Low NOx Emissions Research Articles

Fuel-NO formation mechanism in MILD-oxy combustion of CH4/NH3 fuel blend

MILD-oxy combustion operates conventional oxy-combustion processes under moderate or intense low-oxygen dilution (MILD) condition, which offers a practical approach towards the synergetic control of CO2 and NOx emissions. This numerical work investigates the MILD combustion characteristics and NO emission of CH4/NH3 fuel blend under N2, CO2 and H2O dilutions, aiming at revealing the fuel-NO formation mechanism under MILD-oxy condition. It is found that relative to MILD-N2 combustion, the flame temperature is reduced under MILD-CO2 and MILD-H2O combustion, while the reaction zone is expanded and reduced, respectively, due to the altered co-flow properties. CO2 dilution lowers the radical pool level, while the formation of OH radical is boosted under H2O dilution. Overall NO emission (EINO) under MILD-N2 combustion is the largest while that under MILD-H2O generates the lowest value, showing its low-NOx emissions superiority. NH3 is converted into NO via routes NH3 → NH2 → HNO → NO and NH3 → NH2 → NH → HNO → NO under MILD-CO2 combustion. By contrast, the importance of route NH3 → NH2 → NH → N → NO in producing NO is largely strengthened under MILD-H2O combustion, wherein the direct formation of NO via HNO is inhibited with the enhanced availability of reactive N-containing species, contributing to lower NO emission under MILD-H2O combustion over MILD-CO2 combustion.

Impacts of global change on two tropical, high mountain lakes in Central Mexico

High mountain lakes and their catchments are remote ecosystems in areas with low anthropogenic disturbance. High mountain lakes integrate changes in the atmosphere and catchment areas (e.g., acid rain, airborne pollutants, climate change). The present research analyses long-term datasets of meteorological and limnological variables representing two tropical high mountain lakes, El Sol and La Luna, in Central Mexico to identify the impacts of anthropogenic disturbance (i.e., sentinels of global/climate change). The 54-year meteorological analysis showed marked interannual variability with no statistically significant air temperature or rainfall trends. However, from 2000 to 2018, the air temperature increased by 0.5 °C. Accordingly, the lake water temperature increased (Lake El Sol: 0.8 °C, Lake La Luna: 0.6 °C). Although the rainfall displayed no change, the water level decreased in both lakes (1.5 m), most likely associated with increased evapotranspiration. Unexpectedly, the dissolved inorganic nitrogen (DIN) concentration in the lakes decreased. The initial acid pH rose to close neutrality in Lake La Luna and to alkaline values in Lake El Sol. The latter may be a consequence of the lowered SOx and NOx emissions from governmental regulations promulgated to control atmospheric pollution beginning in 2000 and probably resulting in less acidic deposition. An additional explanation for the lakes' deacidification is the increased deposition of alkaline ions derived from activities at the volcano slopes. Since the atmospheric supply is the primary nitrogen source to high mountain lakes, the DIN concentration decline could reflect the reduction in atmospheric HNO3. Thus, Lakes El Sol and La Luna evidenced global change. Both lakes are inside the same crater and are subjected to similar influences; thus, they showed similar responses to global change (increasing lake water temperatures, declining water levels, higher pH value, and lower DIN concentrations). Nevertheless, their differences (e.g., catchment size, surface area, water volume, water depth, trophic status) influenced the magnitude of the impacts, with higher pH increases recorded in Lake El Sol and higher DIN concentrations in Lake La Luna.

Water-ethanol blending effects on combustion performance of methane-air premixed-flame burners

• Water and ethanol vapors were supplied to a methane-air premixed-flat-flame burner. • Flame stabilization of ceramic-honeycomb and metal-fiber burners were studied. • Simulated CO/NO emissions were compared and radiative heat transfer was examined. • Lifecycle CO 2 and NOx could be reduced with sufficient stability and efficiency. Recently, water injection into the recuperator of a condensing boiler was employed to achieve higher thermal efficiency and lower NOx emission simultaneously. In addition, biofuel blending was adopted to reduce the lifecycle CO 2 emission. In this study, injection of a water-biofuel mixture into a condensing boiler was proposed to provide both advantages simultaneously. Ethanol was selected as the surrogate of a water-miscible biofuel. As an essential step in its technical realization, the combustion performances of methane-air premixed-flame burners were examined with variations in the concentrations of methane, ethanol, and water. Flame stabilization characteristics were examined for two kinds of burners: a ceramic-honeycomb burner and a metal-fiber burner. The effects of water and ethanol blending on CO and NO emissions were examined. The product gas had an orangish light, and its emission spectra and emissive power were investigated. Conclusively, it was shown that blending water with a miscible biofuel (or ethanol) could reduce the lifecycle CO 2 emission and achieve the better combustion performances required for commercial premixed-flame burners of condensing boilers. A mixture of the same water and ethanol vapor volume could reduce NO by 70%, and ethanol blending with methane could increase radiative heat transfer from the product gas by approximately 20%.

Enhanced NOx reduction performance in the I-type radiant tube by combining a novel triple air-staging with flue gas recirculation

In this study, a novel triple air-staged combustion (ASC) with internal flue gas recirculation (FGR) was proposed to further achieve low NOx emissions and high combustion efficiency in the I-type radiant tube (RT). The effects of air vent structural parameters, including secondary air vent gap d (0, 1, 2, 3 mm) and diameter D (2, 4, 6, 8 mm) of tertiary air vents, on NOx emission and thermal efficiency were analyzed by numerical simulation. The results showed that NOx emissions increased from 33.7 to 66.0 mg/m3 as the secondary air vent gap changed from 0 to 3 mm. The thermal efficiency was higher than the original structure only at d=3 mm. When the tertiary air vent diameter increased from 2 to 8 mm, NOx increased from 49.3 to 180.4 mg/m3 and the thermal efficiency changed with a slight variation. On the premise of higher thermal efficiency than the original structure, the NOx at D=2 mm reached the minimum value of 49.3 mg/m3 under all optimized schemes, with a 64.0% reduction.

Chemical looping with oxygen uncoupling of hydrochar in a combined cycle for renewable and low-emission power generation

This work is concerned with design and analysis of a renewable power generation process featured with fast speed of biomass processing, low NOx and SOx emissions and ease of carbon capture. It is based on chemical looping with oxygen uncoupling (CLOU) of hydrochar derived from hydrothermal carbonization (HTC) of biomass. An air reactor and a fuel reactor employing CuO/Cu2O redox reactions are integrated with a combined Brayton and Rankine cycle to generate power. The design simulation is conducted using Aspen Plus V11. By implementing multiple-stage intercooled compression and multiple-stage reheat expansion in the Brayton cycle and multi-pressure steam generation in the Rankine cycle, an overall thermal efficiency of 35.3% on HHV basis without carbon capture (or 32.4% with carbon capture) is obtained.

Experimental study on co-firing characteristics of ammonia with pulverized coal in a staged combustion drop tube furnace

Utilizing ammonia as a co-firing fuel to replace amounts of fossil fuel seems a feasible solution to reduce carbon emissions in existing pulverized coal-fired power plants. However, there are some problems needed to be considered when treating ammonia as a fuel, such as low flame stability, low combustion efficiency, and high NOx emission. In this study, the co-firing characteristics of ammonia with pulverized coal are studied in a drop tube furnace with staged combustion strategy. Results showed that staged combustion would play a key role in reducing NOx emissions by reducing the production of char-NOx and fuel(NH3)-NOx simultaneously. Furthermore, the effects of different ammonia co-firing methods on the flue gas properties and unburned carbon contents were compared to achieve both efficient combustion and low NOx emission. It was found that when ammonia was injected into 300 mm downstream under the condition of 20% co-firing, lower NOx emission and unburnt carbon content than those of pure coal combustion can be achieved. This is probably caused by a combined effect of a high local equivalence ratio of NH3/air and the prominent denitration effect of NH3 in the vicinity of the NH3 downstream injection location. In addition, NOx emissions can be kept at approximately the same level as coal combustion when the co-firing ratio is below 30%. And the influence of reaction temperature on NOx emissions is closely associated with the denitration efficiency of the NH3. Almost no ammonia slip has been detected for any injection methods and co-firing ratio in the studied conditions. Thus, it can be confirmed that ammonia can be used as an alternative fuel to realize CO2 reduction without extensive retrofitting works. And the NOx emission can be reduced by producing a locally NH3 flame zone with a high equivalence ratio as well as ensuring adequate residence time.

Micro-Mixing Combustion for Highly Recuperated Gas Turbines: Effects of Inlet Temperature and Fuel Composition on Combustion Stability and NOx Emissions

Abstract The micromix combustion concept offers an elegant compromise between premixed and nonpremixed combustion. By mixing the fuel and air at the smallest scale possible, one can achieve NOx emissions comparable to premixed combustion while removing the risks of auto-ignition and flashback. Current literature reports multiple micromix designs that achieve low NOx emissions (<10 ppm) with hydrogen or hydrogen-rich fuels at combustor inlet temperatures (CIT) representative of low to medium pressure ratio gas turbines (<650 K). This paper seeks to bridge the gap between current literature and the design requirements of highly recuperated ceramic gas turbines, which should allow low NOx operation with various fuels at combustor inlet temperatures upwards of 900 K. To this end, micromix injection nozzles were designed and tested at small scale to investigate the effects of fuel composition and inlet temperature on combustion stability and NOx emissions. The nozzles were additively manufactured in Inconel 625 having hundreds of holes as small as 0.25 mm. An axial swirler is used to induce recirculation of the products behind the nozzle, which helps stabilize combustion with hydrocarbon fuels due to their longer reaction times and slower flame speeds. Experimental results show that NOx emissions can be decreased down to premixed levels if the jet Damköhler number is kept under a critical value, which requires increasingly smaller holes or higher jet velocities as the inlet temperature increases. Combustion instabilities are observed at low inlet temperatures with hydrocarbons, which are also correlated to the jet Damköhler number.

Combustion of lean ammonia-hydrogen fuel blends in a porous media burner

Ammonia offers a carbon-free fuel alternative for industrial heating, power generation and transportation. However, its low flame speed and high NOx emission represent significant challenges for combustor applications. The present study investigates the use of porous media burners (PMBs) to stabilize lean NH3/H2/air flames. By stabilizing the flame at the interface between two thermally conductive ceramic foams, internal heat recirculation preheats the reactants and increases the flame speed, which increases the combustion stability limits to very lean NH3/H2/air mixtures. Experimental studies are conducted to determine the stability limits of porous media combustion over a wide range of NH3/H2 volume ratios, equivalence ratios and mass fluxes. The NH3/H2 ratio is found to play a significant role with a typical dependency of the equivalence ratio at extinction in the form ϕext∝XH2−0.3. NOx emissions were shown to reach their minimum when the burner was operated near its extinction limit and to increase with increasing mass flux rates. Because unburnt NH3 emissions were also quite low (1400 ppm) at these very lean conditions, this operating regime offers similar or better emissions compared to the commonly employed fuel-rich operation which has been suggested in the literature.

A critical review of the impacts of split injection strategies on the performance, combustion and emissions characteristics of a diesel engine fueled with low carbon alcohol fuels

This paper reviews the impacts of split (pilot/main) injection tactics on the exhaust pollutants, performance, and combustion of compression ignition (CI) engines operated with low carbon alcohols fuels. For the past several years, scholars have been attempting to simultaneously reduce NOx and soot emissions by enhancing combustion, which is accomplished by optimizing the fuel injection technique by using high amount of EGR or low carbon alcohols fuels. Split injection tactics are used in the diesel engines to accomplish the partially premixed combustion (PPC). The prolonged ignition delay (ID) in PPCI engines improves air/fuel mixing, resulting in lower soot and NOx emissions. Furthermore, due to the more substantial charge cooling effect of low carbon alcohol fuels reduce the combustion temperature, resulting in lower NOX emissions. Thus, combining a split injection approach with low-carbon alcohol fuels in a modern CI engine might be a promising option to satisfy stringent exhaust pollution regulations as well as reducing dependency on fossil fuels. The present paper focuses on the work done in the area of split injection strategies with various low-carbon alcohol fuels and the effects of injection pressures on engine performance.

Formulation of optimal bioenergy mixtures from phototrophic and heterotrophic cultures of S. quadricauda and C. pyrenoidosa microalgal strains

The goal of this research is to produce bio-alcohol and biodiesel from S. quadricauda and C. pyrenoidosa microalgae in a sustainable manner using phototrophic and heterotrophic pathways. Experimental and statistical approaches are used to assess the viability of the bioenergy produced as a prospective alternative to diesel fuel. On analysis, it was observed that the %age weight of the dry cell of C. pyrenoidosa cultures on scaling-up resulted lipid content of 44.1%, 46.7%, and 41%, respectively. On scaling up, the cultures’ biomass concentrations reached 5.15 g L −1, 4.24 g L −1, and 4.36 g L −1. Similarly, upscaling S. quadricauda cultures resulted in yield rates of 4.2 g L −1 and 2.96 g L −1 d −1 and a lipid productivity of 2.96 g L −1 d −1, respectively. On engine testing, AA20D80 and AA30D70 blend combinations shows an improvement of 4.84% and 6.9% in BTE with a reduction of 88.6% and 72.41% in fuel consumption. Similarly, as compared to amyl alcohol blends and diesel fuel at full load, CP20D80 and CP30D70 efficiently lower HC, NOx, and smoke emissions. The statistical analysis revealed that the similarity index of the 20% and 30% proportions were ideal in achieving optimal engine output characteristics.

Performance analysis of biomass gasification coupled with ultra-supercritical power generation system

In order to alleviate the impact of coal combustion on the environment, a scheme of biomass gasification in coal-fired ultra-supercritical power plant is proposed and simulated with Aspen plus. Simulation results show that the energy and exergy efficiencies of the coupled system have an increasing-decreasing tendency with the increase of the air/biomass ratio, reaching the maximum value when the air/biomass ratio is 1.6. The energy and exergy efficiencies of the coupled system decrease continuously with the increase of the excess air ratio. The coupled system has the highest energy and exergy efficiencies of 46.89% and 43.13%, which are 2.70% and 1.81% higher than those of an ultra-supercritical coal-fired system, respectively. Meanwhile, the coupled system has low CO2, SO2, and NOx emissions and thus many advantages in terms of environmental performance.

Experimental study and kinetic analysis of the impact of ammonia co-firing ratio on products formation characteristics in ammonia/coal co-firing process

Ammonia is a carbon-free, hydrogen-rich fuel, which is considered as an alternative energy source with high utilization potential. Ammonia/coal co-firing in air-staged coal-fired boilers has attracted the attention of a number of academics in order to meet the aim of low carbon and NOx emissions. In this paper, ammonia/coal co-firing experiments were conducted under air-staged condition in a one-dimensional combustion-temperature controlled down-fired furnace with multi-path air inlets at 1100℃. The influence of ammonia co-firing ratio on NO and CO2 emissions, as well as CO and H2S release characteristics, was studied using Shenhua bituminous coal in air-staged and non-staged combustion conditions. Chemical reaction kinetic calculations were conducted to provide a qualitative analysis of the experimental results. Ammonia plays a complex role in the ammonia/coal co-firing process. An effective CO2 reduction was achieved with ammonia co-firing, and it was found that air-staged combustion is also effective for NO emission control in the ammonia/coal co-firing process. Considering the economy issue and NO emission control, 20–30 % ammonia co-firing ratio is recommended. However, ammonia/coal co-firing can greatly promote the coal gasification reaction, which leads to a higher concentration of CO in the reduction zone in air-staged combustion, compared with coal combustion. Meanwhile, ammonia co-firing has a significant inhibitory effect on the release of H2S from coal and will promote the subsequent decomposition process. This study explains the NO release pattern in ammonia/coal co-firing processes based on experimental results and chemical kinetics calculations, which provides a promising technical advancement direction for the coal-fired power plants to achieve a signification reduction of both CO2 and NO emissions.

A Numerical Study of Decarbonizing Marine Gas Turbine Emissions Through Ammonia/Hydrogen Fuel Blends

The employment of gas turbine in combination with diesel engines and steam generators is a well-known power generation technique in modern marines and ship propulsion. Previously, it rendered its foundations in marine industry through higher power weight ratios and lower NOx emissions if compared to pure diesel engine driven marines. As climate change concerns are becoming more serious, the decarbonization of marine combustion products is becoming of environmental concern. In the present study a modified design of the burner and combustor was suggested to allow for the longer residence time required for releasing the combustion products from the ‘slow’ burning ammonia molecule. Afterwards, the more formidable challenge of relatively higher NOx emissions was treated through analysis of the effect of altering the equivalence ratio, hydrogen blending, increasing the combustor working pressure and staging the combustion. The latest tactic resulted in lowering values of exit NOx to around 30 ppmv, which is a quite promising result.

Experimental investigation on controlling of airflow trajectories and flow-field of down-fired boiler by adding on arch secondary air

Abstract To effectively control the flow and combustion of pulverised coal in down-fired boilers and improve their thermal efficiency while ensuring low NO x emissions, on arch secondary air (OASA) combustion technology and a smoke-tracing analysis were proposed. The aerodynamic characteristics of a 300-MWe Foster Wheeler (FW) down-fired boiler with the OASA ratios (R OASA) of 0%, 10%, 20%, and 30%, the flow trajectories and mixing characteristics of OASA, and fuel-rich coal/air flow (FRA) and fuel-lean coal/air flow (FLA) were studied experimentally. For the R OASA of <10%, FRA and FLA flow to furnace centre soon after they are injected into the furnace. OASA and FLA may mix with FRA before ignition. For the R OASA of 20% or higher, FRA and FLA flow to the near-wall area soon after being injected into the furnace. FRA mixes with OASA after ignition. With the increase in R OASA, the mixing distance between OASA and FRA increases first and then decreases. The penetration depth of coal/air flow and furnace airflow fullness increases. For the R OASA of 30%, the dimensionless penetration depth of coal/air flow can exceed 0.7. Furnace dimensionless airflow fullness at all the dimensionless heights can exceed 0.6. In real operation, the recommended R OASA is between 20 and 30%.

Uniqueness and similarity in flame propagation of pre-dissociated NH3 + air and NH3 + H2 + air mixtures: An experimental and modelling study

Ammonia (NH3) has attracted significant attention as a promising hydrogen carrier and a carbon-free alternative fuel. Partial dissociation could convert part of ammonia to H2 and N2 before injecting the fuel into a combustor, thus overcoming the low reactivity and high NOx emission problems during the NH3 combustion. The pre-dissociated NH3 + air mixture has unburnt species NH3, H2, O2, and N2, the same as more widely investigated NH3 + H2 + air flames, while similarities or differences between these two types of flames have not yet been investigated. In the present work, the laminar burning velocities of pre-dissociated NH3 + air flames at 1 atm and an initial temperature of 298 K have been measured and compared to the scarce data from the literature. Experiments were carried out using the heat flux method at varied dissociation ratio γ and equivalence ratio ϕ. Kinetic simulations were also performed using six recently published or updated mechanisms, while none of the tested mechanisms can accurately reproduce the present results for the pre-dissociated NH3 + air flames over the whole range of the covered conditions, even for those predicting well the NH3 + H2 + air flames. To understand this deficiency, flame temperatures for the two fuel systems were examined, as well as in-depth sensitivity analyses were carried out. Similar conditions between the pre-dissociated NH3 + air and the NH3 + H2 + air flames were found, and a new approach to identifying inconsistent experimental data obtained using the same experimental setup was also suggested and discussed.

Numerical Study of the Effects of Lambda and Injection Timing on RCCI Combustion Mode

Reactivity Controlled Compression Ignition (RCCI), which is a low temperature combustion mode based on the principle of working with dual fuel, has attracted the attention of researchers in recent years due to its advantages such as high thermal efficiency, low NOx and soot emissions, and controllability of combustion. In this study, the effects of injection timing and lambda on RCCI combustion mode were investigated numerically by validating the experimental data with Converge CFD software. A four-cylinder, four-stroke gasoline direct-injection engine with a compression ratio of 9.2 was used in RCCI combustion mode at an engine speed of 1000 rpm. The maximum cylinder pressure also increased and RCCI combustion was advanced while the injection timing was advanced. The highest peak pressure was obtained at SOI=-50°CA aTDC, and the lowest peak pressure was obtained at SOI=-25°CA aTDC. Similarly, the highest peak HRR value was acquired as 213 J/°CA at SOI=-50°CA aTDC. It has been observed that as the lambda decreases, the maximum cylinder pressure increases, and combustion advances. In addition, the heat release rises with a decrease in lambda value. The maximum heat release rate was acquired as 77.91 J/°CA at λ=1.2. The results show that injection timing and lambda have a great influence on RCCI combustion mode and the combustion phase can be controlled with these parameters.

Failures Mapping for Aircraft Electrical Actuation System Health Management

This paper presents the different types of failure that may occur in flight control electrical actuation systems. Within an aircraft, actuation systems are essential to deliver physical actions. Large actuators operate the landing gears and small actuators adjust passenger seats. As developing, aircraft systems have become more electrical to reduce the weight and complexity of hydraulic circuits, which could improve fuel efficiency and lower NOx emissions. Electrical Actuation (EA) are one of those newly electrified systems. It can be categorized into two types, Electro-Hydraulic Actuation (EHA) and Electro-Mechanical Actuation (EMA) systems. Emerging electric and hydrogen fuel aircraft will rely on all-electric actuation. While electrical actuation seems simpler than hydraulic at the systems level, the subsystems and components are more varied and complex. The aim of the overall project is to develop a highly representative Digital Twin (DT) for predictive maintenance of electrical flight control systems. A comprehensive understanding of actuation system failure characteristics is fundamental for effective design and maintenance. This research focuses on the flight control systems including the ailerons, rudders, flaps, spoilers, and related systems. The study uses the Cranfield University Boeing 737 as the basis to elaborate the different types of actuators in the flight control system. The Aircraft Maintenance Manual (AMM) provides a baseline for current maintenance practices, effort, and costs. Equivalent EHA and EMA to replace the 737 systems are evaluated. In this paper, the components and their failure characteristics are elaborated in a matrix. The approach to model these characteristics in DT for aircraft flight control system health management is discussed. This paper contributes to the design, operation and support of aircraft systems.

Experimental evaluation of performance of heavy-duty SI pure methanol engine with EGR

Because methanol can be synthesized from captured CO2 and green H2, it is a promising alternative fuel for reducing carbon emissions from heavy-duty vehicles. Experiments were conducted on a multi-cylinder port fuel injection spark-ignition (SI) heavy-duty pure methanol engine modified from a natural gas engine. The main objective was to evaluate the performance of a heavy-duty SI pure methanol engine and its improvement over the original natural gas engine. The effects of the exhaust gas recirculation (EGR) rate and spark timing on the brake thermal efficiency (BTE), as well as the NOx, CO and HC emissions, were investigated under a wide range of operating conditions. The experimental results indicated that the pure methanol engine had a BTE of > 40% BTE in a brake mean effective pressure (BMEP) range of 1.27–2.17 MPa at speeds ranging from 1000 to 1700 rpm and that the peak BTE was ∼ 41.4%, which was higher than that of the original natural gas engine by an absolute 3%. The EGR tolerance of methanol engine was high and the coefficient of variation of the indicated mean effective pressure (COVIMEP) could be maintained below 2%, even at 31% EGR. With an increase in the EGR rate, the CO emissions first decreased and then increased. The partial oxidation of unburned methanol contributed to higher CO emissions when the EGR rate reached a critical value. Advancing the spark timing increased the HC emissions at a low EGR because of the lower post-oxidation temperature; however, it reduced the HC emissions at a high EGR rate because of the more stable combustion. At a BMEP above 0.87 MPa, the NOx, HC and CO emissions ranged from 0.055 to 1.185 g/kWh, from 2.4 to 13.05 g/kWh, and from 6.36 to 10.88 g/kWh, respectively. The pure methanol engine had lower NOx and CO emissions but higher HC emissions than the original natural gas engine.

Performance, combustion and emission characteristics of a single-cylinder DI diesel engine fuelled with lotus biodiesel-diesel-n-butanol blends

This present experimental work deals with the investigation of lotus biodiesel with different blends in addition to n-butanol as an ignition improver. The blends start with pure diesel (D100), D90L8B2, D80L16B4, D70L24B6 and 6% n-butanol and L100. The experimental results observed at full load condition (100%) the fuel D70L24B6; achieved the fuel efficiency by 28%, whereas D70L24B6 attained the lowest BSFC; decrease in BSFC of 7%. The in-cylinder pressure was 73 bar for the fuel D70L24B6, After the fuel D100, the maximum HRR was 84.1 kJ for the fuel D70L24B6. The lowest CO emissions were found for the fuel L100, which decreases CO emissions by 17.2% compared to D100. The lowest CO2 emissions were found for the fuel D90L8B2 fuel, a decrease of 5.49% compared to D100. The lowest HC emissions were found for the fuel L100, a decrease in HC emission rate by 16.1% compared to pure diesel. The lowest NOx emissions were found for the fuel L100, but there is an increase in NOx emission rate by 8% compared to D100, because of higher oxygen content in biodiesel.

Almond Shells and Exhausted Olive Cake as Fuels for Biomass Domestic Boilers: Optimization, Performance and Pollutant Emissions

The combustion of two non-woody types of biomass (almond shells and exhausted olive cake) in a domestic boiler at different loads was studied in order to evaluate their suitability as fuels. To select the optimal boiler operating conditions (excess air, primary/secondary air ratio and grate vibration), which allows for lower CO and particulate matter emissions for each biomass and load, a statistical design of experiments was performed. Similar optimal operating conditions were found for both fuels at nominal load (excess air: 1.5, primary/secondary air ratio: 20/80), the grate vibration being the only parameter to be modified due to the different ash content (45 and 20 s for almond shells and exhausted olive cake, respectively). At partial load, a slightly higher excess air (1.6) and a higher proportion of primary air (50/50) were needed in the case of almond shells. Results showed higher CO and lower NOx and PM emissions at partial load for both fuels. The high ash content of exhausted olive cake deteriorated its combustion process (accumulated ashes were observed in the fireplace). Gaseous and solid emissions did not fulfil the UNE-EN 303-5 limits for any fuel or condition; although, almond shells seem to be a much more suitable fuel since they could be used just blended with a small quantity of a high-quality biomass or additive. However, exhausted olive cake not only led to a very poor efficiency at partial load (74%), clearly below the minimum required by the standard (77%), but also to an unacceptable pollutant emission level. So, this latter fuel would require a high blending ratio with another type of biomass, pre-treatments for reducing the alkali compounds and/or significant technological modifications allowing for a proper ash handling.