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Ind. Eng. Chem. Res. 2007, 46, 4824-4829
Fire Resistance Characteristics of Plates Containing a High Biomass-Ash Proportion Luis F. Vilches, Carlos Leiva, Jose´ Vale, Joaquı´n Olivares, and Constantino Ferna´ ndez-Pereira* UniVersity of SeVille, School of Industrial Engineering, Department of Chemical and EnVironmental Engineering, Camino de los Descubrimientos s/n E-41092, SeVille, Spain
Ashes produced by the combustion of two types of biomass wastesone generated during the extraction of olive oil (olive pomace) and the other during rice-huskingswere used, without any prior treatment, to make insulating plates with fire-resistant properties by means of simple molding and curing methods. The plates, with a weight composition of 69% olive-pomace ash, 1% rice-husk ash, 29% gypsum, and 1% glass fiber, present a great capacity for water retention and, consequently, when subjected to different thermal tests, showed excellent insulating properties. Furthermore, they also demonstrated acceptable mechanical properties with regard to compressive and bending strengths and resistance to impact, as well as a minimal environmental impact. The plates developed with a high biomass-ash ratio could potentially be useful as components of construction materials used for passive fire protection, such as doors and firewalls in buildings and industrial installations. 1. Introduction Given that there is bound to be an increase in the use of biomass as an energy source in combustion and gasification processes, as well as in co-combustion processes in the near future, new types of ash and slag will be generated in great quantities. Thus, it would be beneficial if efforts were made, using different technologies, to find new uses for these residues.1-4 Compared to coal ash, biomass ash tends to present some different properties, e.g., a high content of alkaline components. These properties may invalidate their use in typical ash recycling applications such as cement and concrete, but they might be useful in new applications such as in insulating products, and consequently, research lines on the recycling of biomass ash in these types of products should be pursued. Some of the commercial products used for thermal insulation or passive fire protection in buildings and industrial installations have a chemical composition and properties like those found in some ash- and slag-based products.5-7 The literature also contains several references to patented fire-resistant materials, which contain a greater or lesser proportion of coal combustion fly ash.8,9 In this paper, a fire-resistant product that comprised mainly ash from the combustion of olive pomace but also containing a small amount of rice-husk combustion ash is described. Olive pomace is the main solid waste resulting from physical oil separation and solvent extraction. In general, the product is 60% pulp and skin and 40% stones (pits or olive husk). Most of the olive pomace produced in Spain is disposed of in large tracts of land with poor-quality soil, resulting in possible contamination; however, there is a trend to find alternatives to land disposal. In the south of Spain, different initiatives concerning the use of olive pomace as an energy source (dry olive pomace has a mean calorific value of 14 600 kJ/kg) are currently underway; they are mainly focused on its combustion in furnaces or boilers in order to generate heat or electricity. Therefore, in addition to the reuse of a waste material, by * Corresponding author. Tel.: +34-954487271. Fax: +34-9554461775. E-mail:
[email protected].
recycling the olive pomace in this way, the environmental impact caused by its disposal is reduced. Rice husk is a byproduct of rice farming and is also produced in large amounts in the south of Spain. Rice husk is commonly used as a fuel in ovens because its calorific value is comparable to that of wood and other similar industrial wastes. When rice husk is subjected to high temperatures in an oxidizing environment, the resulting ash particles present a porous surface with a tunneled structure that increases its surface area.4 The goal pursued in this work was to come up with a plateshaped product that comprised mainly ash. The final composition of the plates was obtained using an optimization process in which two main objectives were taken into account: (1) an improvement in the insulating properties and (2) the need for certain minimal mechanical properties, defined by what the products were ultimately expected to be used for. In the present study, plates made using simple molding and curing methods that comprised mainly biomass ash were subjected to different thermal and mechanical tests with the aim of analyzing their fire-resistance behavior and mechanical properties. A leaching study has also been carried out to better characterize the environmental impact produced by the ash and the final product, as well as to evaluate the recycling potential of this kind of ash. 2. Materials and Methods 2.1. Materials. For this study, two types of ash were used without any pretreatment: ash from the combustion of the residual biomass present in the waste from the olive-oil extraction process (BFA) at a Spanish olive oil company (Aceites Pina, S.A., La Carolina, Spain) and ash from the combustion of rice husks (RHA) in the facilities of a rice producer (Arroces Herba´, Sevilla, Spain). In both cases, the biomass was burned in fixed-grate furnaces from which the ashes were collected. The chemical composition of both types of ash is shown in Table 1. As can be seen, there was a high proportion of potassium components in the composition of the BFA while the RHA had a high content of silica. The very high Si content of the RHA contrasts with its much lower alkaline content (100 µm and >30 wt % of particles >250 µm. Gypsum (G) was used as the binder for the pastes, and glass fiber 2-4 cm long and 20-50 microns in diameter was used to increase the mechanic resistance to bending and fissuring in the mortars. Also, two plates made of commercial products used for passive fire protection were tested in order to compare them. One was a calcium silicate-based product (COM-1) and the other was a gypsum- and vermiculite-based product (COM-2). In both cases, the products contained additives, whose nature was not disclosed by the manufacturers. 2.2. Plate Preparation. The composition used in the plates is shown in Table 2. The solid components shown in the above table were placed in a planetary mixer and were mixed until a homogeneous mixture was achieved. Then water was added to the mixture, and it again was mixed until a homogeneous paste was obtained. The paste obtained was placed in molds 2 cm thick, 28 cm high, and 18 cm wide. The plates were taken out of the molds after 24 h and left to cure at ambient temperature for more than 28 days (average temperature ) 20 °C; average relative humidity ) 45%). This paste was used to make test pieces of different shapes and sizes, which were used in the mechanical tests. A consolidation treatment was performed that has consisted of impregnating the plates and test pieces by immersion in a solution comprising ethyl esters of silicic acid [Si(OEt)4]n and oligomeric polysiloxanes, dissolved in mineral turpentine at a temperature of between 10 and 25 °C for 48 h; after that the samples were left to cure for another 28 days.10 2.3. Insulating Properties. Fire resistance is the property of a material or an assembly to withstand fire or give protection from it.11 The standard fire-resistance test described in Spanish regulation UNE-EN 1363-1,12 which is similar to other widely used international standards, was followed. The experimental setup used to study the insulating capacity of the plates, measuring the temperature in the exposed (Tin) and nonexposed (Tout) surfaces of the plates has been described elsewhere.7
Figure 1. Change in weight (M) and water content (W) of the material before and after the consolidation process.
2.4. Physical and Mechanical Properties. With the aim of characterizing the physical and mechanical properties of the product, the following tests were carried out (all the tests were conducted in duplicate, giving the average of two values): 2.4.1. Water Content. The water content (W) of the material was obtained from a mass balance between the initial water (W0) and the water loss calculated by the differences between the initial weight (M0) and the weight along the curing process (M).13 To identify the weight ratio of the free water to the other kinds of water (the water chemically bound to the gypsum and other crystallization or adsorbed water) in the test material, a thermogravimetric study (TG-SDTA Mettler Toledo 851) was carried out. Samples of 100-200 mg for the TG-SDTA measurements were taken from the surface of the test plates. A heating rate of 10 °C/min was chosen, using air as the purging gas. 2.4.2. Bending and Compressive Strength. The compressive (ASTM D-1633-84) and bending (ASTM D1635-87) strengths of the samples were also evaluated using a compressing test machine (Suzpecar, MEM-102/50 t). The compressive strength tests were performed on 40 mm high, 35 mm diameter cylinders, and bending strength tests were done on 14 cm high test probes with a 4 × 4 cm base. 2.4.3. Surface Hardness (D). The potential applications of the plates in construction materials, which might be subject to impact, caused us to analyze the surface hardness of the material according to UNE 102031:1999. The principle of the method described in that regulation is related to the resistance given by the plate to the penetration of a Shore C durometer. The test was carried out twice on 2 cm thick plates with a surface area of 28 × 18 cm. 2.4.4. Dynamic Modulus of Elasticity. The plates’ dynamic modulus of elasticity (ED) was estimated using the ultrasound technique by which the velocity of the compression wave is related to the ED according to the following expression: ED ) Kν2F, where ν is the velocity of ultrasonic pulse propagation, F is the density, and K is a constant that depends on the material’s Poisson coefficient. The tests were performed on 40 mm high, 35 mm diameter cylinders. The ED is a good index of the stiffness of the material. The greater the ED, the greater is the stiffness/rigidity of the material and, thus, the lesser is the tendency of the plate to curve when it is not subject to a burden. 2.5. Environmental Study. Given that BFA, the main component of the product, has not been highly studied, an environmental study has been carried out to characterize the
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Figure 2. TG and SDTA curves of the material.
ash more completely in order to better evaluate its possible uses. The study involved subjecting the ash to the UNE-EN-12457 test14 as well as subjecting the product to one of the most commonly used leaching tests for monolith samples in the waste management field in Europe, the NEN 7345 diffusion test.15 Metal analysis in UNE-EN and NEN leachates was carried out using atomic absorption spectrophotometry and inductively coupled plasma techniques. 3. Results and Discussion 3.1. Water Content. The water content of fireproof products is very important because the water-latent heat plays an important role in the resistance to the heat propagation, as that is what produces the formation of an evaporation plateau of about 100 °C on the side that is not exposed to the fire. This is the result of an evaporation/condensation front that runs through the material, from the exposed side at high temperatures toward the cold, unexposed area; this evaporation plateau increases the fire resistance of the product, because it produces a delay in the increase in the measured temperatures in the unexposed area. Figure 1 shows the M and W values of the test materials during the curing process, before and after the material’s consolidation stage. The weights in the figure were normalized by M0 and were calculated by adding the amounts of water present in each of the different components and the water used to make the paste. As Figure 1 shows, the weight loss undergone by the material after 28 days, before the consolidation process, was 24% and the final water content of the material may be estimated at 15% of its total initial weight. After consolidation, there was an increase in weight as a result of the absorption of the consolidant. Twenty-eight days after consolidation, the weight of the material was only 2% greater than its weight prior to treatment because nearly all of the solvent evaporated and the consolidant agent was transformed because of the formation of silica gel. The presence of free water in the pastes is a desirable characteristic since the amount of free water in the test materials plays an important role in the resistance to heat propagation, as was mentioned before. Thermogravimetric techniques were used to measure this parameter. The TG-SDTA results for the material after the consolidation stage are plotted in Figure 2. The weight loss of the product up to approximately 250 °C (7%: point A-A′) is due to the evaporation of the free and gypsum hydration water, as
Figure 3. Thermal test of the plates.
evidenced by the endothermic response of the SDTA curve from room temperature to 250 °C. The product weight loss from 250 to 650 °C (6.5%: point B-B′) is due to the evaporation of other crystalline and adsorbed water and the water decomposition of hydroxides, according to the literature related to other similar mortars.16,17 In consequence, the total water content of the material (surface samples of the test material) determined by TG from room temperature to 650 °C amounts for 13.5% and the mass fraction of the free and the gypsum water is evaluated as 52% (7/13.5) of the water total content. Thus, most of the water content of the plates tested was in the form of free water and was chemically bound to the gypsum, which yielded positive effects in the insulating properties of the plates, due to the two hydration molecules of calcium sulfate. 3.2. Insulating Capacity of the Plates. The insulating capacity of the developed product and the commercial plates was calculated by measuring the time necessary for the unexposed side to reach a temperature of 180 °C (t180) with the exposed side subjected to the standard fire-resistance temperature. Figure 3 shows the results obtained in this thermal test for 20 mm thick plates. This figure shows the insulation capacity of the plates, with t180 (vertical arrows) values of over 33 min for thicknesses of 20 mm. This result is considerably better than that for the calcium silicate-based commercial plate (COM-1) and similar to that for the gypsum and vermiculite plate (COM-2), using the same test. The insulating capacity of the product tested is better than that presented by plates of the same thickness, made
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Figure 4. Compressive strength, before and after the consolidation stage.
with a proportion similar to that studied here for coal-combustion fly ash18 (24 min). This result is probably due to the fact that the composition tested has a high capacity for water retention as a result of the high content of alkaline elements in the olive-pomace ash. Finally, it should be pointed out that no smoke was emitted from the plates at any time during the test. 3.3. Physical and Mechanical Properties. Within the optimization process for the composition of the product, we found that samples made exclusively of olive-pomace ash often showed some cracks after being submitted to the thermal test. This could be due to the great overpressures brought about in the inside of the material19 as a consequence of the pressure exerted by the water vapor that is generated during the exposure of the plate to the temperatures reached in the thermal test. Also, it has been found that a high RHA content produces plates with low water retention capacity and worsens the fire-resistance properties of the mixture. However, the presence of a certain amount of rice-husk ash in the material yielded a positive effect and eliminated cracking during the thermal test. Perhaps this is so because the greater particle size of the rice-husk ash and its long, needlelike shape result in a pore-size distribution that lessens the overpressures generated in the product.10 The small proportion of RHA in the optimized composition (1%) might make it appear to be more an additive than a basic component of the product. In fact, very similar behavior has been found in products in which RHA was replaced by other additives, such as vermiculite or perlite. However, these other additives were primary, not secondary, materials, so, although the amount used is small, it is decided to keep the RHA as part of the composition of the product to highlight the recycling of materials. Figure 4 shows the compressive strength (Rc) of test pieces at different times in their formation, before and after the consolidation process. As can be seen, the Rc values are increased in the consolidation process. In this process, the silicic acid esters probably are hydrolyzed when they are put in contact with the atmospheric moisture and are transformed into silica gel and ethyl alcohol that is evaporated during the consolidation process. According to the manufacturer, the silica gel may be responsible for the improvement in the mechanical properties of the plates thanks to the strong binding it produces with the material’s matrix. The product’s resistance to bending (RF) after consolidation was 2.2 MPa, which is a relatively high value in terms of the product application goals as a component of fire-resistant elements like fire doors and firewalls. Furthermore, the contribution of the glass fiber had a very positive effect on the resistance to bending values; it tripled the resistance with respect to the same composition without glass fiber.
Table 3. Comparison between the Mechanical Properties of the Produced Materials and Two Commercial Products
Rc (MPa) RF (MPa) ED (GPa) D (before fire test) (Shore C) D (after fire test) (Shore C) C exposed C nonexposed
COM-1
COM-2
Product
1.9 0.9 1.9 54
3.5 2.3 33
1.7 2.2 2.7 85
8 14
18 26
27 73
In Table 3, a comparison is made between the mechanical properties of the product and those shown by the two commercial products mentioned before. As you can see, the materials resistance of the produced materials is very similar to that found in commercial products. After exposing the test pieces to test temperatures for 60 min, as per the standardized thermal test, there was a significant decrease in the compressive strength (0.2 MPa, in the case of the product). The surface hardness (D) of the material before the fire test was 85 Shore C units. This value is significantly high if compared to the hardness of different commercial gypsum plates when subjected to the same tests: their values ranged from 45 to 70 Shore C units. This result shows that the material developed here can stand superficial impacts of the same order as the gypsum plates do, but without the risk of cracking. After the thermal test, there was a clear decrease in the surface hardness of the exposed side, while on the unexposed side, the decrease was less pronounced. The plate’s elasticity module (ED) values reached 2.7 GPa before the thermal test and 2.3 GPa after the test. These values were considerably less than those shown by the gypsum plates (8-9.5 GPa). 3.4 Environmental Study. In Spain, there is no national legal implemented requirement for the reuse of waste materials in these types of products. Only some regional regulations exist for some combustion residues (different types of slag) used in other kind of applications, apart from the regulations related to the use of pulverized coal fly ash in cement and concrete. Because there is not much information on the environmental impact of BFA, a leachability study was carried out using the UNE-EN-12457 leaching test to compare the leachate concentrations with the limits stated by the EU waste landfill directive (EULFD),20 in which three waste categories are defined, when waste landfill is considered: inert, nonhazardous, and hazardous, depending on the concentration values of different parameters in an aqueous waste leachate. The leaching limit values shown in Table 4 apply for granular (not monolithic) waste acceptable at landfills for inert, nonhazardous, and hazardous waste, calculated at a liquid-to-solid ratio of 10 L/kg for total release.
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Table 4. UNE-EN12457 Leachability of BFA; Criteria for Waste Acceptable at Landfills for Different Wastes According to the EULFD EULFD (mg/kg, dm) element
inert waste
nonhazardous waste
hazardous waste
As Ba Cd Cr (total) Cu Hg Mo Ni Pb Sb Se Zn
0.5 20 0.04 0.5 2 0.01 0.5 0.4 0.5 0.06 0.1 4
2 100 1 10 50 0.2 10 10 10 0.7 0.5 50
25 300 5 70 100 2 30 40 50 5 7 200
BFA 5.7 0.3
