Desulphurization Unit Research Articles

Production of low aromatics and low sulfur diesel in a hydrodesulfurization (HDS) pilot plant unit

The present work investigates the hydrotreating process of a diesel in order to achieve lower sulphur and aromatics content. The entire work was performed in a Hydrodesulphurization (HDS) pilot plant unit located in Chemical Process Engineering Research Institute (CPERI). For the tests, a commercial HDS catalyst (CoMo) was used while the feed was provided by the deep desulphurization unit of a Greek refinery (Motor-Oil refinery). For the determination of diesel aromatics, a method based on the ASTM D- 2549-85 was applied. The objective of the work was to investigate the ability of a typical HDS catalyst for aromatics saturation. The effect of the main hydrotreating operating parameters (T, P, WHSV, H2/Oil ratio) on sulphur and aromatics removal was also investigated. In general, the data showed that the product density and the aromatic and sulphur content of diesel decreased as the temperature or pressure increased or space velocity decreased. It was concluded that with the present catalyst an aromatics saturation degree of up to 40% could be achieved, giving a diesel product with aromatics content of about 20-25% wt. However, high temperatures (>370°C) were required in order to achieve 500 ppmw sulphur in this feedstock.

Mathematical Simulation of Membrane-Absorption for H<sub>2</sub>S Capture from Nature Gas

A dynamic model of mass transfer was developed with mass transfer equation and mass transfer differential equation according to two film theory for the simultaneous transport of hydrogen sulfide through hollow fiber membrane (HFM) contactors while using N-methyldiethanolamine (MDEA) as the chemical solvent. The model results are in excellent agreement with the experimental data. The results indicate that the removal of H2S increased while increasing concentration of MDEA and gas pressure, however, the removal of H2S decreases while increasing gas velocity. The concentration of H2S increases at the same place in the lumen while increasing gas velocity. There is serious decreasing amplitude of axial concentration of H2S during the initial stage, but it slows down at half of the length and a great reduction of H2S concentration in radial direction with the increase of the length. The decreasing amplitude is dropped due to the concentration of H2S decreased in radial direction. The model can indicate H2S removal rate in given operational conditions and offer theory evidence for the design of membrane contactor. Natural gas is believed to play a vital role in the next few decades for industrial and domestic utilization. It is considered as one of the cleanest and safest of all energy sources. However, nature gas is not a pure hydrocarbon and sometimes it has some sour gases such as hydrogen sulfide which has high toxicity. Hydrogen sulfide can not only corrode equipment and transmission pipeline under aerobic and hot humid conditions but also cause catalyst poisoning, even serious threaten the safety of human. Wet desulphurization is widely used for natural gas treatment and aqueous solutions of alkanolamines are often used as absorption solvent. Among these alkanolamines, MDEA as an absorption solvent of acid gases is widely used today because it possesses the characteristics such as higher H2S selectivity, bigger absorption capacity, lower regeneration energy, smaller hot-degradation and lower circulating load. But desulphurization unit can be seriously corroded in the sulfur removal process. On the other hand, these conventional processes such as absorption towers, packed and plate columns possess many disadvantages such as flooding, foam formation, and demand high capital and operating costs. So the technology meets a certain obstacles. Recently, new processes using gas–liquid membrane contactors as gas absorption devices have been a subject of great interest. Among the diversity of membrane geometries available for membrane contactors, hollow-fiber membrane contactors are favored due to their high surface/volume ratio for separation which is 30-50 times compared with traditional absorbers. This type of process offers several practical advantages including low energy and operating costs, simplicity and occupying small area. In addition, membrane contactors as unit equipment can be combined according to actual need. [4~5] used polypropylene hollow fiber membrane as the absorber and MDEA as the chemical solvent for the absorption of H2S via changing operating conditions (e.g. temperature, pressure, the concentration of the solvent, flux of gas-liquid phase) and studied the influence of the changes to mass transfer coefficient and sulfur removal efficiency. The results indicate that the sulfur removal efficiency can be 95% above by optimizing the operating conditions. At home and abroad, comprehensive two-dimensional mathematical models were developed based on differential equation. Wang [6] simulated the absorption of CO2 using different absorption medium in hollow fiber membrane contactors. But they did not consider the effect of mixed gas. Chen [7] modeled the distribution of reactants and products concentration in the shell side in different typ es of reaction. However, the model can not obtain the concentration of H2S in the lumen. Rami Faiz [8] modeled the distribution of acid gas, but the mathematical model was not validated by the experimental work.

Inspection of protective linings using microwave spectroscopy combined with chemometric methods

A novel non-destructive method for quality control of new protective linings on a metal substrate and an assessment of their state during their service life using microwave spectroscopy within a wide frequency range from 6.5 to 13GHz is described. The data obtained were processed by chemometric methods to reduce the dimensionality of the experimental data sets and to help visualise results. Theoretical fundamentals of the method are outlined. Results for the rubber linings with a thickness of 10mm operating in a scrubber of wet flue gas desulphurisation units are presented.

Estimation of the health benefits of controlling air pollution from the Yatağan coal-fired power plant

Abstract The health benefits of the flue-gas desulphurisation units of the Yatagan coal-fired power plant were estimated by using the AirPacts model. The primary pollutant emitted from the stack is sulphur dioxide; however, secondary pollutants are created downstream of the source as a result of chemical reactions involving other species that exist in the surrounding atmosphere. The impact pathway approach was used to quantitatively estimate the level of the health effects caused by sulphur species emission. Health benefits were then assessed by comparing the estimated health impacts with and without the flue-gas desulphurisation equipment. The flue-gas desulphurisation units of the power plant were found to result in, on average, 77 fewer cases of short-term mortality, 2280 fewer cases of long-term mortality, and 88 fewer cases of respiratory hospital admissions annually. In monetary terms, this benefit is equivalent to US $103 million (in 2000 prices) per annum. This benefit exceeds the investment cost (US $80 million).

Flue gas desulphurization for hot recycle Oxyfuel combustion: Experiences from the 30 MW th Oxyfuel pilot plant in Schwarze Pumpe

Significant differences exist in the flue gas composition in hot recycle Oxyfuel conditions as e.g. the high CO 2 partial pressure (>90 vol%, dry), the very high SO 2 concentration and the high water content (approx. 30 vol%). Therefore certain design and operation criteria have to be observed for the flue gas desulphurization with forced oxidation under Oxyfuel combustion conditions. Several performance tests have been executed at the 30 MW th Oxyfuel pilot plant in Schwarze Pumpe to evaluate the main performance parameters and to assess the influence of the major operation parameters. The results show that there are no fundamental problems for the operation of the flue gas desulphurization unit under Oxyfuel combustion conditions. High removal rates could be reached and no negative impact of the high CO 2 partial pressure was observed under the tested operating conditions. No major differences in the gypsum quality have been observed between air firing and Oxyfuel conditions.

Use of FGD gypsum in fire resistant panels

Gypsum from power plant flue gas desulphurization units (FGD gypsum) is a combustion by-product produced in high quantities. In this paper, gypsum panels composed of 100% FGD gypsum from two power plants have been subjected to different physico-chemical (density, pH, humidity), mechanical (flexural and compressive strength, surface hardness, impact resistance), fire resistance and environmental tests (leaching and radioactivity). The results obtained have been compared with the requirements established in some European standards for commercial gypsum and other standards for similar products. In addition, the panels manufactured have been compared with commercial gypsum panels in order to determine the recycling possibilities of this kind of material in this application.

Instantaneous angular speed and power for the diagnosis of single-stage, double-acting reciprocating compressor

Advance condition monitoring systems that use cylinder pressure as a primary feedback variable have been developed. Application has been limited by the cost and difficulties associated with the intrusive cylinder pressure sensor installation. Non-intrusive measurements, the instantaneous angular speed (IAS) and power, have been performed to incorporate the effect of faults in a single-stage, single-cylinder double-acting reciprocating compressor. The study simulates a hydrogen make-up compressor to define a diagnosis criterion that may be used for the condition monitoring of a reciprocating compressor. It involves the development of mathematical models for three physical conditions: excess clearance, cylinder pressure variation, and variation of valves flow area. The simulation is conducted using the torque balance, energy equation, valve dynamics, and flow through the valves. The simulation produced unique effects for each of the physical conditions. The results predicted by the model are validated with the data from the hydrogen make-up compressor located in a Kerosene Desulphurization Unit using the pressure-volume diagram (PV) analysis. This paper presents IAS and power as non-intrusive diagnosis tools. Results showing the effect of faults on the IAS and power trends are presented.

Optimizing make-up flow in a CO2 capture system using CaO

CO2 capture system based on the carbonation/calcination loop, still in its infancy, has gained rapid interest due to promising carbonator CO2 capture efficiency, low sorbent cost and the fact that no flue gases desulphurization unit is needed before entering the system. The sum of these features results in a competitively low cost CO2 capture system. There are different options to design the carbonation loop. In this work, a basic configuration that makes use of two interconnected circulating fluidized beds (carbonator and calciner) has been studied. Among the key variables that influence the performance of these systems, the carbonation conversion of the sorbent and the heat requirement at calciner are the most relevant. Both variables are mainly influenced by CaO/CO2 ratio and make-up flow (purge) of solids. A purge is necessary in order to reduce the sorbent deactivation and to compensate the formation of CaSO4 from the SOx content in the flue gas. Large CaO/CO2 ratios improve the carbonation conversion but also increase the cost of the system due to a more intensive solid circulation. High make-up flow also improves the carbonation conversion and hence the CO2 capture, but increases the heat demand at calciner and the fresh sorbent cost. The aim of this paper is to calculate the optimum make-up flow and CaO/CO2 ratio in order to minimize the capture cost of the system. Independent variables are make-up flow of fresh CaCO3 and CaO/CO2 ratio. The constraint equations are experimental data on carbonation reaction, mass and energy balances, oxygen requirement and fuel composition.

Regional effects and efficiency of flue gas desulphurization in the Carpathian Basin

Although sulphur emissions (mainly as SO 2) have been continuously decreasing over the last 20 years in most western industrialized countries, localized SO 2 problems still exist in conjunction with strong local emission, meteorological, and topographical factors. In this study, the effect of supplementary installed flue gas desulphurization (FGD) units at high-capacity power plants on regional air pollution in the Carpathian Basin is investigated. The dispersion and accumulation of the SO 2 air pollutant are studied with the regional three-dimensional on-line atmosphere-chemistry model REMOTE. The changes in the SO 2 air pollution are investigated by parallel simulations in a case study, where the single modified parameter is the SO 2 emission rate. The results show that FGD units significantly reduce the horizontal and the vertical dispersion of the emitted SO 2, and its transboundary transport, too. Beside the SO 2 removal efficiency, the dispersion and accumulation also depend on the seasonal weather conditions. During winter, the dispersion and accumulation are higher than in other seasons. Due to this phenomenon, higher SO 2 removal efficiency is needed to guarantee similar air quality features like in the other seasons.

Mercury emission control in coal-fired plants: The role of wet scrubbers

When coal is combusted, the combination of the elevated temperature and the volatility of mercury and its compounds results in the presence of gaseous elemental mercury and mercury compounds in the combustion flue gas. In January 2005, the European Commission adopted a mercury strategy that envisages a number of measures to reduce mercury levels in the environment and human exposure. A number of options for mercury removal from coal-fired power plants have been investigated. However, more effort is needed to achieve an efficient and cost-effective technology. The main objective of this work is to investigate the influence of scrubber parameters on mercury removal efficiency to establish effective measures for mercury control. In order to attain these objectives, theoretical predictions based on thermodynamical equilibrium data and lab-scale experimental tests were carried out. The results obtained point to pH and slurry concentration as the most critical parameters for converting FGD (Flue Gas Desulphurization unit) into a multipollutant control technology.

NATIONAL SURVEY ON THE NATURAL RADIOACTIVITY AND 222Rn EXHALATION RATE OF BUILDING MATERIALS IN THE NETHERLANDS

The present study reports on results of a nation-wide survey on the natural radioactivity concentrations and Rn exhalation rates of the prevailing building materials in the Netherlands. In total 100 samples were taken and analyzed for the activity concentrations of Ra, Ra, Th, and K and for their Rn exhalation rate. The sampled materials consisted of gypsum products, aerated concrete, sand-lime and clay bricks, mortars and concrete, representing about 95% of the stony building materials used in the construction of Dutch homes. The laboratory analyses were performed according to two well-documented standard procedures, the interlaboratory reproducibility of which is found to be within 5% on average. The highest radionuclide concentrations were found in a porous inner wall brick to which fly ash was added. The second highest were clay bricks with average Ra and Ra levels around 40 Bq kg. Concrete and mortar show the highest exhalation rates with a fairly broad range of 1 to 13 microBq (kg s). Low natural radioactivity levels are associated with either natural gypsum (products) or gypsum from flue gas desulphurization units, and low exhalation rates with clay bricks. To evaluate the radiological impact the radioactivity concentrations in each sample were combined into a so-called dose factor, representing the absorbed dose rate in a room with a floor, walls and ceiling of 20 cm of the material in question. For that purpose, calculations with the computer codes MCNP, Marmer and MicroShield on the specific absorbed dose rates were incorporated in the paper. The results of these codes corresponded within 6% and average values were calculated at 0.90, 1.10, and 0.080 nGy h per Bq kg for the U series, the Th series, and K, respectively. Model calculations on the external dose rate, based on the incidence of the various building materials in 1,336 living rooms, are in accordance with measured data.

The radiological impact from airborne routine discharges of a modern coal-fired power plant

In this paper the radiological impact from the airborne routine discharges of a modern coal-fired power plant at Langerlo (Belgium) is evaluated. Therefore, the natural radioactivity contents of the coal and the fly-ash discharged were measured. With a bi-Gaussian plume model the maximum annual values of the 226Ra concentration in the air (4.5 nBq/m 3) and of the total deposition (1.5 mBq/m 2) were calculated. The transfer of the radionuclides from air and soil to the biospheric media, exposing man, were modelled and the annual, individual, effective dose to the critical group, after an assumed life span of the power plant of 70 years, was evaluated at 0.05 μSv/y. This is several orders of magnitude lower than the annual doses for most power plants reported in the literature. The flue gas purification system, extended with a denitrification unit and a desulphurisation unit, was found to be the basis for this low impact.

Temporal Evolution of the Sulphur Oxides Emissions from the Greek Electricity Generation Sector

The Greek electricity production sector is based -as far back as the early 60s- on the usage of local lignite and imported heavy-oil. Hence the electricity production process is assumed responsible for a significant contribution to air pollution, including 80% of the national sulphur dioxide emissions. In this context, an extensive and thorough analysis is carried out concerning the SO2 effluents coming from the electricity generation sector during the 1995-2002 period. For this purpose, the available long-term official measurements are taken into consideration and analysed in depth. According to this analysis, the SO2 emissions factor ratio between Southern Greece and Northern Greece lignite fired stations is in the order of 25:1. Additionally, one may definitely state that there is a considerable surcharge of sulphur oxides released by the Greek electricity production system, which although showing a fairly decreasing tendency, is still above the 8.5gr kWh-1 consumed. Finally, the positive contribution of the natural gas, gradually replacing other fuels, and the operation of a new desulphurisation unit in S. Greece are clearly counterbalanced by the significant and constant annual electricity consumption amplification of the last decade.

PEM – Fuel Cell System for Residential Applications

AbstractViessmann is developing a PEM fuel cell system for residential applications. The uncharged PEM fuel cell system has a 2 kW electrical and 3 kW thermal power output. The Viessmann Fuel Processor is characterized by a steam‐reformer/burner combination in which the burner supplies the required heat to the steam reformer unit and the burner exhaust gas is used to heat water. Natural gas is used as fuel, which is fed into the reforming reactor after passing an integrated desulphurisation unit. The low temperature (600 °C) fuel processor is designed on the basis of steam reforming technology. For carbon monoxide removal, a single shift reactor and selective methanisation is used with noble metal catalysts on monoliths. In the shift reactor, carbon monoxide is converted into hydrogen by the water gas shift reaction. The low level of carbon monoxide at the outlet of the shift reactor is further reduced, to approximately 20 ppm, downstream in the methanisation reactor, to meet PEM fuel cell requirements. Since both catalysts work at the same temperature (240 °C), there is no requirement for an additional heat exchanger in the fuel processor. Start up time is less than 30 min.In addition, Viessmann has developed a 2 kW class PEFC stack, without humidification. Reformate and dry air are fed straight to the stack. Due to the dry operation, water produced by the cell reaction rapidly diffuses through the electrolyte membrane. This was achieved by optimising the MEA, the gas flow pattern and the operating conditions. The cathode is operated by an air blower.

Devolatilisation behaviour of petroleum coke under pulverised fuel combustion conditions

The combustion of petroleum coke in large scale facilities has been limited due to its high sulphur content, but the increasing installation of flue-gas desulphurisation units makes possible the firing of petroleum coke either as a primary fuel or blended with coals. This study focuses on the behaviour of three fuel-grade petroleum cokes of different provenance under pulverised fuel combustion conditions. These cokes, ground and sieved 125–20 μm were fed to a drop tube reactor operating at 1300 °C under different atmospheres to produce chars with different combustion degrees. Char reactivity assessment was performed isothermally in a thermobalance at 550 °C and morphology and optical texture of the chars were studied by optical and scanning electron microscopy. Petroleum coke chars are composed of two main types of particles: (i) porous anisotropic particles that passed through a plastic stage and generated either cenospheric or network-like chars and (ii) angular particles with fine-mosaic optical texture that did not swell and show abundant contraction cleats. The relative proportions of both types of particles were very different in the three petroleum coke chars indicating significant differences in their devolatilisation patterns. The morphology and optical texture of the petroleum coke chars were related to their reactivity (as measured in a thermobalance) and to the characteristics (chemical composition and optical texture) of the parent petroleum cokes, in an attempt to understand the implications of their different devolatilisation behaviours on the combustion efficiency.

Inspection of rubber linings operating in flue gas desulphurisation units

Electrochemical impedance spectroscopy (EIS) investigations of a bromobutyl rubber lining were performed under field conditions. The electric capacitance of the lining was determined to be the best indication of the condition of the corrosion protective coating. The method of determination of moisture content is based on the Brasher–Kingsbury equation. Obtained results were compared to the results of destructive tests, performed in the same locations inside the absorber. Conformity of both was stated, allowing application of EIS and the presented computational procedure in the monitoring of the condition of rubber linings.

Anti‐corrosion protection of chimneys and flue gas ducts

Industrial chimneys are a great part of environmental protection in industrial countries. In recent years many of them have been used to carrying away very aggressive gases from boilers and flue gas desulphurisation (FGD) units below acid dew‐point temperature. It is opf very important to modernize the old stacks and protect them against corrosion. The proper anti‐corrosion protection of modern high stacks is also an important technical and economical problem. In this paper the mechanism of acid dew‐point corrosion, as well as construction of industrial chimneys, methods of their anti‐corrosion protection and modernisation are described.

A study of trace element behaviour in two modern coal-fired power plants: II. Trace element balances in two plants equipped with semi-dry flue gas desulphurisation facilities

Two Finnish coal-fired power plants were experimentally investigated with regard to the distribution of environmentally harmful trace elements in the process. The plants were equipped with low NO x burners, an electrostatic precipitator and a semi-dry flue gas desulphurisation unit consisting of a spray-dryer-type reactor and a fabric filter. All the in-going and out-going mass streams were analysed for As, Be, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Se and Tl in the first plant, and for As, Be, Cd, Cr, Hg, Mn, Ni, Pb, Tl and Zn in the other plant. Total atmospheric emissions of trace elements from the plants studied were very low; most of them even orders of magnitudes smaller than 10 μg/m 3 (NTP). The vaporous fraction of the trace elements was found to play a predominant role in the atmospheric emissions. However, the total atmospheric emissions of even the strongly volatile Hg were very low, which can be ascribed to the applied plant technology. Enrichment of the elements into the various ash streams—from the bottom ash up to the particulates downstream of the electrostatic precipitator—was found to be in satisfactory compliance with literature data. Element balances were calculated over the whole process and over the desulphurisation unit for both plants: for the whole process the closure (i.e., ratio output/input) of element balances was within ±30% for all elements studied, except for chromium in plant HB, and for the vast majority of elements it was even within ±20%. The results imply that the sampling techniques and analytical methods developed for this work can be well applied to quantitative mass balance studies in this kind of processes.

Electrochemical investigations of stainless steels used in flue‐gas desulphurization units

Presents the results of direct current polarization and exposure measurements of alloy steels, used as construction materials for flue‐gas desulphurization units. Ascertains that in some cases there is unconformity between results of classic anodic cyclic polarization and results of laboratory exposure. Proposes a modified methodology for the evaluation of the susceptibility of alloy steels to pitting (multiple anodic polarization). Discusses results on the basis of the tested steels’ composition analysis and results of polarization measurements. Determines the conditions in which the pitting resistance equivalent should be used. Tests have been carried out in electrolyte‐simulating conditions in the purified gas zone of the wet flue‐gas desulphurization units. On the basis of the experimental data, determines flexibility for pitting corrosion of investigated materials.

Continuous removal of heavy metals from FGD wastewater in a fluidised bed without sludge generation

A patent pending fluid-bed process has been developed by Krüger for removal of dissolved heavy metals by adsorption/coprecipitation. The waste product is a very compact granulate with a very low water content of 10-20%. Sludge is not produced. The fluid-bed technique was tested at a coal-fired power station with wastewater from the flue gas desulphurisation (FGD) unit for removal of heavy metals from the wastewater. By dosing only potassium permanganate to the wastewater, the content of dissolved nickel, cadmium and zinc was reduced by respectively 99%, 92% and 97% at optimum treatment in one fluid-bed reactor. The weight of waste product produced will constitute less than 25% of the waste product produced by the present traditional chemical precipitation. Chemical costs are approximately 0.6 US$/m3 which are similar for the fluid-bed and present wastewater treatment processes.