Contactless electromagnetic technologies 

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Ecological footprint and source depletion

Ecological footprint metric is widely used in ecosystem accounting. It is a measure of mankind demand on the earth’s ecosystem. In other words, it measures how fast we consume resources and generate waste compared to how fast nature can absorb our waste and generate new re-sources. World population has reached 7.2 billion in 2014 and it has doubled in the last 45 years. For several years population has been growing much faster than many vital non renew-able sources. Developed countries are consuming water, minerals and forest faster than they can regenerate. Animal and plant species are extincting everyday. Two billion people nowa-days live in poverty, more than the world population one hundred years ago. This turns into a scale problem [19]. Not just overpopulation is the only issue but the high levels of energy consumption multiplied by the number of consumers, especially in developed countries. For example, the United States represent 4.5% of the world population and consume 20% of its energy.
Mankind’s energy consumption is the main contributor to release greenhouse gases, in partic-ular CO2 emissions to the atmosphere. Fossil fuels represent 80% of the world energy primary consumption and they are required for global energy needs nowadays [20]. King Hubbert pre-dicted peak oil in the 60s, oil would peak in about 1970 and decline thereafter [21]. The lack of fossil fuels must be replaced to keep current levels of consumption. Depletion of fossil fu-els represents a future challenge, World Coal Institute has determined for coal, oil and gas to last 155, 41 and 65 years respectively at 2006 levels of energy consumption. However, differ-ent studies of economic models to predict reserves of fossil fuels differ and nobody can predict exactly when supplies will be exhausted. This ambiguity in results show a clear controversial theme. Even so, fossil fuels are limited sources, the time left is imponderable [22].

Green tendency

As climate change is one of the major problems of the 21st century, the use of renewable energy sources are being highly promoted. Government policies have been applied in most coun-tries in Europe, Canada and USA to rise the use of renewable energies. These policies involve funding to develop infrastructure and scientific research, with the aim of optimizing the use of natural resources. Biomass has represented 56% of total research on renewable energies, followed by solar 26% and wind 11% between 1979 and 2009. This is because biomass is a very promising energy. Besides the fact that its price is stable and economical as it comes from waste products, the affordable cost of converting biomass into energy in comparison with other RES1 makes biomass even more interesting [23]. It also has the advantage of being stored and continuously used regardless of the climate, while other energies as wind and solar are weather dependent, they can only be produced when the natural resource is available [24]. Biomass con-version efficiencies have been continuously improving in the past years. Gasification and direct combustion are the main conversion techniques to generate electricity and heat using biomass. Direct combustion technique is more widely applied, it represents 90% of all biomass plants in the world. This is because it usually requires less cost investments than gasification tech-nologies. Nowadays forestry and agriculture residues are the principal sources of biomass for electricity and heat generation. In 2010 bioenergy has produced 1313 Mtoe2, wood biomass rep-resents 87%, 9% comes from agricultural crops and 4% from municipal and industrial residues. The total bioenergy consumption is expected to rise to 3271 Mtoe in 2040. Wood is an abundant source present in almost every country. Among the renewable energy sources, wood biomass is a very convenient alternative for heating and electricity production because of the simple conversion technologies [25].

Wood biomass energy for heating

At present, within biomass energy based on power and heat CHP3 generation, wood chips and pellets combustion are the most economically and environmentally convenient options [26]. Biomass co-firing is regarded as one of the most short-term attractive options for power gen-eration. It is based on the combustion of biomass and pulverized coal or gas. Most biomass co-firing systems use existing coal combustion plants so a very low cost investment is required. Replacing old existing coal plants with biomass co-firing has proved to be reducing to almost zero the emissions of CO2 in North America. But the leading trend is the transition to a com-plete carbon-free power generation, which is dedicated biomass combustion, based on pellets or wood chip boilers. These firing systems are commercially available to produce hot water or steam. Their main applications are domestic and district heating systems [27].

Towards a new energy strategy for Europe 2011-2020

Strong dependence on fossil fuels and inefficient use of raw sources contribute to global warm-ing, turn into high energy prices and threats the economic security. The expansion of the world population and the increase of energy consumption will intensify the damage. Therefore the fight against climate change requires drastic plans. Europe is facing a moment of transfor-mation, the crisis has caused recession and nowadays changes are required. There are very optimistic perspectives for this decade and several goals to achieve. Regarding to energy is-sues, 2020 promotes more efficient energy resources, more renewable contribution and more economical competitiveness [11]. The « 20-20-20 » objectives set in the Climate and Energy Pack-age by the European Commission in 2008, represent three key goals that have to be reached by 2020:
• A 20% reduction in emissions of greenhouse gases in the EU, compared to 1990 levels. Trying to reduce them to 30% if conditions are good.
• A 20% increase in participation in the EU energy produced from renewable resources.
• A 20% improvement in energy efficiency in the EU.
Based on these objectives, each country has a national policy to support the energy efficiency and to prevent the climate change. The details for each country are specified in the national action plans for energy efficiency and renewable energy and are defined according to their con-sumption and resources. Figure 1.3 shows the ratio of renewable energy consumption of each country in 2013, and the ratio that must be reached by 2020. Some of the Northern countries have already developed a competitive structure of renewable sources making easier to reach the goal set by the European Union. Others like France, United Kingdom or Netherlands must double or even triple their renewable consumption in relation to 2011 [35].

Freeze drying

This technique is mostly used in alimentary industry, the absence of liquid water and the low temperatures required results in less deterioration of the product so it gives a final product of better quality when compare with oven drying method. Despite of these advantages, freeze-drying is seen as the most expensive process of drying [60]. It consists on freezing the sample at -20 °C which then is dried into a vacuum chamber at ambient air temperature during several days. When applying freeze and oven drying on wood chips, the resulting MC is slightly lower if using freeze drying due to the absence of volatile non-water compounds [58]. Figure 2.1 presents a commercialized model of freeze dryer (Christ ALpha 1-2 LD Freeze dryer) whose ice condenser capacity is 2.5 kg and operating temperature down to −55°C. This process lasts 24 hours for drying a 2-kg sample. There is a wide range of size capacity available [61].

Oven drying

This standard method, EN 14774-1:2009 or ISO/CD 18134-1, is applicable to all solid biofuels. The way of determining the MC is taking a sample of biofuel of about 300 g and drying it in an oven at a temperature of 105°C in air atmosphere until the mass remains constant. This process usually takes 24 hours. Figure 2.2 shows a typical oven applied to the standard method EN 14774-1:2009 or ISO/CD 18134-1. Its use is common in industry and science since provides a precise drying performance. This specific model (Universal Oven UN55plus) works for a temperature range from 5°C above ambient up to 300°C, it holds a 53-L volume and 80-kg maximum loading. Temperature, air flap position and time can be programmed [62].

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Infrared drying

Infrared driers are designed for industrial and professional sectors. They are scale driers that determine the weight while drying. Humidity can be quickly and accurately detected in many different materials such as: powder materials, food, pallets or wood chips. The samples are small, less than 60 g and the time required for wood chips drying is between 7 to 48 minutes [56]. Figure 2.3 presents a moisture analyzer balance based on infrared drying, this model (Infrared Moisture Analyzer MA35M-000115V1) is a simple model that measures samples up to 35 g with 1 mg resolution [63].

Microwave Drying

They are scale driers, like the infrared driers. Unlike conventional oven drying, microwave drying yields accurate results in less than 20 minutes, making it ideal for process control [56]. Figure 2.4 gives a moisture content measurement device based on microwave radiation. Its range of moisture is from 0% to 100% with a 0.2 % bias [64].

Analytical methods

Contrary to thermogravimetric methods, analytical methods take the possible volatile com-pounds lost into account. Avoiding the combustion of treated or impregnated material, analyt-ical methods are chemical methods based on the mixture of the fuel with a chemical element. Azeotropic distillation with water immiscible solvents like toluene or xylene are used to de-termine MC in lignocellulosic biomass. Water distills with the solvent and after condensation, these get separated so the quantity of water can be determined [65]. Xylene distillation ex-periments were performed in reference [58] and compared to results obtained by the standard oven method. The deviation in the results are explained by the amounts of volatile compounds released in the drying method. Karl Fisher method is also used in the determination of MC in wood, dry methanol displaces the water in the sample. Afterward, the water is titrated via the Karl Fisher method. This method is neither affected by possible volatile compounds. Automated Karl fisher titrators are commercially available and they offer excellent measuring accuracy. Figure 2.5 introduces this technology. It can either be used as stand-alone titrator or integrated into an overarching network. The Karl Fisher Titrando family of titrators includes a variety of coulometric, volumetric, and combined titrators, enabling to analyze any water content from 0.001 to 100% [66].

Rapid or indirect methods

The increasing use of wood chips as fuel implies more bulk MC measurements, which are nowadays based on the time-consuming standard methods. Deliveries and contractor num-bers are growing rapidly, expecting accurate delivery control for the bulk fuel. In order to be able to apply feed forward control, moisture content for chips bulk or flow should be measured automatically and online. The importance of saving time and obtaining information on the MC prior to incineration is the main motivation for using rapid test methods for moisture determi-nation. Rapid measurements of the MC are based on using electrical, optical, radiometric or hygrometric methods.

Radiofrequency methods

The dielectric properties of the material depend on its physical properties, especially MC. This fact is utilized in microwave and RF measurements [67]. The term radiofrequency RF theoret-ically represents electromagnetic waves of frequencies between 3 Hz and 300 GHz. However the term microwaves is commonly used for frequencies between 1 GHz and 300 GHz. So ra-diofrequency refers here to frequencies lower than around 1 GHz.

Hygrometrical methods

In the field of moisture measuring, there are two kinds of moisture. The absolute moisture of the material, in this case wood, that indicates the percentage of water content referred to the dry mass. And the relative equilibrium moisture content that indicates the relative moisture of the ambient air counterbalancing the material, in this latter case, the material does not absorb or release any moisture. Air humidity balance method is based on the property of wood of absorbing or releasing water from or to the atmosphere (hygroscopic properties) [83]. When the atmosphere and the material present the same water vapor pressure inside a container, this means that the sample is under constant conditions. As the equilibrium air humidity is a func-tion of the MC of the sample and the temperature, MC can be determined via studying these two parameters. In spite of the accuracy of the humidity balance method, it is not applicable for wood chips because it reaches precision up to 14% MC (wet base) [56]. Sven Hermansson presents the option of determining the MC of wood chips via measuring the oxygen and mois-ture content of the flue gases inside a furnace. The furnace must be equipped with flue-gas condenser. Relative humidity sensors can be used in gases up to 200°C, qualifying the method for this application. Results indicate that the method can predict changes in MC. By knowing the fluctuations in fuel on-line, parameters of the furnace can be regulated to ensure good per-formance, but it has been found that the accuracy of the sensor deteriorates with the presence of water. Condensation phenomena influences the relative humidity measurement so that this method just allows measurements of wood chips with very low MC [84]. Measurements of wood chips based on flue-gas in Sweden consist of determining efficiency via several measurements (inlet air, water steam to the air and combustion, flue-gas) after the combustion chamber and outdoors [57]. Commercialized products based on this technology are available [85]. Figure 2.10 presents an example of humimeter for 12-L sample volume. It covers a range from 5% to 70% water content and its resolution is up to 0.1% water content.

Table of contents :

1 Wood energy sector 
1.1 Critical condition of current energy consumption
1.1.1 Ecological footprint and source depletion
1.1.2 Green tendency
1.2 Wood biomass energy for heating
1.2.1 Interest of the industry
1.2.2 Wood biomass
1.2.3 Boilers
1.2.4 Towards a new energy strategy for Europe 2011-2020
1.2.5 French energy policy
1.3 Industrial process
1.3.1 Production chain
1.3.2 Importance of standards
1.3.3 Standards
2 Background 
2.1 Thermogravimetric methods
2.1.1 Freeze drying
2.1.2 Oven drying
2.1.3 Infrared drying
2.1.4 Microwave Drying
2.2 Analytical methods
2.3 Rapid or indirect methods
2.3.1 Radiofrequency methods
2.3.2 Microwaves methods
2.3.3 Electrostatic methods
2.3.4 Optical methods
2.3.5 Hygrometrical methods
2.3.6 Radiometric methods
2.4 Synthesis
3 Contactless electromagnetic technologies 
3.1 Preliminary physical background
3.1.1 Wood chip dielectric properties
3.1.2 Electromagnetic propagation
3.2 Transmission-Reflection system
3.2.1 Experimental prototype
3.2.2 Optical module Stereo vision method Stereo matching process 3D reconstruction Experimental results and discussion
3.2.3 Electromagnetic module Material Procedure Direct frequency domain analysis Direct time domain analysis Multivariate analysis
3.3 Conclusions
4 In contact technologies 
4.1 Capacitive system
4.1.1 Principle
4.1.2 Experimental system and procedure
4.1.3 Results and discussion
4.1.4 Summary
4.2 Resonator technology
4.2.1 Underlying physics
4.2.2 Antenna resonators Half-wave dipole antenna Quarter-wave monopole antenna Microstrip rectangular antenna In-line half-wave dipole antenna
4.2.3 Simulation analysis
4.2.4 Experimental analysis
4.2.5 Study of half-wave dipole antenna performance
4.2.6 Laboratory-scale system Equipment Methodology Sensor characterization Multivariate analysis Summary
4.3 Full-scale implementation
4.3.1 Bulk measurements in static piles
4.3.2 Bulk measurements in a container
4.3.3 Summary
4.4 Conclusion


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