Asymmetric combustion characteristics and NOx emissions of a down-fired 300 MWe utility boiler at different boiler loads

To investigate the aerodynamic field, cold airflow experiments were conducted under different boiler loads in a cold small-scale model of a down-fired pulverized-coal 300 MW_e utility boiler. At 300 MW_e and 250 MW_e loads, a deflected flow field appeared in the lower furnace. In contrast, at a 150 MW_e load, a U-shaped flow field appeared in regions near the left- and right-side walls in the lower furnace. Concurrently, the regions near the two wing walls adjacent to the front arch had received deflected upward airflow emanating from the region near the rear wall. Moreover, a symmetric W-shaped flow field appeared in the central regions below the front and rear arches.Industrial-sized experiments on the full-scale furnace were also performed at different loads with measurements taken of gas temperatures in the burner region and near the right-side wall, as well as heat fluxes and gas components in the near-wall region. Asymmetric combustion appeared at 300 MW_e and 250 MW_e loads, with large differences arising in gas temperatures, gas components, and heat fluxes between zones near the front and rear walls. At 150 MW_e load, gas temperatures, gas components and heat fluxes are, in general, symmetrically distributed throughout the furnace. By decreasing the load, differences in gas temperatures, gas components, and heat fluxes near the front and rear walls decrease, as did NO_x emissions. Meanwhile, the carbon content in fly ash essentially decreased, yielding an increase in boiler efficiency assisted by a drop in exhaust gas temperature.     

A new experimental apparatus for measurement of spectral emissivity of opaque materials using a reflector as the dummy light source

A new experimental apparatus has been developed, which can be used to accurately measure the spectral emissivity of opaque materials using a reflector as the dummy light source. The experimental apparatus is mainly composed of the optical detection system, heating system, temperature-controlling system, angle-adjusting system and signal-controlling and data-computing system. The optical system works at 1.5 μm and the bandwidth is 20 nm. The sample can be heated up to about 1200 K by the sample heater. The temperature of sample surface is measured by the two highly accurate platinum–rhodium thermocouples and is controlled by means of a microcomputer-controlled proportional–integral–derivative device. The temperature-controlling error is within 2 K over the experimental range from 700 to 1200 K. A reflector is used as the dummy light source to realize the single-wavelength measurements of spectral emissivity. The present apparatus can be used to perform the measurements of spectral emissivity as a function of temperatures and emission angles. The spectral emissivity of several opaque materials has been measured by the apparatus. As an example, the spectral emissivity results of polished aluminum sheet over the temperature range from 788 to 1028 K have been reported here. The analytic dependence between the spectral emissivity and temperature has been determined. The temperature determined by the two thermocouples is employed to assess the accuracy of spectral emissivity measured here. The comparison between the temperatures measured by the thermocouples and those calculated by the emissivity obtained here demonstrates that the results measured by the apparatus achieve much high accuracy and that the proposed measurement technique is reliable.     

Gasification reactivity of biomass chars with CO2

In this study, carbon conversion was calculated from the data obtained with a real-time gas analyzer. In a lab-scale furnace, each biomass sample was pyrolyzed in a nitrogen environment and became biomass char. For preparation of the char, the furnace was electrically heated over 40 min up to the wall temperature of 850 °C, and maintained at the same temperature over 17 min. The furnace was again heated over 3 min to a temperature higher than 850 °C and then CO₂ was injected. The biomass char was then gasified with CO₂ under isothermal conditions. The reactivity of biomass char was investigated at various temperatures and CO₂ concentrations. The VRM (volume reaction model), SCM (shrinking core model), and RPM (random pore model) were used to interpret the experimental data. For each model, the activation energy (E) and pre-exponential factor (A) of the biomass char-CO₂ reaction were determined from gas-analysis data by using the Arrhenius equation. For the RPM, the apparent reaction order was determined. According to this study, it was found that the experimental data agreed better with the RPM than with the other two models. Through BET analyses, it was found that the structural parameter (ψ) of the surface area for the RPM was obtained as 4.22.     

Production of hydrogen by thermal methane splitting in a nozzle-type laboratory-scale solar reactor

A high-temperature solar reactor has been developed for co-producing hydrogen-rich gas and high-grade carbon black (CB) from concentrated solar energy and methane. The approach is based on a single-step thermal decomposition (pyrolysis) of methane without catalysts and without emitting carbon dioxide since solid carbon is sequestered.In the tested reactor, a graphite nozzle absorbs concentrated solar radiation provided by a solar furnace. The heat is then transferred to the reactive flow. The experimental setup, first test results, and effect of operating conditions are described in this paper.The conversion of methane was strongly dependant on the solar furnace power input, on the geometry of the graphite nozzle, on gas flow rates, and on the ratio of inert gas-to-reactive gas. CB was recovered in the carbon trap, and maximum chemical conversion of methane-to-hydrogen and CB was 95%, but typical conversion was in the range 30–90%.     

A combustion concept for oxyfuel processes with low recirculation rate – Experimental validation

Oxyfuel combustion is a technology for Carbon Capture & Storage from coal fired power plants. One drawback is the large necessary amount of recirculation of cold flue gases into the combustion chamber to avoid inadmissible high flame temperatures. The new concept of Controlled Staging with Non-stoichiometric Burners (CSNB) makes a reduction of the recirculation rate possible without inadmissible high flame temperatures. This reduction promises more compact boiler designs. We present in this paper experiments with the new combustion concept in a 3 × 70 kW natural gas combustion test rig with dry flue gas recirculation of 50% of the cold flue gases. The new concept was compared to a reference air combustion case and a reference oxyfuel combustion case with recirculation of 70% of the cold flue gases. FTIR emission spectroscopy measurements allowed the estimation of spectral radiative heat fluxes in the 2–5.5 μm range. The mixing of the gases in the furnace was good as the burnout and the emissions were comparable to the reference cases. The flame temperatures of the CSNB case could be controlled by the burner operation stoichiometry and were also similar to the reference cases. The heat flux in the furnace through radiation to the wall was higher compared to the oxyfuel reference case. This is an effect of the lowered recirculation rate as the mass flow out of the furnace and therefore the sensible heat leaving the furnace decreases. The higher oxygen consumption with lower recirculation rate could be compensated by a lower furnace stoichiometry. This was possible due to better burnout with increased oxygen concentrations in the burner. The results prove that a reduction of the flue gas recirculation rate in oxyfuel natural gas combustion from 70% down to 50% is possible while avoiding inadmissible high flame temperatures with the concept of Controlled Staging with Non-stoichiometric Burners.     

Reduction of energy consumption and pollution emissions for industrial furnace using hydrogen-rich tail gas

Using the recovered tail gas (FG) that consists of 60 mol% (50–70 mol%) of hydrogen gas to replace heavy fuel oil (FO) as furnace fuel was studied. With higher FG/FO ratios, the hydrogen content in the fuel increases so that the volume of flue gas reduces to reduce the furnace internal pressure that leads to slower uprising velocity of the thermal flow in the furnace and hence more efficient thermal transmission in the furnace. The results reveal that complete replacement of fuel oil with the recovered tail gas will reduce about 45.8% of the resulting flue gas, lower the furnace radiation zone temperature by 45 °C, raise the furnace convection zone temperature by 18 °C. Additionally, the annual savings of heavy fuel oil can be 2.3 × 10⁴ m³ heavy fuel oil with the reduction of 53.4 tons SO_x emission, 21.9 tons of NO_x emission and 4.9 × 10⁴ tons of CO₂ emission. Therefore, reusing the recovered tail gas to completely replace heavy fuel oil (FO) as the furnace fuel along with operational adjustments of fresh air flow rate and flue baffle angles will alleviate the discharge of greenhouse gas.     

Radiative heat transfer in enhanced hydrogen outgassing of glass

This paper explores the physical mechanisms responsible for experimental observations that led to the definition of “photo-induced hydrogen outgassing of glass”. Doped borosilicate glass samples were placed inside an evacuated silica tube and heated in a furnace or by an incandescent lamp. It was observed that hydrogen release from the glass sample was faster and stronger when heated by an incandescent lamp than within a furnace. Here, sample and silica tube were modeled as plane-parallel slabs exposed to furnace or to lamp thermal radiation. Combined conduction, radiation, and mass transfer were accounted for by solving the one-dimensional transient mass and energy conservation equations along with the steady-state radiative transfer equation. All properties were found in the literature. The experimental observations can be qualitatively explained based on conventional thermally activated gas diffusion and by carefully accounting for the participation of the silica tube to radiation transfer along with the spectral properties of the silica tube and the glass samples. In brief, the radiation emitted by the incandescent lamp is concentrated between 0.5 and 3.0 μm and reaches directly the sample since the silica tube is nearly transparent for wavelengths up to 3.5 μm. On the contrary, for furnace heating at 400 °C, the silica tube absorbs a large fraction of the incident radiation which reduces the heating rate and the H₂ release rate. However, between 0.8 and 3.2 μm undoped borosilicate does not absorb significantly. Coincidentally, Fe₃O₄ doping increases the absorption coefficient and also reacts with H₂ to form ferrous ions which increase the absorption coefficient of the sample by two orders of magnitude. Thus, doped and reacted samples heat up much faster when exposed to the heating lamp resulting in the observed faster response time and larger H₂ release rate.     

High hydrogen content syngas for biofuels production from biomass air gasification: Experimental evaluation of a char-catalyzed steam reforming unit

This work investigates the performance of a reformer reactor for the upgrading of syngas and char derived from a pilot-scale air gasifier. The proposed setup represents a circular approach for the production of hydrogen-rich syngas from air gasification. Specifically, the reforming-unit was operated under a range of temperatures (from 700 °C to 850 °C) and steam flow rates and for each the improvement in producer gas composition and reducing species yield is evaluated. The results highlight that an increase in hydrogen concentration is obtained at higher temperature, moving from 16.2% to 21.3%, without using steam, and to 45.6%, with steam injection on the char-bed, while CO concentration did not follow a monotonic behavior. Moreover, the gas quality index, defined as a ratio between reducing species and inert species, delivered the highest values at the highest temperatures and steam flow rates. These results provide a guide for future gas quality optimization studies.     

Effect of non-condensable gas on the performance of steam-water ejector in a trigeneration system for hydrogen production: An experimental and numerical study

An ejector containing phase changing gas-liquid flow process acts as a popular and decisive device in multiple industrial applications, including the hydrogen production, electricity production, fuel cells, refrigeration, petroleum industry and desalination systems. However, non-condensable gas is inevitable for the usual operation of phase-changing gas-liquid ejector in the trigeneration or electrolyzer system for hydrogen production, and rarely research is concerned with this issue. In the present study, the effect of non-condensable gas contained in the condensable gas on the characteristics of gas-centered water ejector is presented, with steam, water and air acting as the gas, liquid and non-condensable gas, respectively. Experimentally, the flow rate of steam is controlled to be 1.45 g/s with an absolute pressure of 120 kPa, the air flow rate varies from 0 to 0.14 g/s, resulting in a non-condensable gas concentration ranging from 0 to 9%, and the resulted water flow rate at 100 kPa and 282.15 K changes from 34.7 to 37.3 g/s. Combined with the numerical methods, the performance of ejector expressed in ejected water flow rate was found to increase firstly with a small amount of non-condensable gas, and decrease when the non-condensable gas reaches a certain amount. In addition, the distributions of multiple local flow parameters including pressure, condensation rate and gas volume fraction, velocity and temperature inside the ejector were shown for different non-condensable concentration, by which the mechanism for the change of ejector performance under varying non-condensable concentration was demonstrated. These findings are initiative and insightful for the ejector design optimization in the trigeneration system for hydrogen production and the proposed numerical models can be utilized in analysis and design of steam ejector with non-condensable gas involved.     

Heat and mass transfer analysis of a suspension of reacting particles subjected to concentrated solar radiation – Application to the steam-gasification of carbonaceous materials

A chemical reactor for the steam-gasification of carbonaceous materials (e.g. coal, coke, biomass) using high-temperature solar process heat is modeled by means of a two-phase formulation that couples radiative, convective, and conductive heat transfer to the chemical kinetics for polydisperse suspensions of reacting particles. The governing mass and energy conservation equations are solved by applying advanced Monte–Carlo and finite-volume techniques with smoothing and underrelaxation. Validation is accomplished by comparing the numerically calculated temperatures, product compositions, and chemical conversions with the experimentally measured values obtained from testing a 5 kW solar reactor prototype in a high-flux solar furnace. A unique feature of the reactor concept is that the gas-particle flow is directly exposed to concentrated solar radiation, providing efficient radiative heat transfer to the reaction site for driving the high-temperature highly endothermic process.     

Integrated methods to improve efficiency of furnace burning recovered tail gas

Replacing nature gas (NG) by recovered hydrogen-rich tail gas to fuel a full-scale furnace has been proved to improve furnace efficiency and reduce NO_x formation. Adjusting residual O₂ concentration in flue gases, air preheat temperature and furnace damper angle will further improve the efficiency of the furnace burning the recovered hydrogen-rich tail gas. Prolonging the residence time of the hot gas flow increases the heat release time and reduces the time for the fuel to reach burning temperature so that the furnace efficiency can be improved. On-site test results using a full-scale furnace show that reducing the residual O₂ concentration in flue gases from 4.0 to 3.0 vol.% raises the furnace efficiency by 0.6%. Raising the pre-heated incoming air temperature from 200 °C to 240 °C and reducing furnace damper opening angle from 45° to 39° save 2.4 × 10⁶ and 1.9 × 10⁶ m³ of natural gas, respectively. Integrating the adjustment of flue gas residual O₂ concentration, temperature of incoming air temperature, and furnace damper opening angle will assist industries in achieving higher overall furnace efficiency and reducing carbon dioxide emission.     

Improving furnace energy efficiency through adjustment of damper angle

Numerous furnaces and boilers are extensively used in industrial and commercial facilities to generate thermal energy so that small improvements of the furnace thermal efficiency will amount to tremendous reduction of energy consumption and green gas emission. In this research, the furnace flue damper angle is adjusted to lower the pressure in the furnace for reducing the velocity of hot gas rising in the furnace. This allows more time for the heat to be transferred to the thermal flow that improves the furnace overall thermal efficiency. On the other hand, when the damper angle is adjusted from 45 to 39°, the pressure in the furnace rises from −14.7 to −9.3 mmH₂O, the average fuel volumetric flow rate reduces from 751 to 491 m³/h, and the average temperature lowers from 949 to 909 °C in the radiation section and from 756 to 798 °C in the conventional section. Hence, about 1.7 × 10⁶ m³ of fuel gas consumption can be saved, and 1.9 × 10³ ton of CO₂ emission can be reduced annually. The results confirm that simply by adjusting the flue damper angle of a furnace will achieve significant savings of energy and reduction of carbon dioxide emission.     

The effect of the top and bottom wall temperatures on the laminar natural convection in an air-filled square cavity

The effect of the top and bottom wall temperatures on the natural convection heat transfer characteristics in an air-filled square cavity driven by a difference in the vertical wall temperatures was investigated by measuring the temperature distributions along the heated vertical wall and visualizing the flow patterns in the cavity. The experiments were performed at a horizontal Grashof number of 1.9 × 10⁸. Increasing the top wall temperature resulted in a separated flow region on the top wall, which caused a secondary flow between the separated flow and the boundary layer on the heated vertical wall. This secondary flow had a significant effect on the heat transfer in this region. Changes in the top and bottom wall temperatures changed the temperature gradient and the average temperature of the air outside the thermal boundary layers in the cavity. The local heat transfer along much of the heated vertical wall could be correlated by Nu = C · Ra^0.32, but the constant C increased when the average of the top and bottom wall temperatures increased.     

Combustion and NOx emissions characteristics of a down-fired 660-MWe utility boiler retro-fitted with air-surrounding-fuel concept

Air-surrounding-fuel is a well-known concept used within tangential and wall-fired boilers. Here, we report for the first time on industrial experiments performed to study the effects of this concept on a 660 MW_e full-scale down-fired boiler. Data are reported for the gas temperature distributions along the primary air and coal-mixed flows, furnace temperatures, gas compositions, for example O₂, CO and NO_x, and gas temperatures in the near-wall region. The influence of concentration control valve (CCV) opening on combustion and NO_x emission in the furnace were determined. The results show that the flame stability, temperature distribution, unburnt carbon are influenced by both concentration ratios and fuel-rich flow velocities. As CCV opening increases, NO_x emissions decrease from 2594 mg/m³ to 1895 mg/m³. Considering altogether economic benefits and environmental protection issues, 30% is the optimal value for the CCV opening.     

Dehydrogenation characteristics of lean methane in a thermal reverse-flow reactor

In order to study the dehydrogenation reaction mechanism of ultra-low concentration methane in a thermal reverse-flow reactor, the effects of the cyclic period (120s–240s), the lean methane volume flow (90 Nm³/h to 180 Nm³/h), and the methane concentration (0.2 vol% to 0.8 vol%) on the dehydrogenation performance were studied systematically by using a thermal reverse-flow experimental system. When the methane concentration is 0.2 vol%, the reactor can achieve self-heat maintaining operation. With the increase in the methane concentration, the width of the high-temperature zone, the exhaust gas temperature, the methane conversion rate, and the maximum temperature of the heat-accumulator bed increase. With the increase in the lean methane volume flow, the width of the high-temperature zone, the distance between the center of the high-temperature zone and the center of the reactor, the maximum temperature, the exhaust gas temperature, and the methane conversion rate increase. With the increase in the cyclic period, the exhaust gas temperature and the deviation of the high-temperature zone increase, but the methane conversion rate and the maximum temperature decrease slightly.     

Ceramic/polymer interpenetrating networks exhibiting increased ionic conductivity with temperature control of ion conduction for thermal runaway protection

A novel polymer electrolyte nanostructure consisting of poly(ethylene oxide), PEO, complexed with lithium triflate placed in a nanoporous ceramic membrane was fabricated. The PEO electrolyte was placed in 200 nm pores of AAO filtration membranes and took the form of “sleeves” of polymer lining the 200 nm pores. The confinement of the electrolyte in this nanostructure somewhat increased the ion conduction of the polymer electrolyte compared to a non-confined PEO electrolyte. This increase was attributed to reduction in PEO crystallinity and polymer chain ordering resulting from interaction with the alumina walls of the nanoporous membranes. As expected, when the non-confined PEO sample was heated to the melting temperature, a large increase in ion conduction, resulting from the molten phase, was seen. Interestingly, the polymer confined in nanopores did not exhibit a large increase in conductivity. This was once again attributed to interaction with the alumina walls of the nanoporous membranes, hindering ion motion as compared to molten pure PEO polymer. The value to thermal runaway prevention of lower conductivities after going through the melt will be discussed. TGA studies indicate that the nanocomposite electrolyte is more thermally stable at high temperatures than pure PEO electrolyte. At 450 °C, 80% of the pure PEO was lost to thermal degradation while the nanocomposite electrolyte lost only 30% of its weight by this degradation process. This increased thermal stability was attributed to polymer confining wall interactions and a nanoscale passive thermal management system created by the heat absorbing AAO matrix and the transfer of heat to the gas in the open tubes of the polymer electrolyte.     

Production of hydrogen by simple impingement of a turbulent jet of steam upon a high temperature zirconia surface

The present paper describes the first results of experiments for the production of hydrogen by simple impingement of a turbulent jet of steam on a zirconia surface heated at the focus of an image furnace. Experiments were made by varying the target temperature (2200–2500 K), the steam flow rate and the nozzle to target distance. The reaction is likely to occur in a thin thermal boundary layer close to the surface or by a partially heterogeneous wall dissociation. The results compare favorably with those of other methods using secondary cold turbulent jets of gas for quenching: less steam is wasted (no quenching device) and the available energy for heating the reactants is better used (thermal boundary layer concept).     

PdCu membrane integration and lifetime in the production of hydrogen from methane

PdCu membranes prepared by sequential electroless plating were integrated into a hydrogen production and purification process. Hydrogen was produced from methane through catalytic partial oxidation and wet catalytic partial oxidation with Ni-based catalysts. Membrane permeance was measured with thermal cycles in an inert and hydrogen atmosphere at 673 and 773 K. Permeability was 1.98·10⁻³ mol/(smPa^0.5) at 673 K and 2.62·10⁻³ mol/(smPa^0.5) at 773 K. The optimum sweep gas flow required in the membrane module when operating with hydrogen-containing mixtures was selected. Peak hydrogen recovery was obtained using 15–20% of the feed to the module as sweep gas flow. Membranes were then placed downstream of the hydrogen production reactor. The CO and H₂O percentages fed to the membrane module did not have a major impact on membrane behavior. Around 60–67% of the hydrogen fed to the membrane module was separated, regardless of its composition.     

Experimental investigation of a comfort heating system for a passenger vehicle with an air-cooled engine

This study describes a novel approach utilizing waste heat from the exhaust gas for comfort heating of the passenger compartment of a vehicle with an air-cooled engine. In the devised system, a water stream heated by the hot exhaust gas was sent to the passenger compartment of a commercial minibus with an air-cooled engine, and the system was tested under various operating conditions. Variations of the temperatures at several locations inside the vehicle were monitored while ambient temperatures were −3, 0, 5 and 10 °C and there were various numbers of passengers on board. It is found that the system shows a reasonable heating performance while consuming no extra fuel for this purpose, and experimental data is in good agreement with numerical results based on heat loss calculations. Results show that when the ambient temperature is above 0 °C and the engine speed is above 2500 rpm, the system yielded comfortable compartment temperatures. Compared with alternative methods using extra fuel for comfort heating, the proposed system decreases vehicle operating costs and environmental pollution caused by the heating system as well as causing a lower global warming.     

The development of active vortex generators from shape memory alloys for the convective cooling of heated surfaces

A study of the convective heat transfer enhancement of heated surfaces through the use of active delta wing vortex generators is reported in this paper. The surface-mounted vortex generators (VGs) change their shape to intrude further into the flow at high temperatures to enhance heat transfer, while maintaining a low profile at low temperatures to minimise flow pressure losses. The VGs are made from shape memory alloys and manufactured in a selective laser melting process. Experiments have been carried out in a rectangular duct supplied with laminar-transition air flow. In the test section, a single, and a pair of active delta wing VGs were placed near the leading edge of a heated plate and tested separately for their heat transfer enhancement effects using infrared thermography. The pressure difference across the test section was also measured to determine the pressure drop penalty associated with the obstruction caused by the vortex generators in their active positions. Promising shape memory response was obtained from the active VG samples when their surface temperatures were varied from 20 °C to 65 °C. The vortex generators responded by increasing their angles of attack from 10° to 38° and as the designs were two-way trained, they regained their initial position and shape at a lower temperature. At their activated positions, maximum heat transfer improvements of up to 90% and 80% were achieved by the single and double wings respectively along the downstream direction. The flow pressure losses across the test section, when the wings were activated, increased between 7% and 63% of the losses at their de-activated positions, for the single and double VG respectively.