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Experimental investigation of a 125 kW twin-fire fixed bed gasification pilot plant and comparison to the results of a 2 MW combined heat and power plant (CHP)
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w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Experimental investigation of a 125 kW twin-fire fixed bed gasification pilot plant and comparison to the results of a 2 MW combined heat and power plant (CHP) Robert Kramreiter⁎, Michael Url, Jan Kotik, Hermann Hofbauer Institute of Chemical Engineering, Vienna University of Technology, Getreidemarkt 9/166, A-1060 Vienna, Austria
AR TIC LE I N FO
ABS TR ACT
Article history:
Fixed bed biomass gasification is a promising technology to produce heat and power from a
Received 28 June 2007
renewable energy source. A twin-fire fixed bed gasifier based CHP plant was realized in the
Accepted 2 August 2007
year 2003 in Wr. Neustadt, Austria. Wood chips are used as fuel, which are dried and sieved before being gasified to a low calorific gas of about 5.8 MJ/Nm3dry. Before the clean gas is fed
Keywords:
into a gas engine a cyclone and a RME (rapemethylester)/H2O quench system followed by a
Biomass conversion
wet electrostatic precipitator (ESP) is used for gas cleaning. The CHP plant has a fuel power of
Gasification
2 MWth and an electric output of 550 kWel. As scale up and optimization tool a hot test rig
Fixed bed
with a capacity of 125 kWth was built. Basic parameters like the type of wood chips, power and air distribution were varied to investigate the effect on gas composition, tar content in the producer gas and carbon content in the ash. Additionally a temperature profile over the height of the 125 kW hot test rig was measured. Furthermore, the results from the hot test rig are discussed and compared with the results from the 2 MWth demonstration plant. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
Different biomass gasifier designs have been developed during the last 20 years with the intention to establish an efficient combined heat and power production system based on biomass. The producer gas from the biomass gasifier is cleaned and afterwards fed into a gas engine or turbine which is coupled to an electricity generator. The advantages especially in the small scale range are the high overall biomass utilization and the high electrical efficiencies at the same time. Fixed bed gasifiers are mainly used in the small scale range whereas for larger scale fluidized bed gasifiers are proposed. For combined heat and power production air is normally used as gasification agent [1]. Originally, two different types of fixed bed gasifiers were developed: updraft and downdraft gasifiers. In case of an updraft gasifier the gasification agent is introduced at the bottom and the gas flow is upwards counter-current to the biomass which is
fed from the top. The producer gas leaves the gasifier at the top and the ash is withdrawn at the bottom by a grate. Several zones are created in an updraft fixed bed gasifier which are – starting from the top – the drying, the pyrolysis, the reduction and the oxidation zone. The temperature is increasing from the top to the bottom. The tars are produced mainly in the pyrolysis zone and leave the gasifier together with the producer gas. As there is no zone above the pyrolysis zone which has a higher temperature to thermally destroy the tars a high amount of tars is produced [2–4]. Downdraft gasifiers try to avoid the disadvantage of high tar contents by introducing the gasification agent not at the bottom but at the top or at least at a certain height above the bottom. The main difference to updraft gasifiers is that the gas flows cocurrently downwards with the biomass. This leads to a different order of the reaction zones from top to bottom, namely, drying, pyrolysis, oxidation, and reduction zone with the result of a low tar content in the producer gas. The disadvantage occurs for this
⁎ Corresponding author. Tel.: +43 1 58801 15973; fax: +43 1 58801 15999. E-mail address: [email protected] (R. Kramreiter). 0378-3820/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2007.08.001
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type of gasifier which is a high carbon content in the bottom ash. This can be explained by the reduction zone which is the first zone above the grate [2–4]. A twin-fire fixed bed gasifier is able to combine the advantages in one gasifier and avoid the disadvantage of both systems above. A twin-fire fixed bed gasifier has got two locations for the air introduction: one in the upper part of the fixed bed and another from the bottom via a grate. Therefore, the upper part of the gasifier behaves like a downdraft gasifier and the lower part like an updraft gasifier. Contrary to the downdraft gasifier there will be an additional oxidation zone established above the grate. This should lead to a low tar content in the producer gas and also to an ash with a comparable low carbon content [5]. A lot of developments of different designs of fixed bed gasifiers have been carried out during the last two decades. Among these developments there are all three kinds of the above mentioned gasifiers. Furthermore, two stage gasifiers are currently under development which try to separate the pyrolysis step from the gasification step of the remaining char in two reactors. This leads to an additional high temperature reactor but allows a better controllability of the whole process. Extremely low tar contents suitable for a direct use in gas engines are the result [6]. Demonstration plants for all these systems can be found in different countries in Europe [7]. A 2 MWth plant for a twin-fire fixed bed gasifier was realized in the year 2003 in Wr. Neustadt, Austria. This is the largest plant of that type ever built and
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therefore development work had to be carried out to obtain a successful scale up and operation. As scale up and optimization tools a hot test rig with a capacity of 125 kWth was built, simulation and modelling was carried out, and also a cold flow apparatus to study the moving of the particles through the reactor were used. In this paper the work and the results from the hot test rig are discussed and compared with the 2 MWth demonstration plant.
2.
Experimental
2.1.
Gasification technology of the 2 MW CHP plant
The plant described in the following has been built in Wr. Neustadt/Austria and consists of a so called twin-fire fixed bed gasifier with a fuel power of about 2 MWth, a cyclone and a RME (rapemethylester)/H2O gas cleaning system followed by a wet electrostatic precipitator (ESP). The purified product gas is utilized in a gas engine with a capacity of 550 kWel. The exhaust gas from the gas engine in led into an existing biomass boiler in order to meet the emission limits. The configuration of this plant is illustrated in Fig. 1. The feedstock of this plant is freshly chipped wood from forestry. It is fed from the daily hopper by a moving floor and a screw conveyor into the rotary sieve dryer. In this apparatus the
Fig. 1 – Flow chart of the 2 MWth CHP plant Wr. Neustadt/Austria [14].
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feedstock is conditioned to meet the quite stringent fuel specifications in terms of particle size and water content of the fixed bed gasifier. Preheated air from the district heating boiler is utilized for the drying of the fuel. The exiting air is fed back to the boiler and post combusted to eliminate any smells and dust emitted from the dryer. The sieved small fuel particles are fed also into the boiler and used as additional fuel. The gasifier is a fixed bed type with several reaction zones and combines the advantages of up- and downdraft fixed bed gasifiers (Fig. 2). It is a further development of the original Deutz-twin-fire-gasifier and is based on the experience gained from a pilot plant in Domsland/Germany [8]. Air is supplied at two positions: after the drying zone of the biomass and via the grate from the bottom of the gasifier. In the upper part the gasifier shows downdraft behavior. Feedstock and product gas are moving in the same direction. In the lower part updraft behaviour is obtained. Feedstock and product gas are moving in the opposite directions. Therefore, the gasifier design combines the advantages of a downdraft and an updraft gasifier. This will lead to a producer gas with a low tar content (down draft) and a low carbon content in the ash (up draft). The product gas leaves the reactor at about 650 °C, is firstly cooled in the air pre-heater of the gasifier. A cyclone removes most of the dust and char particles from the hot product gas. High amounts of both, dust and char, can disturb the quench gas cleaning system. Afterwards the hot product gas is quenched instantly to 50 °C. The quench cooler operates with rapemethylester (RME) and also water. The water is sprayed in at first to avoid cracking RME at 650 °C. The sedimentation tank below the quench system separates the two phases, which are RME and water. The reason to utilize RME in the gas cleaning process is due to the better solubility of tar. The accumulated residues (dust and char) at the interface (RME/H2O) of the sedimentation tank will be transferred back into the fixed bed gasifier. For the final gas cleaning a wet ESP is installed. The drain of the wet electrostatic precipitator is put into a slurry tank, which finally is also led back into the gasifier.
Table 1 – Characteristic data of the plant Type of plant Fuel Fuel power Electric output Thermal output Electric efficiency Thermal efficiency Total efficiency
Demonstration plant Wood chips 2000 kW 550 kW 720 kW 27.5% 36.0% 63.5%
After the wet ESP the clean gas is compressed and cooled before being supplied to the gas engine. A Jenbacher gas engine is used for internal combustion to yield 550 kWel of electrical power and 720 kWth of district heating power. Furthermore, the exhaust gas from the gas engine is blown into a biomass boiler. Thereby, the remaining CO and other combustible components are oxidized to meet the emissions limits. Table 1 summarizes the most important characteristic data of the CHP plant.
2.2.
Description of the 125 kW pilot plant
The pilot plant is a scale down from the demonstration plant in Wr. Neustadt described above and was built as an optimization tool for this plant. Basic parameters like air distribution, gas composition, tar content in product gas and carbon content in the ash should be improved. Additionally the type of fuel, water content, and power is varied and investigated. The 125 kWth fix bed gasifier consists of five essential parts: • • • • •
Biomass daily hopper and screw conveyor Twin-fire fixed bed gasifier Rotary grate, ash discharge device and ash hopper Product gas pipe with cyclone Measurement/analysis equipments
The product gas is combusted in a burner and is cleaned of dust and char in an additional second cyclone before the gas is discharged to the environment via the chimney.
2.2.1.
Daily hopper and screw conveyer
The biomass daily hopper and the screw conveyor are illustrated in Fig. 3. The daily hopper has a capacity of 2 m3. A test takes a time of about 15 h. During the run the fuel cannot be refilled because the daily hopper is closed airproofed with a cap on the top. Furthermore the hopper is supplied with an inert sealing gas. A screw conveyer continuously feeds biomass into the gasifier at the top of the reactor. The amount of fuel can be varied by a frequency converter. The biomass flow out of the hopper depends on the bed density and the rotation speed of the drive motor. The mass flow of biomass was in the range of 15–45 kg/h. The level of the wood in the gasifier can be measured with a manual dip stick.
2.2.2.
Fig. 2 – Twin-fire fixed bed gasifier.
Twin-fired fixed bed gasifier
The pilot scale gasifier consists of two concentric pipes. The inner pipe has a diameter of 200 mm and a height of 1050 mm and is about one third shorter than the outer pipe, which has an inner diameter of 300 mm and a height of 1650 mm. On the bottom side of the outer pipe of the gasifier a conical rotary grate is connected by a flange (Figs. 4 and 5). The whole reactor is
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the conical rotary grate. Additional air can be charged to the ash room to be used as barrier air and for cooling the rotary grate (grate air). The product gas leaves the reactor through the annular space. The advantage of this construction is that the primary air is preheated and the heat of the product gas is transferred to the inner pipe to support drying and pyrolysis of the biomass.
2.2.3.
Rotary grate and ash hopper
Fig. 5 shows the grate of the 125 kWth pilot plant. The conical rotary grate has an angle of 60° and 6 fins with a height of 10 mm each. The advantage of this construction is to avoid the formation of bridges and to promote the bed movement. Between the outer wall and the grate there is a slot of 8 mm. The secondary air inlet is just above this slot is. The ash can fall down into the ash chamber through this slot where the ash discharge device is installed which transports the ash into the ash hopper. The ash discharge device is fixed on the same shaft as the rotary grate. Between the ash chamber and the ash hopper a ball valve is installed to be able to exchange the ash hopper during the runs and also to take out ash samples for analysis. Fig. 3 – Biomass daily hopper and screw conveyer.
made of stainless steel and covered outside with an insulation layer. In the inner pipe primary air is injected via eleven nozzles. In the outer pipe secondary air is injected with eight nozzles above
2.2.4.
Complete test facility and test procedures
Tests are performed autothermically (no heat supply by e.g. electrical heating system) at atmospheric pressure and air as gasification agent. The P&I diagram of the 125 kWth twin-fire fixed bed gasifier is illustrated in Fig. 6. The primary air is provided by an air compressor. The secondary and grate air are provided by a compressed air system. These flows are measured
Fig. 4 – Detail of the 125 kWth pilot plant at the TU Vienna.
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Fig. 5 – Grate and ash hopper of the 125 kWth pilot plant.
thermocouples and 10 pressure manometers are used to monitor temperature and pressure profile whereas 7 different thermocouples are installed in the inner pipe for accurate information about the reaction zones. The cyclone is used as a dedusting unit for product gas cleaning. Both, cyclone and product gas pipe are heated electrically and insulated to prevent tar condensation. The tar, dust and gas analysis sampling point is installed between the reactor and the cyclone and is described in detail in the following chapter. For start up preheated primary air is utilized for the ignition of the wood chips. When the desired temperatures are achieved (in the range of 800–1000 °C) the grate can be activated. Then, the primary, secondary and grate airs are adjusted to maintain the desired temperatures. The product gas leaves the reactor at about 650 °C through the annulus. In general, it takes about 2 h to reach a stable operation with respect to the reactor temperatures. At a steady state operation, all parameters are kept constant for at least 60 min for gas sampling and analysis.
2.3. by rotameters. The grate air can be charged to the ash room to be used as sealing gas and for rotary grate cooling. Both air systems (secondary and grate air) offer the advantage to ensure a low carbon content in the ash. In the whole pilot plant 17
Producer gas sampling and measurement procedures
After reaching a steady state operation, producer gas sampling is turned on. A small stream of the producer gas is sucked off through a filter box filled with glass wool. After passing washing bottles filled with RME and a cooler, the clean and cool gas is
Fig. 6 – P&I diagram of the 125 kWth twin-fire fixed bed gasifier of the Vienna University of Technology.
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Table 2 – The test conditions and results of the fix bed gasification Parameter/run
Run_4 Run_5 Run_6 Run_7
Wood chip type Water content [%] Primary air flow [Nm3/h] Secondary air flow [Nm3/h] Grate air flow [Nm3/h] Load range [%] Tar content [g/Nm3dry] Dust content [g/Nm3dry] Biomass consumption (wet) [kg] Grate ash [g] Unburnable part — grate ash [%] Cyclone ash [g] Unburnable part — cyclone ash [%] Total air flow [Nm3] Equivalent ratio [–]
Type 1 11 20–30 0 4–6 b 95 1.2 4.5 192 6227 30 2757 29 224 0.28
Type 1 11 20–30 0–4 4–6 b 95 1.0–1.2 1.5–2.4 159 3074 40 3581 24 190 0.29
Type 2 25 10–28 2–4 4–7 50–88 0.6–2.0 1.0–1.6 185 2380 66 439 39 227 0.36
Type 2 18 18 2 4 65 0.2 0.8 76 868 65 135 47 93 0.33
analysed by a gas chromatograph (model GC 955) with TCD detector using Helium as carrier gas. Gas components like O2, N2, CO2, CO, CH4, C2H4, C2H6, and C3/C4 are detected. The concentration of the H2 is determined by difference to 100%. Additionally, a multi component analyser (Rosemount NGA 2000) is used, measuring O2, H2, CO, CO2, CH4 continuously. The measurement point for tars and particles is located before the cyclone. The producer gas is sampled through a particle filter system consisting of a cyclone and a filter box filled with glass wool. Afterwards, the gas passes through washing bottles and is cooled by a cooling bath with glycol (−20 °C). The bottles are filled with toluene for removing the tar components. The measurement and analysis of the tar and particle is based on the “Tar Guideline” [9]. Further details can be found in [10].
3. 3.1.
Results and discussion Range of operation of the 125 kW pilot plant
The gasifier was designed for a nominal fuel power of 125 kWth. This means that 25 kg/h dry wood chips are fed into the reactor
Fig. 8 – Wood chips type 2.
having a lower heating value (LHV) of the fuel of approximately 18 MJ/kg. For the calculation of the required air for the partial oxidation a stoichiometric oxygen demand calculation for different air ratio was executed: Vair ¼ k
" # 22:41 wC wH wO Nm3 air þ 0:21 12 4 32 kg fuel
The equivalent ratio (λ) was a result of the desired temperature during steady state operation. A lot of test runs were carried out with the 125 kWth pilot plant to understand the behaviour of this gasifier and to get data for comparison and improvement of the 2 MWth demonstration plant in Wr. Neustadt. Table 2 shows the test conditions and main results of 4 selected runs.
3.1.1.
Wood chips used for the runs
Figs. 7 and 8 show the wood chip type 1 and type 2 which were used in the pilot plant. The wood chips had a particles sizes between 11.2 mm to 31.5 mm. Type 1 was a mixture of wood chip from different trees (maple, cottonwood, ash tree) with a water content of ∼11%. The wood particles were long and sharp. Type 2 mostly consisted of beech wood with a water content from 18% to 25%. This type of wood chip had more spherical particles which are better to avoid the formation of bridges in the gasifier. Table 3 shows the elemental analysis of type 1 and type 2.
3.1.2. Pressure drop, total air flow and height of fixed bed versus time Fig. 9 shows the pressure drop, total air flow and level of the fixed bed of the gasifier for run 4 which was carried out with wood chips
Table 3 – Elemental analysis of the dry wood chip type 1 and type 2 Wood chip-type
Fig. 7 – Wood chips type 1.
Type 1 Type 2
Elemental composition [m-%] C
H
O
N
49.9 49.7
6.1 6.1
43.7 43.8
0.3 0.4
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Fig. 9 – Pressure drop, total air flow and bed height of the “run 4” with biomass type 1.
of type 1. As expected the pressure drop increases with rising air flow and height of fixed bed. One goal of this run is to find optimized operating conditions for the gasifier at full load. Thus, the total air flow was increased stepwise over the presented runtime. The drop down of the height of the bed and the pressure at the time 10:50 was not caused by a rotation of the grate. Obviously bridges occurred in the gasifier. When these bridges break down the pressure drop goes down in parallel. This happened several times during this run. At the time 14:30 the pressure drop broke down and stayed nearly constant although the bed height and the air flow were changed. A channel formation in the bed was supposed to be the reason for this. During the whole run the flow of the grate air was 4–6 Nm3/h and no secondary air was introduced in this run,
whereas the temperatures in the range of the grate were at a relative low level (800–850 °C). Fig. 10 shows the pressure drop, total air flow and bed height of the gasifier during run 6 which was carried out with wood chips of type 2. These wood chips had a water content of 25 wt.%. The run was executed in part load between 50 and 88%. The assumed air flow at full load for the conditions in this run is 40 Nm3/h (λ = 0.35). The bed height fluctuations were moderately and the pressure drop changed only according to the total air through flow. The rotary grate was activated every 15–20 min for approximately 5 s. As mentioned already above this type of wood chips had more spherical particles which are better for the flow behavior. Therefore the wood chip type 2 had a better operation behavior
Fig. 10 – Pressure drop, total air flow and bed height of the run 6 with biomass type 2.
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Fig. 11 – Temperature profile inside the gasifier without secondary air taken from run 4.
in the gasifier. The part load operation of the gasifier is also very well and the particle discharge is substantially lower. During the whole run the flow of the grate air is 4–7 Nm3/h and the secondary air is in the range of 2–4 Nm3/h.
3.1.3.
Temperature profiles
Temperature profiles over the gasifier are illustrated in Figs. 11 and 12. Core temperatures of 70–90 °C indicate the drying zone. Temperatures of 200–350 °C were reached in the upper part of the bed near the wall. Consequently, the pyrolysis already begins. In the core zone the volatilization takes place before or shortly after the primary air injection. Due to the temperature difference between the bed in the inner tube and the annulus of 200–300 °C a heat transfer from the wall into the centre occurs. This heat transfer through the bed is very slow.
For better orientation the level of the inner pipe and the primary- and secondary air injection is shown as well in Fig. 11. The first oxidation zone was located in the inner pipe approximately 200 mm below the primary air injection. A second oxidation zone is expected in the lower part above the rotary grate. In this zone the temperature were quite low since no secondary air was used in this run. Only the grate air was introduced with a flow of 4 Nm3/h. Furthermore, the temperature in the upper part of the grate (480 mm) decreases. In this zone and the upper part of the inner pipe obviously endothermic reduction reactions occurred. In the annulus the temperature of the product gas is significantly increased due to the heat from the oxidation zone (up to approx. 700 mm). For drying and pyrolysis above this level the heat of the product gas is exchanged to the inner pipe. According to the construction the
Fig. 12 – Temperature profile inside the gasifier with secondary air taken from run 5.
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Fig. 13 – Composition and LHV of the dry product gas from the steady state run 7.
primary air was preheated with the hot upward streaming product, whereby the product gas was cooled down again. Another temperature profile inside the gasifier taken from run 5 is represented in Fig. 12. In this case the grate air (6 Nm3/h) and additionally the secondary air with a flow rate of 4 Nm3/h were introduced into the gasifier. This led to an increased temperature in the area of the secondary air injection because an oxidation zone was generated close to the rotary grate. Temperatures of about 950 °C were obtained. The temperatures near the grate are significantly higher than the temperature of the annulus.
3.1.4.
Producer gas composition and LHV
A typical producer gas composition and the resulting lower heating value are shown in Fig. 13. In this run (run 7) wood chips of type 2 were used. The chips were dried from a water content of
25 wt.%wet down to 18 wt.%wet before entering the gasifier. The ratio of primary-to secondary and grate air was 3:1 (18 Nm3/ h primary-, 2 Nm3/h secondary-, and 4 Nm3/h grate air). This corresponds to a partial load of 65%. During the steady state run product gas with an average LHV of 5.8 MJ/Nm3dry was produced.
3.1.5.
Tar and particle analysis
The particles in the product gas are defined as the sum of dust (inorganic), char, and tars adsorbed on the particles. Fig. 14 shows the results of the tar and particle analyses of several runs (4, 5, 6 and 7) for different water contents of the biomass. The tar content analyzed was in the range of 0.2–2 g/Nm3dry and a tendency for the amount of tars regarding to the water content cannot be observed. The particle content of the run 5–7 was in the range of approximately 0.8–2.4 g/Nm3dry. The
Fig. 14 – Tar and particle analysis of the runs 4, 5, 6 and 7.
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Fig. 15 – Average product gas composition of the runs 4, 5, 6 and 7 and LHV.
relatively high particle content (4.5 g/Nm3dry) during the run 4 can be explained by the large particle discharge at the end of the gasification period. The run results gathered at the pilot plant (especially those from runs 5, 6, and 7) were in a good accordance compared to the results from operation of the demonstration plant in Wr. Neustadt.
3.1.6.
Discussion of several important parameters
The executed runs showed that the particle shape and the particle size of the wood chips have a big influence on the operation of the gasifier. Consequently, two fractions b11.2 mm N31 mm were sieved out and the wood chip fraction 11.2 mmb × b31.5 was used in the gasifier. According to the size of the plant an upper limit for the particle size of 31.5 mm was assumed. This is in good agreement
with literature [1,11,15]. An influence of the particle shape could be recognized too. As mentioned before the wood chips of type 2 had more spherical particles in comparison to the wood chip type 1 which is better for a smooth flow behavior and to avoid the formation of bridges. Therefore, wood chip type 2 showed a better operational behavior in the gasifier. The grate design and operation and consequently the ash discharge are important figures for a satisfying gasification operation. The high pressure drop in the wood bed promotes channel formation. Therefore, a symmetric flow of the product gas through the bed cannot be secured. This channel formation and the associated raising particle discharge from run 4 (Fig. 9) could be the reason for the sudden pressure drop decrease. Thus higher organic fractions in the ash and higher tar
Fig. 16 – Fuel and chemical power curves during a run of 288 kWel engine power.
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Table 4 – Gas composition (dry gas) Gas components CO H2 CH4 CO2 N2 Higher HC
[Vol.%dry] 24–26 12–15 2.5–3 9–11 47–49 0.5–0.9
Table 5 – Pollutants in the raw and clean gas (tar in the producer gas is GC-MS determined)
Tar Char Dust
3.2. contents in the product gas are found. Between run 4 and run 5 the grate slot was reduced. The unburnable part in the ash could be increased form 30 to 40% and the organic part was reduced as well. The amount of the solid residue in case of wood chips of type 2 was also lower. Fig. 15 shows the average product gas composition and the heating values of 4 selected runs (4, 5, 6 and 7) using wood chips of type 1 and type 2 with different water contents. The lower heating value decreased with increased water content of the wood chips. Based on the increasing combustion the CO2 amount was also increased and the H2 and CO amounts in the product gas were decreased, respectively. This is of course also well known from literature [12]. If the water content N35 wt.%wet the gasifier could not be operated efficiently [13]. In the series of runs no dependence of the tar in the product gas on the water content could be observed (Fig. 14). The average water content in the product gas ranged from 10–14%. The highest optimization potential of the gasifier lies in the air flow variation. The gas quality and the efficiency can be raised significantly by adjusting the air distribution. For a better biomass conversion in the upper part of the gasifier more heat should be supplied through the second oxidation zone. Temperatures above 1100 °C should be avoided because of the danger of destroying the grate. As the ash melting point was also in this temperature range slagging could be observed at these high temperatures. The particle carry over (cyclone ash) at full load was very high (Table 2). At part load operation (50–90%) the gasifier operated very well and the particle carry over was substantially lower.
Raw gas [mg/Nm3dry]
Clean gas [mg/Nm3dry]
1180–2000 1500–2200 66–140
180–240 Unverifiable ∼0.7
Operation experience of the 2 MW CHP plant
The flow sheet of the total plant is shown in Fig. 1. So far the drying, gasification and gas cleaning were operated for 2650 h and the gas engine for 2100 h.
3.2.1.
Operation performance of the plant
Fig. 16 shows the fuel and chemical power from a run with 288 kWel engine power in September 2006. This is a partial load of 52% of the designed full load. The data being taken directly from the process control system (PCS) of the plant. As it can be seen that a constant gasifier power is possible which is necessary for an efficient gas engine operation.
3.2.2.
Producer gas composition and LHV
By this selected run of 288 kWel engine power the water content of the wood chips after the dryer is ∼ 21 wt.%wet. A producer gas with an average LHV of 5.8–6.2 MJ/Nm3dry is produced in the gasifier. A typical gas composition (referring to dry gas) is given in Table 4. Fig. 17 shows the gas composition of one selected run (288 kWel) produced by the gasifier. It can be seen that the gas composition is quite stable, which leads to smooth operation of the gas engine.
3.2.3.
Pollutants in the raw and clean gas
Obviously, the tar and dust levels in the raw gas are too high for a direct application in the gas engine and requires further reduction. Table 5 shows the pollutants of the raw and clean gas respectively. It can be seen that the dust content after the wet ESP reaches a very low level which means that the wet ESP shows an excellent separation efficiency.
Fig. 17 – Composition of the dry product gas during a run of 288 kWel engine power.
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Fig. 18 – Single components of tar in the dry product gas (GC-MS determined).
The tar components in the raw as well as the clean gas are illustrated in Fig. 18. The phenols and molecules with lower size were separated nearly completely. The small molecules are more or less harmless for the gas engine, because the condensation of these components is not taking place. Fig. 19 shows a GC chromatogram of one typical tar sample. The BTX (benzene, toluene and xylene) aromatics have a lower boiling point and partly remain in the gas. These components are not problematic for further usage. Moreover, these aromatics are increasing the lower heating value and the anti-knocking properties of the gas The stable gas conditions
as well as the low pollutant loadings resulted in excellent operation behaviour of the total plant as well as the gas engine.
3.2.4.
Efficiencies
The following Table 6 reflects the most relevant efficiencies of the plant operation in September 2006 at 288 kWel engine power, calculated with the reconciled data set in IPSEpro and compared to those from September 2004 at 350 kWel engine power. Comments on the operation in 2006: Because of the partial load on the gas engine the necessary cooling capacity was reduced to approx. 60% of the value at full load of only ∼411 kWth
Fig. 19 – GC chromatogram of the typical sample from the Wr. Neustadt plant.
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Table 6 – Plant efficiencies related to input fuel power from September 2006/2004
Table 7 – Results between the CHP plant and the hot test rig
Efficiency related to fuel power [%]
Parameter
ηchem, plant ηel, brutto ηel, netto ηQ, district heat ηfuel utilization,
brutto
09/2006
09/2004
68.1 23.2 20.5 33.0 56.2
77.0 27.1 24.6 36.6 63.7
(design value: 720 kWth). The overall plant efficiencies (electrical and thermal) was thereby significantly reduced. The electrical efficiencies as well as the chemical efficiency are being marked by the high water content (∼21 wt.%wet) of the biomass. An evaluation of the plant performance from September 2004 [12] outlines the potential of the CHP plant at partial load when operated with a well dried biomass (water content ∼12 wt.%wet).
4.
Conclusion
In this study a twin-fire fixed bed gasifier based CHP plant was realized in the year 2003 in Wr. Neustadt, Austria. Wood chips are used as fuel, which are dried and sieved before being gasified to a low calorific gas of about 5.8 MJ/Nm3dry. The CHP plant has a fuel power of 2 MWth and an electric output of 550 kWel. As scale up and optimization tools a hot test rig with a capacity of 125 kWth was built. Basic parameters like type of wood chips, power and air distribution were varied to investigate gas composition, tar content in producer gas and carbon content in the ash. For comparison the results of the 2 MWth CHP plant and the 125 kWth hot test rig are illustrated in Table 7. Between both plants a good accordance can bee seen. Therefore, the hot test rig is a good optimization tool for the CHP plant. This type of plant provides the possibility to convert biomass with a high efficiency into heat and power. The plant was operated at part load of about 52% which corresponds to 288 kWel engine power. The relatively high water content (∼21 wt.%wet) of the biomass has a big influence of the efficiency. This was mainly the reason for comparable low cold gas efficiency of 68.1%, thermal efficiency of 33.0%, and the electric efficiency of 23.2%.
Acknowledgements The authors are grateful for the financial support from RENET Austria (Knet/Kind — public funds program, Austria).
REFERENCES [1] R.E.H. Sims, The brilliance of bioenergy in business and practice, James & James, London, 2002. [2] H. Hofbauer, M. Kaltschmitt, Thermochemische Umwandlung, in: M. Kaltschmitt, H. Hartmann (Eds.), Energie aus Biomasse: Grundlage, Techniken und Verfahren, Springer, Berlin, 2001, pp. 427–507.
Maximum fuel power [kW] Biomass water content [wt.%wet] Lower heating value [MJ/Nm3dry] Load range [%] CO [vol.%dry] CO2 [vol.%dry] CH4 [vol.%dry] H2 [vol.%dry] Tar concentration [g/Nm3dry, raw gas]
Hot test rig
CHP plant
125 11–25 5.6–6.3 50–95 20–23 12–14 2–2.5 16–19 0.2–2
2000 ∼ 21 5.8–6.2 52 24–26 9–11 2.5–3 12.15 1.2–2
[3] H.A.M. Knoef, Practical aspects of biomass gasification, in: H.A.M. Knoef (Ed.), Handbook Biomass Gasification. BTG biomass technology group B.V., Netherlands, 2005, pp. 13–37. [4] H.A.M. Knoef, Status of small-scale biomass gasification and prospects, in: H.A.M. Knoef (Ed.), Handbook Biomass Gasification. BTG biomass technology group B.V., Netherlands, 2005, pp. 38–75. [5] J. Schmoll, J. Penz, C. Aichernig, H. Tremmel, R. Kramreiter, J. Kotik, The Wr. Neustadt Project, Biomass gasification for an efficient provision of electricity and fuels — state of knowledge 2007, Leipzig, 2007. [6] F. Lettner, Fixed bed gasification — state of technology, Biomass gasification for an efficient provision of electricity and fuels — state of knowledge 2007, Leipzig, 2007. [7] K.W. Kwant, Status of countries participating in the IEA Bioenergy gasification activity, International Energy Agency Report, 2004. [8] W. Baaske, K. Jörg, J. Schmoll, Biomassevergasungsanlage Wiener Neustadt, Gutenberg, Leipzig, 2004. [9] J.P.A. Neeft, H.A.M. Knoef, K. Zielke, K. Sjöström, P. Hasler, P.A. Simell, M.A. Dorrington, L. Thomas, N. Abatzoglou, S. Deutsch, C. Greil, G.J. Buffinga, C. Brage, M. Suomalainen, Guideline for sampling and analysis of tar and particles in biomass producer gases, Energy project EEN5-1999-00507 (Tar protocol), Version 3.3., 2003. [10] Höftberger, E., 2005. In-situ CO2-adsorption in a dual fluidized bed biomass steam gasifier to produce a hydrogen rich gas. Ph.D.-thesis, Vienna University of Technology, Institute of Chemical Engineering. [11] H. Timmerer, F. Lettner, Anlagensicherheit und Genehmigung von Biomasse-Vergasungsanlagen, Leitfaden für Betreiber, Hersteller und Behörden, Bericht aus Umweltforschung 45/2005, 2005. [12] Müller, A., 2004. Energetische und exergetische Evaluierung des Biomasse-Vergasungskonzept „Civitas Nova“. Master thesis, Vienna University of Technology, Institute of Chemical Engineering. [13] M. Klemm, R. Wilhelm, Erzeugung eines andwendungsgerechten Brenngases aus Biomasse in einem System aus Gleichstromvergaser und Heißentteerung, LIES Abschlussworkshop 2005, TU Dresden, 2005, p. 25—37. [14] M. Bolhàr-Nordenkampf, K. Jörg, W. Baaske, J. Schmoll, G. Herdin, H. Hofbauer, Extension of a biomass district heating plant with a twin-fire fixed bed gasifier, 2nd World Conference on Biomass for Energy and Industrie, Rom, 2004. [15] M. Kaltschmitt, H. Hartmann (Eds.), Energie aus Biomasse: Grundlagen, Techniken und Verfahren, Springer, Berlin, 2001.
w w w. e l s e v i e r. c o m / l o c a t e / f u p r o c
Experimental investigation of a 125 kW twin-fire fixed bed gasification pilot plant and comparison to the results of a 2 MW combined heat and power plant (CHP) Robert Kramreiter⁎, Michael Url, Jan Kotik, Hermann Hofbauer Institute of Chemical Engineering, Vienna University of Technology, Getreidemarkt 9/166, A-1060 Vienna, Austria
AR TIC LE I N FO
ABS TR ACT
Article history:
Fixed bed biomass gasification is a promising technology to produce heat and power from a
Received 28 June 2007
renewable energy source. A twin-fire fixed bed gasifier based CHP plant was realized in the
Accepted 2 August 2007
year 2003 in Wr. Neustadt, Austria. Wood chips are used as fuel, which are dried and sieved before being gasified to a low calorific gas of about 5.8 MJ/Nm3dry. Before the clean gas is fed
Keywords:
into a gas engine a cyclone and a RME (rapemethylester)/H2O quench system followed by a
Biomass conversion
wet electrostatic precipitator (ESP) is used for gas cleaning. The CHP plant has a fuel power of
Gasification
2 MWth and an electric output of 550 kWel. As scale up and optimization tool a hot test rig
Fixed bed
with a capacity of 125 kWth was built. Basic parameters like the type of wood chips, power and air distribution were varied to investigate the effect on gas composition, tar content in the producer gas and carbon content in the ash. Additionally a temperature profile over the height of the 125 kW hot test rig was measured. Furthermore, the results from the hot test rig are discussed and compared with the results from the 2 MWth demonstration plant. © 2007 Elsevier B.V. All rights reserved.
1.
Introduction
Different biomass gasifier designs have been developed during the last 20 years with the intention to establish an efficient combined heat and power production system based on biomass. The producer gas from the biomass gasifier is cleaned and afterwards fed into a gas engine or turbine which is coupled to an electricity generator. The advantages especially in the small scale range are the high overall biomass utilization and the high electrical efficiencies at the same time. Fixed bed gasifiers are mainly used in the small scale range whereas for larger scale fluidized bed gasifiers are proposed. For combined heat and power production air is normally used as gasification agent [1]. Originally, two different types of fixed bed gasifiers were developed: updraft and downdraft gasifiers. In case of an updraft gasifier the gasification agent is introduced at the bottom and the gas flow is upwards counter-current to the biomass which is
fed from the top. The producer gas leaves the gasifier at the top and the ash is withdrawn at the bottom by a grate. Several zones are created in an updraft fixed bed gasifier which are – starting from the top – the drying, the pyrolysis, the reduction and the oxidation zone. The temperature is increasing from the top to the bottom. The tars are produced mainly in the pyrolysis zone and leave the gasifier together with the producer gas. As there is no zone above the pyrolysis zone which has a higher temperature to thermally destroy the tars a high amount of tars is produced [2–4]. Downdraft gasifiers try to avoid the disadvantage of high tar contents by introducing the gasification agent not at the bottom but at the top or at least at a certain height above the bottom. The main difference to updraft gasifiers is that the gas flows cocurrently downwards with the biomass. This leads to a different order of the reaction zones from top to bottom, namely, drying, pyrolysis, oxidation, and reduction zone with the result of a low tar content in the producer gas. The disadvantage occurs for this
⁎ Corresponding author. Tel.: +43 1 58801 15973; fax: +43 1 58801 15999. E-mail address: [email protected] (R. Kramreiter). 0378-3820/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.fuproc.2007.08.001
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type of gasifier which is a high carbon content in the bottom ash. This can be explained by the reduction zone which is the first zone above the grate [2–4]. A twin-fire fixed bed gasifier is able to combine the advantages in one gasifier and avoid the disadvantage of both systems above. A twin-fire fixed bed gasifier has got two locations for the air introduction: one in the upper part of the fixed bed and another from the bottom via a grate. Therefore, the upper part of the gasifier behaves like a downdraft gasifier and the lower part like an updraft gasifier. Contrary to the downdraft gasifier there will be an additional oxidation zone established above the grate. This should lead to a low tar content in the producer gas and also to an ash with a comparable low carbon content [5]. A lot of developments of different designs of fixed bed gasifiers have been carried out during the last two decades. Among these developments there are all three kinds of the above mentioned gasifiers. Furthermore, two stage gasifiers are currently under development which try to separate the pyrolysis step from the gasification step of the remaining char in two reactors. This leads to an additional high temperature reactor but allows a better controllability of the whole process. Extremely low tar contents suitable for a direct use in gas engines are the result [6]. Demonstration plants for all these systems can be found in different countries in Europe [7]. A 2 MWth plant for a twin-fire fixed bed gasifier was realized in the year 2003 in Wr. Neustadt, Austria. This is the largest plant of that type ever built and
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therefore development work had to be carried out to obtain a successful scale up and operation. As scale up and optimization tools a hot test rig with a capacity of 125 kWth was built, simulation and modelling was carried out, and also a cold flow apparatus to study the moving of the particles through the reactor were used. In this paper the work and the results from the hot test rig are discussed and compared with the 2 MWth demonstration plant.
2.
Experimental
2.1.
Gasification technology of the 2 MW CHP plant
The plant described in the following has been built in Wr. Neustadt/Austria and consists of a so called twin-fire fixed bed gasifier with a fuel power of about 2 MWth, a cyclone and a RME (rapemethylester)/H2O gas cleaning system followed by a wet electrostatic precipitator (ESP). The purified product gas is utilized in a gas engine with a capacity of 550 kWel. The exhaust gas from the gas engine in led into an existing biomass boiler in order to meet the emission limits. The configuration of this plant is illustrated in Fig. 1. The feedstock of this plant is freshly chipped wood from forestry. It is fed from the daily hopper by a moving floor and a screw conveyor into the rotary sieve dryer. In this apparatus the
Fig. 1 – Flow chart of the 2 MWth CHP plant Wr. Neustadt/Austria [14].
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feedstock is conditioned to meet the quite stringent fuel specifications in terms of particle size and water content of the fixed bed gasifier. Preheated air from the district heating boiler is utilized for the drying of the fuel. The exiting air is fed back to the boiler and post combusted to eliminate any smells and dust emitted from the dryer. The sieved small fuel particles are fed also into the boiler and used as additional fuel. The gasifier is a fixed bed type with several reaction zones and combines the advantages of up- and downdraft fixed bed gasifiers (Fig. 2). It is a further development of the original Deutz-twin-fire-gasifier and is based on the experience gained from a pilot plant in Domsland/Germany [8]. Air is supplied at two positions: after the drying zone of the biomass and via the grate from the bottom of the gasifier. In the upper part the gasifier shows downdraft behavior. Feedstock and product gas are moving in the same direction. In the lower part updraft behaviour is obtained. Feedstock and product gas are moving in the opposite directions. Therefore, the gasifier design combines the advantages of a downdraft and an updraft gasifier. This will lead to a producer gas with a low tar content (down draft) and a low carbon content in the ash (up draft). The product gas leaves the reactor at about 650 °C, is firstly cooled in the air pre-heater of the gasifier. A cyclone removes most of the dust and char particles from the hot product gas. High amounts of both, dust and char, can disturb the quench gas cleaning system. Afterwards the hot product gas is quenched instantly to 50 °C. The quench cooler operates with rapemethylester (RME) and also water. The water is sprayed in at first to avoid cracking RME at 650 °C. The sedimentation tank below the quench system separates the two phases, which are RME and water. The reason to utilize RME in the gas cleaning process is due to the better solubility of tar. The accumulated residues (dust and char) at the interface (RME/H2O) of the sedimentation tank will be transferred back into the fixed bed gasifier. For the final gas cleaning a wet ESP is installed. The drain of the wet electrostatic precipitator is put into a slurry tank, which finally is also led back into the gasifier.
Table 1 – Characteristic data of the plant Type of plant Fuel Fuel power Electric output Thermal output Electric efficiency Thermal efficiency Total efficiency
Demonstration plant Wood chips 2000 kW 550 kW 720 kW 27.5% 36.0% 63.5%
After the wet ESP the clean gas is compressed and cooled before being supplied to the gas engine. A Jenbacher gas engine is used for internal combustion to yield 550 kWel of electrical power and 720 kWth of district heating power. Furthermore, the exhaust gas from the gas engine is blown into a biomass boiler. Thereby, the remaining CO and other combustible components are oxidized to meet the emissions limits. Table 1 summarizes the most important characteristic data of the CHP plant.
2.2.
Description of the 125 kW pilot plant
The pilot plant is a scale down from the demonstration plant in Wr. Neustadt described above and was built as an optimization tool for this plant. Basic parameters like air distribution, gas composition, tar content in product gas and carbon content in the ash should be improved. Additionally the type of fuel, water content, and power is varied and investigated. The 125 kWth fix bed gasifier consists of five essential parts: • • • • •
Biomass daily hopper and screw conveyor Twin-fire fixed bed gasifier Rotary grate, ash discharge device and ash hopper Product gas pipe with cyclone Measurement/analysis equipments
The product gas is combusted in a burner and is cleaned of dust and char in an additional second cyclone before the gas is discharged to the environment via the chimney.
2.2.1.
Daily hopper and screw conveyer
The biomass daily hopper and the screw conveyor are illustrated in Fig. 3. The daily hopper has a capacity of 2 m3. A test takes a time of about 15 h. During the run the fuel cannot be refilled because the daily hopper is closed airproofed with a cap on the top. Furthermore the hopper is supplied with an inert sealing gas. A screw conveyer continuously feeds biomass into the gasifier at the top of the reactor. The amount of fuel can be varied by a frequency converter. The biomass flow out of the hopper depends on the bed density and the rotation speed of the drive motor. The mass flow of biomass was in the range of 15–45 kg/h. The level of the wood in the gasifier can be measured with a manual dip stick.
2.2.2.
Fig. 2 – Twin-fire fixed bed gasifier.
Twin-fired fixed bed gasifier
The pilot scale gasifier consists of two concentric pipes. The inner pipe has a diameter of 200 mm and a height of 1050 mm and is about one third shorter than the outer pipe, which has an inner diameter of 300 mm and a height of 1650 mm. On the bottom side of the outer pipe of the gasifier a conical rotary grate is connected by a flange (Figs. 4 and 5). The whole reactor is
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the conical rotary grate. Additional air can be charged to the ash room to be used as barrier air and for cooling the rotary grate (grate air). The product gas leaves the reactor through the annular space. The advantage of this construction is that the primary air is preheated and the heat of the product gas is transferred to the inner pipe to support drying and pyrolysis of the biomass.
2.2.3.
Rotary grate and ash hopper
Fig. 5 shows the grate of the 125 kWth pilot plant. The conical rotary grate has an angle of 60° and 6 fins with a height of 10 mm each. The advantage of this construction is to avoid the formation of bridges and to promote the bed movement. Between the outer wall and the grate there is a slot of 8 mm. The secondary air inlet is just above this slot is. The ash can fall down into the ash chamber through this slot where the ash discharge device is installed which transports the ash into the ash hopper. The ash discharge device is fixed on the same shaft as the rotary grate. Between the ash chamber and the ash hopper a ball valve is installed to be able to exchange the ash hopper during the runs and also to take out ash samples for analysis. Fig. 3 – Biomass daily hopper and screw conveyer.
made of stainless steel and covered outside with an insulation layer. In the inner pipe primary air is injected via eleven nozzles. In the outer pipe secondary air is injected with eight nozzles above
2.2.4.
Complete test facility and test procedures
Tests are performed autothermically (no heat supply by e.g. electrical heating system) at atmospheric pressure and air as gasification agent. The P&I diagram of the 125 kWth twin-fire fixed bed gasifier is illustrated in Fig. 6. The primary air is provided by an air compressor. The secondary and grate air are provided by a compressed air system. These flows are measured
Fig. 4 – Detail of the 125 kWth pilot plant at the TU Vienna.
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Fig. 5 – Grate and ash hopper of the 125 kWth pilot plant.
thermocouples and 10 pressure manometers are used to monitor temperature and pressure profile whereas 7 different thermocouples are installed in the inner pipe for accurate information about the reaction zones. The cyclone is used as a dedusting unit for product gas cleaning. Both, cyclone and product gas pipe are heated electrically and insulated to prevent tar condensation. The tar, dust and gas analysis sampling point is installed between the reactor and the cyclone and is described in detail in the following chapter. For start up preheated primary air is utilized for the ignition of the wood chips. When the desired temperatures are achieved (in the range of 800–1000 °C) the grate can be activated. Then, the primary, secondary and grate airs are adjusted to maintain the desired temperatures. The product gas leaves the reactor at about 650 °C through the annulus. In general, it takes about 2 h to reach a stable operation with respect to the reactor temperatures. At a steady state operation, all parameters are kept constant for at least 60 min for gas sampling and analysis.
2.3. by rotameters. The grate air can be charged to the ash room to be used as sealing gas and for rotary grate cooling. Both air systems (secondary and grate air) offer the advantage to ensure a low carbon content in the ash. In the whole pilot plant 17
Producer gas sampling and measurement procedures
After reaching a steady state operation, producer gas sampling is turned on. A small stream of the producer gas is sucked off through a filter box filled with glass wool. After passing washing bottles filled with RME and a cooler, the clean and cool gas is
Fig. 6 – P&I diagram of the 125 kWth twin-fire fixed bed gasifier of the Vienna University of Technology.
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Table 2 – The test conditions and results of the fix bed gasification Parameter/run
Run_4 Run_5 Run_6 Run_7
Wood chip type Water content [%] Primary air flow [Nm3/h] Secondary air flow [Nm3/h] Grate air flow [Nm3/h] Load range [%] Tar content [g/Nm3dry] Dust content [g/Nm3dry] Biomass consumption (wet) [kg] Grate ash [g] Unburnable part — grate ash [%] Cyclone ash [g] Unburnable part — cyclone ash [%] Total air flow [Nm3] Equivalent ratio [–]
Type 1 11 20–30 0 4–6 b 95 1.2 4.5 192 6227 30 2757 29 224 0.28
Type 1 11 20–30 0–4 4–6 b 95 1.0–1.2 1.5–2.4 159 3074 40 3581 24 190 0.29
Type 2 25 10–28 2–4 4–7 50–88 0.6–2.0 1.0–1.6 185 2380 66 439 39 227 0.36
Type 2 18 18 2 4 65 0.2 0.8 76 868 65 135 47 93 0.33
analysed by a gas chromatograph (model GC 955) with TCD detector using Helium as carrier gas. Gas components like O2, N2, CO2, CO, CH4, C2H4, C2H6, and C3/C4 are detected. The concentration of the H2 is determined by difference to 100%. Additionally, a multi component analyser (Rosemount NGA 2000) is used, measuring O2, H2, CO, CO2, CH4 continuously. The measurement point for tars and particles is located before the cyclone. The producer gas is sampled through a particle filter system consisting of a cyclone and a filter box filled with glass wool. Afterwards, the gas passes through washing bottles and is cooled by a cooling bath with glycol (−20 °C). The bottles are filled with toluene for removing the tar components. The measurement and analysis of the tar and particle is based on the “Tar Guideline” [9]. Further details can be found in [10].
3. 3.1.
Results and discussion Range of operation of the 125 kW pilot plant
The gasifier was designed for a nominal fuel power of 125 kWth. This means that 25 kg/h dry wood chips are fed into the reactor
Fig. 8 – Wood chips type 2.
having a lower heating value (LHV) of the fuel of approximately 18 MJ/kg. For the calculation of the required air for the partial oxidation a stoichiometric oxygen demand calculation for different air ratio was executed: Vair ¼ k
" # 22:41 wC wH wO Nm3 air þ 0:21 12 4 32 kg fuel
The equivalent ratio (λ) was a result of the desired temperature during steady state operation. A lot of test runs were carried out with the 125 kWth pilot plant to understand the behaviour of this gasifier and to get data for comparison and improvement of the 2 MWth demonstration plant in Wr. Neustadt. Table 2 shows the test conditions and main results of 4 selected runs.
3.1.1.
Wood chips used for the runs
Figs. 7 and 8 show the wood chip type 1 and type 2 which were used in the pilot plant. The wood chips had a particles sizes between 11.2 mm to 31.5 mm. Type 1 was a mixture of wood chip from different trees (maple, cottonwood, ash tree) with a water content of ∼11%. The wood particles were long and sharp. Type 2 mostly consisted of beech wood with a water content from 18% to 25%. This type of wood chip had more spherical particles which are better to avoid the formation of bridges in the gasifier. Table 3 shows the elemental analysis of type 1 and type 2.
3.1.2. Pressure drop, total air flow and height of fixed bed versus time Fig. 9 shows the pressure drop, total air flow and level of the fixed bed of the gasifier for run 4 which was carried out with wood chips
Table 3 – Elemental analysis of the dry wood chip type 1 and type 2 Wood chip-type
Fig. 7 – Wood chips type 1.
Type 1 Type 2
Elemental composition [m-%] C
H
O
N
49.9 49.7
6.1 6.1
43.7 43.8
0.3 0.4
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Fig. 9 – Pressure drop, total air flow and bed height of the “run 4” with biomass type 1.
of type 1. As expected the pressure drop increases with rising air flow and height of fixed bed. One goal of this run is to find optimized operating conditions for the gasifier at full load. Thus, the total air flow was increased stepwise over the presented runtime. The drop down of the height of the bed and the pressure at the time 10:50 was not caused by a rotation of the grate. Obviously bridges occurred in the gasifier. When these bridges break down the pressure drop goes down in parallel. This happened several times during this run. At the time 14:30 the pressure drop broke down and stayed nearly constant although the bed height and the air flow were changed. A channel formation in the bed was supposed to be the reason for this. During the whole run the flow of the grate air was 4–6 Nm3/h and no secondary air was introduced in this run,
whereas the temperatures in the range of the grate were at a relative low level (800–850 °C). Fig. 10 shows the pressure drop, total air flow and bed height of the gasifier during run 6 which was carried out with wood chips of type 2. These wood chips had a water content of 25 wt.%. The run was executed in part load between 50 and 88%. The assumed air flow at full load for the conditions in this run is 40 Nm3/h (λ = 0.35). The bed height fluctuations were moderately and the pressure drop changed only according to the total air through flow. The rotary grate was activated every 15–20 min for approximately 5 s. As mentioned already above this type of wood chips had more spherical particles which are better for the flow behavior. Therefore the wood chip type 2 had a better operation behavior
Fig. 10 – Pressure drop, total air flow and bed height of the run 6 with biomass type 2.
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Fig. 11 – Temperature profile inside the gasifier without secondary air taken from run 4.
in the gasifier. The part load operation of the gasifier is also very well and the particle discharge is substantially lower. During the whole run the flow of the grate air is 4–7 Nm3/h and the secondary air is in the range of 2–4 Nm3/h.
3.1.3.
Temperature profiles
Temperature profiles over the gasifier are illustrated in Figs. 11 and 12. Core temperatures of 70–90 °C indicate the drying zone. Temperatures of 200–350 °C were reached in the upper part of the bed near the wall. Consequently, the pyrolysis already begins. In the core zone the volatilization takes place before or shortly after the primary air injection. Due to the temperature difference between the bed in the inner tube and the annulus of 200–300 °C a heat transfer from the wall into the centre occurs. This heat transfer through the bed is very slow.
For better orientation the level of the inner pipe and the primary- and secondary air injection is shown as well in Fig. 11. The first oxidation zone was located in the inner pipe approximately 200 mm below the primary air injection. A second oxidation zone is expected in the lower part above the rotary grate. In this zone the temperature were quite low since no secondary air was used in this run. Only the grate air was introduced with a flow of 4 Nm3/h. Furthermore, the temperature in the upper part of the grate (480 mm) decreases. In this zone and the upper part of the inner pipe obviously endothermic reduction reactions occurred. In the annulus the temperature of the product gas is significantly increased due to the heat from the oxidation zone (up to approx. 700 mm). For drying and pyrolysis above this level the heat of the product gas is exchanged to the inner pipe. According to the construction the
Fig. 12 – Temperature profile inside the gasifier with secondary air taken from run 5.
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Fig. 13 – Composition and LHV of the dry product gas from the steady state run 7.
primary air was preheated with the hot upward streaming product, whereby the product gas was cooled down again. Another temperature profile inside the gasifier taken from run 5 is represented in Fig. 12. In this case the grate air (6 Nm3/h) and additionally the secondary air with a flow rate of 4 Nm3/h were introduced into the gasifier. This led to an increased temperature in the area of the secondary air injection because an oxidation zone was generated close to the rotary grate. Temperatures of about 950 °C were obtained. The temperatures near the grate are significantly higher than the temperature of the annulus.
3.1.4.
Producer gas composition and LHV
A typical producer gas composition and the resulting lower heating value are shown in Fig. 13. In this run (run 7) wood chips of type 2 were used. The chips were dried from a water content of
25 wt.%wet down to 18 wt.%wet before entering the gasifier. The ratio of primary-to secondary and grate air was 3:1 (18 Nm3/ h primary-, 2 Nm3/h secondary-, and 4 Nm3/h grate air). This corresponds to a partial load of 65%. During the steady state run product gas with an average LHV of 5.8 MJ/Nm3dry was produced.
3.1.5.
Tar and particle analysis
The particles in the product gas are defined as the sum of dust (inorganic), char, and tars adsorbed on the particles. Fig. 14 shows the results of the tar and particle analyses of several runs (4, 5, 6 and 7) for different water contents of the biomass. The tar content analyzed was in the range of 0.2–2 g/Nm3dry and a tendency for the amount of tars regarding to the water content cannot be observed. The particle content of the run 5–7 was in the range of approximately 0.8–2.4 g/Nm3dry. The
Fig. 14 – Tar and particle analysis of the runs 4, 5, 6 and 7.
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Fig. 15 – Average product gas composition of the runs 4, 5, 6 and 7 and LHV.
relatively high particle content (4.5 g/Nm3dry) during the run 4 can be explained by the large particle discharge at the end of the gasification period. The run results gathered at the pilot plant (especially those from runs 5, 6, and 7) were in a good accordance compared to the results from operation of the demonstration plant in Wr. Neustadt.
3.1.6.
Discussion of several important parameters
The executed runs showed that the particle shape and the particle size of the wood chips have a big influence on the operation of the gasifier. Consequently, two fractions b11.2 mm N31 mm were sieved out and the wood chip fraction 11.2 mmb × b31.5 was used in the gasifier. According to the size of the plant an upper limit for the particle size of 31.5 mm was assumed. This is in good agreement
with literature [1,11,15]. An influence of the particle shape could be recognized too. As mentioned before the wood chips of type 2 had more spherical particles in comparison to the wood chip type 1 which is better for a smooth flow behavior and to avoid the formation of bridges. Therefore, wood chip type 2 showed a better operational behavior in the gasifier. The grate design and operation and consequently the ash discharge are important figures for a satisfying gasification operation. The high pressure drop in the wood bed promotes channel formation. Therefore, a symmetric flow of the product gas through the bed cannot be secured. This channel formation and the associated raising particle discharge from run 4 (Fig. 9) could be the reason for the sudden pressure drop decrease. Thus higher organic fractions in the ash and higher tar
Fig. 16 – Fuel and chemical power curves during a run of 288 kWel engine power.
100
F U E L P RO C ES S IN G T EC HN O L O G Y 8 9 (2 0 0 8) 9 0–1 0 2
Table 4 – Gas composition (dry gas) Gas components CO H2 CH4 CO2 N2 Higher HC
[Vol.%dry] 24–26 12–15 2.5–3 9–11 47–49 0.5–0.9
Table 5 – Pollutants in the raw and clean gas (tar in the producer gas is GC-MS determined)
Tar Char Dust
3.2. contents in the product gas are found. Between run 4 and run 5 the grate slot was reduced. The unburnable part in the ash could be increased form 30 to 40% and the organic part was reduced as well. The amount of the solid residue in case of wood chips of type 2 was also lower. Fig. 15 shows the average product gas composition and the heating values of 4 selected runs (4, 5, 6 and 7) using wood chips of type 1 and type 2 with different water contents. The lower heating value decreased with increased water content of the wood chips. Based on the increasing combustion the CO2 amount was also increased and the H2 and CO amounts in the product gas were decreased, respectively. This is of course also well known from literature [12]. If the water content N35 wt.%wet the gasifier could not be operated efficiently [13]. In the series of runs no dependence of the tar in the product gas on the water content could be observed (Fig. 14). The average water content in the product gas ranged from 10–14%. The highest optimization potential of the gasifier lies in the air flow variation. The gas quality and the efficiency can be raised significantly by adjusting the air distribution. For a better biomass conversion in the upper part of the gasifier more heat should be supplied through the second oxidation zone. Temperatures above 1100 °C should be avoided because of the danger of destroying the grate. As the ash melting point was also in this temperature range slagging could be observed at these high temperatures. The particle carry over (cyclone ash) at full load was very high (Table 2). At part load operation (50–90%) the gasifier operated very well and the particle carry over was substantially lower.
Raw gas [mg/Nm3dry]
Clean gas [mg/Nm3dry]
1180–2000 1500–2200 66–140
180–240 Unverifiable ∼0.7
Operation experience of the 2 MW CHP plant
The flow sheet of the total plant is shown in Fig. 1. So far the drying, gasification and gas cleaning were operated for 2650 h and the gas engine for 2100 h.
3.2.1.
Operation performance of the plant
Fig. 16 shows the fuel and chemical power from a run with 288 kWel engine power in September 2006. This is a partial load of 52% of the designed full load. The data being taken directly from the process control system (PCS) of the plant. As it can be seen that a constant gasifier power is possible which is necessary for an efficient gas engine operation.
3.2.2.
Producer gas composition and LHV
By this selected run of 288 kWel engine power the water content of the wood chips after the dryer is ∼ 21 wt.%wet. A producer gas with an average LHV of 5.8–6.2 MJ/Nm3dry is produced in the gasifier. A typical gas composition (referring to dry gas) is given in Table 4. Fig. 17 shows the gas composition of one selected run (288 kWel) produced by the gasifier. It can be seen that the gas composition is quite stable, which leads to smooth operation of the gas engine.
3.2.3.
Pollutants in the raw and clean gas
Obviously, the tar and dust levels in the raw gas are too high for a direct application in the gas engine and requires further reduction. Table 5 shows the pollutants of the raw and clean gas respectively. It can be seen that the dust content after the wet ESP reaches a very low level which means that the wet ESP shows an excellent separation efficiency.
Fig. 17 – Composition of the dry product gas during a run of 288 kWel engine power.
F U E L P RO C ES S IN G T EC H NOL O G Y 8 9 (2 0 0 8) 9 0–1 0 2
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Fig. 18 – Single components of tar in the dry product gas (GC-MS determined).
The tar components in the raw as well as the clean gas are illustrated in Fig. 18. The phenols and molecules with lower size were separated nearly completely. The small molecules are more or less harmless for the gas engine, because the condensation of these components is not taking place. Fig. 19 shows a GC chromatogram of one typical tar sample. The BTX (benzene, toluene and xylene) aromatics have a lower boiling point and partly remain in the gas. These components are not problematic for further usage. Moreover, these aromatics are increasing the lower heating value and the anti-knocking properties of the gas The stable gas conditions
as well as the low pollutant loadings resulted in excellent operation behaviour of the total plant as well as the gas engine.
3.2.4.
Efficiencies
The following Table 6 reflects the most relevant efficiencies of the plant operation in September 2006 at 288 kWel engine power, calculated with the reconciled data set in IPSEpro and compared to those from September 2004 at 350 kWel engine power. Comments on the operation in 2006: Because of the partial load on the gas engine the necessary cooling capacity was reduced to approx. 60% of the value at full load of only ∼411 kWth
Fig. 19 – GC chromatogram of the typical sample from the Wr. Neustadt plant.
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F U E L P RO C ES S IN G T EC HN O L O G Y 8 9 (2 0 0 8) 9 0–1 0 2
Table 6 – Plant efficiencies related to input fuel power from September 2006/2004
Table 7 – Results between the CHP plant and the hot test rig
Efficiency related to fuel power [%]
Parameter
ηchem, plant ηel, brutto ηel, netto ηQ, district heat ηfuel utilization,
brutto
09/2006
09/2004
68.1 23.2 20.5 33.0 56.2
77.0 27.1 24.6 36.6 63.7
(design value: 720 kWth). The overall plant efficiencies (electrical and thermal) was thereby significantly reduced. The electrical efficiencies as well as the chemical efficiency are being marked by the high water content (∼21 wt.%wet) of the biomass. An evaluation of the plant performance from September 2004 [12] outlines the potential of the CHP plant at partial load when operated with a well dried biomass (water content ∼12 wt.%wet).
4.
Conclusion
In this study a twin-fire fixed bed gasifier based CHP plant was realized in the year 2003 in Wr. Neustadt, Austria. Wood chips are used as fuel, which are dried and sieved before being gasified to a low calorific gas of about 5.8 MJ/Nm3dry. The CHP plant has a fuel power of 2 MWth and an electric output of 550 kWel. As scale up and optimization tools a hot test rig with a capacity of 125 kWth was built. Basic parameters like type of wood chips, power and air distribution were varied to investigate gas composition, tar content in producer gas and carbon content in the ash. For comparison the results of the 2 MWth CHP plant and the 125 kWth hot test rig are illustrated in Table 7. Between both plants a good accordance can bee seen. Therefore, the hot test rig is a good optimization tool for the CHP plant. This type of plant provides the possibility to convert biomass with a high efficiency into heat and power. The plant was operated at part load of about 52% which corresponds to 288 kWel engine power. The relatively high water content (∼21 wt.%wet) of the biomass has a big influence of the efficiency. This was mainly the reason for comparable low cold gas efficiency of 68.1%, thermal efficiency of 33.0%, and the electric efficiency of 23.2%.
Acknowledgements The authors are grateful for the financial support from RENET Austria (Knet/Kind — public funds program, Austria).
REFERENCES [1] R.E.H. Sims, The brilliance of bioenergy in business and practice, James & James, London, 2002. [2] H. Hofbauer, M. Kaltschmitt, Thermochemische Umwandlung, in: M. Kaltschmitt, H. Hartmann (Eds.), Energie aus Biomasse: Grundlage, Techniken und Verfahren, Springer, Berlin, 2001, pp. 427–507.
Maximum fuel power [kW] Biomass water content [wt.%wet] Lower heating value [MJ/Nm3dry] Load range [%] CO [vol.%dry] CO2 [vol.%dry] CH4 [vol.%dry] H2 [vol.%dry] Tar concentration [g/Nm3dry, raw gas]
Hot test rig
CHP plant
125 11–25 5.6–6.3 50–95 20–23 12–14 2–2.5 16–19 0.2–2
2000 ∼ 21 5.8–6.2 52 24–26 9–11 2.5–3 12.15 1.2–2
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