Updraft gasifier with moisturization means

Abstract

 The invention relates to an updraft gasifier (4) for a biomass gasification system (2) to generate gasifier product gas to be burned in a combustion unit (6), which gasifier (4) comprises:
● a first inlet (9) for biomass in the upper area (12) of the gasifier (4), and
● a first outlet (11) for ash in the lower area (13) of the gasifier (4), and
● a second inlet (14) for a gasification agent in the lower area (13) of the gasifier (4), and
● a second outlet (15) for the gasifier product gas in the upper area (12) of the gasifier (4),

wherein, the gasifier furthermore comprises:
● a temperature sensor (19) to measure the temperature of the gasifier product gas in the upper area (12) of the gasifier (4), and
● moisturization means (20) to moisture the biomass, and
● a control circuit (21) to activate the moisturization means (20) to moister the biomass in case the temperature of the gasifier product gas exceeds a maximum temperature.

Description

[0001] The invention relates to an updraft gasifier for a biomass gasification system to generate gasifier product gas to be burned in a combustion unit, which gasifier comprises:

● a first inlet for biomass in the upper area of the gasifier, and

● a first outlet for ash in the lower area of the gasifier, and

● a second inlet for a gasification agent in the lower area of the gasifier, and

● a second outlet for the generated gasifier product gas in the upper area of the gasifier.

[0002] Document WO 2007/081296 discloses a cross draft gasifier for biomass that uses a downdraft and an updraft mode to produce gasifier product gas, which can be burned in a combustion unit. The disclosed gasifier comprises a first inlet for municipal waste in the upper area of the gasifier and a first outlet of the gasifier for the ash at the bottom of the gasifier. The gasifier furthermore comprises a second inlet for preheated air or oxygen in the lower area of the gasifier and a second outlet in the upper area of the gasifier for syngas as gasifier product gas generated in the gasifier. The gasifier product gas at the outlet of the gasifier comprises dust and tar residues, which need to be kept at a low percentage level of the gasifier product gas as they may result in damage at the internal combustion engine unit.

[0003] To achieve a low percentage level of tar in the gasifier product gas WO 2007/081296 discloses that a number of parameters need to be maintained within certain limits. These include: particle size distribution, moisture content, ash content, volatile matter content, heating value, bulk density and feedstock composition. The heating value is kept in a range be providing preheated air as needed. It is furthermore disclosed that the moisture content of the biomass needs to be kept in a 10% to 20% range to achieve this low level of tar in the gasifier product gas.

[0004] This known gasifier comprises the disadvantage that there is a need of a high number of sensors and control circuits to keep all these parameters in the preferred rang an still the result is not satisfying as there is a high percentage of tar in the gasifier product gas. A biomass gasification system that uses this known cross draft gasifier therefore needs an expensive system to separate the tar and dust from the rest of the gasification product gas before burning in the combustion unit.

[0005] Thus in view of the foregoing it is desirable to have an improved biomass gasifier that provides a more effective way to cope with dust and tar residues from the gasifier product gas before burning.

[0006] The inventors realized that there is a need to keep the temperature of the generated gasifier product gas in the upper area of the gasifier below a maximum temperature to ensure that the tar in the gasifier product gas is captured in moisture drops. The gasification product gas at the second outlet of the gasifier has to have a temperature in the range of about 70 to 80 degrees Celsius. This temperature and other process parameters enable equilibrium between vaporisation and condensation of water and tar in upper area of the updraft gasifier. The tar residues in the gasifier product gas are captured in moisture drops of nebular. This nebula consists of liquid tar dissolved in the water coming from the wood chips moisture. These moisture drops get in contact with pipes connected to the second outlet of the gasifier and condense on the inner walls of the pipes and run along the declined pipes towards the combustion unit. With the moisture drops, the tar from the gasifier product gas is flushed away towards the combustion unit where it is burned residue-free together with the gasification product gas.

[0007] Based on this knowledge a gasifier according to the invention is characterized in, that the gasifier furthermore comprises:

● a temperature sensor to measure the temperature of the gasifier product gas in the upper area of the gasifier, and

● moisturization means to moisture the biomass, and

● a control circuit to activate the moisturization means to moister the biomass in case the temperature of the gasifier product gas exceeds a maximum temperature.

[0008] This updraft gasifier comprises the advantage that the tar and dust residues in the gasifier product are kept in a condition that enables an easy and cost effective way to separate and to keep them in the gasifier, convert them to ash and to take them out as grate ash from the gasifier. Consequently, the product gas and then after combustion the flue gas has a very low dust content. Further details and advantages of this updraft gasifier will become more apparent in the following description and the accompanying drawings.

Figure 1 is a perspective view of a Stirling plant with the preferred biomass gasifier of this invention.

Figure 2 is a side view of the Stirling plant of Figure 1.

Figure 3 is a side view of the gasifier of the Stirling plant of Figure 1.

[0009] Figure 1 is a perspective view and Figure 2 is a side view of a Stirling plant 1 to generate heat and electricity from biomass. The Stirling plant 1 comprises a biomass gasification system 2 to generate heat, which heat to some extent is transformed into electricity by four Stirling engines 3. The biomass gasification system 2 comprises an updraft gasifier 4, from which gasifier product gas is transported through pipes 5 to four combustion units 6 where the gasifier product gas is burnt to heat up a transfer medium used to drive the Stirling engines 3.

[0010] Figure 3 is a side view of the gasifier 4 of the biomass gasification system 2. The gasifier 4 is built as updraft gasifier. In principle the updraft gasifier is a standing tubular reactor 7 which consists essentially out of cylindrical steel shell isolated and lined with bricks on the inner side. Additional the reactor 7 is isolated on the outer side as well, to avoid hot surfaces. The gasifier 4 comprises a wood chip conveyor 8 connected to a first inlet 9 of the gasifier 4 as fuel feed and an ash conveyer 10 connected to a first outlet 11 of the gasifier 4 as fuel discharge.

[0011] The wood chip conveyor 8 feeds the gasifier 4 through the lateral first inlet 9 which is arranged in the upper area of the gasifier 4. The feeding is monitored by a filling level indicator. The filling level indicator consists out of a rotating blade in the filling space mounted on a geared motor. The filling level is periodically checked and if necessary the fuel feed activated.

[0012] The gasification in the updraft gasifier 4 takes place in a counterflow principle, thereby the fuel passes the gasifier 4 from its upper area 12 to its lower area 13, while the gasification agent enters the gasifier 4 at a second inlet 14 at the bottom of the gasifier 4 and leaves the gasifier 4 as gasification product gas at a second outlet 15 in the upper area of the gasifier 4. Thereby different zones are formed in the gasifier 4. Basically four zones are formed, the drying zone, the pyrolysis zone, the reduction zone and the oxidation zone. As a gasification agent, a defined mixture of flue gas and air is used. The gasification agent is preheated in a gas pre-heater of the biomass gasification system 2 and afterwards fed at the second inlet 14 at the bottom of the gasifier 4.

[0013] The biomass gasification system 2 comprises a manifold system 16 to connect the second outlet 15 of the gasifier 4 with pipes 5 for the four combustion units 6 of the biomass gasification system 2. The biomass gasification system 2 is setup in such a way, that there is a route of transportation R of the gasifier product gas from the outlet 15 of the gasifier 4 through the manifold system 16 and the pipes 5 into inlets 18 of the combustion units 6. The highest or top points of these routes of transportation R is the manifold system 16 from where there is a decline W of the pipes 5 into the combustion units 6 in the direction of the route of transportation R and from where there is a decline through the manifold system 16 to the outlet 15 of the gasifier 4 towards the route of transportation R.

[0014] The gasifier 4 comprises a temperature sensor 19 to measure the temperature of the gasifier product gas in the upper area 12 of the gasifier 4. The gasifier 4 furthermore comprises moisturization means that are realized by a sprinkler system 20 arranged at the chip conveyor 8 to moisture the biomass in the chip conveyor 8. A control circuit 20 is connected to the temperature sensor 19 and the sprinkler system 20 and is arranged to activate the sprinkler system 20 in case the measured temperature exceeds a maximum temperature stored in the control circuit 20. The appropriate maximum temperature to be stored in the control circuit might depend on different circumstances like for instance the kind of biomass fed into the gasifier or contents of the gasifier product gas allowed to be burned in the combustion unit.

[0015] During the gasification process in the gasifier 4 the moisture of the biomass (wood chips) is typically in a preferred range of 20-55%. The gasification product gas at the second outlet 15 of the gasifier 4 has to be kept in a temperature range of about 70 to 80 degrees Celsius. This temperature and other process parameters enable equilibrium between vaporisation and condensation of water and tar in the upper area of the updraft gasifier 4. The tar residues in the gasifier product gas are captured in moisture drops of nebular. This nebula consists of liquid tar dissolved in the water coming from the wood chips moisture. These moisture drops get in contact with the pipes 5 and condense on the inner walls of the pipes 5 and run along the pipes 5 towards the combustion units 6. With the moisture drops, the tar from the gasifier product gas is flushed away towards the combustion units 6 where it is burned residue-free together with the gasification product gas. The pipes 5 are insulated to avoid that too much of the moisture drops condensate on the inner walls of the pipes 5. This keeps the amount of condensate in a preferred range.

[0016] In case the temperature of the gasifier product gas exceeds the maximum temperature defined within the range of 75-100 degrees Celsius, the control circuit 21 activates the sprinkler system 20. As a result the sprinkler system 20 moistures the biomass in the chip conveyor 8 with steam or water droplets. This biomass with an increased moisture content is transported by the chip conveyor 8 into the gasifier 4 what increases the moister content in the gasification process what reduces the temperature of the gasifier product gas by the cold due to evaporation.

[0017] As a result the advantage is achieved, that tar and dust residues in the gasifier product gas are kept in a status of nebula during the whole process what enables, that the amount of dust in the gasifier product gas that enters the combustion units 6 is reduced dramatically. This helps to reduce the dust in the emissions of the biomass gasification system 2. The biomass gasification system 2 therefore is characterized by a very low amount of dust in the emission which can be less then 20mg/m³ or even less than 10mg/m³. This makes the use of the biomass gasification system 2 outstanding environment-friendly.

[0018] The control circuit 21 deactivates the sprinkler system 20, in case a deactivation criteria is met. As this process to reduce the temperature of the gasifier product gas by the cold due to evaporation takes some time it has proven to be advantageous to use fixed time duration as deactivation criteria. As an example, the sprinkler system 20 could moisture the biomass in the chip conveyor 8 for a given time duration of e.g. 120 seconds or 5 minutes, while the chip conveyor transports the moistured chips into the gasifier 4. An alternative deactivation criteria would be, that the temperature of the gasifier product gas fell below a minimum temperature of e.g. 60 or 75 degrees Celsius.

[0019] A further deactivation criteria would be, that a moisture sensor attached to the gasifier to measure the moisture of the chips in the gasifier 4 exceeds a maximum moisture limit. The control circuit 21 could use only one of these deactivation criteria or two or more of these deactivation criteria to decide about when to deactivate the sprinkler system 20.

[0020] In another embodiment of the invention, the sprinkler system is mounted at the ceiling of the gasifier to moister the biomass already in the gasifier. This solution has the advantage that the steam or water droplets from the sprinkler system immediately increase the humidity of gasifier product gas and the biomass in the gasifier. Apart from this additional advantage the same advantages are achieved with a gasifier of this embodiment as explained for the gasifier 4.

[0021] Furthermore it has proven to be advantageous to supply the water trough the mechanical leveler of the gasifier.

[0022] The biomass gasification system 2 with the combustion units 6 may be used in the Stirling plant 1, but could be used in a heating system of a building as well. In both embodiments the combustion units 6 heat up a transfer medium (e.g. water or air). In the Stirling plant 1 this transfer medium is used to drive the Stirling engine 3 and in a heating system the transfer medium would be used to heat the radiators in one or more buildings. Combined systems may be used as well.

[0023] In a biomass gasification system where tar would be accumulated in the gasifier, the gasification process would be hampered or even stopped. It is therefore essential that the tar residues in the gasification product gas are not accumulated within the gasifier 4. Based on the fact that the pipes 5 that transport the gasification product gas from the valves 17 to the combustion units 6 are declined towards the combustion units 6, the tar residues are flushed away from the gasifier 4 towards the combustion units 6 and burned there. This avoids any maintenance work to remove accumulated tar from the biomass gasification system 2 as the tar is not accumulated in the system.

[0024] Without departing from the gist of the invention, the preferred temperature range could be in a different range of e.g. 60 to 100 degrees Celsius with maximum and minimum temperatures adapted to it

[0025] It has to be stated that instead of the conveyer 8, the gasifier could comprise a hydraulic piston or other means to transport the biomass through the first inlet into the gasifier.

Claims

1. Updraft gasifier (4) for a biomass gasification system (2) to generate gasifier product gas to be burned in a combustion unit (6), which gasifier (4) comprises:

● a first inlet (9) for biomass in the upper area (12) of the gasifier (4), and

● a first outlet (11) for ash in the lower area (13) of the gasifier (4), and

● a second inlet (14) for a gasification agent in the lower area (13) of the gasifier (4), and

● a second outlet (15) for the gasifier product gas in the upper area (12) of the gasifier (4),

characterized in, that the gasifier furthermore comprises:

● a temperature sensor (19) to measure the temperature of the gasifier product gas in the upper area (12) of the gasifier (4), and

● moisturization means (20) to moisture the biomass, and

● a control circuit (21) to activate the moisturization means (20) to moister the biomass in case the temperature of the gasifier product gas exceeds a maximum temperature.

2. Updraft gasifier (4) of claim 1 characterized in, that the gasifier (4) comprises a conveyer (8) to transport the biomass through the first inlet (9) into the gasifier (4) and that the moisturization means (20) are arranged to moisture the biomass in the conveyer (8).

3. Updraft gasifier (4) of claim 1 characterized in, that the moisturization means (20) are arranged in the upper area (12) of the gasifier (4) to moisture the biomass in the gasifier (4).

4. Updraft gasifier (4) according to any of the preceding claims characterized in, that the control circuit (21) deactivates the moisturization means (20), if one or more of the following deactivation criteria are met:

● moisturization time limit reached;

● temperature of the generate gasifier product gas falls below a minimum temperature;

● moisture in the generated gasifier product gas exceeds a maximum moisture limit.

5. Updraft gasifier (4) of claim 4 characterized in, that

● the maximum temperature is about 100°C and preferred about 75°C;

● the minimum temperature is about 60°C and preferred about 75°C

6. Stirling plant (1) to generate heat and electricity from biomass which plant (1) comprises at least one Stirling motor (3) to transform heat into electricity, characterized in, that the plant (1) furthermore comprises an updraft gasifier (4) in a biomass gasification system (2) to generate gasifier product gas to be burned in a combustion unit (6) according to any of the preceding claims to provide the heat to fuel the at least one Stirling motor (3).

7. Stirling plant (1) of claim 6 characterized in, that it comprises at least one pipe (5) to transport the gasifier product gas from the second outlet (15) of the gasifier (4) to an inlet (18) of the combustion unit (6), and that the at least one pipe (5) is declined (W) from the gasifier (4) to the combustion unit (6) to transport fractions of tar in the gasifier product gas towards the combustion unit (6), and preferred into the combustion unit (6).

Drawing