TASK 3.4--IMPACTS OF COFIRING BIOMASS WITH FOSSIL FUELS

Zygarlicke, Christopher J. ; McCollor, Donald P. ; Eylands, Kurt E. ; Hetland, Melanie D. ; Musich, Mark A. ; Crocker, Charlene R. ; Dahl, Jonas ; Laducer, Stacie

University of North Dakota

关键词: Biomass;    Hydroxides;    Subbituminous Coal;    01 Coal, Lignite, And Peat;    Bituminous Coal;   

DOI  :  10.2172/828064
RP-ID  :  NONE
RP-ID  :  FC26-98FT40320
RP-ID  :  828064

美国|英语

来源: UNT Digital Library

【 摘 要 】

With a major worldwide effort now ongoing to reduce greenhouse gas emissions, cofiring of renewable biomass fuels at conventional coal-fired utilities is seen as one of the lower-cost options to achieve such reductions. The Energy & Environmental Research Center has undertaken a fundamental study to address the viability of cofiring biomass with coal in a pulverized coal (pc)-fired boiler for power production. Wheat straw, alfalfa stems, and hybrid poplar were selected as candidate biomass materials for blending at a 20 wt% level with an Illinois bituminous coal and an Absaloka subbituminous coal. The biomass materials were found to be easily processed by shredding and pulverizing to a size suitable for cofiring with pc in a bench-scale downfired furnace. A literature investigation was undertaken on mineral uptake and storage by plants considered for biomass cofiring in order to understand the modes of occurrence of inorganic elements in plant matter. Sixteen essential elements, C, H, O, N, P, K, Ca, Mg, S, Zn, Cu, Fe, Mn, B, Mo, and Cl, are found throughout plants. The predominant inorganic elements are K and Ca, which are essential to the function of all plant cells and will, therefore, be evenly distributed throughout the nonreproductive, aerial portions of herbaceous biomass. Some inorganic constituents, e.g., N, P, Ca, and Cl, are organically associated and incorporated into the structure of the plant. Cell vacuoles are the repository for excess ions in the plant. Minerals deposited in these ubiquitous organelles are expected to be most easily leached from dry material. Other elements may not have specific functions within the plant, but are nevertheless absorbed and fill a need, such as silica. Other elements, such as Na, are nonessential, but are deposited throughout the plant. Their concentration will depend entirely on extrinsic factors regulating their availability in the soil solution, i.e., moisture and soil content. Similarly, Cl content is determined less by the needs of the plant than by the availability in the soil solution; in addition to occurring naturally, Cl is present in excess as the anion complement in K fertilizer applications. An analysis was performed on existing data for switchgrass samples from ten different farms in the south-central portion of Iowa, with the goal of determining correlations between switchgrass elemental composition and geographical and seasonal changes so as to identify factors that influence the elemental composition of biomass. The most important factors in determining levels of various chemical compounds were found to be seasonal and geographical differences related to soil conditions. Combustion testing was performed to obtain deposits typical of boiler fouling and slagging conditions as well as fly ash. Analysis methods using computer-controlled scanning electron microscopy and chemical fractionation were applied to determine the composition and association of inorganic materials in the biomass samples. Modified sample preparation techniques and mineral quantification procedures using cluster analysis were developed to characterize the inorganic material in these samples. Each of the biomass types exhibited different inorganic associations in the fuel as well as in the deposits and fly ash. Morphological analyses of the wheat straw show elongated 10-30-{micro}m amorphous silica particles or phytoliths in the wheat straw structure. Alkali such as potassium, calcium, and sodium is organically bound and dispersed in the organic structure of the biomass materials. Combustion test results showed that the blends fed quite evenly, with good burnout. Significant slag deposit formation was observed for the 100% wheat straw, compared to bituminous and subbituminous coals burned under similar conditions. Although growing rapidly, the fouling deposits of the biomass and coal-biomass blends were significantly weaker than those of the coals. Fouling was only slightly worse for the 100% wheat straw fuel compared to the coals. The wheat straw ash was found to show the greatest similarity from the fuel to the ash analyzed. A high percentage of particles from both fuel and ash samples contained both Si and K. While Cl was a significant component in the fuel, very little was detected in the ash sample.