Small Boilers Research Articles

Application of different <i>k</i>-<i>ε</i> turbulence models on combustion process modelling in small-scale pellet stoves for household heating

Constant demand for development and construction improvement of small scale pellet stoves and boilers (nominal power up to 50 kWth) for household heating leads to conclusion that appropriate upgrading should be done on that field. Optimisation of stove construction should be performed based on results of experimental tests, as well as results of numerical analysis and mathematical modelling of combustion process. This is compulsory, because stove construction shall comply with demands for energy and environmental characteristics defined in appropriate quality standards. Besides, stove should operate with pellets of different quality, produced from various biomass raw materials. Considering that numerical analysis is less time consuming process than experimental tests, nowadays it is more often used for development and improvement of combustion appliance construction. As the turbulence modelling is one of the key parts in the eligible numerical analysis, the aim of the research presented in this paper is to define the influence of different k-ε turbulence models and their application on defined CFD mathematical model of combustion process in small-scale pellet stove for household heating.

Chemical and Physical Analysis of Different Ash Fractions from Small Biomass Boilers

AbstractDue to the growing number of small biomass boilers in residential buildings, the ash amounts from these plants are increasing. In order to find a beneficial use for these ashes, chemical and physical analyses were carried out on different ash fractions of different boilers. The results enable a comparison of the used boilers and fuels. As a result, it was shown that the chemical composition and physical properties strongly depend on the type of the ash, the fuel, and the boiler. The main focus of the investigations was on assessing whether the ashes were to be classified as fertilizer or waste. Accordingly, the results of the analyses were compared with the respective limit values for these cases. In addition, further possible applications were evaluated with regard to their suitability, and related current problems were identified.

Biomass early stage combustion in a small size boiler: experimental and numerical analysis

The increment in the world energy consumption and the necessity for a sustainable industrial production, indicate that renewable resources may be key actors for future development. In this scenario, biomass appears fundamental for the smooth transition from fossil fuels to lower carbon footprint technologies, and as a moderator agent within the renewable market. The small size biomass combustion application appears as suitable for smart grid and distributed generation applications, but it is necessary to improve the design tools capabilities and the experimental knowledge of these systems. The present work aims at investigating the thermal behaviour of a 140 kW fixed-bed boiler sited at the Biomass to Energy Research Centre (CRIBE) of the University of Pisa and fed with woodchips. Experimental activities were conducted in order to acquire thermal and chemical data. Moreover, a computational fluid dynamic model was developed and validated. Attention was paid to the fixed bed analysis, and the results showed a good model prediction capability, with respect to the reduced computational demand required.

Theoretical model and preliminary design of an innovative wet scrubber for the separation of fine particulate matter produced by biomass combustion in small size boilers

Fine particulate matter (PM) emission from biomass boilers for non-industrial heating represents one of the most important causes (together with the transport sector) of air pollution, in particular during winter. Separation technologies for fine PM are already well-known and adopted on an industrial scale, as a consequence of strict limits set by national and international regulations. On domestic boilers, the same technologies utilized on an industrial scale are not feasible due to high investment costs. Moreover, the emission limits for small size biomass boilers are higher than for industrial boilers, so high efficiency separation technologies are not needed, and are sometimes not present at all. The main goal of the paper is the development and testing of a mathematical model that is able to foresee the PM removal efficiency of a wet scrubber device. After an experimental validation based on several tests, it was possible to approach the preliminary design of an innovative wet scrubber, which is described in the paper. The main characteristics are (i) removal efficiency over 99.9%, (ii) specific energy consumption under 36 kJ m−3, which is an industrial reference, and (iii) relatively low investment, operation and maintenance costs.

Dobór parametrów eksploatacyjnych elektrofiltru obniżającego niską emisję pyłów pochodzących ze spalania paliw stałych

The article presents the results of research aimed at developing the construction of an ESP (electrostatic precipitator), as well as the performance and selection of operating parameters of the ESP for household applications. The object of the experiment was the ESP prototype, designed and made by the authors, assigned to be placed in a gas pass of a detached house. A simulation of dustiness caused by burning solid fuels has been done. The experiment has been carried out for two different degrees of dust concentration at the ESP inlet, by controlling the given voltage. The results proved that the proposed constructional solution of the ESP significantly limits low emission PM2.5 and PM10 dust emitted during the process of burning solid fuels: coal and/or biomass in boilers and fireplaces used in households or in small local boiler houses.

An inhouse code for simulating heat recovery from boilers to heat water

The current tendency in energy domain is to reduce fuel consumption in favor of sustainable energy approaches. In this frame, the present work suggests an efficient way of heat recovery from boilers using concentric tube. The motivation behind the suggested concept is that it could be considered the cheapest, easiest to construct and simplest to use among all the existing heat recovery systems. In other words, the goal is to suggest a technique that could be utilized by a wider range of users regardless their technical level. Another advantage of the proposed concept is that is can be applied even on small scale boilers. With this in mind, a numerical tool is also developed allowing to make pre-studies to optimize the geometric parameters such as diameters and length, as well as to perform post-studies that allows to optimize operational parameters such as flow rates and fluids configurations. Furthermore, an experimental study is carried-out to validate the numerical results of the adopted heat exchanger. It was shown that water can be heated up to 100 °C depending on the flow rate and that the recovered heat increases through a rational function.

A New Mixture Refrigerant for Space Heating Air Source Heat Pump: Theoretical Modelling and Performance Analysis

Air source heat pump (ASHP) is becoming a substitute for small coal boilers in rural residential buildings of North China. However, the application of ASHP faces challenges of heating capacity, energy efficiency, ozone depletion potential (ODP) and global warming potential (GWP). Proper refrigerant is a key factor influences the performance of ASHP. In this paper, a new mixture refrigerant named RHR-1 is proposed, which aimed to improve energy efficiency, eliminate ODP and reduce GWP of ASHP refrigerant. The performance of RHR-1 was analyzed and compared with commonly used refrigerants including R134a, R410A, R407C and R22 in terms of heating coefficient of performance (COPh), compression ratio (CR) and discharging temperature (DT). The results show that, under the design cases, where supply water temperatures vary from 35 °C to 50 °C and outdoor air temperatures vary from −15 °C to 15 °C, the COPh of RHR-1 are in the range of 2.43–4.93. The COPh of RHR-1 is higher than other candidates when the supply water temperature is above 40 °C. The CR and DT of RHR-1 are in medium levels of the compared samples. A logarithmic regression equation was deduced to get the relationship of COPh with temperature difference between supply water and outdoor air which suggested the temperature difference should be controlled within 47.5 °C to get reasonable COPh. In addition, RHR-1 has no ODP, and its GWP is 279, which is much lower than other candidates. RHR-1 could be a reasonable refrigerant used in ASHP for space heating in North China.

Wet oxidation and absorption procedure for NOx removal

Abstract The removal of NO x with low oxidation degree, low temperature, and high vapor content from small coal-fired boiler emission through selective catalytic reduction is difficult. This study investigated liquid oxidation and absorption to remove NO x systematically. Thermodynamic calculation showed that the equilibrium constants of the NO oxidation and NO x absorption reaction are high; thus, NO x treatment by liquid oxidation and absorption is feasible. Four oxidants (NaClO2, NaClO, H 2 O2, and KMnO4) and four absorbents (Ca(OH)2, CaCO3, NaOH, and Na2CO3) were used to study the reaction mechanism of the oxidation and absorption processes. Results show that NaClO2 solution is the best oxidant, and the optimum pH value of the oxidation reaction is 5–6. The oxidation degree reaches 100% when the concentration of the NaClO2 solution is 1.0%, and the oxidation degree can still reach approximately 50% when the concentration is 0.1%. Ca(OH)2 slurry has the best absorption performance among the four kinds of absorbents. The NO x removal efficiency of Ca(OH)2 slurry can reach approximately 70% when the oxidation degree is 50%. The removal efficiency is up to 80% when the oxidation degree is 100%.

Chemical Composition Characteristics of PM2.5 Emitted by Medium and Small Capacity Coal-fired Boilers in the Yangtze River Delta Region

Seven small and medium coal-fired boilers were selected to analyze the emission factors and chemical composition characteristics of PM2.5, and the efficiency of the dust collection unit in removing PM2.5, OC, and EC of different particle sizes. At the outlet of the dust collection unit, PM2.5 mass emission factor was around 0.047-0.283 g·kg-1 with an average of (0.146±0.081) g·kg-1. SO42- was the most abundant ionic component, followed by NH4+ and Ca2+. S was the most abundant among all the elements, followed by Al, Ca, and Fe. The contents of OC and EC fluctuated greatly, and the ratio of OC and EC produced by each boiler was greater than one. In the range of the measured particle sizes, the mass concentration of PM2.5 showed a singlet or bimodal distribution, and the peak value appeared at 0.07-0.12 μm and greater than 1 μm in bimodal distribution. The mass concentration of OC showed a bimodal or trimodal distribution, and the peak value appeared at 0.04 μm, 0.20-0.31 μm, and greater than 1 μm in trimodal distribution. The efficiencies of the dust collection unit in removing PM2.5, OC, and EC were 66%, 53%, and 23%, respectively.

Simulation of Velocity Field in a Pulverized Coal Fired Boiler

This paper selects a 58MW pulverized coal boiler as the research object, three dimensional physical modelling and mathematical modelling are established for the boiler furnace, according to the set of reasonable hypothesis and boundary conditions, the Fluent software is used to simulate the velocity field in the furnace under six operating conditions. Through the numerical simulation results, the optimal operating conditions of this kind of boiler are discussed, which can provide data reference for improving the energy efficiency of small and medium-sized coalfired boilers.

Control strategy study on frequency and peak-load regulation of coal-fired power plant based on boiler heat storage capacity

Nowadays, quantity of coal-fired power plant and its single unit capacity are greatly improved in China, and power grid’s frequency and peak-load regulation range become wider. Based on the basic regulation theory and unit’s characteristics, this paper indicates the limitations of unit’s original control strategies and such limitations have produced great damages to coal-fired boilers through assessment of 2 years statistical data of boiler tube explosion in regional power grid. Under field tests, the quantified boiler heat storage capacity of six typical thermal power units in the power grid is provided. Comparisons of these six coal-fired boilers are made for getting the relationships between BHSC and boilers’ tube explosion ratio under the in-depth frequency and peak-load regulation. The results show that unit’s boiler heat storage capacity varies greatly amongst boilers with different types and is inversely proportional to boiler’s installed capacity and steam parameter class, and boiler heat storage capacity is inversely proportional to boiler’s tube explosion ratio, which is in conformity with the theory analysis. After that, in-depth frequency and peak-load regulation tests of thermal power units are carried out, respectively. The results show pulverized coal-fired boiler with small boiler heat storage capacity is not suitable for in-depth frequency and peak-load regulation for the safety of power grid and unit itself, while the circulating fluidized bed boiler and pulverized coal-fired boiler with larger boiler heat storage capacity have better adaptability for these functions. Some effective control strategies of frequency and peak-load regulation are presented in load’s rate and range by boiler heat storage capacity of units.

The effect of papermaking sludge as an additive to biomass pellets on the final quality of the fuel

The ever increasing demand on top quality solid biofuel in form of biopellets is limited by the sources and properties of raw materials. The best quality biopellets are suitable for burning in small scale boilers up to 100 kWt. The most limiting factor of this fuel is the ash melting temperature. Therefore the use of cheaper and readily available phytomass as a raw material is considerably restrictive. The research in this paper is based on the theory of the chemical process of ash creation and on the effect of additives on this process. The objective of the study is to find and apply an effective and available additive to produce the best quality phytomass-based fuel pellets, suitable for small scale boilers. A by-product of the paper recycling process, papermaking sludge was used as the most effective additive to wheat straw. This is due to the high content of calcite and kaolin in the sludge. It was also confirmed by experiments that the composition of sludge including cellulose fibres very positively affects the physical and mechanical properties of the composite pellets. Our research deals with an experiment concerning an appropriate production process of composite fuel based on wheat straw. Different weight proportions of papermaking sludge were added and the effect of the sludge on the fuel parameters, especially on the ash melting behaviour, was observed. The final quality of the composite fuel was thoroughly tested by physical, mechanical and thermal methods, and its properties were compared to European and International Standards for Biofuels. The results of the experiments demonstrate that the composite pellets produced by the above-mentioned process have a high level of fuel quality and, due to high ash melting temperature, they are suitable for small scale boilers. The results have shown that the appropriate range of papermaking sludge content for producing high quality wheat straw composite pellets ranges from 10% to 20%.

Combustion of wood-chips in a small-scale fixed-bed boiler: Validation of the numerical model through in-flame measurements

Experimental campaigns and numerical simulations were carried out to investigate the effect of air distribution on the performance of a small-scale fixed-bed boiler fed with biomass. Despite the small dimension of the fire pit, temperature measurements just above it showed a large spatial variation, thus indicating that modeling approaches based on perfectly stirred reactor conditions for the biomass bed are not suited. Moreover these approaches, often used for small boilers, suffer of high uncertainty related to the inlet turbulence levels that affect the mixing of reactants and thus reaction rates. Instead the representation of the biomass bed as a porous medium with defined sources and sinks of chemical species and energy, may provide a cheap and effective strategy to take into account the distribution of the primary air and overcome uncertainties on inlet turbulence. Results are encouraging with a good prediction of the trends of the available measurements. Significant discrepancies were noticed in some locations, hence calling for further efforts to improve the description of the kinetics as well as of the biomass dimensional distribution needed for the definition of the porous medium.

Areas of high energy efficiency of energy supply systems with cogeneration heat pump installations of small power and peak electric boilers

Areas of high energy efficiency of energy supply systems with cogeneration heat pump installations of small power and peak electric boilers

The impact of co-combustion of polyethylene plastics and wood in a small residential boiler on emissions of gaseous pollutants, particulate matter, PAHs and 1,3,5- triphenylbenzene

The aim of this study was to simulate a banned but widely spread practice of co-combustion of plastic with wood in a small residential boiler and to quantify its impact on emissions of gaseous pollutants, particulate matter (PM), polycyclic aromatic hydrocarbons (PAHs), and 1,3,5-triphenylbenzene (135TPB), a new tracer of polyethylene plastic combustion. Supermarket polyethylene shopping bags (PE) and polyethylene terephthalate bottles (PET) were burnt as supplementary fuels with beech logs (BL) in an old-type 20 kW over-fire boiler both at a nominal and reduced heat output.An impact of co-combustion was more pronounced at the nominal heat output: an increase in emissions of PM, total organic carbon (TOC), toxic equivalent (TEQ) of 7 carcinogenic PAHs (c-PAHs) and a higher ratio of c-PAHs TEQ in particulate phase was observed during co-combustion of both plastics.135TPB was found in emissions from both plastics both at a nominal and reduced output. In contrast to findings reported in the literature, 135TPB was a dominant compound detected by mass spectrometry on m/z 306 exclusively in emissions from co-combustion of PE. Surprisingly, six other even more abundant compounds of unknown identity were found on this m/z in emissions from co-combustion of PET. One of these unknown compounds was identified as p-quaterphenyl (pQ). Principal component analysis revealed strong correlation among 135TPB, pQ and five unknown compounds. pQ seems to be suitable tracers of polyethylene terephthalate plastic co-combustion, while 135TPB proved its suitability to be an all-purpose tracer of polyethylene plastics combustion.

Experimental investigation on the fixed bed of a small size biomass boiler

During the last decades, the increase of the world energy consumption promoted the renewable resource development and use. Together with wind, solar and hydro energy, biomasses play a key role in the reduction of the industrial environmental impact; moreover, the biomass combustion systems are very attractive for micro-generation purpose. In this paper, experimental tests on a 140kW small size fixed bed biomass boiler were carried out. The main goal was to study the thermal behavior and some chemical products, such as CO, CO2, Methane and Ethylene, from the combusting fixed bed of the system. In fact, despite the wide amount of literature for the laboratory scale systems, the commercial scale boilers have been seldom studied by the experimental point of view. The data were obtained by varying the operational parameters of the boiler, that are the air excess and the secondary to primary air feeding ratio. Furthermore, the collected data were analyzed and the relationship between the thermal-chemical data and the control variables was discussed.

Adaptation of hard coal with high sinterability for solid fuel boilers in residential heating systems

Solid fuels boilers with low emission and modern construction, which are used for residential heating, require coal fuel with low sinterability (RI < 15). This paper presents a process for adapting coal with high sinterability for use in small boilers in residential heating systems. This solution allows for the combustion of coals with an RI of 50 without causing negative effects (in terms of combustion stability) in typical burners used in heating devices. The proposed solution involves the delivery of fuel particles coated with an oil film (waste oil and canola oil) to the combustion chamber. Tests were performed using a commercial-scale boiler with a 17–23 kW output, equipped with a retort burner and a dosing device, which applied oil to the coal in the boiler. Tests were performed during the combustion of hard coals characterized by different sinterability, with and without various streams of oils. Emission issues are also analysed. Impregnation of the coal by canola oil caused slightly lower NOx emissions, but higher CO and dust emissions. Analyses of the basic quality parameters (i.e., coal structure after thermal treatment) show that changes in the coal properties also occurred. Based on these studies, a mechanism that explains the beneficial effect of oil presence on high sinterable coal (HSC) combustion is proposed.

Effects of internal flue gas recirculation rate on the NOx emission in a methane/air premixed flame

This study fundamentally investigates the effects of internal flue gas recirculation (IFGR) on the combustion characteristics of methane/air mixtures, particularly on the NOx formation. To induce IFGR, a prototype tubular flame burner with internal recirculation has been made, in which 12 recirculation paths are designed. The IFGR rate is controlled by varying the number (n) of opening recirculation paths (n = 0-12). To quantify IFGR rate and furthermore evaluate its influence on the NOx formation, the recirculated gas volume is determined by measurements of velocity distribution with a Particle Image Velocimetry (PIV) system. The results illustrate that by increasing n, the flame length increases while flame luminosity weakens, steady tubular flame range in equivalence ratio shrinks, and the flame temperature decreases. Temperature of recirculated burned gas drops significantly before diluting the fresh reactants. Detailed observations show that the measured IFGR rate is about 13% (convert to STP) when all the recirculation paths are opened (n = 12), in which the NOx concentration significantly decreases to almost a half. Owing to IFGR, the local oxygen mole fraction and equivalence ratio in the reaction zone decrease, which directly reduce the flame temperature and oxygen atom concentration, resulting in a significant decrease of thermal type NO formation. Variations of species concentrations with IFGR rate have been examined, and the obtained relations provide a useful guide for the operation of small boilers adopting IFGR, which can reduce NOx emission without great cost and space.

Thermal energy recovery via integrated small scale boiler and superheater

Abstract A small-scale exhaust energy recovery system has been designed and tested using MEMS (Micro-Electro Mechanical System) fabrication techniques in combination with Additive Manufacturing. The system extracts thermal energy from low temperature exhaust flows with a porous copper heat exchanger and passes that energy to a unified superheater/boiler. MEMS fabrication techniques were utilized for the boiler and Additive Manufacturing (3D printing) was used to combine with the superheater. Capillary channels were utilized within the boiler to achieve internal working fluid pumping action. The phase changed working fluid was then passed to the superheater for superheat. The prototype system was tested in two configurations: first with boiler atop exhaust heat exchanger and second with combined boiler and superheater working in combination. The boiler operating without superheater captured 66% of incoming thermal energy. Operating in combination with the superheater showed a 10% increase in thermal absorption capability at working fluid flow rate of 3 mg/s. Working fluid was superheated successfully from saturation temperature of 76 °C–85 °C. Increased head loss associated with the superheater and boiler combination limited flow rates. The system achieves measurable superheat of working fluid when applied to the real-world challenge of thermal energy scavenging.

Experimental investigation on air displacement and air excess effect on CO, CO2 and NO emissions of a small size fixed bed biomass boiler

Abstract Biomass has a key role in the future of the energetic scenario, since it is a programmable renewable energy source which is suitable either to generate power and/or produce heat. This paper deals with biomass combustion processes in small size boilers, which is suited either for large residential buildings, or for small size industrial users. This kind of boilers is different from small pellet stoves and fireplaces and from large utility size boilers. In particular, those small size boilers have been seldom studied in the scientific literature about the combustion kinetics and pollutant formation mechanisms. Hence, the authors present the thermo-chemical data collected from a 140 kWth chipped wood boiler at the University of Pisa. The effect of the air excess and the ratio between and secondary feeding air and primary feeding air mass flow rate, on CO, CO2 and NOx emissions was investigated. Results have shown that the air excess has the main influence on the pollutant produced. A comparison of the measured data with those presented in the literature and the discrepancies with other studies are also discussed.