https://www.et.byu.edu/~tom/classes/733/Biomass/Jenkins.pdf

Combustion properties of biomass

Modeling chemical and physical processes of wood and biomass pyrolysis

Biomass combustion in fluidized bed boilers: Potential problems and remedies

An overview of the organic and inorganic phase composition of biomass

Role of organic amendments on enhanced bioremediation of heavy metal(loid) contaminated soils

The removal of troublesome elements in biomass to reduce slagging and fouling in furnaces and other thermal conversion systems was tested by washing (leaching) the fuel with water. Rice straw and wheat straw were washed by various techniques and analyzed for composition and ash fusibility. Potassium, sodium, and chlorine were easily removed in both tap and distilled water. Total ash was reduced by about 10% in rice straw and up to 68% in wheat straw, although washing was more effective in increasing ash fusion temperatures for rice straw than for wheat straw due to the higher initial silica concentrations in rice straw. Untreated straw ash which fused below 1000°C was observed to become more refractory at higher temperatures when washed. Scanning electron microscopy of untreated and treated rice straw ashed at 1000°C revealed all untreated ash particles to be fused and glassy, while treated particles remained unfused, were heavily depleted in most elements other than Si, and displayed structures characteristic of original cellular morphology. The fusion temperatures of the straw ash were consistent with predicted temperatures from alkali oxide-silica phase systems based on the observed concentrations of elements in the ash. A simple attempt at simulating a possible full scale washing process was carried out by spraying the surface of a bed of straw with water for an arbitrary time of 1 min. This proved less effective in removing alkali metals and chlorine than soaking the samples in water, flushing water through them in a more controlled manner, or leaving the straw exposed in the field to natural precipitation. Electrical conductivity measurements of leachate revealed that extraction was mostly complete after application of 0.04 l g−1, equivalent to 24 mm of precipitation over uniformly spread rice straw. Full scale furnace experiments have not yet been conducted, and issues involving the practical application of the technique require further investigation, but these results suggest that fouling rates should decline for treated fuels compared to untreated fuels in conventional and advanced biomass power systems.

On the properties of washed straw

The simple leaching of straw, grass (e.g. switchgrass) and other types of biomass with water is shown to result in substantial improvements to the combustion behavior. Although the economic feasibility of leaching is case specific, leaching does extract large amounts of alkali metals and chlorine which improve the fouling, slagging, and burning characteristics of biomass fuels, especially grasses and straws. Increases in the fusion temperature of ash arise from extraction of alkali elements by leaching, and decreases are, in general, observed in the amount of volatile ash. Pilot combustion tests showed lower rates of fouling when burning leached rice straw compared to unleached fresh material. Pyrolysis rates obtained via dynamic thermogravimetric analysis are slower for leached materials, probably as a result of a reduction in catalytic effects due to alkali metal salts, although volatile matter emission is observed to terminate earlier under isothermal heating for leached compared to unleached fuel. For the highly refractory fuel--rice straw--improvements in the ignition and burning characteristics of single straws have been observed to accompany leaching, possibly as a result of lower chlorine activity in terminating free radical chain reactions.

Combustion Characteristics of Leached Biomass

Slagging and fouling of boilers remains one of the most important obstacles to the widespread use of herbaceous and annual growth biomass in conventional power plants and steam generators. High concentration of alkali and alkaline earth metals in combination with silica and other compounds leads to the rapid build-up of tenacious deposits on grates and heat exchange surfaces. This study investigates the removal of inorganic compounds prior to firing by relatively simple techniques and discusses potential applications in the biomass power industry. Rice and wheat straw were leached by various techniques, and concentrations of major inorganic species determined in the solid and liquid phases. Total ash contents as well as concentrations of potassium and other alkali and alkaline earth species were reduced substantially. Softening and fusion of the leached fuel ash were observed to occur at higher temperatures than for the untreated fuel ash. Full-scale experiments are pending, but laboratory results suggest that fouling rates should decline for treated fuels compared to untreated fuels in conventional and advanced biomass power systems.

J.B. Wei

Papers

On the properties of washed straw

Combustion Characteristics of Leached Biomass

Removal of inorganic elements to improve biomass combustion properties