power) and heat production systems, while also taking advantage of the more economically attractive chemicals markets. There is no obvious best technology. Fluid beds offer robust and scalable reactors, but the problem of heat transfer at large-scales in not yet proven. Circulating fluid beds and transported beds may overcome the heat transfer problem but scaling is not yet proven and there is an added problem of char attrition. Intensive mechanical devices such as ablative and rotating cone reactors offer advantages of compactness and absence of fluidising gas, but may suffer from scaling problems and always the problems associated with moving parts at high temperature. 10

11 There are specific challenges facing fast pyrolysis that relate to technology, product and applications including: Uncertainty of cost of bio-oil Availability: there are limited supplies for testing and development of applications There is a lack of standards for use and distribution of bio-oil and inconsistent quality inhibits wider usage; considerable work is required to characterize and standardize these liquids and develop a wider range of energy applications Bio-oil is incompatible with conventional fuels Users are unfamiliar with this material Dedicated fuel handling systems are needed Pyrolysis as a technology does not enjoy a good image The most important issues that need to be addressed are: Long term operation of large scale reactor systems Cost demonstration Improving product quality including setting norms and standards for producers and users Environment health and safety issues in handling, transport and usage Encouragement for developers to implement processes and users to implement applications Information dissemination 4. GASIFICATION TECHNOLOGIES Various technologies are employed for the gasification of solid fuels. The basic classification of these processes is carried out by considering the basic reactor principles that are applied: fixed-bed systems fluidized-bed systems entrained flow systems. Differentiation is based on the means of supporting the biomass in the reactor vessel, the direction of flow of both the biomass and oxidant, and the way heat is supplied to the reactor. All these processes can be operated at ambient or increased pressure and serve the purpose of thermochemical conversion of solid biomass (as a rule unprocessed forest wood chips) into a secondary fuel in a gaseous state, i.e. wood gas or producer gas. For thermochemical conversion it is necessary to supply sufficiently solid biomass and gasification media as well as to provide a pressure and temperature regime by means of suitable gas control and geometrical construction of the reactor. Pure oxygen, atmospheric oxygen or water vapour is used as gasification media. Here the gasification medium fulfils the function of a reaction partner in partial oxidation as well as reduction, which leads to the generation of the producer gas along with drying and pyrolysis processes in the reactor UTILISATION OF PRODUCT GAS The main application at presence of product gas from gasification is found in direct or indirect combustion to generate power with co-production of heat. Due to the typical complex composition of product gases, more advanced applications are not first choice, as the syngas components H 2 and CO represent only approx. 50% of the energetic value of a product gas. The only exception is the production of synthetic natural gas (SNG), as in that case the presence of (as much as possible) CH 4 is beneficial to achieve high overall yields. Therefore, in the context of SNG production, product gas is sometimes referred to as methane-rich synthesis gas. 11

12 Co-firing The most straightforward application of product gas is co-firing in existing coal power plants by injecting the product gas in the combustion zone of the coal boiler. Co-firing percentages up to 10% (on energy basis) are feasible without the need for substantial modifications of the coal boiler. Critical issue in co-firing is the impact of the biomass ash on the quality of the boiler fly and bottom ash. The application of the fly and bottom ash in construction and cement production often sets the specifications for the amount and type of biomass that can be co-fired. Examples of biomass co-firing plants are the AMER 85 MW t circulating fluidised bed (CFB) gasifier in the Essent power plant in Geertruidenberg (the Netherlands) and the Foster Wheeler CFB gasifiers in Lahti (Finland) and Ruien (Electrabel power plant, Belgium) Combined heat and power (CHP) In combined heat and power (CHP) plants the product gas is fired on a gas engine. Modified gas engines can run without problems on most product gases even those from air blown gasification that have calorific values of approx. 5-6 MJ Nm -3. Typically, the energetic output is one-third electricity and two-third heat. The main technical challenge in the implementation of integrated biomass gasification CHP plants has been, and still is, the removal of tar from the product gas. Regarding the presence of tar in product gas, it may be stated that tar is equivalent to a major economic penalty in biomass gasification. Tar aerosols and deposits lead to more frequent maintenance and repair of especially gas cleaning equipment and resultantly lower plant capacity factors. This leads to a decrease of revenues or to higher investments, as some equipment will be installed in duplicate to overcome standstills. Furthermore, removal of tar components from the process wastewater requires considerable investments that can even be dramatic as some tar components show poisoning behaviour in biologic wastewater treatment systems. The few technological successful biomass gasification plants, e.g. the plants in Güssing and Harboøre (Denmark), have in common that they result from long development trajectories and that the technologies are neither simple nor cheap. Interesting development for small-scale gasification power production is the externally fired Stirling engine. In the Figura 6 a engine Stirling pyrolysis plant with a nominal output of 35 kw electric (20% efficiency) and 110 kw thermal energy and 110 kw BioCha. Up to 28% efficiency is aimed at by improving the process and scaling up to 150 kw (e). The system does not require a wood gas cleaning or flue gas filtration. Figura 6 Biomass gasifier with a Stirling engine of Stirling DK in Lyngby, Denmark: a) layout; b) burner and stirling engine. 12

13 4.2. GAS CLEANING In biomass gasification plants the system components are used to clean process gas for the purpose of attaining specific clean gas quality parameters of the generated producer gas for subsequent utilization. The presently used gas cleaning systems are based on the application of processes adapted from de-dusting and washing of process gas, which interact to fulfil the requirements of biomass gasification plants (dust and tar loading). The systems are differentiated according to the following factors: the pollutant fraction in the process gas (dust, tar, heavy metals, alkali or alkaline earth metals, permanent pollutant gas, etc.) to be eliminated; process media (dry, wet, half-dry, etc.); separation mechanisms; operating temperatures and pressures; separating efficiency. The guarantee of certain separation efficiencies is a fundamental condition for the operation of a gas conditioning unit as well as gas utilization, since particle loads with regard to condensed hydrocarbon, heavy metal as well as alkaline and alkaline earth metal compounds can lead to malfunctions and impairments of diverse components of gas utilization systems (gas mixer, gas control and safety system, exhaust gas turbocharger, oxidation catalytic converter, etc,). Figura 7 Typical separation efficiencies of gas cleaning systems. 13

14 Figura 8 Gasification plant with fixed bed (80kwe) of ENEA in Trisaia, Italy. 14