Use of Biomass Gasification Fly Ash in Lightweight Plasterboard

Energy & Fuels 2007, 21, 361-367

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Use of Biomass Gasification Fly Ash in Lightweight Plasterboard C. Leiva, A. Go´mez-Barea, L. F. Vilches, P. Ollero, J. Vale, and C. Ferna´ndez-Pereira* Chemical and EnVironmental Engineering Department, Escuela Superior de Ingenieros, UniVersity of SeVille, Camino de los Descubrimientos s/n E-41092, SeVille, Spain ReceiVed June 7, 2006

Fly ash from biomass gasification differs considerably from conventional combustion ashes. Gasification ash might contain high concentrations of unburned carbon and harmful, soluble compounds. This restricts direct ash utilization and makes some pretreatment necessary, representing a significant share of the overall operating cost of gasification-based systems for energy production. Therefore, economic methods for the management of this type of ash without any pretreatment are attractive. In this paper, we present an initial study on new lightweight plasterboard made of gasification ash. Our goal was to come up with a product that satisfies two basic requirements: (1) the product is mostly made of fly ash, and (2) the product enables the utilization of ash without any pretreatment. We manufactured plates by means of low-cost moulding and curing methods, using ash percentages of up to 60% (w/w). Gypsum and additives (vermiculite and fiber) were added to provide the plates with acceptable mechanical and thermal properties. The fly ash used was generated in a fluidized-bed pilot plant for processing orujillo, a byproduct of the olive oil industry. Mechanical, thermal, and environmental properties of ash gasification plates were studied and compared with commercial plates. The results lead us to conclude that the plates could be used commercially as fire-resistant construction boards, and therefore, the potential for commercial development is promising. In addition, the overall environmental benefit of waste gasification plus ash utilization of a difficult fly ash makes the overall process attractive.

1. Introduction Because of the growing concern for future energy supplies and for limiting CO2 emissions, the use of a renewable energy source, such as a residual biomass for heat and power production, has generated interest. A thermochemical process for the treatment of biomass and waste is one of the alternatives currently being developed. Gasification, in particular, offers a combination of flexibility, efficiency, and environmental acceptability that is essential for meeting future energy requirements.1 Gasification has considerable benefits over direct combustion because the feedstock is converted to a gaseous fuel, which significantly increases the opportunities for using biomass as an energy source. This process leads to a fuel gas suitable for cofiring in existing boilers and also, when sufficiently cleaned, for feeding efficient gas engines and gas turbines that generate electricity and as a raw gas for the synthesis of liquid fuels or chemicals. Gasification also offers potential environmental advantages because the fuel gas produced by gasifiers is lower in both volume and temperature than the fully combusted product from a combustor. These characteristics provide an opportunity to clean and condition the fuel gas prior to use. Combustion of the resulting gaseous fuel can be more accurately controlled than combustion of solid biomass. As a result, the overall emissions from gasification-based power systems, particularly those of NOx, can be reduced.2 Basic gasification technology exists today, but the technical and financial competitiveness of large-scale gasification has to * To whom correspondence should be addressed. Telephone: +34954487271. Fax: +34-9554461775. E-mail: [email protected]. (1) Bridgwater, A. V. The Technical and Economic-Feasibility of Biomass Gasification for Power-Generation. Fuel 1995, 74, 631-653. (2) Stevens, D. J. Hot Gas Conditioning: Recent Progress with LargerScale Biomass Gasification Systems (NREL/SR-510-29952), 2001.

be improved to be comparable to the latest generation of fossilfuel-fired power plants. Fluidized-bed gasification of biomass and waste fuels has been successfully demonstrated for a wide variety of feedstocks. Costs related to fly ash disposal represent a significant share of the overall operation costs for gasificationbased energy production. Gasification fly ashes differ considerably from conventional combustion ashes. Unburned carbon is generally abundantly present, typically comprising 10-60% of the ash mass. Ashes also contain polycyclic aromatic hydrocarbon (PAH) compounds and, in the case of waste gasification, chlorine and heavy metals as well. This makes the management of gasification fly ash a challenging task, especially for contaminated fuels. In practice, the utilization methods developed for ashes from coal combustion processes cannot be directly transferred to gasification ashes. For instance, the utilization in the cement and concrete industry without pretreatment is not possible for most gasification ashes. The need for pretreatment has a direct impact on the final cost of the overall process. Consequently, lessening the cost of the biomass and waste gasification by developing sustainable and economical methods for ash utilization and management is an urgent need. In many developing countries, because of the increasing cost of raw materials and the constant reduction of natural resources, the use of waste materials is a potential alternative in the construction industry. Waste materials, when properly processed, have shown themselves to be effective as construction materials and readily meet design specifications. The development of new composites made of waste combined with binders offers economic and environmental advantages. However, the amount of ashes added to blends is typically under 50% by weight.3-6 Most of the coal combustion ashes are utilized in building (3) Vilches, L. F.; Leiva, C.; Vale, J.; Ferna´ndez-Pereira, C. Insulating Capacity of Fly Ash Pastes Used for Passive Protection against Fire. Cem. Concr. Compos. 2005, 27, 776-781.

10.1021/ef060260n CCC: $37.00 © 2007 American Chemical Society Published on Web 11/24/2006

362 Energy & Fuels, Vol. 21, No. 1, 2007

industry applications, including the use as an addition to concrete and as a constituent and raw material for cement. In addition, coal combustion ashes are used as an aggregate or binder in road construction, as mineral fillers, and as fertilizers. Studies on utilization routes for fly ash from the biomass gasification process are, however, very limited.7,8 Three main utilization categories can be identified for the gasification ashes derived from biomass and waste.9 (1) Use as a fuel if the carbon content is high enough: cofiring in coal/ biomass-fired power plants, firing in a dedicated boiler, replacement fuel in smelters/incinerators, and firing in cement kilns; (2) use in construction: filler in asphalt or asphalt-like products, an additive in concrete manufacturing, bulk construction, and raw building material, lightweight bricks, fire-resistant material, and a stabilization/solidification agent; and (3) use in agriculture: directly as a fertilizer or as a soil improver. A landfill may also be considered a form of utilization. However, landfill costs are expected to increase in the near future. This paper aims to increase knowledge about the direct utilization of fly ash from biomass/waste gasification in fluidized beds. In particular, it deals with a preliminary study on the use of biomass ash as the main component of lightweight construction material with fire-resistant properties. The ashes were generated in a 150 kWth bubbling fluidized-bed pilot plant that processed an agroindustrial waste called “orujillo”.10 Orujillo is a residue of the olive oil extraction industry. Approximately 2 megatons/year of this biowaste is generated in Spain. The generation of this biomass residue is concentrated in Andalusia, a region in southern Spain with a production of 1.2 megatons/ year. This fuel has a high heating value but contains a high amount of alkali metals (primarily potassium), which can cause serious deposit formation and agglomeration in high-temperature processes, such as fluidized-bed gasification and combustion. Fly ashes without any previous treatment were used in different proportions for the preparation of plasterboards. One of the aims of this study was to use high percentages of ash in blends. Boards with proportions of up to 80% were tested. 2. Experimental Section 2.1. Materials. Ash. Biomass gasification ashes generated at the DIQA pilot plant (University of Seville, Spain) were used for the present study. Pilot plant gasification tests were carried out at atmospheric pressure and temperatures of 700-820 °C to assess the technical viability of gasifying solid olive oil residues (olive oil pomace), also called “orujillo”. Different air flow rates and air/ biomass ratios were used, giving an air factor (the actual incoming air over the stoichiometric air) within the range of 0.17-0.31. A series of parameters, such as gasification efficiency, thermal efficiency, gas yield, overall carbon conversion, gas quality, and (4) Vilches, L. F.; Ferna´ndez-Pereira, C.; Olivares del Valle, J.; Rodriguez Pin˜ero, M. A.; Vale, J. Development of New Fire-Proof Products Made from Coal Fly Ash: The CEFYR Project. J. Chem. Technol. Biotechnol. 2002, 77, 361-366. (5) Vilches, L. F.; Ferna´ndez-Pereira, C.; Olivares del Valle, J.; Vale, J. Recycling Potential of Coal Fly Ash and Titanium Waste as New FireProof Products. Chem. Eng. J. 2003, 95, 155-161. (6) Leiva, C.; Vilches, L. F.; Ferna´ndez-Pereira, C.; Vale, J. Influence of the Type of Ash on the Fire Resistance Characteristics of Ash-Enriched Mortars. Fuel 2005, 84, 1433-1439. (7) Holt, E.; Raivio, P. Use of Gasification Residues in Aerated Autoclaved Concrete. Cem. Concr. Res. 2005, 35, 796-802. (8) Holt, E.; Raivio, P. Use of Gasification Residues in Compacted Concrete Pasing Blocks. Cem. Concr. Res. 2006, 36, 441-448. (9) Gasash Project. Improvement of the Economics of Biomass/Waste Gasification by Higher Carbon Conversion and Advanced Ash Management. (ENK5-2001-00635), 2005 (10) Go´mez-Barea, A.; Arjona, R.; Ollero, P. Pilot Plant Gasification of Olive Stone: A Technical Assessment. Energy Fuels 2005, 19, 598-605.

LeiVa et al. Table 1. Chemical Compositions of Fuel, Bed Material, and Ash parameter

fuel (orujillo) (wt %)

LHV (MJ/kg) HHV (MJ/kg) C H N O moisture ash (fuel) volatile matter fixed carbon

14.09 15.02 36.57 3.71 1.00 27.32 10.82 20.41 53.34 15.44

parameter

ofite (wt %)

fly ash (wt %)

moisture loss on ignition CaO MgO Fe2O3 Al2O3 SiO2 K2O Na2O

0.47 0.64 11.15 7.90 9.15 13.61 53.93 0.48 3.49

1.31 18.24 23.23 8.10 5.53 8.53 43.48 6.83 1.51

Table 2. Fly Ash Size Distribution particle-size distribution (µm)

ash (wt %)

>300 300-150 150-90 90-75 75-45 50 wt %); (2) satisfactory mechanical properties (specified by the requirements demanded for the final product); and (3) acceptable fire-resistant behavior. The composition selected is shown in Table 3. The solid components shown in Table 3 were placed in a planetary mixer. The blend was mixed until a homogeneous mixture was achieved. Afterward, water was gradually added and mechanically blended until a homogeneous paste was obtained. The paste was placed in 2 cm thick, 28 cm high, and 18 cm wide moulds. After 24 h, the paste was removed from the moulds 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 employed in the mechanical tests. In comparison to other residues of similar characteristics, the water/solid ratio used in the blend was high.4,5,6 This enabled us to obtain a relatively easy to mould product. 2.3. Physical and Chemical Properties. To characterize the physical and mechanical properties of the product under study, the following tests were carried out. CompressiVe Strength. The compressive strength12 of samples was evaluated using a compressing test machine (Suzpecar, MEM(12) ASTM E 761-86. Compressive Strength of the Fire-Resistive Material Applied to Structural Member.

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LeiVa et al.

Figure 3. Change in weight (M/M0) and water content (Wc) during the curing process. Table 4. Physical and Chemical Properties of the Plates F (kg/m3)

pH

A (%)

Rc (MPa)

SH (shore C)

652

9.95

67.4

0.43

28

102/50 t). Compressive strength tests were carried out on 40 mm high, 35 mm diameter cylinders; for bending strength assessment, 14 cm high test probes with a 4 × 4 cm base were used. The compressive strength was measured before and after the fire test. Surface Hardness. Potential applications of plates in construction materials require resistance to impact. Thus, we analyzed the surface hardness of the material. The method described by Spanish standard UNE-EN 10203113 allows for the quantification of the hardness based on the resistance 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. The surface hardness was also measured in samples before and after the fire test. pH. The pH was measured according to European standards.14 A sample of 2 g was taken from the plate and was dissolved into 20 g of water. After 5 min, the pH of the solution was measured. Water Content. Water content is important in fire-resistant materials, because water-latent heat plays an important role in resistance to heat propagation. The resistance is mainly caused by the formation of an evaporation plateau on the side not exposed to fire, at around 100 °C (at atmospheric pressure). This resistance is the result of an evaporation-condensation front propagated through the material, which runs from the exposed side (at high temperature) toward the unexposed area (a low temperature). This evaporation plateau produces a delay in the rise in temperature on the unexposed area. Consequently, it increases the overall fire resistance of the product. The water content of the material was obtained by measuring the mass of the sample during the curing process.15 Water Absorption Capacity (A). This parameter was measured according to European standards.14 The mass of the plate test piece was measured before its immersion in water (M1), whose temperature was between 21 and 25 °C. After 120 min, the plate was taken out of the water and drained for 5 min and its mass was measured (M2). The value of water absortion (A) is calculated by A)

M 2 - M1 × 100 M1

Mass Loss. The change of mass with temperature was determined using a Mettler-Toledo TG 851 thermogravimetric apparatus under a flowing nitrogen atmosphere. The temperature reached 800 °C (13) UNE-EN 102031:1999. Gypsum Building Plaster. Physical and Mechanical Test Methods. (14) UNE-EN 12859. Gypsum Blocks. Definitions, Requirements, and Test Methods. (15) Asako, Y.; Otaka, T.; Yamaguchi, Y. Fire Resistance Characteristics of Materials with Polymer Gels Which Absorb Aqueous Solution of Calcium Chloride. Numer. Heat Transfer, Part A 2004, 45, 49-66.

at a heating rate of 30 °C/min.6 The mass of specimens ranged from 80 to 110 mg. 2.4. Fire-Resistant Properties. The standard fire-resistant test is described in European regulations.16 The standard used is similar to other widely used international standards and is the result of observations and analysis of real fires. To simulate conditions of fire exposition, the regulation requires one of the sides of the protective material to be exposed to heat according to a standard temperature curve defined by the following equation: T ) 20 + 345 log10(1 + 8t) where T is the oven temperature for the tests (in °C) and t is the time (in minutes) from the beginning of the test. To study the fire resistance of the plates, several special modifications were made to an existing oven.3,4 This setup enabled us to measure and register the temperature of the exposed surface of the plate (hot surface, Tin). To measure the temperature inside the oven, an S-type thermocouple was placed there. This thermocouple was used to regulate the temperature of the oven by means of a proportional controller. On the unexposed face (cold surface, Tout), the temperature was registered by means of a Pt-100 probe with a stainless-steel contact surface (see Figure 2). To analyze the fire resistance of the plates in a similar way to that in the European standards, the time necessary for Tout to reach 180 °C (t180) was taken as a reference. 2.5. Environmental Study. Potential uses for the plates developed include construction materials such as fire doors and wallboards. In these types of commodities, the fire-resistant product is usually contained within metallic sheets or protected by a special type of paint. Direct contact with people or other materials are not expected. However, there are few environmental studies on biomass gasification ash in the literature, and consequently, we decided to further characterize this construction material. The environmental assessment enables us to better evaluate further potential uses. The study involved subjecting the ash to two well-known leaching tests, the DIN 38414-S417 and the CEN/TS 14405 column test,18 as well as subjecting the product to one of the most commonly used leaching tests for monolith samples in the waste management field, the NEN 7345 diffusion test.19 Atomic absorption spectrophotom(16) UNE-EN 1363-1:2000. Fire Resistance Test. Part 1: General Requirements. (17) DIN 38414 S4. German Standard Procedure for Water, Wastewater, and Sediment Testing. Group S (Sludge and Sediment). Determination of Leachability (S4); Institu¨t fu¨r Normung, Berlin, Germany, 1984. (18) CEN/TS 14405. Characterization of Waste-Leaching Behaviour Tests-Up-Flow Percolation Test (under Specified Conditions); European Committee for Standardization CEN, 2004. (19) NEN 7345. Leaching Characteristics of Soil an Stony Building and Waste MaterialssLeaching TestsDetermination of the Leaching of Inorganic Components from Building and Monolithic Waste Materials with the Diffusion Test.

Biomass Gasification Fly Ash in Lightweight Plasterboard

Energy & Fuels, Vol. 21, No. 1, 2007 365 Table 5. Mechanical Properties of the Plate after the Fire Test F (kg/m3)

Rc (MPa)

SH (shore C)

524

0.02

unexposed side 9 exposed side 4

Table 6. Metals Limits for DIN Leachates According to the Order on Slag Valorization (µg/L)

limit fly ash

Cd

CrVI

Crtotal

Ni

Cu

Zn

As

Hg

Pb

100