Biomass gasification system with an updraft gasifier

Abstract

 The invention relates to a biomass gasification system (2) to generate gasifier product gas to be burned in a combustion unit (6), which system comprises a gasifier (4) with a first inlet (9) for biomass, and a second inlet (14) for gasification agent and an outlet (15) for the generated gasifier product gas, and a combustion unit (6) with an inlet (20) for the gasifier product gas from the gasifier (4), and at least one pipe (5) to transport the gasifier product gas from the outlet (15) of the gasifier (4) to the inlet (20) of the combustion unit (6), wherein the gasifier (4) is an updraft gasifier with its outlet (15) for the gasifier product gas at the top end (12) of the gasifier (4), and that the at least one pipe (5) is declined 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), and that the combustion unit (6) is constructed to heat a transfer medium.

Description

[0001] The invention relates to a biomass gasification system to generate gasifier product gas to be burned in a combustion unit, which system comprises a gasifier with a first inlet for biomass, and a second inlet for gasification agent, and an outlet for the generated gasifier product gas, and a combustion unit with an inlet for the gasifier product gas from the gasifier, and at least one pipe to transport the gasifier product gas from the outlet of the gasifier to the inlet of the combustion unit.

[0002] Document US2008/089503 A1 discloses a wood gasification system that gasifies forest wood chips into a gasifier product gas which can be burned in an internal combustion engine to mechanically drive a device. The disclosed system comprises a downdraft gasifier with an inlet for the forest wood chips at the top end of the gasifier and an outlet of gasifier product gas at the bottom end of the gasifier. The gasifier product gas at the outlet of the gasifier comprises dust and tar residues which need to be separated from the gas to avoid damage at the internal combustion engine.

[0003] To achieve a separation of the tar residues from the gas, the known system uses a ceramic filter that may be heated up to 300 degrees Celsius and more to clean the filter. This known system comprises the disadvantage that there is a need to clean the filter in fixed intervals and replace it from time to time. This is time and cost intensive and reduces the effectiveness of this wood gasification system.

[0004] Thus in view of the foregoing it is desirable to have an improved biomass gasification system that provides a more effective way to cope with dust and tar residues from the gasifier product gas before burning. A biomass gasification system according to the invention is characterized in, that the gasifier is an updraft gasifier with its outlet for the gasifier product gas at the top end of the gasifier, and that the at least one pipe is declined from the gasifier to the combustion unit to transport fractions of tar in the gasifier product gas towards the combustion unit, and preferred into the combustion unit, and that the combustion unit is constructed to heat a transfer medium.

[0005] Due to the fact that the biomass gasification system according to the invention uses an - as such known - updraft gasifier, the advantage is achieved to have a reduced amount of dust at the outlet of the gasifier for the gasifier product gas compared to the downdraft gasifier of above prior art system. Furthermore, in an updraft gasification process the tar residues in the gasifier product gas at the outlet of the gasifier are nebulous. The nebula consists of liquid tar which is dissolved in water coming from the wood chips moisture. These moisture drops get in contact with the inner wall of the pipe and run along the pipe towards where it is declined. The complete mixture of gasifier product gas and condensate, which consists of the mixture of tar and water, is running to the combustion chamber, where it is burned in the combustion unit. Finally, the gasifier product gas is burnt in a combustion unit that is constructed to heat a transfer medium. Compared to a internal combustion engine according to the state of the art system, there is no problem to burn the tar without damages in such a combustion unit as there are no moving parts in the combustion unit (like a piston) involved. All these measures together enable that the dust in the emission of the biomass gasification system is very low due to the reduced dust in the gasifier product gas, what makes it outstanding environment-friendly.

[0006] The gasification process in the gasifier would be hampered or even stopped in case that too much of the condensate, water tar mixture, would flow back into the gasifier. It is therefore essential that the tar residues in the gasification product gas are not accumulated within the gasifier or gasifier product gas tube. According to the invention the pipe that transports the gasification product gas from the gasifier to the combustion unit is declined towards the combustion unit. This provides the advantage that the tar residues are flushed away from the gasifier towards the combustion unit.

[0007] It is in particular advantageous, that the pipe from the gasifier to the combustion unit is declined into the combustion unit. This enables that the tar residues in the gasifier product gas are not accumulated, but constantly burned residue-free in the combustion unit. As the tar comprises a high heating value the overall efficiency of the biomass gasification system is increased by burning the tar residues. Furthermore any maintenance work to remove accumulated tar from the biomass gasification system is avoided, what reduces costs.

[0008] Further details and advantages of this biomass gasification system 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 gasification system of this invention.

Figure 2 is a side view of the biomass gasification system of Figure 1.

Figure 3 is a side view of the gasifier of the biomass gasification system of Figure 1.

Figure 4 is a top view of a manifold system of the biomass gasification system of Figure 1.

Figure 5 is a sectional view of the manifold system of Figure 4.

[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 one 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 an 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. 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 top area 12 to its bottom 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 an outlet 15 at the top end 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 outlet 15 of the gasifier 4 with pipes 5 for the four combustion units 6 of the biomass gasification system 2. The manifold system 16 of the biomass gasification system 2 is shown in more detail in a top view in Figure 4 and a sectional view in Figure 5. The manifold system 16 comprises a pneumatic actuated gas valve 17 for each of the pipes 5 to the four combustion units 6. With these valves 17 the flow of gasifier product gas from the gasifier 4 to each of the combustion units 6 may be opened or closed to use more or less of the four Stirling engines 3 to adopt to the actual demand of product gas to establish the needed load. The manifold system 16 comprises a further pneumatic actuated gas valve 18 to open or close the outlet 15 of the gasifier 4 for all combustion units 6. The manifold system 16 furthermore comprises a middle chamber 19 to which all valves 17 and 18 are connected to ensure a continues and proper distributed flow with only minor turbulences in the stream of gasifier product gas from the outlet 15 into the pipes 5.

[0014] 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 valve 18 and the middle chamber 19 and the valves 17 and the pipes 5 into inlets 20 of the combustion units 6. The highest or top points of these routes of transportation R are the valves 17 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 middle chamber 19 to the outlet 15 of the gasifier 4 towards the route of transportation R.

[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 outlet 15 of the gasifier 4 has a temperature in the range of 70 to 80 degrees Celsius. This temperature and other process parameters enable equilibrium between vaporisation and condensation of water and tar in top 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] As a result the advantage is achieved, that the amount of dust in the gasifier product gas that enters the combustion units 6 is reduced dramatically, what 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.

[0017] 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 motors 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. As the combustion units 6 do not comprise moving parts like in a internal combustion engine, there is no problem to burn the gasifier product gas together with the condensate of tar and water without any damage of the combustion units 6.

[0018] 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.

[0019] As the major part of the length of this route of transportation R is from the valves 17 through the pipes 5 into the combustion units 6, more or less all tar that condenses with the moisture drops during this route of transportation R flows into the combustion units 6. Therefore only a very small amount of tar flows back into the gasifier 4. This tar moves with the biomass down the reactor 7 and gets evaporated again and then moves upwards in the gasifier 4 with the gasifier product gas. Therefore no tar is accumulated in the biomass gasification system 2.

[0020] To ensure that there is a continuous flow of moisture and tar along the inner walls of the pipes 5 there needs to be a minimum decline W of at least one degree (angular measurement). A minimum degree W of three to ten degrees has proven to be the preferred range of decline W of the pipes 5 to ensure that tar gets flushed into the combustion units 6.

[0021] In a less preferred embodiment, the lowest point of the route of transportation would be at the inlet to the product gas burner close before the combustion unit. Any such accumulation of tar would cause some maintenance work to empty the tar reservoir from time to time, but still it would be ensured that the tar cannot accumulate in the pipes 5.

[0022] There is an advantage to connect more than one combustion units and Stirling engines to a gasifier to use the amount of gasifier product gas generated by the one gasifier in the most efficient way. This advantage is achieved by the manifold system that partitions the gasifier product gas for the different combustion units. The scope of the invention is not limited by the number of combustion units and Stirling engines that may be connected to the gasifier via the manifold system.

Claims

1. Biomass gasification system (2) to generate gasifier product gas to be burned in a combustion unit (6), which system comprises a

• gasifier (4) with a first inlet (9) for biomass, and a second inlet (14) for gasification agent and an outlet (15) for the generated gasifier product gas, and a

• combustion unit (6) with an inlet (20) for the gasifier product gas from the gasifier (4), and at least one

• pipe (5) to transport the gasifier product gas from the outlet (15) of the gasifier (4) to the inlet (20) of the combustion unit (6),
characterized in, that

• the gasifier (4) is an updraft gasifier with its outlet (15) for the gasifier product gas at the top end (12) of the gasifier (4), 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), and that

• the combustion unit (6) is constructed to heat a transfer medium.

2. Biomass gasification system (2) of claim 1 characterized in, that at least one valve (17) connected to the gasifier (4) at the outlet (15) of the gasifier product gas is the top point of a route of transportation of the gasifier product gas from the gasifier (4) to the combustion unit (6), and that the inlet (20) of the combustion unit (6) for the gasifier product gas is the lowest point of this route of transportation.

3. Biomass gasification system (2) of claim 2 characterized in, that the at least one valve (17) is part of a manifold system (16) connected to the outlet (15) of the gasifier (4) to partition the gasifier product gas for at least two combustion units (6) of the biomass gasification system (2), and that the manifold system (16) comprises a middle chamber (19) to which the valves (17) are connected, and that there is a decline from the valves (17) through the middle chamber (19) to the outlet (15) of the gasifier (4) towards the route of transportation of the gasifier product gas.

4. Biomass gasification system (2) according to any of the preceding claims characterized in, that the at least one pipe (5) is declined (W) with at least one and preferred with at least three to ten degrees.

5. Biomass gasification system according to any of the preceding claims characterized in, that the combustion unit is used to heat a transport medium of a heating system of a building.

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 a biomass gasification system (2) according to any of the preceding claims to provide the heat to fuel the at least one Stirling motor (3).

Drawing