Larry L. Baxter

The behavior of inorganic material in biomass-fired power boilers : field and laboratory experiences

This paper highlights some of the major findings of the Alkali Deposits Investigation, a collaborative effort to understand the causes of unmanageable ash deposits in biomass-fired electric power boilers. A group of interested industrial institutions and the US DOE Energy Efficiency and Renewable Energy Offices Biomass Power Program through the National Renewable Energy Laboratory jointly sponsored the project. The industries contributed both funding and, in most cases, use of facilities to the project and included Mendota Biomass Power and Woodland Biomass Power (both associated with Thermo Electron Energy Systems), CMS Generation Operating (formerly Hydra-Co Operations), Wheelabrator, Shasta and Hudson Energy, Sithe Energy, Delano Energy, the Electric Power Research Institute, Foster Wheeler Development, and Elkraft Power of Denmark. Research contracts with Thomas R. Miles Consulting Design Engineers, Sandia National Laboratories, and The National Renewable Energy Laboratories provided the government portion of the funding. In addition, the University of California at Davis and the Bureau of Mines performed significant work in close collaboration with the other researchers. This summary highlights the major findings of the project more thoroughly discussed in a recent report [2]. We highlight fuel properties, bench-scale combustion tests, a framework for considering ash deposition processes, pilot-scale tests of biomass fuels, and field tests in commercially operating biomass power generation stations. Detailed chemical analyses of 11 biomass fuels representing a broad cross-section of commercially available fuels reveal their properties that relate to ash deposition tendencies. The fuels fall into three broad categories: (1) straws and grasses (herbaceous materials); (2) pits, shells, hulls and other agricultural by-products of a generally ligneous nature; and (3) woods and recycle fuels of commercial interest. Woods and wood-derived products represent the most commonly used biomass fuels. Herbaceous fuels contain silicon and potassium as their principal ash-forming constituents. They are also commonly high in chlorine relative to other biomass fuels. These properties portend potentially severe ash deposition problems at high or moderate combustion temperatures. The primary sources of these problems are shown to be: (1) the reaction of alkali with silica to form alkali silicates that melt or soften at low temperatures (can be lower than 700°C, depending on composition), and (2) the reaction of alkali with sulfur to form alkali sulfates on combustor heat transfer surfaces. Alkali material plays a central role in both processes. The mobility of alkali material, defined as its ability to come in physical contact with other materials, is measured using chemical extractive techniques. Potassium is the dominant source of alkali in most biomass fuels. The analyses below indicate that essentially, all of the biologically occurring alkali, in particular potassium, has high mobility. The non-biologically occurring alkali is present as soil contaminants and additives to the fuels, such as clay fillers used in paper production. This non-biologically occurring alkali exhibits far lower mobility than the biological fraction. The relative amounts of biologically vs. non-biologically occurring material depend on fuel type and fuel handling. In the fuels investigated here, the dominant form of alkali was biologically occurring potassium. Some traditional indicators of deposit behavior, most notably ash fusion temperatures, poorly predict ash behavior compared with a more mechanistic interpretation of the data. Many of the agricultural by-products also contain high potassium concentrations with equally high potassium mobility. Some woods, on the other hand, contain far less ash overall, differing by as much as a factor of 40 from high-ash straws, for example. In addition, the ash-forming constituents contain greater amounts of calcium with

The implications of chlorine-associated corrosion on the operation of biomass-fired boilers

Abstract The design of new biomass-fired power plants with increased steam temperature raises concerns of high-temperature corrosion. The high potassium and chlorine contents in many biomasses are potentially harmful elements with regard to corrosion. This paper condenses the current knowledge of chlorine-induced, high-temperature corrosion and describes the potential corrosion problems associated with burning biomass fuels either alone or in blends with coal, for electricity production. Chlorine may cause accelerated corrosion resulting in increased oxidation, metal wastage, internal attack, void formations, and loose non-adherent scales. The partial pressure of HCl in a biomass-derived flue gas, is not high enough to cause severe gas-phase corrosion attacks, but may provide scale failure and increased sulfidation of water walls in areas where locally reducing conditions occur due to poor combustion and flame impingement. The most severe corrosion problems in biomass-fired systems are expected to occur due to Cl-rich deposits formed on superheater tubes. The presence of alkali chloride salts in deposits may cause accelerated corrosion well below the melting point of the salt. The corrosion can be severe in air but may be further enhanced by SO 2 which may cause intra-deposit sulfation of the alkali chlorides liberating HCl or Cl 2 gas close to the metal surface. In case the metal surface temperature becomes high enough for molten phases to form in the deposit, the corrosion may be even further enhanced.

Ash deposition during biomass and coal combustion: A mechanistic approach

Abstract The variability in both inorganic and organic properties of biomass fuels is large. This paper discusses combustion-driven transformations and deposition of inorganic material found in solid fuels, with a focus on the formation of deposits and their properties. A small number of mechanisms is used to describe both the transformations and deposition. The discussion below outlines this mechanistic approach to describing the fate of inorganic material in solid fuels with a particular focus on the mechanisms of ash deposition. This mechanistic approach has the potential of embracing a large range of fuel variations, combustor types, and operating conditions without the need of developing extensive databases or testing procedures for each new situation. The approach has been successfully demonstrated for coal combustion, and examples from coal experiments will be used as illustrations. The same methodology and logic can be applied to biomass combustion. A comparison of coal and biomass is briefly presented, including the chemical structures and the modes of occurrence of inorganic material in the fuels. The major mechanisms of ash deposition during combustion of coal and biomass are related to the types of inorganic material in the fuel and the combustion conditions. The effects of fuel (biomass or coal) characteristics and combustor operating conditions on ash deposit properties such as tenacity, emissivity, thermal conductivity, morphology, strength, chemical composition, viscosity, and rate of growth are discussed. A mechanistic model describing ash deposition in solid-fuel combustors is presented and used to postulate characteristics of ash deposits formed in biomass combustors.

Boiler deposits from firing biomass fuels

Abstract Alkali in the ash of annual crop biomass fuels creates serious fouling and slagging in conventional boilers. Even with the use of sorbents and other additives, power plants can fire only limited amounts of these fuels in combination with wood. The National Renewable Energy Laboratory (NREL). U.S. Department of Energy (DOE), and the biomass power industry conducted eight full-scale firing tests and several laboratory experiments to study the nature and occurrence of deposits. The goal was to increase the quantities of these biofuels which can be used. This paper describes the results of the laboratory and power plant tests which included: tracking and analyzing fuels and deposits by various methods; recording operating conditions; and extensive laboratory testing. These analyses have advanced the understanding of the role of minerals in the combustion of biomass, and their occurrence in biofuels. Deposits occur as a result of the boiler design, fuel properties and boiler operation. The limited furnace volume and high flue gas exit temperatures of most biomass boilers promote slag or deposits from biofuels which contain significant amounts of alkali, sulfur or chlorine and silica. All annual growth, whether from urban tree trimmings, annual crops and residues or energy crops contains sufficient volatile alkali, 0.34 kg GJ − (0.8 lb MMBtu −1 ) or more, to melt in combustion or vaporize and condense on boiler tubes and refractory. Special boiler designs are required for annual crops, including grasses and straws. Addition of magnesium oxide and other additives may be necessary to inhibit alkali volatilization while burning these biofuels.

Char fragmentation and fly ash formation during pulverized-coal combustion

Abstract Experimental measurements of char and fly ash size distributions are reported in the size range from approximately 0.5 to 100 μm for three coals, ranging in rank from high-volatile bituminous coal to lignite. These measurements are coupled with a theoretical model of fly ash formation to determine the extent of char fragmentation as a function of initial char particle size. These data reveal several mechanistic aspects of fragmentation. The extent of fragmentation is strongly dependent or size and coal rank. Bituminous coals may form over 100 fragments per char particle at large initial char particle sizes (above 80 μm) and less than 10 at small initial char particle sizes (less than 20 μm). Lignites fragment less extensively, with the number of fragments per original char particle being less than 5 at all particle sizes. These results partially resolve some apparent discrepancies in published studies of fragmentation.

Impact of Mineral Impurities in Solid Fuel Combustion

Keynote Papers. Section 1: Mineral Matter, Ash and Slag Characterisation. Section 2: The Use of Low-Rank and Low-Grade Coals and Cofiring. Section 3: Case Studies in Conventional and Advanced Plant. Section 4: Studies at Rig Scale (Including Corrosion). Section 5: Developments in Advanced Coal Technologies. Section 6: Modelling.

Applications of advanced technology to ash-related problems in boilers

Ash Formation, Deposition, Corrosion and Erosion in Conventional Boilers (S. Benson). Research Needs of the Power Industry (R. DeSollar). Evolutionary Changes in Furnace Combustion Conditions Which Affect Ash Deposition in Modern Boilers (D. Fitzgerald). The UK Collaborative Research Program on Slagging Pulverized Coalfired Boilers: Summary of Findings (W. Gibb). Ash Deposit Properties and Radiative Transfer in Coal Fired Plant: Current Understanding and New Developments (T. Wall). Rates and Mechanisms of Strength Development in Lowtemperature Ash Deposits (J.P. Hurley). Deposit Formation During the CoCombustion of Coal-Biomass Blends (K. Hein). Quantification of Deposit Formation Rates as Functions of Operating Conditions and Fraction of Biomass Fuel Used in a Converted PC Boiler (100 MW) (A. Nordin). In situ Measurements of Boiler Ash Deposit Emissivity and Temperature in a Pilotscale Combustion Facility (D. Shaw). Erosion Oxidation of Carbon Steel and Deposition of Particles on a Tube During Combustion of Coalbased Fuels in an Industrial Boiler (J. Xie). Laboratory Measurements of Alkali Metal Containing Vapors Released During Biomass Combustion (D. Dayton). An Improved Ash Fusion Test (C. Coin). 31 additional articles. Index.

A mechanistic description of ash deposition during pulverized coal combustion : predictions compared with observations

Abstract A mechanistic model of ash deposition is based on the transformations of mineral species in coal during transport of particles through an arbitrary combustion environment. Quantitative predictions include the elemental composition of boiler ash deposits as a function of location, operating conditions and coal type. Qualitative predictions relating to practical aspects of boiler operation are also included. Model predictions are compared with experimental results at pilot and utility scales. A three-week test burn of a Wyoming coal in a power plant boiler designed for midwestern and eastern coal is described. Data reported include deposit accumulation rate, strength, morphology, removability, emissivity and elemental composition. Similar data are also reported for the Sandia multifuel combustor, a pilot-scale facility. Deposits from the Wyoming coal accumulated at about the same rate as those from the fuel used previously. The deposits from the Wyoming coal were granular and friable. They were easily removed from boiler heat transfer surfaces by normal soot blowing practices. They were light-coloured and highly reflective. All these qualitative trends are consistent with model predictions. The measured elemental composition of the ash deposits from the Wyoming coal is within ~ 5% (absolute) of the predicted composition.

Influence of ash deposit chemistry and structure on physical and transport properties

Boiler ash deposits generated during combustion of coal, biomass, black liquor, and energetic materials affect both the net plant efficiency and operating strategy of essentially all boilers. Such deposits decrease convective and radiative heat exchange with boiler heat transfer surfaces. In many cases, even a small amount of ash on a surface decreases local heat transfer rates by factors of three or more. Apart from their impact on heat transfer, ash deposits in boilers represent potential operational problems and boiler maintenance issues, including plugging, tube wastage (erosion and corrosion), and structural damage. This report relates the chemistry and microstructural properties of ash deposits to their physical and transport properties. Deposit emissivity, thermal conductivity, tenacity, and strength relate quantitatively to deposit microstructure and chemistry. This paper presents data and algorithms illustrating the accuracy and limitations of such relationships.

IGNITION BEHAVIOR OF LIVE CALIFORNIA CHAPARRAL LEAVES

Current forest fire models are largely empirical correlations based on data from beds of dead vegetation. Improvement in model capabilities is sought by developing models of the combustion of live fuels. A facility was developed to determine the combustion behavior of small samples of live fuels, consisting of a flat-flame burner on a moveable platform. Qualitative and quantitative combustion data are presented for representative samples of California chaparral: manzanita (Arctostaphylos parryana); oak (Quercus berberidifolia); ceanothus (Ceanothus crassifolius), and chamise (Adenostoma fasciculatum). Times to ignition were significantly influenced by shape effects, whereas ignition temperature was more dependent on chemical composition.