It should be noted that a considerable amount of the pertinent prior art in respect of this technology is disclosed in various patent applications filed designating the present applicant as inventor. These include PCT WO2010/072830 entitled to “A dual fuel boiler” and European patent specification number EP 2 187 122 entitled “A granular fuel-fired boiler brazier”.
One of the major problems with such granular fuel burning boilers is the removal of ash contents from the boiler. Much of the granular fuel is normally pelleted wood. Unfortunately, such pelleted wood contains impurities and what is effectively sand which is ingested through the bark of the tree as it grows. When it burns, the ash content is made up of relatively soft combustion products, almost pure carbon dust and what is a vitrified clinker, very similar to glass. It should be appreciated that the ash usually weighs somewhat of the order of 0.5% of the weight of the fuel and the vitrified clinker is somewhat of the order of 1 to 2% by weight of the ash. Thus, it is a relatively small proportion of the combustion products. However, because of its effect, it is found to be a not insignificant component of the ash contents. What happens is that this vitrified clinker forms a skin over the base of the brazier and prevents air being delivered up into the burning fuel from underneath. The way in which the combustion products are removed from the brazier is to move the base of the brazier away from it's side walls to allow the combustion products to fall out of the brazier. There are considerable problems with this as the vitrified clinker does not fall through the brazier but bridges the brazier retaining the softer ash. A particularly useful means of breaking up this vitrified clinker on discharging the brazier is described in the co-pending British Patent Application Number 0821060.1, filed by my company However, even with this extremely efficient invention, we have found some slight, albeit minor, problems with the operation of this fragmentation means and the present application is directed towards attending to this. With many other solid granular fuels, this problem is exacerbated. It is hard to over emphasise the difficulties that such vitrified ash causes.
A further problem with these boilers for burning granular fuel is that they are usually arranged so that there is a hood over the brazier, which hood is essential to retain the burning products to ensure that they burn correctly before being dissipated out of the boiler. A typical example of such a hood is described in GB Patent Specification Number 22 274 162 A (Jonathon Greenall). Even with such hoods, there is a problem in that the amount of carbon monoxide (CO) in the boiler flue, on discharge, is relatively large, at best being somewhat of the order of 100 ppm and, at worst, 1000 ppm or even more. Clearly, if this could be improved on, it would be advantageous. Ideally, the amount of carbon monoxide should not exceed 100 ppm. A further problem with these hoods is that they disintegrate fairly rapidly in use and require constant replacement. The problem is that to be effective, the hood must trap hot combustion gases to ensure adequate combustion before delivery out of the brazier enclosure. Since the hood will be directly above the brazier and the gases will naturally rise to impinge against the hood, the hood is under severe stress. A typical solution to this problem is to provide a hollow hood such as described in the aforementioned GB Patent Specification Number 22 274 162. However, there are still considerable problems in producing an efficient construction of such a hood which will ensure minimal amounts of carbon monoxide.
Another problem with these boilers for burning granular fuels such as wood pellets is the necessity to make sure that the granular fuel is delivered onto the burning fuel bed as gently as possible so as not to disturb the burning fuel already there. The problem is that if the burning fuel within the brazier is disturbed, then the lighter ash will be prematurely delivered out of the brazier into the boiler itself or more usually into the ash pan, rather than being retained for subsequent delivery to an ash pan on complete combustion taking place. Any incompletely combusted fuel which is light can also be delivered out of the boiler enclosure into the ash pan where it will smoulder producing carbon monoxide. Any ash delivered into the boiler itself will almost certainly form a thin layer on the inside of the boiler reducing the heat transfer properties. Accordingly, the less disturbance of the burning fuel the better. Accordingly, disturbing the burning fuel also reduces the efficiency of the burning operation and generates more carbon monoxide.
Another problem with these boilers is to ensure that there is adequate air available for combustion. It has been found, for example, that with these constructions of braziers, air escaping out of the brazier is a major problem. Further, the escaping air almost certainly is mixed with incomplete combustion gases, further reducing the efficiency of the boiler. Additionally, it has been found essential to ensure that adequate air is provided into the centre of the brazier and also into the hot combustion gases as they rise out of the brazier. The efficient control of the air is an essential requirement for optimum burning conditions and thus optimum operation of the boiler.
Another problem that has been identified is the need to provide boilers of different heat outputs which causes difficulties in that braziers of different sizes have to be provided.
A problem with present constructions of such boilers is that they are often not cleaned adequately or indeed, more importantly, at the right time intervals. If the boiler is relatively inefficiently operated, then very quickly a skin of ash builds up on the boiler walls, reducing the heat transfer capacity of the boiler. A further problem is that very often the brazier is not emptied frequently enough. This is particularly a problem when the boiler is operating somewhat close to maximum capacity.
An additional problem that has been noted with these boilers is the amount of carbon monoxide and other incomplete combustion gases which are delivered out of the boiler flue into the atmosphere during start-up. This is particularly a problem when the boiler is working at well under normal capacity as it is starting up and shutting down a considerable number of times in any period of operation. Again something needs to be done to improve the efficiency of combustion during start-up.
What has to be emphasised about most of these problems is that they are not of themselves when taken singularly seen as being of major importance however it is the cumulative effect of these problems that has led to a less than satisfactory performance in the market place.
Heretofore, while granular fuel fired boilers such as wood pellet boilers have been welcomed and installed on quite a large scale in many countries, they have not, by any means, been successful. This unfortunately has led to a very bad reputation for such granular fuel burning boilers and indeed their abandonment by many of their original champions.
The initial problem related to the indifferent quality of granular fuel and particularly wood pellets. Most of the problems encountered with the fuels and their storage have been generally resolved. It was felt by many that when these problems were solved the granular fuel burning boilers would operate satisfactorily. Unfortunately, that was not the case. It just merely highlighted the remaining problems which have now come to prominence. Indeed, we believe that many of these problems were not fully appreciated by those in the industry heretofore. Many were aware that, for example, there was too much ash build up within the boiler, relatively large percentages of carbon monoxide in the exhaust flue and so on, without appreciating the reason for these. These often apparently minor problems were largely centred round the handling of the combustion products and the fuel. The boilers must operate at required efficiency without requiring constant attention and maintenance by the householder. Until these problems are solved, the clear advantages of using a granular fuel burning boiler will not be appreciated by the consumer.