Emissions from small-scale energy production using co-combustion of biofuel and the dry fraction of household waste

In sparsely populated rural areas, recycling of household waste might not always be the most environmentally advantageous solution due to the total amount of transport involved. In this study, an alternative approach to recycling has been tested using efficient small-scale biofuel boilers for co-combustion of biofuel and high-energy waste. The dry combustible fraction of source-sorted household waste was mixed with the energy crop reed canary-grass (Phalaris Arundinacea L.), and combusted in both a 5-kW pilot scale reactor and a biofuel boiler with 140-180 kW output capacity, in the form of pellets and briquettes, respectively. The chlorine content of the waste fraction was 0.2%, most of which originated from plastics. The HCl emissions exceeded levels stipulated in new EU-directives, but levels of equal magnitude were also generated from combustion of the pure biofuel. Addition of waste to the biofuel did not give any apparent increase in emissions of organic compounds. Dioxin levels were close to stipulated limits. With further refinement of combustion equipment, small-scale co-combustion systems have the potential to comply with emission regulations.     

Trace element partitioning in ashes from boilers firing pure wood or mixtures of solid waste with respect to fuel composition,chlorine content and temperature

Trace element partitioning in solid waste (household waste, industrial waste, waste wood chips and waste mixtures) incineration residues was investigated. Samples of fly ash and bottom ash were collected from six incineration facilities across Sweden including two grate fired and four fluidized bed incinerators, to have a variation in the input fuel composition (from pure biofuel to mixture of waste) and different temperature boiler conditions. As trace element concentrations in the input waste at the same facilities have already been analyzed, the present study focuses on the concentration of trace elements in the waste fuel, their distribution in the incineration residues with respect to chlorine content of waste and combustion temperature.Results indicate that Zn, Cu and Pb are dominating trace elements in the waste fuel. Highly volatile elements mercury and cadmium are mainly found in fly ash in all cases; 2/3 of lead also end up in fly ash while Zn, As and Sb show a large variation in distribution with most of them residing in the fly ash. Lithophilic elements such as copper and chromium are mainly found in bottom ash from grate fired facilities while partition mostly into fly ash from fluidized bed incinerators, especially for plants fuelled by waste wood or ordinary wood chips. There is no specific correlation between input concentration of an element in the waste fuel and fraction partitioned to fly ash. Temperature and chlorine content have significant effects on partitioning characteristics by increasing the formation and vaporization of highly volatile metal chlorides. Zinc and cadmium concentrations in fly ash increase with the incineration temperature.     

Decreased PCDD/F formation when co-firing a waste fuel and biomass in a CFB boiler by addition of sulphates or municipal sewage sludge

Polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) are formed during waste incineration and in waste-to-energy boilers. Incomplete combustion, too short residence times at low combustion temperatures (<700 °C), incineration of electronic waste and plastic waste containing chlorine are all factors influencing the formation of PCDD/Fs in boilers. The impact of chlorine and catalysing metals (such as copper and iron) in the fuel on PCDD/F formation was studied in a 12 MW_th circulating fluidised bed (CFB) boiler. The PCDD/F concentrations in the raw gas after the convection pass of the boiler and in the fly ashes were compared. The fuel types were a so-called clean biomass with low content of chlorine, biomass with enhanced content of chlorine from supply of PVC, and solid recovered fuel (SRF) which is a waste fuel containing higher concentrations of both chlorine, and catalysing metals. The PCDD/F formation increased for the biomass with enhanced chlorine content and it was significantly reduced in the raw gas as well as in the fly ashes by injection of ammonium sulphate. A link, the alkali chloride track, is demonstrated between the level of alkali chlorides in the gas phase, the chlorine content in the deposits in the convection pass and finally the PCDD/F formation. The formation of PCDD/Fs was also significantly reduced during co-combustion of SRF with municipal sewage sludge (MSS) compared to when SRF was fired without MSS as additional fuel.     

Combustion studies of high moisture content waste in a fluidised bed

The combustion of three high moisture content waste materials in a fluidised bed combustor has been investigated and a comparison with co-firing of these materials with coal in the same combustor has been made. Waste materials burnt were olive oil waste, municipal solid waste and potato, which is representative of vegetable waste. Mixtures of up to 20% mass concentration water in the waste were fed to the combustor. Above that value the moisture content was too high to sustain combustion without addition of coal. Measurements of CO, NOx, SO2 temperatures were made and the carbon combustion efficiency evaluated. Co-firing with coal resulted in markedly higher combustion efficiencies with an increase of approximately 10-80% when burning the simulated MSW. However, this was much lower than the value of 93% when coal was burnt on its own. It was also much lower than the value obtained, average 90%, when co-firing potato and olive oil waste with coal and there was little difference in the combustion efficiency between the two types of waste and with increasing moisture content. It was concluded that the high ash content of the simulated MSW 26%, compared with 5% in the other two waste materials resulted in slower burning and consequently the char particles were elutriated from the bed without being fully burnt. In term of gaseous emissions during co-combustion, CO emission is relatively insensitive to change in waste fraction. While emission of SO2 can be reduced as the waste fraction increases as a result of fuel-S dilution. But in terms of percent fuel-S converted, it is actually increased by increasing waste fraction. Emissions of NO and N2O increase slightly with MSW fraction.     

PHYSICO-CHEMICAL CHARACTERIZATION AND LEACHING OF DESULPHURIZATION COAL FLY ASH

Fly ash produced by coal combustion using two types of desulphurization process were studied: a conventional pulverized coal boiler equipped with lime injection (PCL ash), and a circulating fluidized bed combustion boiler with limestone injection (CFBC ash). The ashes were characterized completely: granulometry, morphology, mineralogy, chemical composition and behaviour to water contact. Both PCL ash and CFBC ash present similar features: fine granulometry, presence of anhydrite phase and sulphate content. However, PCL ash also shows lots of spherical particles, unlike CFBC ash, and a much higher lime content, due to the lower desulphurization rate in PC boilers. Unlike CFBC ash, most of the trace elements in PCL ash show an inverse concentration–particle size dependence. Leachates obtained from both samples are rich in soluble salts [CaSO₄and Ca(OH)₂] and arsenic and selenium are prevented from solubilizing by high lime content. In wetted PCL ash, the formation of ettringite crystals stabilizes calcium and sulphate ions. Simultaneously, arsenate, selenate and chromate anions are trapped in the crystal. CFBC ash does not really harden because the lime content is too low. However, the leached selenium concentration is cut down in wetted CFBC ash samples.     

Characterization of a thermal power plant air heater washing waste: a case study from Iran.

In Iran most of the electricity is generated by thermal power plants. As a result of fuel oil burning in winter time, the air heaters of the boilers have to be washed and cleaned frequently. The wastewater originating from air heater washing is then treated in an effluent treatment plant by chemical precipitation followed by dewatering of the sludge produced. The resulting waste is classified as specific industrial waste that should be characterized in detail under the Waste Management Act of Iran. The quantity of this waste produced in the studied power plant is about 20 tonnes year(-1). In the present investigation, the first to be carried out in Iran, seven composite samples of dewatered sludge from air heater washing wastewater treatment were subjected to investigation of the physical properties, chemical composition and leaching properties. The most likely pollutants that were of concern in this study were heavy and other hazardous metals (Cd, Co, Cr, Mn, Ni, Pb, Zn and V). The results revealed that mean pH, wet and dry density and moisture content of the waste were 6.31, 1532 kg m(-30, 1879 kg m(-3) and 15.35%, respectively. Magnetite, SiO2, P2O5, CaO, Al2O3 and MgO were the main constituents of the waste with a weight percentage order of 68.88, 5.91, 3.39, 2.64, 2.59 and 1.76%, respectively. The toxicity characteristic leaching procedure test results for some heavy and other hazardous metals showed that mean elemental concentrations of Cd, Co, Cr, Mn, Ni, Pb, V and Zn in leachate were 0.06, 1.55, 5.49, 36.32, 209.10, 0.58, 314.06 and 24.84 mg L(-1), respectively. According to the Waste Management Act of Iran this waste should be classified as hazardous and should be disposed of in accordance with hazardous waste disposal regulations.     

Fundamental characteristics of input waste of small MSW incinerators in Korea

Waste incineration in a small incinerator is a simple and convenient way of treating waste discharged from small areas or from large facilities and buildings such as business centers, marketplaces, factories, and military units. Despite their ostensible advantages, however, many small incinerators frequently suffer from serious problems, e.g., unsystematic waste feeding, unstable combustion, deficient air pollution control devices, and consequently, environmental pollution. To obtain a better understanding of the characterization of wastes in small incinerators, we investigated a series of fundamental characteristics, i.e., physical composition, bulk density, proximate and ultimate analysis, potential energy content, and so on. The main waste components in small incinerators were identified as paper and plastic; the proportion of food waste was less than that in large incinerators. Especially, a low ratio of food waste had a strong influence on other waste characteristics, e.g., lower moisture content and bulk density, and higher potential energy. On the other hand, in contrast with that of HCl, there was no distinguishable linear relationship between Cl content in waste and PCDD/DF concentration in combustion gas.     

Combined incineration of industrial wastes with in-plant residues in fluidized-bed utility boilers--decision relevant factors.

In Austria more than 50% of the high-calorific industrial residues and wastes generated are utilized for energy recovery in industrial utility boilers. This study investigated full-scale trials of combined incineration of in-plant residues with various industrial wastes. These trials were carried out in order to learn how the alternatively used fuel influences the incineration process itself as well as the quantity and quality of the various incineration products. The currently used fuel, which consisted of in-plant residues as well as externally acquired waste wood and the refuse-derived fuel (RDF) mixtures used during the full-scale trials are characterized in terms of material composition as well as chemical and physical parameters. An input-output mass balance for the incineration plant (two fluidized bed combustion units, 20 and 30 MW, respectively) has been established, based on the data collected during the full-scale incineration trials. Furthermore, pollutant concentrations in the off-gas as well as the solid incineration residue are reported. It is not only the pollutant content but also a variety of other internal as well as external factors that have to be considered if a company is to decide whether or not to thermally utilize specific waste types. Therefore a strengths and weaknesses profile for several types of waste and the specific industrial boiler is also presented.     

Co-combustion of waste from olive oil production with coal in a fluidised bed

Waste from olive oil production was co-fired with coal in a fluidised bed combustor to study the feasibility of using this waste as an energy source. The combustion efficiency and CO emission were investigated and compared to those of burning 100% of coal. Olive oil waste with up to 20% mass concentration can be co-fired with coal in a fluidised bed combustor designed for coal combustion with a maximum drop of efficiency of 5%. A 10% olive oil waste concentration gave a lower CO emission than 100% coal firing due to improved combustion in the freeboard region. A 20% olive oil waste mixture gave a higher CO emission than both 100% coal firing and 10% olive oil waste mixture, but the combustion efficiency was higher than the 10% olive oil waste mixture due to lower elutriation from the bed.     

A review of municipal solid waste composition and quantities in Poland

A review of results of the research involving the quantitative and composition analyses of household waste conducted in Poland in recent years is presented in this paper. For these analyses various methodologies have been employed, as there is not one obligatory methodology how to characterise municipal solid waste. The results of the research in large Polish cities indicate great variability, which is difficult to be reasonably explained. This situation is affected by a number of factors, such as various methods and places for sample collection, various methods for tests, fractional character of most of the studies (studies carried out only in some periods during a year). Consequently, it is neither possible to measurably compare their results, nor to generalise them. Hence, within this article only individual data for large cities is provided. It is therefore necessary to standardise methodology for such analyses in Europe, taking into account local variability (such as different housing patterns, climate and waste collection schemes) to allow comparison of results.Reported yearly household waste generation in Polish cities varies from 238 to 309 kg per inhabitant. Biodegradable waste is a strongly dominated fraction in household waste from Polish cities, followed by paper/cardboard and plastics. Historical data shows that waste composition has undergone profound changes, the most significant being an increase of the share of plastics and decrease of fine fraction. The presented data indicates that waste composition strongly depends on the type of housing and its heating system. In the new multi-family buildings with central heating the share of paper and plastics is higher than in old houses with individual heating. In the latter ones the share of fine fraction is higher.     

The composition,heating value and renewable share of the energy content of mixed municipal solid waste in Finland

For the estimation of greenhouse gas emissions from waste incineration it is essential to know the share of the renewable energy content of the combusted waste. The composition and heating value information is generally available, but the renewable energy share or heating values of different fractions of waste have rarely been determined.In this study, data from Finnish studies concerning the composition and energy content of mixed MSW were collected, new experimental data on the compositions, heating values and renewable share of energy were presented and the results were compared to the estimations concluded from earlier international studies. In the town of Lappeenranta in south-eastern Finland, the share of renewable energy ranged between 25% and 34% in the energy content tests implemented for two sample trucks. The heating values of the waste and fractions of plastic waste were high in the samples compared to the earlier studies in Finland. These high values were caused by good source separation and led to a low share of renewable energy content in the waste. The results showed that in mixed municipal solid waste the renewable share of the energy content can be significantly lower than the general assumptions (50–60%) when the source separation of organic waste, paper and cardboard is carried out successfully. The number of samples was however small for making extensive conclusions on the results concerning the heating values and renewable share of energy and additional research is needed for this purpose.     

Evolution of heavy metals in municipal solid waste during bio-drying and implications of their subsequent transfer during combustion

Bio-drying has been applied to improve the heating value of municipal solid waste (MSW) prior to combustion. In the present study, evolution of heavy metals in MSW during bio-drying and subsequent combustion was studied using one aerobic and two combined hydrolytic-aerobic scenarios. Heavy metals were concentrated during bio-drying and transformed between different metal fractions, namely the exchangeable, carbonate-bound, iron- and manganese-oxides-bound, organic-matter-bound and residual fractions. The amounts of heavy metals per kg of bio-dried MSW transferred into combustion flue gas increased with bio-drying time, primarily due to metals enrichment from organics degradation. Because of their volatility, the partitioning ratios of As and Hg in flue gas remained stable so that bio-drying and heavy metal speciation had little effect on their transfer and partitioning during combustion. In contrast, the partitioning ratios of Pb, Zn and Cu tended to increase after bio-drying, which likely enhanced their release potential during combustion.     

Variation in deformation properties of processed MSWI bottom ash: results from triaxial tests

This study is part of a larger study of the mechanical properties of processed municipal solid waste incinerator bottom ash. The aim was to investigate the variation in deformation properties of the ash for future use in unbound road layers. The effect of the material variation was analysed in particular. Specimens of bottom ash from four different incinerator plants and four sampling periods over a period of one year were tested by means of cyclic load triaxial tests. The results showed that there were variations in the deformation properties of the materials. Although there were significant differences between incinerator plants, the seasonal fluctuations were not significant. The differences were mainly due to the organic matter content. For the cyclic stress levels used, the resilient modulus ranged between 60 and 140 MPa, which is comparable to that of sand, but the plastic/permanent deformation was lower than for sand. It was also shown that the organic content has a limiting effect on the resilient modulus. For the material studied, the resilient modulus increased by 50% when the content of organic matter was halved.     

Determination of renewable energy yield from mixed waste material from the use of novel image analysis methods

Two novel techniques are presented in this study which together aim to provide a system able to determine the renewable energy potential of mixed waste materials. An image analysis tool was applied to two waste samples prepared using known quantities of source-segregated recyclable materials. The technique was used to determine the composition of the wastes, where through the use of waste component properties the biogenic content of the samples was calculated. The percentage renewable energy determined by image analysis for each sample was accurate to within 5% of the actual values calculated. Microwave-based multiple-point imaging (AutoHarvest) was used to demonstrate the ability of such a technique to determine the moisture content of mixed samples. This proof-of-concept experiment was shown to produce moisture measurement accurate to within 10%. Overall, the image analysis tool was able to determine the renewable energy potential of the mixed samples, and the AutoHarvest should enable the net calorific value calculations through the provision of moisture content measurements. The proposed system is suitable for combustion facilities, and enables the operator to understand the renewable energy potential of the waste prior to combustion.     

Co-combustion of agricultural residues with coal in a fluidized bed combustor

Power generation from biomass is an attractive technology that utilizes agricultural residual waste. In order to explain the behavior of biomass-fired fluidized bed incinerator, biomass sources from agricultural residues (rice husk and palm kernel) were co-fired with coal in a 0.15m diameter and 2.3m high fluidized bed combustor. The combustion efficiency and carbon monoxide emissions were studied and compared with those for pure coal combustion. Co-combustion of a mixture of biomass with coal in a fluidized bed combustor designed for coal combustion increased combustion efficiency up to 20% depending upon excess air levels. Observed carbon monoxide levels fluctuated between 200 and 900 ppm with the addition of coal. It is evident from this research that efficient co-firing of biomass with coal can be achieved with minimal modifications to existing coal-fired boilers.     

Legislative and environmental issues on the use of ash from coal and municipal sewage sludge co-firing as construction material

For the economy of any co-firing process, it is important that the common waste management options of ash remain practical. Ash from bituminous coal combustion is typically handed to the construction industry. This paper describes the current European legislation on use of ash for construction purposes. Also, it presents an experimental study on the suitability of fly ash from combustion of mixtures of bituminous coal and municipal sewage sludge as additive to cement and concrete, and for use in open-air construction works, based on the ash chemical composition and the characteristics of the extract of the ash. Presently, two European standards forbid the use of ash from co-firing as additive to cement or concrete. This study shows that ash derived from coal and sewage sludge co-firing contains generally less unburned carbon, alkali, magnesium oxide, chlorine, and sulfate than coal ash. Only the concentration of free lime in mixed ash is higher than in coal, even though, at least up to 25% of the thermal input, still below the requirements of the standards. This ash also meets the requirements for the use of fly ash in open-air construction works--concentration and mobility of few elements--although this management option is forbidden to ash from co-firing. The leaching of Cd, Cr, Cu, Ni, Pb and Zn was investigated with three leaching tests. The concentration of these metals in the extracts was below the detection limit in most cases. The concentration of Cu and Zn in the extract from fly ash was found to increase with increasing share of sewage sludge in the fuel mixture. However, the concentration of these two metals in the extract is not regulated. This study indicates that excluding a priori the use of ash from co-firing as a suitable additive for construction material could cause an unnecessary burden on the environment, since probably ash would have to be disposed of in landfill. However, allowing this requires the modification of current European standards to include limitations on all elements and compounds, absent in coal but which might be present in other fuels, that are deleterious for the quality of construction materials.     

沸腾炉大气污染物的生成与排放特征分析

以20台沸腾炉(功率小于等于60 MW)的燃料特性分析数据和大气污染物的排放实测数据为基础,利用统计分析方法,研究了燃烧过程中排放的颗粒物(PM)、SO_2和NO_x初始排放浓度的影响因素,分析了沸腾炉PM、SO_2和NO_x排放现状,探讨了我国中小型沸腾炉PM、SO_2和NO_x排放管理控制的潜力和可行性.实验结果表明:在锅炉运行负荷大于等于80%的条件下,中小型沸腾炉PM的初始排放浓度基本上不受锅炉出力、过量空气系数和燃煤灰分含量的影响;燃煤的硫含量越高,SO_2初始排放浓度越高;过量空气系数越大,燃煤挥发分越低,NO_x初始排放浓度越高.     

Comparing Waste-to-Energy technologies by applying energy system analysis

Even when policies of waste prevention, re-use and recycling are prioritised a fraction of waste will still be left which can be used for energy recovery. This article asks the question: How to utilise waste for energy in the best way seen from an energy system perspective? Eight different Waste-to-Energy technologies are compared with a focus on fuel efficiency, CO₂ reductions and costs. The comparison is carried out by conducting detailed energy system analyses of the present as well as a potential future Danish energy system with a large share of combined heat and power as well as wind power. The study shows potential of using waste for the production of transport fuels. Biogas and thermal gasification technologies are hence interesting alternatives to waste incineration and it is recommended to support the use of biogas based on manure and organic waste. It is also recommended to support research into gasification of waste without the addition of coal and biomass. Together the two solutions may contribute to alternate use of one third of the waste which is currently incinerated. The remaining fractions should still be incinerated with priority to combined heat and power plants with high electric efficiency.     

Residues characterisation from the fluidised bed combustion of East London’s solid recovered fuel

Waste thermal treatment in Europe is moving towards the utilisation of the combustible output of mechanical, biological treatment (MBT) plants. The standardisation of solid recovered fuels (SRF) is expected to support this trend and increase the amount of the generated combustion residues. In this work, the residues and especially the fly ashes from the fluidised bed combustion (FBC) of East London’s NCV 3, Cl 2, and Hg 1 class SRF, are characterised. The following toxicity indicators have been studied: leachable chlorine, organochlorides expressed as pentachlorobenzene and hexachlorobenzene, and the heavy metals Cu, Cr, Cd, Zn, Ni, and Pb. Furthermore the mineralogical pattern of the ashes has been studied by means of XRD and SEM–EDS. The results suggest that these SRF derived ashes have significantly lower quantities of Cu, Cd, Pb, Zn, leachable Cl, and organochlorides when compared to other literature values from traditional waste thermal treatment applications. This fact highlights the importance of modern separation technologies employed in MBT plants for the removal of components rich in metals and chlorine from the combustible output fraction of SRF resulting to less hazardous residues.     

Numerical simulation of municipal solid waste combustion in a novel two-stage reciprocating incinerator

A mathematical model was presented in this paper for the combustion of municipal solid waste in a novel two-stage reciprocating grate furnace. Numerical simulations were performed to predict the temperature, the flow and the species distributions in the furnace, with practical operational conditions taken into account. The calculated results agree well with the test data, and the burning behavior of municipal solid waste in the novel two-stage reciprocating incinerator can be demonstrated well. The thickness of waste bed, the initial moisture content, the excessive air coefficient and the secondary air are the major factors that influence the combustion process. If the initial moisture content of waste is high, both the heat value of waste and the temperature inside incinerator are low, and less oxygen is necessary for combustion. The air supply rate and the primary air distribution along the grate should be adjusted according to the initial moisture content of the waste. A reasonable bed thickness and an adequate excessive air coefficient can keep a higher temperature, promote the burnout of combustibles, and consequently reduce the emission of dioxin pollutants. When the total air supply is constant, reducing primary air and introducing secondary air properly can enhance turbulence and mixing, prolong the residence time of flue gas, and promote the complete combustion of combustibles. This study provides an important reference for optimizing the design and operation of municipal solid wastes furnace.