Optimizing the energy efficiency of conventional multi-cylinder dryers in the paper industry

The paper industry is, with about 6% of the total worldwide industrial energy use, an energy-intensive industry. The drying section is with approximately 50% the largest energy consumer in a paper mill, energy use in this section is mainly heat use. Several options to decrease heat use in conventional multi-cylinder drying sections are investigated, calculating the effect on energy use. Optimization measures include a) decreasing the amount of water evaporation by applying additives in higher consistencies and by lowering the water viscosity, b) decreasing the heat use of water evaporation by increasing the dew point temperature of the dryer and c) increasing the amount of heat recovery by using exhaust air to not only pre-heat the incoming air but also to increase process water temperatures. These could all be achieved by retrofitting and/or choosing different processing conditions in existing factories. The combined thermal heat saving potential due to the optimization actions is 1.3 GJ_h/t paper (or 32% of the drying section’s heat use) as compared to the reference situation.     

Potential use of geothermal energy sources for the production of lithium-ion batteries

The lithium-ion battery is one of the most promising technologies for energy storage in many recent and emerging applications. However, the cost of lithium-ion batteries limits their penetration in the public market. Energy input is a significant cost driver for lithium batteries due to both the electrical and thermal energy required in the production process. The drying process requires 45–57% of the energy consumption of the production process according to a model presented in this paper. The model is used as a base for quantifying the energy and temperatures at each step, as replacing electric energy with thermal energy is considered. In Iceland, it is possible to use geothermal steam as a thermal resource in the drying process. The most feasible type of dryer and heating method for lithium batteries would be a tray dryer (batch) using a conduction heating method under vacuum operation. Replacing conventional heat sources with heat from geothermal steam in Iceland, we can lower the energy cost to 0.008USD/Ah from 0.13USD/Ah based on average European energy prices. The energy expenditure after 15 years operation could be close to 2% of total expenditure using this renewable resource, down from 12 to 15% in other European countries. According to our profitability model, the internal rate of return of this project will increase from 11% to 23% by replacing the energy source. The impact on carbon emissions amounts to 393.4–215.1 g/Ah lower releases of CO₂ per year, which is only 2–5% of carbon emissions related to battery production using traditional energy sources.     

Superheated steam drying of sawdust in continuous feed spouted beds – A design perspective

Spouted bed drying technology shows promising results for the drying of unscreened sawdust in superheated steam. In this paper, the experiences from designing, running and evaluating two spouted bed continuous feed dryers are presented. Stable running conditions and drying results have been achieved. This has been particularly important for sawdust that will be compressed into pellets or briquettes. The spouted bed superheated steam dryer also shows high potential for energy efficient integration into sawmills. Our recommendation is thus, to use the outlet steam temperature as the control parameter for the outlet moisture content. A drying rate above and one below the fibre saturation level, can be identified. Visual observations through the viewing glass in the drying zone in both the dryers clearly showed that not all of the material participated in the spout at all times; there were, however, no indications of dead zones. A heat transfer analysis indicated that only about 70% of the surface area of the material was in thermal contact with the steam. This paper sums up the experiences regarding drying properties, control and system properties obtained when sawdust is dried using superheated steam as the drying medium. Further work on standardised dryers in series or in parallel is necessary to increase the capacity in the spouted bed dryer.     

Solar dryer with thermal energy storage systems for drying agricultural food products: A review

Developing efficient and cost effective solar dryer with thermal energy storage system for continuous drying of agricultural food products at steady state and moderate temperature (40–75 °C) has become potentially a viable substitute for fossil fuel in much of the developing world. Solar energy storage can reduce the time between energy supply and energy demand, thereby playing a vital role in energy conservation. The rural and urban populations, depend mainly, on non-commercial fuels to meet their energy needs. Solar drying is one possible solution but its acceptance has been limited partially due to some barriers. A great deal of experimental work over the last few decades has already demonstrated that agricultural products can be satisfactorily dehydrated using solar energy. Various designs of small-scale solar dryers having thermal energy storage have been developed in the recent past, mainly for drying agricultural food products. Therefore, in this review paper, an attempt has been taken to summarize the past and current research in the field of thermal energy storage technology in materials as sensible and latent heat in solar dryers for drying of agricultural food products. With the storage unit, agricultural food materials can be dried at late evening, while late evening drying was not possible with a normal solar dryer. So that, solar dryer with storage unit is very beneficial for the humans and as well as for the energy conservation.     

Energy and Exergy Analysis of a Novel Efficient Combined Process by Hydrothermal Degradation and Superheated Steam Drying of Degradable Organic Wastes

This paper considers the combination of hydrothermal degradation(HTD)and superheated steam(SHS)drying indisposal and processing of degradable organic wastes in municipal solid wastes(MSW).In SHS drying, a fractionof dryer thermal energy input can be recovered and used to satisfy the heat requirement in maintaining the HTDoperating temperature.Both energy and exergy analysis are applied to the combined process.The analysis coversranges of dryer inlet temperatures of 202.38-234.19℃ and feed water content of 32.5-65%.Thermal energyanalysis shows that the combination of HTD and SHS drying can achieve thermal energy self-sufficiency(TES)by manipulating process variables.The exergy analysis indicates the location,type,and magnitude of the exergylosses during the whole process by applying the second law of thermodynamics.     

Minimization of total drying costs for a continuous packed-bed biomass dryer operating at an integrated chemical pulp and paper mill

This paper presents a MILP (Mixed Integer Linear Programming) model for a continuous packed-bed biomass dryer. The model minimizes total drying costs, including both capital and operational costs, of the dryer. Heated air, which flows through a biomass bed and perforated conveyor, is used as drying gas by the dryer. We define the dryer size with the help of an experimentally-measured characteristic drying curve. The MILP model is tested in a case study where the dryer is assumed to have been installed at a Scandinavian pulp and paper mill. There are three different heat sources available for the heating of drying air: warm water at 60 °C, warm water at 80 °C and backpressure steam at the pressure of 0.4 MPa. The results indicate that, in practice, the use of only low-temperature warm water flows for heating of drying air is the most economic method when their prices are low (below 1 € MWh⁻¹ in the case study). Warm water flows are usually waste heat from pulp and paper mills and their prices are low compared to the price of back-pressure steam (typically from 10 to 15 € MWh⁻¹). The use of steam for drying may be reasonable if the price of warm water is for some reason clearly higher than the price of a typical waste-heat stream. The MILP model presented in this work can be used for minimizing drying costs of any material (not only biomass) dried in a continuous packed-bed dryer if the characteristic drying curve of the material is available.     

Evaluation of energy efficiency in biofuel drying by means of energy and exergy analyses

The calculation of heat consumption is based on the First Law and it gives quantitative information about the energy used in drying. However, it does not pay any attention to the quality of the energy used in drying. To take into account the quality of the energy, attention must be paid to the Second Law, too. Especially in those cases where the energy used in drying may be converted to mechanical work, it is important to consider the Second Law is. In this paper, the energy efficiency of biofuel drying in a pulp and paper mill is evaluated on the basis of energy and exergy analysis. The evaluation is based on the determination of the heat consumption and the irreversibility rate for energy and exergy analysis, respectively. The evaluation methods are applied to two different drying systems, single-stage-drying with partial recycle of spent air, and multi-stage-drying. Both drying systems are also provided with a heat recovery unit in which the inlet air is pre-heated using the outlet air of the dryer. There are two alternative heat sources available for the drying energy, steam at a pressure of 3 bar and water at a temperature of 80 °C. The results show that the heat consumption is only dependent to a small extent on the heat source type or the drying system. On the other hand, the irreversibility rate depends to a considerable on the heat source and the drying system.     

How process and power plant changes can increase industrial cogeneration: The case of kraft pulp mills

Industrial cogeneration can be substantially increased in energy intensive process industries, such as pulp and paper, by making process and operating changes such as reducing water use, minimizing effluent discharge, generating chemicals on-site and drying biomass fuels. The economic benefits of cogeneration are demonstrated by comparing three cases. The first is the no generation case in which steam is generated for process use and electric power is purchased from the utility. The second is the thermal match case in which steam is generated at a pressure substantially higher than needed for process and passed through a turbine to generate electric power before being used to meet the plant's thermal demands. In the third case, the minimum cogeneration case, more steam is produced than required by the plant. The additional steam is expanded through the turbine-generator to a condenser and generates additional electricity which can be sold. The study is based upon the conceptual design of a hypothetical 1000 tons/day bleached kraft pulp mill scheduled to begin operation in the United States in 1985, but the general approach and conclusions are applicable to a wide variety of industries with high energy demands.     

Performance of indirect through pass natural convective solar crop dryer with phase change thermal energy storage

A state-of-the-art solar crop dryer was developed with thermal energy storage to maintain continuity of drying of herbs for their colour and flavour vulnerability. The dryer consists of flat plate solar collector, packed bed phase change energy storage, drying plenum with crop trays and natural ventilation system. Dryer is designed with a maximum collector area of 1.5 m², six crop trays with an effective area of 0.50 × 0.75 m², can hold 12 kg of fresh leafy herbs. The dryer is attached with a packed bed thermal energy storage having capacity of 50 kg phase change material (PCM). The drying system works in such a manner that phase change material stores the thermal energy during sun shine hours and releases the latent and sensible heat after sunset, thus dryer is effectively operative for next 5–6 h. The temperature in drying chamber was observed 6 °C higher than the ambient temperature after sunshine hours till the mid night during the month of June at Jodhpur. Economic performance of the dryer was analysed with return on capital and simple payback period as 0.65 and 1.57 year respectively on optimum cost of raw material and product sale price.     

A review on energy and exergy analysis of solar dying systems

Solar energy is a clean, abundant and freely available renewable energy sources. Energy and exergy analysis of solar thermal devices has drawn considerable interest among the researchers across the world. Solar drying is the promising option to utilize low grade energy to dry agricultural produces. Exergy analysis is a tool to access the efficient usage of solar energy. It is the property of the system, which gives the maximum power that can be distracted from the system when it is brought to a thermodynamic equilibrium state from a reference state. Using exergy analysis, based on the first and second laws of thermodynamics, it is possible to infer the true potential of different kinds of energies. In this paper, a holistic approach on energy and exergy analysis of solar dryer with case studies has been made.     

Extending existing combined heat and power plants for synthetic natural gas production

In this work, integration of a synthetic natural gas (SNG) production process with an existing biomass CHP steam power cycle is investigated. The paper assesses two different biomass feedstock drying technologies—steam drying and low‐temperature air drying—for the SNG process. Using pinch technology, different levels of thermal integration between the steam power cycle and the SNG process are evaluated. The base case cold gas efficiency for the SNG process is 69.4% based on the lower heating value of wet fuel. The isolated SNG‐related electricity production is increased by a factor of 2.5 for the steam dryer alternative, and tenfold for the low‐temperature air dryer when increasing the thermal integration. The cold gas efficiency is not affected by the changes. Based on an analysis of changes to turbine steam flow, the integration of SNG production with an existing steam power cycle is deemed technically feasible. Copyright © 2011 John Wiley & Sons, Ltd.     

Review of solar assisted heat pump drying systems for agricultural and marine products

Combining solar energy and heat pump technology is a very attractive concept. It is able to eliminate some difficulties and disadvantages of using solar dryer systems or solely using heat pump drying separately. Solar assisted heat pump drying systems have been studied and applied since the last decades in order to increase the quality of products where low temperature and well-controlled drying conditions are needed. This paper reviewed studies on the advances in solar heat pump drying systems. Results and observation from the studies of solar assisted heat pump dryer systems indicated that for heat sensitive materials; improved quality control, reduced energy consumption, high coefficient of performance and high thermal efficiency of the dryer were achieved. The way forward and future directions in R&D in this field are further research regarding theoretical and experimental analysis as well as for the replacement of conventional solar dryer or heat pump dryer with solar assisted heat pump drying systems and solar assisted chemical and ground source heat pump dryers which should present energy efficient applications of the technologies.     

Specific energy consumption in microwave drying of garlic cloves

The convective and microwave-convective drying of garlic cloves was carried out in a laboratory scale microwave dryer, which was developed for this purpose. The specific energy consumption involved in the two drying processes was estimated from the energy supplied to the various components of the dryer during the drying period. The specific energy consumption was computed by dividing the total energy supplied by amount of water removed during the drying process. The specific energy consumption in convective drying of garlic cloves at 70 °C temperature and 1.0 m/s air velocity was estimated as 85.45 MJ/kg of water evaporated. The increase in air velocity increased the energy consumption. The specific energy consumption at 40 W of microwave power output, 70 °C air temperature and 1.0 m/s air velocity was 26.32 MJ/kg of water removed, resulting in about a 70% energy saving as compared to convective drying processes. The drying time increased with increase in air velocity in microwave-convective drying process; a trend reverse to what was observed in convective drying process of garlic cloves.     

Comparison of energy consumption and specific energy requirements of different methods for drying mushroom slices

Energy consumption for drying of mushroom slices was evaluated using various drying methods including hot air, microwave, vacuum, infrared, microwave-vacuum and hot air-infrared. Results of data analysis showed that the lowest and highest energy consumption levels in drying mushroom slices were associated with microwave and vacuum dryers, respectively. The use of vacuum in conjunction with microwave drying increased energy consumption relative to microwave drying alone. Energy consumption in the hot air dryer showed a downward trend with increasing temperature and an upward trend with increasing air velocity. In drying mushroom using infrared radiation, it was observed that increased air velocity increases drying time and consequently the amount of consumed energy. Using a combination of hot air and infrared drying decreased energy consumption relative to infrared drying alone and increased it relative to hot air drying. In the combined microwave-vacuum dryer, drying time and consequently energy consumption decreased in comparison to the vacuum dryer. Hot air-infrared drying of mushroom slices proved to have the lowest energy consumption.     

Improving the energy efficiency of conventional tumbler clothes drying systems

This paper discusses the drying processes of four different designs of household clothes tumbler dryer using electric power input. The relative advantages and disadvantages and the energy analyses of the dryers are presented. A parametric study was conducted to assess the effects of different variables on energy efficiency and specific moisture extraction rate. It is shown that the conventional tumbler dryer technology can be improved significantly by the use of heat‐recovery heat exchangers. Copyright © 2001 John Wiley & Sons, Ltd.     

Re‐circulating heat pump assisted continuous bed drying and energy analysis

Specialty crops such as ginseng, herbs and echinacea need to be dried at low temperatures (30–35°C) for product quality optimization. A drying system that is both energy efficient and preserves product quality is desired. A re‐circulating heat pump continuous bed dryer system was designed, constructed and field‐tested for this purpose. The heat pump dryer system was experimentally evaluated using several potential herbal and medicinal crops such as alfalfa, catnip, wormwood, red clover, portulaca, dandelion and ginseng. These crops were dried either in chopped, sliced or whole form, depending on the part of the plant (aerial or root). The specific moisture extraction rates (SMER) obtained for various crops were in the range of 0.06–0.61 kg kWh^?1. It took 5 days and 190 kWh of energy to reduce the average moisture content of ginseng roots below 10% (wb), while commercial dryers currently in use would take on an average 14 days and 244 kWh of energy at comparable loading rates. The re‐circulating nature of the heat pump dryer made it 22% more energy efficient and resulted in 65% reduced drying time compared to conventional dryers incorporating electric coil heaters. Copyright © 2005 John Wiley & Sons, Ltd.     

Energy aspects in paper mills utilising future technology

Developments in paper machinery have produced or are producing solutions to enhance water removal and improve paper properties. These include impingement drying and impulse technology. However, both methods also use energy in some other form than steam to bring about drying. On paper machines, where a large share of conventional drying is replaced with new technologies, the energy system should be reconstructed to achieve the increased power-to-heat ratio with better efficiency than with the conventional steam power plant. CHP-solutions based on diesel and gas turbine processes were compared against the steam boiler based process to assess their potential for increasing thermal efficiency in combined power and heat production. The calculated results confirm that the efficiency of heat and power production can be significantly increased in the selected cases by utilising either diesel or gas turbine technology.     

Dehumidifier batch drying—effect of heat‐losses and air‐leakage

The performance of a dehumidifier dryer is influenced strongly by the operating temperature and humidity. This paper shows how heat conduction and air leakage losses can cause the temperature of a dehumidifier kiln to collapse in a batch drying process, resulting in increased drying time and energy use. By means of a dynamic simulation model it is shown that heat loss due to an uninsulated floor alone may be sufficient to prevent a kiln reaching its normal operating temperature. It is shown that the effect of heat losses is exacerbated when the dehumidifier capacity is modulated for humidity control. Auxiliary heating can prevent temperature collapse in a poorly insulated kiln. This maintains the drying speed but adds considerably to the energy used. To avoid these difficulties the insulation and air seals of a dehumidifier dryer should be appropriate to the power dissipated by the dehumidifier and fans. An example is presented in which sealing and insulating the kiln yields a reduction of 44 per cent in the drying time, a reduction of 32 per cent in energy use, and an increase of 168 per cent in net operating revenue. Copyright © 2000 John Wiley & Sons, Ltd.     

Energy efficiency improvements in longan drying practice

This paper presents and evaluates methods of improvement of energy utilization and reduction of energy cost in conventional unpeeled longan drying. Existing dryers were modified into a new dryer arrangement. Performance in terms of specific energy utilization, thermal efficiency and operating cost indices for both traditional and new designs was evaluated. Results showed that the modified dryer yielded an average thermal efficiency of 0.35, compared to 0.29 for the existing dryer. For the same mass of dried longan produced, specific energy utilization and fuel cost were reduced by more than 16% and 80%, respectively. The improvement was attributed to fuel switching from liquefied petroleum gas to wood, heat recovery via hot air recirculation, better temperature and humidity control, and thermal insulation. The new dryer with improved design and better energy efficiency was estimated to have payback period less than 3 years.