Review of influence of combustion method on properties of RHA:

The main factors in the various combustion and gasification processes that determine the type of ash produced are time, temperature and turbulence. These effect all chemical changes that occur in the combustion process including the way the ash morphology is altered. A broad explanation of combustion techniques was given in Section 5.2. Specific chemical and physical properties of ash, taken from literature accounts, are described below. Appendix A compiles the chemical analyses of rice husk ash from the literature review, going back several decades. It also includes the analyses of two samples of RHA (one bottom ash and one fly ash sample) obtained specifically for this study. In most of the analyses there were no details of combustion or analytical

techniques, making it impossible to associate the chemistry of ash directly with a specific combustion technique. This lack of information may be due in part to many of the analyses having been conducted under laboratory conditions, and also due to the commercial sensitivity of giving exacting technological specifications alongside chemical analyses of ash. A summary of all the ash analyses is at the end of this chapter.

A study in 1972 compared a range of data for ash composition. The wide range of values was as a result of the variation of purity of the samples and the accuracy of the analytical procedures used. However, since there is no information on different combustion techniques employed in the husk combustion, or analytical techniques used, it is difficult to tell whether any of the reported ranges in chemistries seen could be attributed to particular combustion techniques. The patent filed by P.K.Mehta, for producing RHA of a quality ideally suited to the cement market, describes burning the husk continuously at a low temperature to preserve the amorphous nature of the silica.

The method utilizes the fly ash after its separation from the flue gases by a multi cyclone separator. Commonly, in the production of highly amorphous ash, low temperatures and fairly long “burntimes” are used, as for Mehta’s patent. Other work in India has also concentrated on this technique, and has shown how a two-stage process of combustion could control the chemical and physical properties of the resultant ash, increasing its pozzolanic activity by taking the husk through a carbonizing process without “flaming”. This type of burning was shown to produce a fine white ash which did not ‘carbonize’. By comparison, a “normal” combustion process (taking the furnace from room temperature up to the fixed burning temperature, where it was held until combustion was completed) produced a black colored ash. This same study compared the RHA in terms of electrical conductivity and compressive strength tests with concrete. The electrical conductivity is an effective measure of the amorphousness of the ash and showed that the “slow-burn” process produced significantly more amorphous ash. Similar results were found in a study in Guyana to ascertain the relationship between operation conditions and ash chemistry produced in terms of ash colour, carbon content and ‘silica activity index’ (a measure of

its pozzalanicity). Comparing ‘5-hour’ with ‘7-hour’ burn times showed higher LOI in the shorter burn-time experiments (~6%) compared with ~3% LOI for longer burn times. In addition, higher percentages of silica were produced over longer combustion periods although no details were given concerning the percentage of amorphous and crystalline ash.

3.3.6.1 Fixed grate boilers:

None of the reports in the literature made specific reference to conventional grate (fixed- or moving-grate) technology, and although reference to “normal” or

“conventional” boilers may well be a reference to a grated boiler we cannot assume this in terms of the reported ash properties. However, a sample of ash (“Patum”) from a fixed grate boiler in Thailand was analyzed, the results of which are given in analysis 31 in Appendix 1. A significant difference between this and other ash samples is the large grain size, with 50% of the sample larger than 0.425mmsq/hole sieve. Compared with the circulating fluidized bed RHA (see “Fortum” ash analysis below) the Patum RHA showed a higher LOI (4.1% versus 2.2%), a higher total carbon content (3% versus 0.5%) and higher crystalline silica content as one would expect comparing the two technologies. The coarseness of the ash samples has market significance, because for the majority of marketable purposes (steel, cement, absorbent etc) a fine material is preferred, and the grinding of husks before combustion or RHA after combustion adds a significant cost to the process.

3.3.6.2 Fluidized bed:

Since FBCs have a more uniform and lower combustion temperature than stoker boilers, it is possible that such boilers produce less crystalline ash. Burning husk in a fluidized bed burner has been found to give mainly amorphous ash with a high specific surface area. Best results have been obtained by controlling the temperature of the burner via fuel feed rate, with the air supply set at an optimum velocity of 15m/s and the temperature set at an optimum 750ºC. Comparing the properties of this ash for

pozzolanic reactivity with Portland cement, with ash obtained from conventional combustion techniques, (although no description of “conventional” combustion techniques was given) gave excellent results in terms of its compressive strength. There was no information giving of the proportion of amorphous to crystalline ash although the silica ‘recovery’ was high (97.6%) and the carbon content ranged from 1-4%.

3.3.6.3 Circulating Fluidized Bed (CFB):

The only RHA sample available that can be unequivocally assigned to combustion by a circulating fluidized bed is “Fortum”, a sample obtained specifically for this study. The sample is a very fine material, with approximately 50% by volume passing through a 0.150mm sq/hole sieve. It is a pale grey ash (see Plate 12b) compared with the coarser bottom ash from Patum (Plate 12a). It has a low carbon content of 0.5% as is often, although not always, the case with pale colored ash. Generally one would expect more amorphous ash from CFB combustion since the time spent at higher temperatures tends to be short, and due to the suspended nature of the fuel the temperature is evenly distributed and does not result in extremely high temperature “hot-spots”. However, the analysis of the Fortum ash reveals a fairly high crystalline silica content of 33%

crystobalite and 20% (transitional amorphous to crystalline) quartz. In terms of trace elements the Fortum and Patum samples exhibit similar concentrations.

3.3.6.4 Grate versus ‘conventional’:

The National Research Institute in Chatham, UK is conducting a two year long investigation into improving the boiler efficiency of rice furnaces in Bangladesh, with a view to producing RHA of a consistent quality to sell to the cement industry. The NRI have conducted a series of experiments both on the RHA itself and also on blocks made with varying proportions of RHA, substituting for cement, to examine changes in its strength properties. The NRI obtained several samples, mainly from two types of boiler (grate and conventional), however no additional information about the exact type and operating conditions were taken. The results so far show a clear correlation between the

types of ash produced, in terms of crystalline vs. amorphous silica content, and the boiler type. The average percentage of crystalline silica in the ash was 75.1% and 17.45%

for grated and conventional furnaces respectively.

3.3.6.5 Gasification:

Literature sources reviewed to date focus more on the relatively high un-burnt carbon content of char/ash from gasifier without providing data on the relative amounts of amorphous to crystalline ash. The un-burnt carbon can exceed 40%. This would preclude use of the ash for other than low cost uses and may explain why no extensive beneficial use of gasifier ash has been found. Joseph investigated the combustion processes necessary to burn husks under controlled conditions such that the ash remains mainly amorphous and that the C content is reduced to below 15%, in order that it can be used as an additive in lime bricks or cement. The findings from this fairly early research concluded that combustion through gasification, rather than through a vortex furnace produced the better quality ash, and the quality of the ash was further

“improved” by varying the gasification conditions. Significantly and so far unreported from other publications, they found that variations in collection methods and ash cooling significantly affected the properties and characteristics of the ash. Once collected from the gasification system carbon burnout occurred over the proceeding four days in the concrete ash pit, the carbon content of the ash after four days had dropped to 7-10% from 26% immediately after collection.

3.3.6.6 Additional Technology:

Torftech, a Canadian based company, supplies Torbed® reactor based rice hull combustion systems. The technology provides ash with a high percentage of amorphous silica for use in the concrete and cement industries. They are able to produce low carbon, high surface area low crystalline ash by maintaining the temperature of their expanded bed reactors below 850ºC (at 830ºC with an estimated

residence time of approximately 5 minutes, no crystallization occurs). The technology has been a joint venture between Torftech and the University of Western Ontario.