BIOMASS GASIFICATION IN DOWNDRAFT REACTOR FOR POWER GENERATION

Transcription

1 BIOMASS GASIFICATION IN DOWNDRAFT REACTOR FOR POWER GENERATION N. CERONE, L. CONTUZZI, S. CAVALIERE, F. ZIMBARDI, G. BRACCIO ENEA, Dipartimento Tecnologie per l'energia, Fonti Rinnovabili e Risparmio Energetico, Sezione Biomasse Rotondella (MT) 75026, Italy SUMMARY: A plant based on a 2 m 3 downdraft air-blown reactor, integrated with an internal combustion engine (ICE) and an electricity generator, was run to assess the specific parameters for the ligno-cellulosic biomass conversion to power. Experimental tests were carried out to quantify the performance of this system and include: matter and energy flows (to assess the material and energy balances), conversion efficiency and specific fuel consumption. Two types of biomasses were used: oak and turkey oak, both loaded as cubes of side 0.1 m. The low heating value (LHV) of the dry gas was calculated from the gas composition and resulted in 977 kcal/nm 3 for turkey oak and 884 kcal/nm 3 for oak; this difference showed the dependence of the produced gas composition on the used biomass, probably caused by different ash content and composition. The overall energetic gasification efficiency ranged from 0.69 to 0.79, increasing with the degree of solid conversion. The cleaned produced gas was used to feed the modified diesel engine which, in turn, was connected to a generator for electrical power generation. 1. INTRODUCTION The growing importance of biomass as a modern energy source calls for substantial R&D efforts on biomass from both the energy and the environment point of view (McKendry, 2002). The aims are to carry out a comprehensive energy-environment assessment of biomass. As part of the efforts, the gasification is interesting to produce combustible fuel gas for thermal and power generation applications and to reduce environmental pollutions (Bridgwater, 1995). One of the major issues in biomass gasification is to deal with the tar formed during the process (Devi & al 2003). Fixed bed downdraft gasifier configuration has led to the advantage of minimal tar contamination in the product gas compared to other types of reactors; therefore, extensive gas cleaning is not needed for coupling with an internal combustion engine. Moreover, this technology has a good reliability and the process is easy to operate (Zainal & al 2002; Midilli & al, 2001). The aim of this work was to assess the specific parameters of such system coupled with an internal combustion engine and an electricity generator for an overall production of 30 kw el (Larson, 1993).

2 2. EXPERIMENTAL APPARATUS In this work the pyrolysis and gasification plant built at the ENEA Trisaia Research Centre, named PIGA, was used. This apparatus is constituted by: Fixed bed downdraft reactor; Scrubber, demister and biomass filter for the gas cooling and cleaning; Volumetric ICE, synchronous alternator and electrical resistance to dissipate produced energy. The schematic diagram of the plant is shown in Figure 1. P 2 T 1 P 1 T T air T T 2 Q v,gas 5 To t he engine 6 Figure 1. Schematic diagram of the plant PI.GA.: 1) Downdraft Gasifier; 2) Cyclone; 3) Scrubber; 4) Demister; 5) Filter-biomass; 6) Balance. T and P indicate points of temperature and pressure measurement respectively. 2.1 Gasifier The gasifier used in the tests is a fixed bed downdraft, usually operated by using air as gasification medium. The schematic diagram of the gasifier is shown in Figure 2. It is shaped like a cylinder two meters high with a diameter of one meter and is made in carbon steel doublesided wall. All surfaces subject to high temperature, heavy heat stress and chemical aggression are made of stainless steel of thickness proportional to the actual condition of work. At the hot zone, a thermal insulation is placed to reduce the transmission of heat outwards. A serie of 6 thermocouples are positioned along the height of the gasifier, from the cone reaction chamber (h=0) to the top of the gasifier (h= 1.4 m), to monitor the gasifier thermal profile. The produced gas in the area of reduction, where the gasification reactions occur, passes outside the jacket of the gasifier and exchanges heat with the interior, where is the loaded biomass, and with the external environment (the gasifier is not adiabatic). The average temperature of the produced gas at the output of the gasifier is within C. The gas sampling points are positioned on the top of the reactor placed on the gas output. In this type of reactor the combustion products

3 are concurrent with biomass and the chamber of the burner has complied with a double cone reaction.the downdraft gasifier has been run in batch mode by loading biomass having the shape of a cube with side 0.1 m. Out SYNGAS BIOMASS Gasification medium: AIR CONE REACTION gas ash Figure 2. Schematic diagram of the downdraft gasifier 2.2 Cleaning section The hot gas obtained from the gasifier, contains particulate matter and tar as impurities that have to be removed before the gas is supplied to the engine. The cooling and cleaning section consists of the following 4 stages: Two cyclones 2 m high and with a diameter of 0.4 m that remove dust and most of the particulate matter present in the gas; The water scrubber that cleans and cools the gas from the tars and is shaped like a parallelepiped 1.70 m high with the basis of 0.45x0.8 m. Fresh water was loaded at beginning of the test and recycled at the head of the scrubber for the duration of the test, then analysed and disposed; The demister that removes the droplets of liquid gas drag and consists of a tube bundle composed of 6 elements arranged in series. The apparatus is 1.3 m high, 1.3 m wide and 0.8 m long. The removed liquid is collected in a tank located on the lower part; The filter-biomass that dries and removes any remaining particles and is shaped like a cylinder 0.8 m wide and 1.6 m high. Wood chips or coarse sawdust are used as filter and at the end of their cycle of use they can be gasified in the reactor. 2.3 Power section This section is constituted by: 13.8 l diesel engine (FIAT IVECO) modified Otto cycle for gas feeding; 25 kva synchronous alternator (TESSARI);

4 A set of electrical resistances to dissipate the produced energy in a water bath up to 24 kw by steps of 1-3 kw. 3. EXPERIMENTAL PROCEDURE Five trials were carried out for each biomass, each of them lasting about five hours. A typical test consisted of the following steps: 3.1 Biomass loading The two woods were provided by a local mill as regular cubic pieces of 0.1 m side. About 65 kg of biomass (10-15% of moisture) were loaded manually and randomly in the gasifier, then the gasifier was closed with a cover which was tight with springs. 3.2 Ignition The ignition took place through a wick soaked with naphtha introduced in the main air supply. At this stage the feed of air was ensured by a centrifugal fan connected to the main air supply that provided a slight overpressure. The procedure was sometimes sped up by introducing in the ignition area a small quantity of char. 3.3 Gasification at not stationary auto thermal conditions In the initial stage the produced gas was sent in a primary torch directly from the gasifier: it was necessary to wait for that the gas quality to be sufficient to be sent to the engine, otherwise the low quality of the gas (tars, water steam) could have overloaded the filters (Hasler and Nussbaumer, 1999). This was empirically deducted by observing the colour and shape of the flame that became light blue and stable. At this point gas was sent through the filter and then through a pipe connected to a secondary torch, still bypassing the engine. After switching to this other torch, the gas was ready to be sent to the engine. 3.4 Gasification at stationary auto thermal conditions The plant was fed through the aspirations of the engine, which is in light depression; the value of the depression was variable, reached the maximum value at the output of the plant and was dependent on the level of the generated power. Different parameters were monitored during the tests by taking readings every 5 minutes. Temperatures and pressures were measured at the input and output of each device. For the gasifier, the inner profile of temperature was determined. Flow rate of the produced gas and air were also measured through differential pressure. The amount of gasified biomass was evaluated by a balance and the weight change with the progress of gasification was determined. The composition of the produced gas was determined by gas- chromatography. Sampling of gas were made every fifteen minutes, after the biomass filter and before the engine, where the temperature ranged bewteen C and the absolute pressure was 86,600 Pa. 3.5 Engine running and power generation The RPM is the parameter monitored for the ICE. The electrical power produced by the alternator was assessed through the inclusion of several resistances in a bath (from 1 kw and 3 kw) to dissipate the produced energy. The maximum applicable load was 24 kw.

5 4. EXPERIMENTAL RESULTS AND DISCUSSION The composition of two biomasses, as obtained by the CEN/TS procedures, is reported in the Table 1. Turkey Oak has a lower content of ash and a corresponding higher calorific value and fixed carbon (Sutton, 2001; Demirbas, 2004)). The profile of temperature was used to assess when the auto thermal condition were reached: this happened after one hour from the ignition and corresponded to the consumption of about 20% of the loaded biomass. The compositions of the produced gas during the tests and for increasing electrical load are reported in Table 2. As the gasification proceeded, for both biomasses, the H 2 and CO 2 concentration decreased, while the CO concentration increased. The change of the composition was due to the change of the gasified solid; indeed the first reactions of pyrolysis led to a solid containing more C, which is a condition promoting the Boudouard reaction (Giltrapa 2003). The heating value of the produced gas was calculated on the basis of the composition and resulted on average 977 kcal/nm 3 and 884 kcal/nm 3 for Turkey Oak and Oak respectively. The overall results of the tests are reported in Table 3. The energetic yield of gasification, defined as: V LHVgas η gas. = M LHV biom. (where V is the total volume of produced gas, M is the gasified biomass) increased with the proceeding of the gasification from 0.69 to 0.79 for both biomasses. The overall energetic efficiency of the process defined as: E η total = M LHVbiom (where E is the produced electrical power) reached the value of 0.12 when the best coupled conditions between gasifier and engine were used. This value is lower than the typical value reported for the conversion of biomass to electricity, probably because of the oversized engine respect to the gasifier potential. The filtering section removed flying ash, tar, water steam corresponding on the whole to 3-4% of the fed biomass; this value was calculated from the mass balance having considered the total gas produced, the ash remained in the reactor and the biomass composition. Table 1. Analysis of biomasses Oak Turkey Oak Volatiles (w %) Fixed carbon (w %) LHV (kj/kg) Ash (w %) CaO % (in ash) K 2 O % (in ash) Na 2 O % (in ash) MgO % (in ash)

6 Table 2. Gas composition from Turkey Oak and Oak after cleaning (% vol) Turkey Oak Oak 12 kw (a) 18 kw (b) 24 kw (c) 12 kw (a) 18 kw (b) 24 kw (c) H CO CH CO O N (a) Average values measured from t=0 to t=30 min; (b) from t=35 to t=65 min; (c) from t=70 to t=105 min Table 3. Main operation parameters for Turkey Oak and Oak conversion Turkey Oak Oak 12 kw 18 kw 24 kw 12 kw 18 kw 24 kw Gasified Biomass, kg Total LHV of the gasified biomass, kwh Volume of produced gas, m Volume air of gasification, m Total LHV of produced gas, kwh Average flow of produced gas, Nm 3 /h Average flow of air, Nm 3 /h Electric energy, kwh CONCLUSIONS The examined configuration of the downdraft gasifier integrated with a modified diesel engine and an electrical generator showed a good reliability. The repeatability of the results was verified on the produced gas composition and flow rate during 5 tests for each biomass in the same experimental conditions. The use of different types of biomass did not lead to different results from the operational point of view and had only slight differences in the gas composition and the corresponding calculated LHV, 977 kcal/nm 3 for Turkey Oak and 884 kcal/nm 3 for Oak. The overall energetic gasification efficiency ranged from 0.69 to 0.79, increasing with the degree of solid conversion. ACKNOWLEDGEMENTS We thank the technicians of the chemical lab for their analytical work. REFERENCES Bridgwater A. V. (1995) The technical and economic feasibility of biomass gasification for power generation, Fuel, 74, pp Devi L., Ptasinski K. J. and Janssen F. J. J. G. (2003) A review of the primary measures for tar

7 elimination in biomass gasification processes, Biomass and Bioenergy, 24, pp Demirbas A. (2004) Combustion characteristics of different biomass fuels, Progress in Energy and Combustion Science, 30, pp Giltrapa D. L., McKibbinb R. and Barnes G. R. G. (2003) A steady state model of gas-char reactions in a downdraft biomass gasifier, Solar Energy, 74, pp Hasler P., and Nussbaumer Th. (1999) Gas cleaning for IC engine applications from fixed bed biomass gasification, Biomass and Bioenergy, 16, pp Larson E. D. (1993) Technology for Electricity and Fuels from Biomass, Annual Review of Energy and the Environment, 18, pp McKendry P. (2002) Energy production from biomass (part 1): overview of biomass, Bioresource Technology, 83, pp Midilli A., Dogru M., Howarth C. R., Ayhan T. (2001) Hydrogen production from hazelnut shell by applying air-blown downdraft gasification technique - International Journal of Hydrogen Energy, Elsevier Sutton D., Kelleher B. and Ross Julian R. H. (2001) Review of literature on catalysts for biomass gasification, Fuel Processing Technology, 73, pp Zainal Z. A.,, Ali Rifau, Quadir G. A. and Seetharamu K. N. (2002) Experimental investigation of a downdraft biomass gasifier, Biomass and Bioenergy, 23, pp