Natural Gas Air Research Articles

Experimental and numerical study of multiple injection effects on combustion and emission characteristics of natural gas–diesel dual-fuel engine

Natural gas–diesel dual-fuel engines can significantly reduce soot emissions and mitigate the oil crisis. However, some drawbacks exist including low thermal efficiency, as well as high methane and carbon monoxide emissions. Multiple injection plays an important role in improving the performance and reducing emissions in dual-fuel engines. This paper presents an experimental and numerical study of multiple injection effects on the combustion and emission characteristics of a natural gas–diesel dual-fuel engine at low load condition. The results showed that, when the first diesel injection timing is advanced, the indicated thermal efficiency and nitrogen oxide emissions increased first and then decreased, while the maximum pressure rise rate, carbon monoxide and methane emissions reduced initially and then increased. When the first diesel injection ratio increased, combustion rate of natural gas–air mixture increased, methane and carbon monoxide emission decreased, and nitrogen oxide emission increased. When the first diesel injection timing was relatively rearward, the combustion process showed a distinct four-stage heat release characteristic. The proportion of premixed combustion of the first injected diesel and the duration of flame propagation of natural gas had considerable influence on methane emission. When the first diesel injection timing was relatively forward, the first injected diesel played a premixed role, and the start of combustion was controlled by the second injected diesel. A relatively high indicated thermal efficiency can be obtained under this injection strategy. The methane was mainly distributed in the cylinder wall and crevice area at exhaust valve opening due to the limitation of flame propagation under the multiple injection strategy.

Environmental Characteristics of Infrared Burners with a Catalytic Radiation Screen

The possibility of providing an appreciable decrease in carbon monoxide concentrations in products due to the combustion of natural gas-air mixtures in infrared burners by installing a radiation gauze screen with a catalytic Pd-Ce/Al2O3 coating over the matrix is demonstrated. When permeable matrices with radiation gauze screens with and without catalytic coatings are used, the concentration of nitrogen oxides in combustion products was less than 20 ppm within a range of specific combustion powers from 20 to 100 W/cm2. The use of a screen with a catalytic coating provides record low CO concentrations in combustion products at a level of 3–5 ppm within a range of specific combustion powers from 30 to 60 W/cm2. The catalytic coating on the gauze screen that is developed retains its serviceability through multiple cycles of heating to temperatures of 1400 K.

Effect of diesel injection timing on the combustion of natural gas/diesel dual-fuel engine at low-high load and low-high speed conditions

Past research has shown that advancing diesel injection timing is a promising approach to decrease the unburned methane and greenhouse gas (GHG) emissions of natural gas/diesel dual-fuel engines at lower engine loads. However, this benefit may not persist under medium to high load-low speed conditions. To explore this, the present paper uses experiments and detailed computational fluid dynamic (CFD) modeling to investigate the impacts of diesel injection timing on the combustion and emissions performance of a heavy-duty natural gas/diesel dual-fuel engine under four different engine load-speed conditions. The results showed that advancing diesel injection timing increases the peak pressure, thermal efficiency, and NOx emissions for all examined engine load-speed conditions. Advancing diesel injection timing also significantly decreases the unburned methane and CO2-equivalent (GHG) emissions of the dual-fuel engine under low load-low speed and medium load-high speed conditions. The concentration of OH and CH4 revealed that the central part of the combustion chamber is the main source of the unburned methane emissions under low load-low speed and medium load-high speed conditions, and advancing diesel injection timing significantly improves the combustion of natural gas-air mixture in this region. However, advancing diesel injection timing slightly increases the unburned methane emissions trapped in the crevice volume. However, this slight increase in the unburned methane emissions in the crevice volume is much lower than its significant decrease in the central region of the combustion chamber. At medium to high load-low speed conditions, there is almost no unburned methane in the central part of the combustion chamber, and the crevice region is considered as the main source of unburned methane emissions. As a result, advancing diesel injection timing does not improve the combustion of natural gas-air mixture in the central part of the combustion chamber but slightly increases the unburned methane trapped in the crevice region. This is the main reason that advancing diesel injection timing slightly increases the unburned methane emissions under medium to high load-low speed conditions. Overall, advancing diesel injection timing significantly increases thermal efficiency and decreases the unburned methane and GHG emissions under low load-low speed and medium load-high speed conditions. It improves the thermal efficiency under medium to high load-low speed conditions, but comes at the expense of increased methane and unchanged GHG emissions.

Influence of Injection Parameters and Operating Conditions on Ignition and Combustion in Dual-Fuel Engines

Dual-fuel (DF) engines offer great fuel flexibility since they can either run on gaseous or liquid fuels. In the case of diesel pilot-ignited DF engines, the main source of energy is provided by gaseous fuel, whereas the diesel fuel acts only as an ignition source. Therefore, a proper auto-ignition of the pilot fuel is of utmost importance for combustion in DF engines. However, auto-ignition of the pilot fuel suffers from lower compression temperatures of Miller or Atkinson valve timings. These valve timings are applied to increase efficiency and lower nitrogen oxide (NOx) engine emissions. In order to improve the ignition, it is necessary to understand which parameters influence the ignition in DF engines. For this purpose, experiments were conducted and the influence of parameters, such as injection pressure, pilot fuel quantity, compression temperature, and air–fuel (A/F) equivalence ratio of the homogenous natural gas–air mixture were investigated. The experiments were performed on a periodically chargeable combustion cell using optical high-speed recordings and thermodynamic measurement techniques for pressure and temperature. The study reveals that the quality of the diesel pilot ignition in terms of short ignition delay and a high number of ignited sprays significantly depends on the injection parameters and operating conditions. In most cases, the pilot fuel suffers from too high dilution due to its small quantity and long ignition delays. This results in a small number of ignited sprays and consequently leads to longer combustion durations. Furthermore, the experiments confirm that the natural gas of the background mixture influences the auto-ignition of the diesel pilot oil.

REDUCTION OF NITROGEN OXIDES EFFLUENTS WITHIN AN OPEN FLAME BY INPUT THE WATER VAPOUR INTO COMBUSTION AIR FLOW

The state-of-the art the problem concerning reduction the NOx formation by humidification the combustion air is briefly consideredan, alized and discussed. The proper technologies of combustion the wet inflammable mixtures are considered for the cases of using the water vapour pump (WVP), Maisotsenko cycle (M-cycle) and suitable facilities along with the cases of operation the combined systems with heat recovery due transfer the water vapour energy to working medium (combustion products). Both environmental and power advantages of the systems of «wet combustion» have been demonstrated accordingly data collected in some earlier researches. The results of experimental researches of burning process under conditions of input the water vapour into the gas–air mixture to be supplied for combustion are presented in the paper in frame of consideration the results of own authors’ tests. Given information concerning the investigation of environmental consequences of impact the water vapour within the inflammable mixture on nitrogen oxides NOx value in combustion products ([NOx] concentration) has been generalized. The techniques of experimental researches, the design of firing rig and instrumentation, used in the tests have been described. The distinction of ignition the natural gas-air mixture and of stable flame maintenance for case of free vertical flame operation have been evaluated in dependence on inflammable mixture’s composition along with studying of impact a moisture content — H2O concentration in initial gas-air mixture. Comparative evaluations of NO value within an open flame in dependence on temperature and on composition of initial natural gas — air mixture (ratio «air : gas») have been carried out. An opportunity of two-times reduction of maximum NOmax concentration within a flame by initial gas-air mixture enrichment with water vapour has been proven. Bibl. 23, Fig.10, Tab.1.

INFLUENCE OF THE METHOD OF NATURAL GAS SUPPLYING ON GAS DYNAMICS AND HEAT TRANSFER IN AIR TUYERE OF BLAST FURNACE

The use of natural gas reduces the amount of coke to produce pig iron. The flow of hot blast pushes natural gas to the surface of blowing channel in the conventional tuyere. Natural gas is poorly mixed with natural gas. This causes incomplete natural gas combustion and pyrolysis. Therefore, the problem of the completeness of combustion of natural gas is relevant. One way to improve the mixing of natural gas and hot blast is the gas pipe in the blow channel. However, dynamics and change of the thermal state of the tuyere are understudied for this option. The possibility of ignition of natural gas inside tuyeres must also be taken into account. The authors have investigated the inf­luence of the method of natural gas supplying on gas dynamics and heat transfer in air tuyere of a blast furnace with the help of modeling in ANSYS Fluent 15.0.7. Simplifying assumptions were adopted. Only fluid inside the blowing channel is considered as the modeling zone, and the processes of heat transfer to the water of cooling system are considered in the extended boundary conditions. A simplified diagram of the computational domain was created in DesignModeler and a computational mesh – in AnsysMeshing. The boundary conditions were set for blowing, natural gas, and also for the border of the fluid with copper walls. The calculations were carried out for half of the tuyere. It is shown that under the given conditions of flow of air and natural gas, combustion inside tuyere with extended to mid-channel gas blowing nozzle does not occur, and natural gas is mixed with the hot air. Improving the mixing of natural gas and hot air, one side, reduces heat flow at the exit of the blowing channel and the average temperature of the gas mixture, on the other side, creates conditions for complete combustion of natural gas outside the tuyere.

Modeling of Thermal Behavior of Raw Natural Gas Air Coolers

When gas is being prepared for a long-range transportation, it passes through air cooling units (ACUs) after compressing; there, hot gas passing through finned tubes is cooled with air streams. ACU's mode of operation shall ensure a certain value of gas temperature at the ACU's outlet. At that, when cooling raw gas, temperature distribution along all the tubes shall be known to prevent local hydrate formation. The paper proposes a mathematical model allowing one to obtain a thermal field distribution inside the ACU and study influence of various factors onto it.

Modeling of Hydrate Formation Mode in Raw Natural Gas Air Coolers

Air cooling units (ACU) are used at all the gas fields for cooling natural gas after compressing. When using ACUs on raw (wet) gas in a low temperature condition, there is a danger of hydrate plug formation in the heat exchanging tubes of the ACU. To predict possible hydrate formation, a mathematical model of the air cooler thermal behavior used in the control system shall adequately calculate not only gas temperature at the cooler's outlet, but also a dew point value, a temperature at which condensation, as well as the gas hydrate formation point, onsets. This paper proposes a mathematical model allowing one to determine the pressure in the air cooler which makes hydrate formation for a given gas composition possible.

Determining Total Radiative Intensity in Combustion Gases Using an Optical Measurement

A method is presented whereby spectral radiation measurements made in a combustion flue gas can be used with a spectral gas absorption model to calculate gas temperature, H2O concentration, CO2 concentration, and total radiation intensity for the gas. Measured spectral intensities from a natural gas-air flame in a 150 kWth furnace were used in conjunction with a spectral gas absorption model to calculate gas temperature and H2O concentration. The measured spectral intensities matched spectral intensities predicted by a one-dimensional intensity model when peaks were shifted and convolved to account for FTIR biases. On the basis of a successful prediction of intensities in the measured range of 1.709–2.128 μm, the calibrated intensity model was used to predict intensities for various wavelength bands including H2O and CO2 relative contributions. The total intensity for a wavelength range of 1–50 μm for the conditions studied was 10 659 W/m2/sr, with an equivalent total gas emissivity of 0.163.

A numerical investigation on NO2 formation reaction pathway in a natural gas–diesel dual fuel engine

This paper numerically investigated the NO2 formation pathway in a natural gas (NG)–diesel dual fuel engine using a computational fluid dynamics (CFD) model CONVERGE. The fuel chemistry coupled was a reduced primary reference fuel (PRF) mechanism consisting of 45 species and 142 reactions including the NOx mechanism from GRI chemistry. The NO2 formation pathway was investigated by examining the rate of production (ROP) of key species dominating NO2 formation in each cell. The ROP of the key species was further processed to derive the representative creation reactions (RCR) of NO2 (RCRNO2). The simulation revealed that the NO2 produced during main combustion stage was formed in the hot combustion products of n-heptane spray through reaction pathway C7H15 → CH2O → HCO → H → O → NO2 and the interface between the hot combustion products and NG–air mixture dominated by the HO2 produced through reaction pathway CH4 → CH3 → CH2O → HCO → HO2. The NO2 formed in hot combustion products during main combustion stage was later on destructed to NO and was not able to survive through the expansion process. In comparison, the NO2 formed during the post combustion expansion process was dominated by HO2 radical formed in the interface between the NO-containing combustion products and unburned NG–air mixture. It was concluded that the increased conversion from NO to NO2 in a NG–diesel dual fuel engine was due to the increased HO2 produced through the reaction path: CH4 → CH3 → CH2O → HCO → HO2 in the post combustion stage. The availability of methane necessary for the production of HO2 after the completion of the main combustion process was the main factor contributing to the significantly increased NO2 emissions from NG–diesel dual fuel engines. The research aiming at reducing NO2 emissions from dual fuel engines should focus on the approaches capable of significantly improving the combustion efficiency of NG.

Numerical and wind tunnel investigation of Hot Air Recirculation across Liquefied Natural Gas Air Cooled Heat Exchangers

Air Cooled Heat Exchangers (ACHE)s are used for heat rejection in Liquefied Natural Gas (LNG) plants. Their thermal performance decreases under elevated ambient temperatures and windy conditions as exhaust air recirculates back into the ACHE units causing Hot Air Recirculation (HAR). This paper investigates the effect of various incident wind speed and directions on HAR. Understanding HAR helps to avoid undesirable work conditions. Three dimensional Computational Fluid Dynamics (CFD) studies were utilized to simulate the airflow around a full scale LNG plant. The simulations clearly captured HAR between parallel ACHE banks (Cross-HAR) and within a single ACHE unit (Self-HAR). Wind tunnel smoke visualization was used to qualitatively assess the CFD model. The impact of removing downstream ACHE protection screens on the HAR was computationally investigated showing an exhaust air temperature reduction of 5 °C. Different HAR mitigation methods were proposed using different configurations of side winglets. Horizontal winglets were shown to be more effective than vertical winglets as they decreased intake and exhaust air temperatures by about 5 °C and 8 °C respectively and increased exhaust air velocity by 1.2 m/s. This work provides a thorough understanding of HAR around ACHEs and proposes mitigation methods to reduce its effects on plant production.

Design of Compressed Natural Gas-Air Mixer for Dual Fuel Engine Using Three-Dimensional Computational Fluid Dynamics Modeling

In diesel—compressed natural gas (CNG) dual fuel systems, the CNG is generally inducted into the intake manifold by a CNG mixer mounted at the intake manifold, while the diesel fuel is directly injected into the engine cylinder using a diesel fuel injector system. The poor mixing performance of gaseous mixers is among the causes of unsatisfactory engine performance and lethal exhaust emissions. Based on an existing mixer model, four different models of mixers with 29 cases were created in this study to investigate the effects of the diameter, location, and number of holes inside the existing mixer on the homogeneity and distribution of the mixture. A computational fluid dynamics analysis software was used to check the flow behavior of the CNG and air inside the existing and new mixer models, with the new model being fixed on a 3.2 L engine. These models were examined depending on the maximum speed of the engine (4000 rpm), the full-opened valve, and the stoichiometric air–fuel ratio (34.6). Compared with the new mixer models, the existing mixer model shows a non-uniform methane and air distribution. Model 4/case 26 shows a uniform distribution of the CNG-air mixture with the best homogeneity. This model was then examined to check the flow characteristics of CNG and air at different engine speeds (1000, 2000, 3000, and 4000 rpm). Model 4/case 26 also shows a stoichiometric air–fuel ratio depending on the engine speed.

Assessment of Finite Rate Chemistry Large Eddy Simulation Combustion Models

Large Eddy Simulations (LES) of a swirl-stabilized natural gas-air flame in a laboratory gas turbine combustor is performed using six different LES combustion models to provide a head-to-head comparative study. More specifically, six finite rate chemistry models, including the thickened flame model, the partially stirred reactor model, the approximate deconvolution model and the stochastic fields model have been studied. The LES predictions are compared against experimental data including velocity, temperature and major species concentrations measured using Particle Image Velocimetry (PIV), OH Planar Laser-Induced Fluorescence (OH-PLIF), OH chemiluminescence imaging and one-dimensional laser Raman scattering. Based on previous results a skeletal methane-air reaction mechanism based on the well-known Smooke and Giovangigli mechanism was used in this work. Two computational grids of about 7 and 56 million cells, respectively, are used to quantify the influence of grid resolution. The overall flow and flame structures appear similar for all LES combustion models studied and agree well with experimental still and video images. Takeno flame index and chemical explosives mode analysis suggest that the flame is premixed and resides within the thin reaction zone. The LES results show good agreement with the experimental data for the axial velocity, temperature and major species, but differences due to the choice of LES combustion model are observed and discussed. Furthermore, the intrinsic flame structure and the flame dynamics are similarly predicted by all LES combustion models examined. Within this range of models, there is no strong case for deciding which model performs the best.

Experimental study on a comparison of typical premixed combustible gas-air flame propagation in a horizontal rectangular closed duct

Research surrounding premixed flame propagation in ducts has a history of more than one hundred years. Most previous studies focus on the tulip flame formation and flame acceleration in pure gas fuel-air flame. However, the premixed natural gas-air flame may show different behaviors and pressure dynamics due to its unique composition. Natural gas, methane and acetylene are chosen here to conduct a comparison study on different flame behaviors and pressure dynamics, and to explore the influence of different compositions on premixed flame dynamics. The characteristics of flame front and pressure dynamics are recorded using high-speed schlieren photography and a pressure transducer, respectively. The results indicate that the compositions of the gas mixture greatly influence flame behaviors and pressure. Acetylene has the fastest flame tip speed and the highest pressure, while natural gas has a faster flame tip speed and higher pressure than methane. The Bychkov theory for predicting the flame skirt motion is verified, and the results indicate that the experimental data coincide well with theory in the case of equivalence ratios close to 1.00. Moreover, the Bychkov theory is able to predict flame skirt motion for acetylene, even outside of the best suitable expansion ratio range of 6<E<8.

Modeling study of the acceleration of ignition in ethane–air and natural gas–air mixtures via photochemical excitation of oxygen molecules

The paper addresses the study of the influence of laser-induced excitation of O2 molecules to the b1Δg+ electronic state and their photodissociation by laser photons on the ignition of ethane and natural gas in air. The extended reaction mechanism for the oxidation of CH4–C2H6–air mixture comprising excited O2(a1Δg), O2(b1Σg+) molecules and O(1D) atoms has been built. This mechanism includes recent ab initio data on the interaction of C2H6 with O2(a1Δg) molecule. The cross section of laser radiation absorption at given wavelength was calculated in a detailed manner by taking into account the effects of temperature, pressure and contribution of several nearest spectral lines. The computations have shown that both considered photochemical methods accelerated the ignition and decreased the ignition temperature. They are much more effective in the ignition enhancement than mere heating the mixture by equivalent input energy. It has been revealed that the photodissociation of O2 molecules by laser photons with λI = 193.3nm was somewhat more effective in accelerating the ignition of the C2H6–air mixture, especially, at low temperatures (T0 < 850K) and high pressure (P0 = 10atm). However, at smaller pressure (P0 = 1atm) both for ethane and for natural gas, the excitation of O2 molecules to the singlet sigma state provides higher reduction in the induction time and ignition temperature compared to their photodissociation.

Flame Holding in the Premixing Zone of a Gas Turbine Model Combustor After Forced Ignition of H2–Natural Gas–Air Mixtures

Flashback (FB) and self-ignition in the premixing zone of typical gas turbine swirl combustors in lean premixed operation are immanent risks and can lead to damage and failure of components. Thus, steady combustion in the premixing zone must be avoided under all circumstances. This study experimentally investigates the flame holding propensity of fuel injectors in the swirler of a gas turbine model combustor with premixing of H2–natural gas (NG)–air mixtures under atmospheric pressure and proposes a model to predict the limit for safe operation. The A2EV swirler concept exhibits a hollow, thick walled conical structure with four tangential slots. Four fuel injector geometries were tested. One of them injects the fuel orthogonal to the air flow in the slots (jet-in-crossflow injector, JICI). Three injector types introduce the fuel almost isokinetic to the air flow at the trailing edge of the swirler slots (trailing edge injector, TEI). A cylindrical duct and a window in the swirler made of quartz glass allow the application of optical diagnostics (OH* chemiluminescence and planar laser induced fluorescence of the OH radical (OH-PLIF)) inside the swirler. The fuel–air mixture was ignited with a focused single laser pulse during steady operation. The position of ignition was located inside the swirler in proximity to a fuel injection hole. If the flame was washed out of the premixing zone not later than 4 s after the ignition, the operation point was defined as safe. Operation points were investigated at three air mass flows, three air ratios, two air preheat temperatures (573 K and 673 K), and 40 to 100 percent per volume hydrogen in the fuel composed of hydrogen and natural gas. The determined safety limit for atmospheric pressure yields a similarity rule based on a critical Damköhler number. Application of the proposed rule at conditions typical for gas turbines leads to these safety limits for the A2EV burner: With the TEIs, the swirler can safely operate with up to 80 percent per volume hydrogen content in the fuel at an air ratio of two. With the JIC injector, safe operation at stoichiometric conditions and 95 percent per volume hydrogen is possible.

Effects of non-steady state discharge plasma on natural gas combustion: Flammability limits, flame behavior and hydrogen production

The effects of a non-steady state plasma discharge on flammability limits and flame structure of air–natural gas mixtures are investigated. As plasma power increases, flame structure is changed and flammable range is extended. In the absence of a visible flame, a higher hydrogen production is observed, revealing that the discharge is a source of molecular hydrogen. The reduction on hydrogen production inside the flammable range suggests a burning of hydrogen in the flame.

Hydrogen production from algae biomass in rich natural gas-air filtration combustion

This paper examines the hydrogen production from the combustion of rich natural gas-air mixtures in a porous medium composed of aleatoric algae biomass particles and alumina spheres. Combustion wave temperature, propagation rate and products (H2, CO, CO2, CH4) were measured as a function of algae volume fraction in the hybrid bed (0–100%) and its moisture content (0–100%). Three types of algae were analyzed: Ulva lactuca, Lessonia trabeculata, and Lessonia nigrescens. Experimental results show that combustion temperature and hydrogen yields increases with an increase of volume algae fraction in the hybrid bed and an increase of moisture content in the algae. The hydrogen concentration from L. trabeculata was found to be essentially higher than L. nigrescens and U. lactuca, achieved 9.56% with a 100% of moisture content. The maximum energy return over energy invested (EROI) of the process was 50% for L. nigrescens with 50% of algae volume fraction in the hybrid bed and 5% of moisture content.

Exergetic Analysis of Triple-Level Pressure Combined Power Plant with Supplementary Firing

AbstractThis work presents a thermodynamic analysis of a triple-level pressure combined power plant with supplementary firing. The supplementary firing is a technological development for improving and increasing the output power of a combined-cycle (CC) power plant. In Mexico, there is a plant operating with this configuration, which generates a net power of 1,170 MW, with a power ratio of the gas turbine (GT) to the steam turbine (ST) of 1.35∶1. This plant is composed of four combined-cycle units with supplementary firing, each providing an output power of 292.5 MW. The energetic and exergetic analyses of the combined cycle with and without supplementary firing are pursued, taking into account the combustion process of natural gas and humid air. These analyses are also carried out for the heat recovery steam generator, considering the waste gases of the gas turbine and of the supplementary firing as the energy sources. The obtained results are fuel, steam, and humid air flows; excess of air; gas and stea...

Experimental and theoretical analysis of effects of equivalence ratio on mixture properties, combustion, thermal efficiency and exhaust emissions of a pilot-ignited NG engine at low loads

This paper presents the effects of equivalence ratio (Φ) on mixture properties, combustion, thermal efficiency and exhaust emissions of a pilot-ignited natural gas (NG) engine using diesel as an ignition source at low loads. Φ was changed in a wide range (0.4–0.9) by changing throttle opening with other combustion boundaries being unchanged with different NG percentage energy substitutions (PES) at 1335rpm and 25% load. The experiments were arranged based on an electronically controlled heavy-duty, turbocharged, intercooler, 6-cylinder and four-stroke pilot-ignited NG engine. Thermodynamic properties of the NG–air mixture with decreasing air were analyzed theoretically using thermodynamic relations and ideal gas equations. In addition, the time-sequenced characteristic and the heat release rate (HRR)-imbalanced characteristic were found in the engine combustion process, with their quantitative indicators being defined. The results show that as Φ increases, the density, the specific heat ratio and the heat capacity of the NG–air mixture decrease, but the gas constant and the specific heat capacity increase. CA10 moves far away from top dead centre (TDC), CA50 moves near TDC first and then far away with increasing Φ. Three kinds of profiles of HRR curves with different time-sequenced and HRR-imbalanced characteristics appear as Φ increases. The engine thermal efficiency increases first and then decreases with increasing Φ. In addition, as Φ increases, total hydrocarbons (THC) emissions decrease significantly (decrease by 97% as Φ increases from 0.45 to 0.91 with 90% PES) and nitrogen oxides (NOx) emissions increase first and then decrease.