Grate Ash Research Articles

Fuel Improvement Measures for Particulate Matter Emission Reduction during Corn Cob Combustion

Fuel-related measures and modernization of small-scale combustion units has become the focus of attention in the renewable heat generation sector, as a means to promote local biomass utilization and fuel-flexibility while meeting strict environmental legislative requirements. With the aim to mitigate total particulate matter emissions and ash-associated problems characteristic of crop residue combustion, (1) corn cob pellets (with and without kaolin and binder) as well as (2) fuel blends with wood pellets were combusted in a pellet oven under full load. Results show that additivation or fuel blending (e.g., 50 wt. % wood and 50 wt. % corn cob pellets) reduce total particulate and CO-emissions by 48 to 60 wt. % and 64 to 89 wt. %, respectively, in comparison to baseline emissions from non-additivized corn cob pellets. Kaolin prevented sintering of corn cob ash. However, considerable grate ash entrainment was observed. TPM consists of a “primary network”—polyhedral and spherical particles approximately 1 μm in diameter (mainly KCl), and a “secondary network” built on top of the primary network, consisting of square-prism-shaped particles of approximately 200 nm in diameter. KCl and K2SO4 are main compounds in particles from corn cob and wood pellet combustion, respectively. Effective measures demonstrated within this study should be complemented with low-cost coarse ash removal systems.

Thermal behavior and kinetics of municipal solid waste during pyrolysis and combustion process

The thermal behavior of municipal solid waste (MSW) in south China during pyrolysis and combustion processes was investigated via thermogravimetric analysis. In order to investigate the effect of metal oxides in the grate ash, the pyrolysis and combustion experiments were carried out with and without additives (ZnO, Fe2O3, CuO and Al2O3). Different artificial atmospheres (O2: N2 = 1:9, O2: N2 = 3:7, O2: N2 = 5:5, O2: N2 = 7:3) were adopted to investigate the oxygen-enriched combustion characteristics. Results showed that all additives facilitated the decomposition of MSW, while the CuO prohibited the decomposition of biowaste matter. The same effort was applied on the combustion process. All additives had a catalytic effect on ignition and combustion characteristics of MSW, except for biowaste volatile combustion of MSW with CuO. The oxygen-enriched combustion conditions reduced the ignition temperature of MSW and provided more help to the combustion process of MSW. Based on several compositions parallel reaction mechanism, the kinetic parameters were obtained and confirmed that both some additives and high oxygen concentration improved MSW combustion characteristics.

Toxicity of waste gasification bottom ash leachate

Toxicity of waste gasification bottom ash leachate from landfill lysimeters (112m3) was studied over three years. The leachate of grate incineration bottom ash from a parallel setup was used as reference material. Three aquatic organisms (bioluminescent bacteria, green algae and water flea) were used to study acute toxicity. In addition, an ethoxyresorufin-O-deethylase (EROD) assay was performed with mouse hepatoma cells to indicate the presence of organic contaminants. Concentrations of 14 elements and 15 PAH compounds were determined to characterise leachate. Gasification ash leachate had a high pH (9.2–12.4) and assays with and without pH adjustment to neutral were used. Gasification ash leachate was acutely toxic (EC50 0.09–62 vol-%) in all assays except in the algae assay with pH adjustment. The gasification ash toxicity lasted the entire study period and was at maximum after two years of disposal both in water flea (EC50 0.09 vol-%) and in algae assays (EC50 7.5 vol-%). The grate ash leachate showed decreasing toxicity during the first two years of disposal in water flea and algae assays, which then tapered off. Both in the grate ash and in the gasification ash leachates EROD-activity increased during the first two years of disposal and then tapered off, the highest inductions were observed with the gasification ash leachate. The higher toxicity of the gasification ash leachate was probably related to direct and indirect effects of high pH and to lower levels of TOC and DOC compared to the grate ash leachate. The grate ash leachate toxicity was similar to that previously reported in literature, therefore, confirming that used setup was both comparable and reliable.

Leachate formation and characteristics from gasification and grate incineration bottom ash under landfill conditions

Characteristics and formation of leachates from waste gasification and grate firing bottom ash were studied using continuous field measurements from 112m3 lysimeters embedded into landfill body for three years. In addition, the total element concentrations of the fresh ash were analysed and laboratory batch tests were performed to study leachate composition. The three-year continuous flow measurement showed that about one fifth of the leachates were formed, when the flow rate was >200l/d, covering <3.5% of the study time. After three years, the liquid/solid-ratio for the quenched grate ash was 1 (l/kg (d.m.)) and for the initially dry gasification ash 0.4 (l/kg (d.m.)). The low initial water and residual carbon content of the gasification ash kept the leachate pH at a high level (>13) major part of the study. In the grate ash leachate pH was lower (<8) due to the presence of organic carbon and biodegradation indicated by biological oxygen demand and redox potential measurements. In the gasification ash the high pH probably delayed leaching of major elements such as Ca, therefore, raising the need for a longer after-care period. The high pH also explains the higher leaching of As from the gasification ash compared to the grate ash both in the batch test and under landfill conditions.

Element budgets of forest biomass combustion and ash fertilisation – A Danish case-study

Harvest of forest biomass for energy production may lead to a significant export of nutrients from the forest. Ash spreading and recycling of nutrients from wood chip combustion to the forest has come into focus as a means for counteracting the nutrient export. This study was carried out to examine the retention of various elements in the different ash fractions and utilise the nutrient recovery to evaluate the fertiliser quality of the examined ash. The mass and element flux of wood chips, bottom ash, cyclone fly ash and condensation sludge at Ebeltoft central heating plant was studied over a four day period in spring 2005. On average, 19 ton wood chips (dry weight) were combusted each day. The combustion of the wood chips produced 0.70% ash and sludge (dry weight). The ash and sludge dry matter was distributed as 81% fly ash, 16% bottom and residual grate ash and 3% sludge solid phase. Substantial amounts of nutrients were retained in the fly ash (P, Ca, Mg, Mn and Cu have a recovery higher than 60% and K, S and Fe have a recovery higher than 30%). The recovery of elements in the bottom ash was smaller. The added recovery of the usable fractions of ashes (both fly ash and bottom ash) exceeded 75% for the nutrients P, Ca, Mn and Mg. Both these ash fractions should be considered for fertilisation.

Weathering of gasification and grate bottom ash in anaerobic conditions

The effect of anaerobic conditions on weathering of gasification and grate bottom ash were studied in laboratory lysimeters. The two parallel lysimeters containing the same ash were run in anaerobic conditions for 322 days, after which one was aerated for 132 days. The lysimeters were watered throughout the study and the quality of leachates and changes in the binding of elements into ash were observed. The results show that organic carbon content and initial moisture of ashes are the key parameters affecting the weathering of ashes. In the grate ash the biodegradation of organic carbon produced enough CO 2 to regulate pH. In contrast the dry gasification ash, containing little organic carbon, was not carbonated under anaerobic conditions and the pH decreased only after aeration was started. During the aeration the CO 2 absorption capacity was not reached, indicating that intense aeration would be needed to fully carbonate gasification ash. The results indicate that in common weathering practice the main emissions-reducing processes are leaching and carbonation due to CO 2 from biodegradation. The results of the aeration study suggest that the role of atmospheric CO 2 in the weathering process was insignificant.

Wastes based glasses and glass-ceramics

Actually, the inertization, recovery and valorisation of the wastes coming from municipal and industrial processes are the most important goals from the environmental and economical point of view. An alternative technology capable to overcome the problem of the dishomogeneity of the raw material chemical composition is the vitrification process that is able to increase the homogeneity and the constancy of the chemical composition of the system and to modulate the properties in order to address the reutilization of the waste. Moreover, the glasses obtained subjected to different controlled thermal treatments, can be transformed in semy-cristalline material (named glass-ceramics) with improved properties with respect to the parent amorphous materials. In this review the tailoring, preparation and characterization of glasses and glass-ceramics obtained starting from municipal incinerator grate ash, coal and steel fly ashes and glass cullet are described.

Thermal and chemical behaviour of different glasses containing steel fly ash and their transformation into glass-ceramics

Abstract Chemically inert and different coloured glasses were prepared by inserting up to 10 wt.% steel plant fly ash into different kinds of inorganic matrices. Three different matrices were tested: municipal incinerator grate ash, glass cullet and a low cost CMAS devetrifiable glass. The experimental techniques used to characterise both the starting and the heat treated (glass-ceramics) glasses were differential thermal analysis, durability and release tests, X-ray diffraction, scanning electron microscopy and colour analysis (CIELab method). The results reached suggest the possibility to recover the steel ash both as a starting material in glass and glass-ceramic formulations and as a colouring agent in place of a pure iron oxide into a colourless base system.

The possibility to recycle solid residues of the municipal waste incineration into a ceramic tile body

The feasibility of utilising solid residues of the municipal wastes incineration such as grate (bottom) ash and electrofilter and sleeve filter fly ash, in combination with a porcelain stoneware body, in the preparation of tiles has been investigated. While the chemical, mineralogical, thermal and rheological characterisation of the waste raw materials carries out some problems arising by using fly ash, these seem to be overcome with the bottom ash. The introduction of up to 20 wt% of this powder into the ceramic body does not substantially change the mineralogical and thermal behaviour of the product.

Oxyanions forming elements in waste combustion -partitioning of antimony

Theoretical considerations attribute a high volatility in waste combustion to antimony. Experimental investigations in a test facility, including spiking experiments, indicate that a substantial amount of antimony remains in the grate ash. The competing chemical reaction controlling this behaviour is presumably the formation of antimonates. The experiments documented the expected promoting effect of fuel bed temperature and of chlorine inventory in waste on the actual thermal mobility of antimony.

Oxyanions forming elements in waste combustion- partitioning of antimony

Theoretical considerations attribute a high volatility in waste combustion to antimony. Experimental investigations in a test facility, including spiking experiments, indicate that a substantial amount of antimony remains in the grate ash. The competing chemical reaction controlling this behaviour is presumably the formation of antimonates. The experiments documented the expected promoting effect of fuel bed temperature and of chlorine inventory in waste on the actual thermal mobility of antimony.

Determination of polychlorinated dibenzo- p-dioxins and dibenzo-furans in solid residues from wood combustion by HRGC/HRMS

PCDD/PCDF were determined in solid samples from wood combustion. The samples included grate ashes, bottom ashes, furnace ashes as well as fly and cyclone ashes. The solid waste samples were classified into bottom and fly ash from native wood and bottom and fly ash from waste wood. For each of the four classes concentration distribution patterns from individual congeners, the sums of PCDD/PCDF and the international toxicity equivalents (I-TEQ) values are given. The I-TEQ levels of fly ash from waste wood burning can be approximately up to two thousand times higher than the values from fly ashes of natural wood. The I-TEQ levels in bottom ashes from waste wood combustion systems are as low as the corresponding ashes from the combustion of native wood. Grate ash samples from waste wood combustion systems with low carbon burnout show high levels of PCDD/PCDF.

Combustion products of biosludge from pulp mill

Polychlorinated phenols (PCP), catechols (PCC), guaiacols (PCG), benzenes (PCBz), biphenyls (PCB), dibenzo-p-dioxins (PCDD) and dibenzofurans (PCDF) were analysed in flue gas, fly ash, grate ash and biosludge samples. Samples were collected from a Finnish pulp mill, where biosludge is used in a power station for energy production.

A review of organic emissions from selected combustion processes

The 309 organic compounds reported in the literature as emissions from selected combustion processes are tabulated, with 109 originating from coal combustion, 213 from waste incineration and 69 from coal/refuse combustion. The largest percentage of components have been reported to be present in the grate ash from coal combustion, in the stack emissions from waste incineration and in the fly ash from coal/refuse combustion. Quantitative data for specific compounds are very incomplete, even for the more common components such as polycyclic aromatic hydrocarbons and polychlorinated biphenyls.

Corrosion and Deposits from Combustion of Solid Waste—Part VI: Processed Refuse as a Supplementary Fuel in a Stoker-Fired Boiler

The utilization of shredded and magnetically separated municipal refuse to supplement high-sulfur coal as fuel in a stoker-fired boiler was investigated, using the facilities of the Columbus, Ohio, Municipal Electric Plant. Corrosion probe exposures were used to show the effectiveness of cofiring with high-sulfur coal to reduce chloride corrosion of boiler tube metals by refuse. Reduced emissions of SO2 from the high-sulfur coal also resulted from dilution of the coal with refuse and by action of alkaline components of the refuse. It was demonstrated that 700 hr corrosion rates with refuse and high-sulfur coal were 5–10 times less than those with bulk refuse burning, and approximated those from coal alone. In some runs sulfur dioxide emissions were reduced by about ten percent more than the refuse dilution factor. The cofiring with refuse had no significant effect on grate ash composition, but the ash fusion temperature was lowered about 100°F.