The invention concerns a system comprising a ventilation system, a biomass pellets storage facility and a leak tight pellets feed-in unit between the pellets storage facility and a pellets combustion unit.
Biomass pellets are usually made from dried and milled sawdust and wood shavings that have been compressed into pellets with exemplary dimensions of 10 to 20 mm in length and 3 to 12 mm in diameter. Usually they do not contain any additives or binders. Wood pellet boilers are used in homes and businesses as an alternative to oil or gas fired boilers. They are also being installed to replace coal-fired boilers. In general pellets from biomass create carbon monoxide (CO) while they are stored in storage facilities.
Pellets are usually stored in large sealed containers or in concrete storage rooms that are almost air tight. The pellets storage facility is usually connected via a feeder, for example a screw feeder, to the boiler of a combustion unit. Alternatively, the storage tank can be mounted over the boiler for gravity feeding. Due to the enclosed nature of these storage tanks or rooms, the atmosphere inside can become oxygen depleted and a toxic atmosphere containing carbon monoxide can accumulate. The reason therefore is so far not fully explained, but it is assumed that auto-oxidation processes are responsible for carbon monoxide production from wood pellets.
Even small quantities of wood pellets can produce life-threatening quantities of carbon monoxide in a confined space and that there are various factors that will affect the amount of carbon monoxide produced like storage duration, storage temperature, type of wood used for pellets, available oxygen content, surface area of pellets as well as amount of mechanical abrasion of the pellets during storage.
Consequently the carbon monoxide concentration is increasing over time when the storage facilities are not ventilated properly. Incorporated in this is a risk on human safety in case of the pellets storage facilities have to be inspected. Recently several people were critically injured or died because they entered such storage facilities while the carbon monoxide concentration was above critical levels due to a lack of ventilation.
Carbon monoxide can kill quickly without warning. It is a colourless, odourless and tasteless gas that is highly toxic. When carbon monoxide enters a human body, it prevents the blood from bringing oxygen to cells, tissues and organs.
It follows from the foregoing that it is desirable to have an improved ventilation system for pellets storage facilities that ensures low carbon dioxide concentration within the wood pellet storage facility and avoids safety risks due to carbon dioxide contamination.
Aim of the invention is to provide a system for pellets storage facilities that overcomes the disadvantages known in the state of the art. This problem is solved by the system according to the preamble of claim 1, as shown in the document
A system according to the invention comprises a biomass pellets storage facility, a leak tight pellets feed-in unit between the pellets storage facility and a pellets combustion unit, and a separate venting unit connecting the pellets storage facility to the pellets combustion unit. Thus a ventilation of the storage facility to reduce the harmful carbon monoxide (CO) content in the facility is provided by the pellets combustion unit. Combustion supply air that is required to operate the pellets combustion unit is supplied via the venting unit from the storage facility. Accordingly carbon monoxide that has formed in the storage facility while storing the biomass pellets like wood pellets or wood chips is transported via the venting unit into the pellets combustion unit and is co-combusted or vented through the burner together with the pellets.
In a further advantageous embodiment of the invention, within the venting unit, a ventilation supply air of the pellets storage facility is sucked as combustion supply air into the pellets combustion unit. The combustion supply air that is needed to operate the pellets combustion unit is here sucked out of the pellets storage facility. Carbon monoxide that is generated from pellets during storage is sucked as well as the needed combustion supply air into the pellets combustion unit. Advantageously the CO emissions of the pellets storage facility are further oxidized to carbon dioxide in the combustion unit and are sent to a chimney of the combustion unit together with the flue gas of the combustion.
Usefully at a system according to the invention, the pellets storage facility is equipped with at least one direct connection line and/or one indirect connection line to bring in ventilation supply air from an outside to the storage facility. The connection of the pellets storage facility to ambient air in the surroundings outside of a building like a combined biomass heat and power station can be realised either via a direct connection line to bring in ventilation supply air from the outside directly into the storage facility, or via an indirect connection line for instance from the outside to a boiler room or boiler house, respectively, and then further to the pellets storage facility. Within the scope of the invention it is also possible that one or several direct connection lines for ventilation supply air as well as one or several indirect connection lines are combined to safeguard that a sufficient amount of ventilation supply air of ambient air is transported to the pellets storage.
According to another advantageous embodiment of the invention, the venting unit and/or at least one direct connection line and/or indirect connection line is/are equipped with a suction device and/or a ventilation device. It might be required and is within the scope of the invention that suction devices like a pump and/or ventilation devices like a fan are installed in the direct and/or indirect connection lines to generate a reduced air pressure within the pellets storage facility compared to the ambient atmospheric pressure. Also the venting unit might be equipped with at least one suction and/or venting device to ensure that carbon monoxide formed within the storage facility is transported together with the combustion supply air into the combustion unit. This at least one suction device and/or ventilation device operates mechanically and is powered electrically, mechanically or pneumatically according to the regulations in force.
Advantageously, the venting unit is integrated in the feed-in unit. Thus unintended leakage and back-mixing of carbon monoxide-enriched combustion supply air from the combustion unit to the storage facility is successfully avoided. In a preferred embodiment of the invention an air pressure in the pellets storage facility is lower than an atmospheric pressure of the ambient air. Advantageously unintended leakage of ventilation air with an enhanced CO content out of the pellets storage facility is avoided.
Usefully an inventive system features an air pressure in the venting unit that is lower than an air pressure in the pellets storage facility. With an air pressure difference or pressure gradient, respectively, between the pellets storage facility and the venting unit it is ensured that combustion supply air with an enhanced CO content due to the stored pelletized material is sucked from the storage facility into the combustion unit. Consequently unintended leakage of carbon monoxide out of the pellets storage facility can be avoided.
Advantageously with a system according to the invention a carbon monoxide content in the pellets storage facility can be kept below a certain limiting value.
Further details and advantages of this system according to the invention will become more apparent in the following description and the accompanying drawing.
The figure depicts a schematic view of a combined biomass heat and power station with a ventilation system 1. A typical setup of the system according to the invention comprises the ventilation system 1, a pellets storage facility 2, a pellets feed-in unit 3 that feeds the pellets from the pellets storage facility 2 in direction 4 to a pellets combustion unit 5, and a venting unit 6 connecting the storage facility 2 to the pellets combustion unit 5. The level of the stored pellets on the lower section of the pellets storage facility 2 is indicated in the figure. Within the venting unit 6 the ventilation supply air is transported in direction 7 from the pellets storage facility 2 as combustion supply air to the pellets combustion unit 5. In a combustion chamber within the pellets combustion unit 5 the wood pellets are combusted together with the combustion supply air and together with the carbon monoxide (CO) formed within the pellets storage facility 2. The off-gas of the combustion unit 5 is then lead as flue gas 8 to a chimney 9 and emitted to the surroundings 10.
The connection of the pellets storage facility 2 to ambient air 10 in the surroundings outside of a combined biomass heat and power station 11 can be realised either via a direct connection line 12 for ventilation supply air 13 from the outside 10 to the storage facility 2, or via an indirect connection line 15 from the outside 10 to a boiler room or boiler house 14, respectively, and further to the pellets storage facility 2. Within the scope of the invention it is also possible that one or several direct connection lines 12 for ventilation supply air 13 as well as one or several indirect connection lines 15 are combined to safeguard a sufficient amount of ventilation supply air 13 of ambient air is transported to the pellets storage 2.
Other alternatives of the connection to ambient air are as well possible but not shown in the figure. For example the venting air of the storage facility 2 could be taken from the boiler room 14 and then be sucked to the boiler 5. In that case the boiler room 14 needs to be vented to the surrounding 10. While the pellets combustion unit 5 is in operation the combustion supply air 7 is sucked out of the pellets storage facility 2. Thus the carbon monoxide formed in the pellets storage facility 2 is exhausted together with combustion supply air in direction 7 into the pellets combustion unit 5. Hereby the carbon monoxide (CO) is oxidised to carbon dioxide (CO2) within a burner of the combustion unit 5 where the wood chips are combusted due to the high temperature and the presence of oxygen.
Since the combustion air is taken from the pellets storage facility 2 and due to the fact that the storage facility 2 is directly 12 and/or indirectly 15 via the boiler house 14 connected to ambient air 10, the storage facility 2 is sufficiently ventilated and the risk of a high carbon monoxide concentration within the pellets storage facility 2 is significantly reduced. Additionally the environmentally harmful carbon monoxide formed during pellets storage is oxidised to carbon dioxide and emitted only after previous co-combustion in the combustion chamber. This is less harmful to the environment compared with regular ventilation of carbon monoxide laden off-air of the pellets storage facilities as known in the art.
Effective and sufficient ventilation between the pellets storage facility 2 and the pellets combustion unit 5 via the pellets feed-in unit 3 is in most cases not possible since the pellets feed-in unit 3 is usually completely filled with wood chips. The feed-in unit 3 typically comprises a screw conveyor that is also filled quasi gas-tight with pellets that are moved in direction 4 into the combustion unit 5. Accordingly a separate venting unit 6 between the pellets storage facility 2 and the pellets combustion unit 5 is realised, as is depicted in the pellets storage facility 2 and the pellets combustion unit 5 is realised, as is depicted in the figure.
In the figure also the direct air supply connection 12 as well as the indirect air supply connection 15 are equipped with a ventilation device 17 to define the direction of ventilation supply air 13 and avoid leakage of carbon monoxide-enriched ventilation air from the pellets storage facility 2 to the outside 10. The air supply connection 12 might in many cases also be used to refill the storage from time to time with pellets and is therefore not necessarily only a ventilation device.
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.