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

Abstract

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 Office's 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 less silicon. The total amount of potassium in wood is much lower than in straws, although this is not necessarily the case when expressed as a fraction of total ash. Calcium reacts with sulfur to form sulfates in ways analogous to potassium, but the lower mobility and vapor pressure of calcium and the structural properties of the deposits it forms are both more favorable to sustained furnace operation than the ashes formed from straws and grasses. Chlorine is shown to be a major factor in deposit formation. Chlorine facilitates the mobility of many inorganic compounds, in particular potassium. Potassium chloride is among the most stable high-temperature, gas-phase, alkali-containing species. Chlorine concentration often dictates the amount of alkali vaporized during combustion more strongly than the alkali concentration in the fuel. In most cases, the chlorine appears to play a shuttle role, facilitating the transport of alkali from the fuel to surfaces, where the alkali often forms sulfates. In the absence of sulfur, chlorides often reside on the surface. In the absence of chlorine, alkali hydroxides are the major stable gas-phase species in moist, oxidizing environments (combustion gases). Bench-scale combustion tests coupled with mass spectrometry techniques reveal the major species evolving from biomass samples during combustion. The fuels subjected to these combustion tests were also examined by chemical analyses. The tests indicate that the major release of alkali material occurs during the char combustion phase and that the primary form of stable alkali-bearing off-gases corresponds with thermodynamic estimates of product stability and vapor pressure. Investigations were performed varying temperature, oxygen concentration, and moisture levels and revealed thermodynamically consistent results; the amount of alkali vaporized increases with temperature and the amount of hydroxide formed increases with increasing moisture content. Details of other species released are also presented. Thermogravimetric and differential thermogravimetric analyses of the samples were also performed, the results of which prove to be marginally useful to predicting ash deposition. A conceptual framework expresses ash deposition as a combination of four mechanisms: inertial impaction, thermophoretic deposition, condensation, and chemical reaction is presented. This conceptual framework serves to organize the remaining discussion and observations of ash deposition at both pilot and commercial scale. The influences of boiler design, boiler operating conditions, and fuel properties on ash deposit behavior reveal themselves by emphasizing or reducing the role of one or more of these mechanisms and thereby changing deposit composition, phase, and properties. Pilot-scale investigations on the standard suite of fuels were carried out in an entrained-flow furnace. Isokinetically sampled fly ash samples and deposits collected on instrumented, temperature-regulated probes simulating both water-walls and convection pass provide the basis for in situ and subsequent ex situ examination of deposits. Deposit properties reveal spatial dependencies on probe surfaces that are consistent with both the commercial-scale tests and the conceptual framework for deposit growth and property development. In addition, changes in deposit composition with time, temperature, and other operation-relevant variables exhibit the same consistencies with commercial operation and the conceptual framework. Fuels containing high alkali and silica concentrations form alkali silicates that melt or sinter at low temperatures. The rates of deposit growth and sintering/melting increase with increasing temperature and chlorine concentration but are high at all boiler-relevant temperatures and chlorine concentrations for the straws and grasses. Surfaces exposed to impacting particles can accumulate silica and alkali silicates at very rapid rates. Surfaces exposed to combustion gases but less exposed to particle impaction show evidence of thermophoretic accumulation of deposits, vapor condensation, and sulfation of condensed alkali-laden vapors. These processes lead to deposits with markedly different properties (high reflectivity, modest thickness and growth rate) compared to the impacted regions. Fuels containing little alkali or silica indicate far less deposit growth and development of more manageable deposits, by which we mean soot blowers and boiler maintenance techniques are able to sustain operation of a facility for periods of many months without unscheduled shutdowns. Commercial-scale investigations using nearly every type of commercially significant biomass boiler design provide full scale data for analysis and comparison with pilot-scale results. Bubbling fluid beds, circulating fluid beds, and various grate-based combustors fed by stokers, augers, and a cigar burner provided the data for comparison. Fuel types ranged from wood-derived material with blended agricultural by-products to straws, sometimes blended with urban wood fuel. Many of the commercial-scale experiments were conducted in the context of commercial operation and employed varying compositions in the fuel. In all cases, comparisons of deposit composition with position in the boiler, type of deposit surface, position on the surface, and fuel properties reveal complete consistency with the fuel analyses, bench-scale combustion results, pilot-scale results, and the conceptual framework. Fuels containing high alkali and silica fractions exhibited the same rapid accumulation of ash deposits with the same sintered/molten character as was observed in the pilot-scale tests. Advanced mineralogical examinations of selected deposits indicate chemical compositions consistent with the conceptual framework and are presented as appendix material. The deposit properties are consistent with the conceptual framework for their formation and the observed bench-scale combustion results. Fuels with less alkali, chlorine, and silica, with less total ash-forming material, and with higher calcium contents exhibit more manageable ash deposits. Wood and non-recyclable paper generated the most manageable deposits. This report provides highlights of a more detailed analysis of fuel property, operating condition, and boiler design issues that dictate as deposit formation and property development [1, 2]. The span of investigations from bench-top experiments to commercial operation and observations, including both practical illustrations and theoretical background, provides a self-consistent and reasonably robust basis to understand the qualitative nature of ash deposit formation in biomass boilers. While there remain many quantitative details to be pursued to complete our understanding, this project encapsulates essentially all of the conceptual aspects of the issue. It provides a basis for understanding and potentially resolving the technical and environmental issues associated with ash deposition during biomass combustion.