Systems interactions analysis for the energy efficiency improvement of a Kraft process

Several techniques are available to improve the energy performance of a process (internal heat recovery, water reutilization, condensates return, energy upgrading and conversion, elimination of non-isothermal mixing). They are applied to specific energy systems on the utility or process side (steam production and distribution, hot or cold water networks, process heat sources and sinks). Since those systems are interconnected, actions taken on one of them may have effects on another. These effects can be positive (synergies) or negative (counter-actions). A systematic, stepwise methodology has been developed to ensure that synergies are exploited and counter-actions avoided, and is presented. It has been validated by application to an existing Kraft pulping mill. Key performance indicators and the evolution of the thermal composite curves were used to monitor progress as the successive steps of the methodology were implemented. It was found that the combined direct and indirect effects of water reutilization constituted the most important source of potential energy savings. Water reutilization also reduced the need for additional purchased heat exchanger area. Overall, the water intake by the mill could be reduced by 33% and steam savings could be 26% of current production. This would liberate sufficient steam production capacity for the installation of a 44.4 MW cogeneration unit.     

Industrial use of geothermal energy at the Tasman pulp & paper co. ltd's mill, Kawerau, New Zealand

Tasman Pulp & Paper Co. Ltd's mill at Kawerau has an annual production capacity of 380,000 tonnes of Newsprint and 200,000 tonnes of Kraft pulp. The mill electrical requirement amounts to 128MW. Geothermal steam has a significant impact on the mill energy balance and contributes around 35% of the mills steam requirements. There are five geothermal to clean steam heat exchangers with a total capacity of 140t/hr of 345kPa saturated process steam and two boiler feedwater heaters which supply two chemical recovery boilers and three power boilers. Additional geothermal steam is used to heat combustion air and operate shatter sprays at the recovery boilers. A 10MVA geothermal turbo-alternator exhausts steam to a black liquor pre-evaporator and a heat exchanger to heat clean process water. All the available geothermal condensate is collected and treated in a condensate recovery plant to meet quality specifications for boiler feedwater. This meets all of the feedwater requirements of the geothermal heat exchangers plus the make up for the recovery and power boilers. Geothermal water separated in the borefield is used by Bay of Plenty Electricity in two Ormat binary cycle turbines which generate 2.0MW nett. The discharge from the binary cycle plant is also processed in an experimental pilot scale Precipitated Silica plant to recover silca which is used as a newsprint additive. The remainder is discharged through a serpentine canal to the Tarawera river.     

Assessment of the possibilities of electricity and heat co-generation from biomass in Romania's case

The assessment of different systems for electricity production from biomass in Romanian specific conditions shown the most suitable system for electricity (and heat) (co) generation from biomass is the system based on boiler and steam turbine of 2 MW running on wood-wastes (bark). A pulp and paper mill - S.C. “LETEA” S.A Bacau - which needed electricity and heat, and, in the same time, had large amount of bark from technological process, was found as the most suitable location for a demonstrative project.     

Exergoeconomic analysis of condenser type heat exchangers

In this study, an exergoeconomic analysis of condenser type parallel flow heat exchangers is presented. Exergy losses of the heat exchanger and investment and operation expenses related to this are determined with functions of steam mass flow rate and water exit temperature at constant values of thermal power of the heat exchanger at 75240 W, cold water mass flow rate and temperature. The inlet temperature of water is 18 °C and exit temperatures of water are varied from 25 °C to 36 °C. The values of temperature and pressure of saturated steam in the condenser are given to be T_con=47 ° C and P_con=10.53 kPa. Constant environment conditions are assumed. Annual operation hour and unit price of electrical energy are taken into account for determination of the annual operation expenses. Investment expenses are obtained according to the variation of heat capacity rate and logarithmic mean temperature difference and also heat exchanger dimension determined for each situation. The present analysis is hoped to be useful in determining the effective parameters for the most appropriate exergy losses together with operating conditions and in finding the optimum working points for the condenser type heat exchangers.     

Analysis of a mechanical pulp and paper mill using advanced composite curves

In this study a mechanical pulp and paper mill is analysed using advanced composite curves. It is a graphical pinch-based approach that takes into account the existing heat exchanger network and the utilities actually used at the mill. The possibilities of making a cost-effective heat exchanger retrofit in an operating mechanical pulp and paper mill are discussed. The study also shows that the advanced composite curves can be used to describe the amount of nonisothermal mixing taking place in the process.     

Energy audit and conservation opportunities for pyroprocessing unit of a typical dry process cement plant

Cement production process has been highly energy and cost intensive. The cement plant requires 8784 h per year of the total operating hours to produce 640,809 tonnes of clinker. To achieve effective and efficient energy management scheme, thermal energy audit analysis was employed on the pyroprocessing unit of the cement plant. Fuel combustion generates the bulk of the thermal energy for the process, amounting to 95.48% (4164.02 kJ/kg_cl) of the total thermal energy input. Thermal efficiency of the unit stands at 41%, below 50–54% achieved in modern plants. The exhaust gases and kiln shell heat energy losses are in significant quantity, amounting to 27.9% and 11.97% of the total heat input respectively. To enhance the energy performance of the unit, heat losses conservation systems are considered. Waste heat recovery steam generator (WHRSG) and Secondary kiln shell were studied. Power and thermal energy savings of 42.88 MWh/year and 5.30 MW can be achieved respectively. Financial benefits for use of the conservation methods are substantial. Environmental benefit of 14.10% reduction in Greenhouse gases (GHG) emissions could be achieved.     

Energy efficiency improvement of dryer section heat recovery systems in paper machines – A case study

Modern paper machines are equipped with heat recovery systems that transfer heat from the humid exhaust air of the paper machine’s dryer section to different process streams. As a result of process changes, the heat recovery systems may operate in conditions far from the original design point, creating a significant potential for energy efficiency improvement. In this paper we demonstrate this potential with a case study of three operating paper machines. Both operational and structural improvement opportunities are examined. Since the existing retrofit methodologies for heat exchanger networks can not be applied to cases with condensing air, we use thermodynamic simulation models presented earlier to assess the effects of possible changes on the existing heat recovery systems. In order to reduce the required processing time of the simulation models, only a limited number of pre-screened retrofit designs are considered. The pre-screening is carried out on the basis of guidelines presented earlier. The analysis in the case mill revealed savings of 110 GWh/a in process heat with profitable investments. According to the follow-up study, the investments carried out have resulted in 12% lower fuel use and 24% lower CO₂ emissions. The results imply that all operating paper machines should be similarly examined.     

Binary conversion cycles for concentrating solar power technology

It is recognized that the temperature potential of concentrated solar energy is much higher than needed by standard conversion cycles. High temperature solar receivers are in the development stage hopefully leading to the use of solarized gas turbines or of solar combined cycles. These systems are analyzed and taken as a reference standard. Binary alkali-metal steam cycles are shown to be intrinsically more efficient than combined cycles owing to their fully condensing nature. Even at top temperatures of about 600 °C typical for steam cycles the binary cycle allows, in principle, a significant efficiency gain (49.5% against 43% of a steam cycle). However, the binary high temperature systems are investigated featuring either a direct vaporization of the metal within the receiver or a liquid receiver cooling loop with the working fluid vaporized in a proper heat exchanger.With reference to the second option, the computed efficiency is 56% at a top cooling loop temperature of 1000 °C (the same efficiency is attained in a direct vaporization loop at 720 °C). A 60% thermal efficiency is within the potential of the technology. The above figures can be compared with a combined cycle efficiency of 50% at 1200 °C turbine inlet temperature.Available alkali metals are reviewed for the use of working fluid: potassium being the best known fluid but rubidium (or cesium) offering, in perspective, a better overall performance. Material problems connected with the containment of alkali metals at high temperature are reviewed. Experimental evidence suggests that up to 800–850 °C stainless steel is an adequate material, while for higher temperatures, up to 1200 °C, refractory metals should be used.With reference to heat storage the availability of appropriate high temperature substances either as liquids or as melting solids, storing energy as sensible or as latent heat respectively, is discussed.Finally the critical issue of metal vapour turbine design is considered. The results of a number of computations are presented giving the basic geometrical data of some potassium, rubidium and cesium expanders. Rotor diameters tend to be comparatively large. With reference to a 50 MW overall plant output the maximum tip diameter is 3.9 m for a potassium and 2.8 m for a rubidium turbine.     

Reconstruction of a heat exchanger network under industrial constraints — the case of a crude distillation unit

Heat exchanger network retrofit using a pinch based approach is presented. In this approach, the criterion of minimum sensitivity of heat exchanger to fouling effects is accounted for. The present paper introduces this criterion without explaining its details that are described in the literature. A summary is given of HEN reconstruction in a crude distillation unit processing 4.2 million ton crude oil per year. While the total heat quantity of hot streams is 110 MW, the heat recovery in the existing HEN is 60 MW. Using Pinch Analysis, the target value of heat recovery at ΔT_min=10 K was determined at 91 MW. Measurements were carried out on the existing HEN with the aim to determine the influence of fouling effects on the heat transfer in the exchangers. Taking local constraints including fouling into account, HEN reconstruction was proposed. The heat savings in the reconstructed HEN was estimated at 75 MW.     

Integration of a cogeneration unit into a kraft pulping process

The integration of cogeneration with other measures that impact the power production capacity in a Canadian Kraft pulping mill is studied. Those measures are removal of pressure reduction valves, adjustment of the steam pressure level, biomass boiler capacity, and reduction in process energy demand. CADSIM Plus software is used to simulate the cogeneration plant. The dynamic behavior of the process during start-up and its effect on electricity generation are also considered. It is shown that by replacing the PRVs with turbines, 14.4 MW of power can be generated. Moreover, by implementing cogeneration units and process measures to recover 23% of internal energy, 44.5 MW of electricity can be generated in addition to shutting down the existing bunker oil boiler. Therefore, implementation of cogeneration in the pulp and paper industry is technically possible and it offers significant economic advantages. A cost analysis of the complete project gives a simple payback time of less than a year.     

Targeting for energy efficiency and improved energy collaboration between different companies using total site analysis (TSA)

Rising fuel prices, increasing costs associated with emissions of green house gases and the threat of global warming make efficient use of energy more and more important. Industrial clusters have the potential to significantly increase energy efficiency by energy collaboration. In this paper Sweden’s largest chemical cluster is analysed using the total site analysis (TSA) method. TSA delivers targets for the amount of utility consumed and generated through excess energy recovery by the different processes. The method enables investigation of opportunities to deliver waste heat from one process to another using a common utility system.The cluster consists of 5 chemical companies producing a variety of products, including polyethylene (PE), polyvinyl chloride (PVC), amines, ethylene, oxygen/nitrogen and plasticisers. The companies already work together by exchanging material streams. In this study the potential for energy collaboration is analysed in order to reach an industrial symbiosis. The overall heating and cooling demands of the site are around 442 MW and 953 MW, respectively. 122 MW of heat is produced in boilers and delivered to the processes.TSA is used to stepwise design a site-wide utility system which improves energy efficiency. It is shown that heat recovery in the cluster can be increased by 129 MW, i.e. the current utility demand could be completely eliminated and further 7 MW excess steam can be made available. The proposed retrofitted utility system involves the introduction of a site-wide hot water circuit, increased recovery of low pressure steam and shifting of heating steam pressure to lower levels in a number heat exchangers when possible. Qualitative evaluation of the suggested measures shows that 60 MW of the savings potential could to be achieved with moderate changes to the process utility system corresponding to 50% of the heat produced from purchased fuel in the boilers of the cluster.Further analysis showed that after implementation of the suggested energy efficiency measures there is still a large excess of heat at temperatures of up to 137 °C.     

Simulation of an unglazed collector system for domestic hot water and space heating and cooling

This paper details modeling assumptions and simulation results for an unglazed collector system supplying domestic hot water, space heating, and space cooling loads. Collectors are modeled using unglazed collector test results. Variation of savings with collector area, storage volume, heat exchanger size, and wind for the Albuquerque, NM climate are shown. Over the storage-to-collector ratio range of 40–640 l/m² collector, annual savings varies only ±15%. Cooling is sensitive to heat exchanger size, and heating is sensitive to wind velocity. At a collector area of 23 m², the unglazed system meets about 56% of the annual total energy demand, saving 25.9 $/m² yr for an all-electric home. For the 23 m² area, savings for a cold/damp (Madison) and a hot/humid (Miami) climate are 64% and 56%, respectively, of the savings in Albuquerque.     

Energy optimization in a pulp and paper mill cogeneration facility

Alarmingly low pulp prices in early 2009 left pulp and paper mills across North America desperate for any way to improve thin profit margins. One solution that continues to gain popularity among the industry is improved energy management systems for cogeneration systems, which use steam for two purposes – to provide heat for the pulping process and to generate electricity for sale to regional providers. This paper presents an energy optimization algorithm for use in a pulp and paper mill cogeneration system. The algorithm is applicable to a number of popular mill configurations, power sale contracts, and fuel purchasing scenarios. The method is also extended to address weather-dependent cooling limitations encountered by a mill cogeneration facility, in which case an iterative solution is proposed in order to maintain convexity of the optimization problem. Results are presented in the form of three case studies.     

Energy recovery by pinch technology

This paper shows how the application of pinch technology can lead towards great energy savings. The heat exchanger network of a nitric acid plant has been studied and it was found that it is possible to reduce requirements for cooling water and medium pressure steam. In order to enable these savings, three heat exchangers should be replaced with new ones. Energy consumption in steam power system increases slightly. However, the final result is a reduction of energy costs and a payback time of 14.5 months.     

Studies on gas–solid heat transfer during pneumatic conveying

Interactions between solids and gas during pneumatic conveying can be utilized for variety of applications including flash drying, solids preheating etc. Experiments on air–solid heat transfer were carried out in a vertical pneumatic conveying heat exchanger of 54 mm inside diameter, using gypsum as the solid material. The effect of solids feed rate (0.6–9.9 g/s), air velocity (4.21–6.47 m/s) and particle size (231–722.5 μm) on air–solid heat transfer rate, heat transfer area and air–solid heat transfer coefficient has been studied. Empirical correlations have been proposed for the prediction of Nusselt number based on the present experimental data. The proposed correlations predict Nusselt number within an error of ±15% for the present data.     

Modeling an industrial energy system: Perspectives on regional heat cooperation

Through energy efficiency measures, it is possible to reduce heat surplus in the pulp and paper industry. Yet pulp and paper mills situated in countries with a heat demand for residential and commercial buildings for the major part of the year are potential heat suppliers. However, striving to utilize the heat within the mills for efficient energy use could conflict with the delivery of excess heat to a district heating system. As part of a project to optimize a regional energy system, a sulfate pulp mill situated in central Sweden is analyzed, focusing on providing heat and electricity to the mill and its surrounding energy systems. An energy system optimization method based on mixed integer linear programming is used for studying energy system measures on an aggregated level. An extended system, where the mill is integrated in a regional heat market (HM), is evaluated in parallel with the present system. The use of either hot sewage or a heat pump for heat deliveries is analyzed along with process integration measures. The benefits of adding a condensing unit to the back-pressure steam turbine are also investigated. The results show that the use of hot sewage or a heat pump for heat deliveries is beneficial only in combination with extended heat deliveries to an HM. Process integration measures are beneficial and even increase the benefit of selling more heat for district heating. Adding a condensing turbine unit is most beneficial in combination with extended heat deliveries and process integration. Copyright © 2007 John Wiley & Sons, Ltd.     

Financial viabilities of husk-fueled steam engines as an energy-saving technology in Thai rice mills

Rice husk generated as a by-product of rice milling process can be utilized as an energy source for rice mills. The advantages of applying the steam engine as a power source for rice mills are discussed. An economic model was developed to find out the internal rate of return, IRR, on the investment in steam engine’s as an energy-saving technology in Thai rice mills. Based on the technical and economic data presented in this study, rice mills from 45 to 120 t d⁻¹ in size are financially feasible for investments in steam engines as an energy-saving technology for the mills. The maximum affordable husk prices at the different levels of mill use for various sizes of rice mill were analyzed and the economic performance of higher efficiency technology was also tested in this study.     

A Scandinavian chemical wood pulp mill. Part 1. Energy audit aiming at efficiency measures

A Swedish wood-pulp mill is surveyed in terms of energy supply and use in order to determine the energy-saving potential. Conservation measures are of increasing interest to Swedish industry, as energy prices have continued to rise in recent years. The electricity price particularly increased after the deregulation of the Scandinavian electricity market in 1996. The deregulation expanded to all of the EU in July 2004, which may increase the Swedish electricity price further until it reaches the generally higher European price level. Furthermore, oil prices have increased and the emissions trading scheme for CO₂ adds to the incentive to reduce oil consumption. The energy system at the surveyed pulp mill is described in terms of electricity and process heat production and use. The total energy-saving potential is estimated and some saving points are identified. The heat that today is wasted at the mill has been surveyed in order to find potential for heat integration or heat export. The result shows that the mill probably could become self-sufficient in electricity. Particularly important in that endeavour is updating old pumps.     

CaBr2 hydrolysis for HBr production using a direct sparging contactor

The calcium–bromine cycle being investigated is a novel continuous hybrid cycle for hydrogen production employing both heat and electricity. Calcium bromide (CaBr₂) hydrolysis generates hydrogen bromide (HBr) which is electrolyzed to produce hydrogen. The CaBr₂ hydrolysis at 1050 K (777 °C) is endothermic with the heat of reaction δG_T = 181.5 KJ/mol (43.38 kcal/mol) and the Gibbs free energy change is positive at 99.6 kJ/mol (23.81 kcal/mol). What makes this hydrolysis reaction attractive is both its rate and that well over half the thermodynamic requirements for water-splitting heat of reaction of δG_T = 285.8 KJ/mol (68.32 kcal/mol) are supplied at this stage using heat rather than electricity. Molten-phase calcium bromide reactors may overcome the technical barriers associated with earlier hydrolysis approaches using supported solid-phase calcium bromide studied in the Japanese UT-3 cycle. Before constructing the experiment two design concepts were evaluated using COMSOL™ multi-physics models; 1) the first involved sparging steam into a calcium-bromide melt, while 2) the second considered a “spray-dryer” contactor spraying molten calcium bromide counter-currently to upward-flowing steam. A recent paper describes this work [6]. These studies indicated that sparging steam into a calcium-bromide melt is more feasible than spraying molten calcium bromide droplets into steam. Hence, an experimental sparging hydrolysis reactor using a mullite tube (ID 70 mm) was constructed capable of holding 0.3–0.5 kg (1.5–2.5 × 10⁻³ kg mol) CaBr₂ forming a melt with a maximum 0.08 m (8 cm) depth. Sparging steam at a steam rate of 0.02 mol/mol of CaBr₂ per minute (1.2–2.3 × 10⁻⁵ kg/s), into this molten bath promptly yielded HBr in a stable operation that converted up to 25% of the calcium bromide. The kinetic constant derived from the experimental data was 2.17 × 10⁻¹² kmol s⁻¹ m⁻² MPa⁻¹ for the hydrolysis reaction. The conversion rate is highly dependent on melt depth and the design for steam sparging. This experimental data provides a basis for designing a larger-scale sparging hydrolysis reactor for the calcium bromide thermochemical cycle where the endothermic heat of reaction can be effectively supplied by heat transfer coils embedded in the melt.     

Design and analysis of a solar tower power plant integrated with thermal energy storage system for cogeneration

In the current study, a solar tower–based energy system integrated with a thermal energy storage option is offered to supply both the electricity and freshwater through distillation and reverse osmosis technologies. A high‐temperature thermal energy storage subsystem using molten salt is considered for the effective and efficient operation of the integrated system. The molten salt is heated up to 565°C through passing the solar tower. The thermal energy storage tanks are designed to store heat up to 12 hours. The temperature variations in the storage tanks are studied and compared accordingly for evaluation. The effect of operating temperatures on the freshwater production and overall system efficiency is determined. About 24.46 MW electricity is generated in the steam turbine under sunny conditions. Furthermore, the storage subsystem stores heat during sunny hours to utilize later in cloudy hours and night time. The produced power decreases to 20.17 MW in discharging hours due to temperature decrease in the tank. The electricity generated by the system is then used to produce freshwater through the reverse osmosis units and also to supply electricity for the residential use. A total flowrate of 240.02 kg/s freshwater is obtained by distillation and reverse osmosis subsystems.