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Qualitative comparison of the temperature gradients produced by microwave and conventional heating. Unlike conventional heating, the temperature inside a material heated by microwave is usually higher than the temperature of the surrounding atmosphere.

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Microwave potential for bioenergy production


Dr Beatriz Fidalgo Fernandez

Despite investigations of microwave heating and biomass conversion, large-scale application of microwaves is very limited. Future industrial deployment of microwave technology depends on the understanding of the heating mechanism and the scalability options, as Dr Beatriz Fidalgo Fernandez explains.

The design and manufacturing of magnetrons for the generation of microwave radiation goes back to the first half of the 20th century. Although their ability to heat materials was documented, the research focus was on the application of microwaves in military navigation and communication devices. It was not until the 1980s that the use of household microwave ovens rose massively, contributing to the “culinary revolution” of ready-to-cook meals and safe and rapid defrosting of food. Nowadays, microwave ovens are considered indispensable appliances in many homes due to their convenience and speed compare hob cookers and conventional ovens. Thus, the penetration rate of microwave oven in the UK increased from approximately 74% in 1995 to 91% in 2014. Despite the wide presence of microwave radiation in the everyday life, the understanding of the microwave heating mechanism and the recognition of its huge potential to be applied to diverse industrial processes is very limited.

What is Microwave Heating?

Microwaves are a non-ionising electromagnetic radiation that lies between infrared and radio waves, in the range of the electromagnetic spectrum limited by the frequencies between 0.3 and 300 GHz. The usual operating frequency for domestic and industrial microwave applications is 2.45 GHz or 915 MHz to avoid interference with radar and telecommunication frequencies.

High-frequency radiation such as microwaves causes dielectric heating. The electric field component of the microwave radiation interacts with the charged particles of the irradiated material. Heating up of the material occurs when the particles are not free to move and try to align with the alternating field. This phenomenon is known as dielectric polarisation. A common misconception is to believe that materials need to have significant water content in order to be heated by microwaves. There are in fact two types of polarisation mechanisms involved in microwave heating: dipolar polarisation and space charge polarization. Water and other polar fluids (ethanol for example) heat due to dipolar polarisation, which occurs in dielectrics that have induced or permanent dipoles. Space charge polarisation occurs in dielectric materials with charged particles (electrons, ions, etc.) which are free to move in a delimited region. This mechanism predominates in dielectric solids such as most carbon materials and some inorganic oxides and sulphides, and it is of great relevance in the case of microwave application to conversion of fuels and biofuels.

Microwave heating is the direct conversion of microwave energy into thermal energy within the heated material, rather than a heat transfer as in the case of conventional heating. This volumetric nature of microwave heating is its distinctive feature. A number of advantages of microwave heating over conventional heating are consequence of these core differences in the heating mechanisms. Some of these advantages can be easily identified by any person who has ever used a microwave oven: rapid heating, quick start-up and stoppage or reduced processing time. Other advantages may not be so evident but are extremely important when microwaves are applied to industrial processes: non-contacting heating, higher level of safety and automation, reduction in the size of equipment and higher flexibility or selective heating of materials.

Microwave Heating in Industrial Processing and Research

The number of applications of microwave processing has significantly increased over the last decades. Nevertheless, these industrial applications are relatively low compared to the enormous potential shown by microwave heating. During the 1980s microwave-assisted organic chemistry attracted great interest as laboratory technique due to the dramatic reduction in reaction times and the improved selectivity towards desired products. After three decades, microwave heating applied to organic synthesis has become a mature technology and is used in academia and more recently in chemical, pharmaceutical and biochemical industries with commercially available microwave chemistry equipment. Due to the potential energy savings and enhanced efficiency, microwaves has also been successfully applied to other industrial processes such as food processing, sterilisation and pasteurisation, wood drying, rubber vulcanization, and processing of ceramics, minerals and metallic materials. Moreover, extensive investigation has been recently carried out on the use of microwave radiation in diverse processes such as ceramic and polymer processing, metallurgy and mineral processing, contaminated soil remediation, waste management, production, modification and regeneration of activated carbons, upgrading of fractions of petroleum, gas-phase catalytic reactions (dry reforming of CH4, H2S decomposition, NOx and SO2 reduction, or CH4 oxidative coupling), or thermal conversion of biomass and upgrading of biofuels.

Microwave Heating and Bioenergy Processes

The investigation of microwave-assisted biomass and biofuel processing in order to achieve improvements with respect to conventional approaches has increased significantly over the last decade. Microwave heating has been applied to almost any process involving biomass conversion with varying success. The aim of nearly all the studies has been to improve product quality and quantity, reduce processing time, and ultimately increase the overall efficiency of the process.

Driven by the established benefits of the application of microwaves to organic synthesis, studies on the use of microwave heating in bioethanol, biodiesel and biogas production have been carried out. In the case of bioethanol production, research has been focused on microwave pretreatment. Microwave-assisted hydrolysis of different lignocellulosic biomass have been proven to increase bioethanol yields due to enhanced degradation of lignin, cellulose and hemicellulose, and increased enzymatic susceptibility of the materials. Microwaves have also been studied as pretreatment method of various organic wastes and sludge to be processed by anaerobic digestion, enhancing larger production of methane. Moreover, microwave pretreatment favours dewatering, reduces foaming and diminishes the amount of pathogens in the sludge. Microwave-assisted biodiesel production has been widely studied. Microwaves has been successfully applied to biomass transesterification reactions both in batch and continuous set-ups; shorter processing times, higher yields and higher product purities have been obtained compared to those achieved under conventional heating. And the use of microwave radiation prior lipid extraction has been found to enhance the efficiency of the extraction processes. Despite these advantages, some reports have questioned the actual energy savings when using microwave heating in all these processes, and have highlighted the need to optimise each particular process and to carry out a thorough assessment of energy usage.

Microwave-assisted thermochemical conversion of biomass and biofuels has been far more investigated than the biochemical processes. Microwave-assisted pyrolysis of a wide range of biomass feedstocks is the process that has attracted more attention. In general, raw biomass is not good microwave absorber; meaning that it is not easily heated by microwave radiation. Its dielectric properties improve as the pyrolysis proceeds and the carbon content of the remaining residue increases. In fact, the ability of char obtained from biomass pyrolysis to be heated by the microwaves is very good and it can be used as microwave receptor to indirectly heat the biomass up to the temperatures required for thermal decomposition. Other materials such as activated carbon or metal oxides have also been used as microwave receptors. As happens under conventional heating, microwave pyrolysis gives rise to liquid, gas and solid fractions, and the proportion in which the three fractions are produced depends on the operating conditions.

Microwave-assisted pyrolysis for biochar production is carried out at low temperatures (i.e. lower input microwave power). The solid residue produced by this process has been observed to exhibit a good potential for gas adsorption and higher energy content than the char produced by conventionally-heated pyrolysis. However, lower energy efficiency has been reported in the case of microwave pyrolysis. It should be noted though that this efficiency depends on the suitability of microwave oven design for the process requirements, which may not be optimum when considering lab-scale rigs.

Various studies have established the technical feasibility of microwave pyrolysis for bio-oil (liquid fraction) production, which is carried out at high heating rates and moderates temperatures. Since the operating conditions, the biomass feedstock and the reactor configuration have an effect on the yield and composition of the bio-oil produced, it is difficult to establish general conclusions. Nevertheless, most researchers have observed that the quantity and quality of the bio-oils produced under microwave heating are significantly better than those under conventional pyrolysis. Less polycyclic aromatic hydrocarbons, increasing amount of phenolic compounds and a larger degree of deoxygenation have been observed in the bio-oils produced under microwave heating. This improved composition of the bio-oil is actually related to the selective heating provided by the microwaves. The microwave process can be carried out flowing a cold sweep gas (which is barely heated by the microwaves) and so the released volatiles are cooled and condensed rapidly. The secondary reactions of the liquid precursors due to high temperatures are largely avoided and much more compounds are preserved in the final bio-oil. The implications of the enhanced composition of the bio-oil produced from microwave pyrolysis are significant since one of the main issues on the production of pyrolysis bio-oil is its high oxygen content. Moreover, few preliminary results have shown the effectiveness of microwave heating applied to ex-situ upgrading of bio-oil.

Microwave-assisted pyrolysis for gas production is carried out at high temperature which favours secondary reactions between vapours and solid residue. The yield of the gas produced from microwave pyrolysis has been found to be larger than that obtained from conventional pyrolysis. Not only more gas is produced but also its composition is higher in valuable synthesis gas (H2 + CO) and lower in undesirable greenhouse gases (CH4 and CO).

The change in the gas composition is explained based on the enhancement of CO2 gasification of the char and methane decomposition into H2. Microwave radiation is known to have the potential to increase reaction rate, selectivity and yield of catalytic heterogeneous reactions (as in the case of CO2 gasification and CH4 decomposition) due to the random formation of microplamas throughout the solid residue, in which temperature is much higher than the average temperature. In addition to the improved composition of the gas, it has been reported that less char and tar are produced under these gasification conditions. The lower amount of tar can potentially simplified subsequent gas cleaning operations, improving the overall process efficiency. Microwave-assisted steam and CO2 gasification of biomass has also been investigated, although to less extent than pyrolysis. Actually, microwave gasfication has been mainly carried out over biochar produced from conventional pyrolysis. High conversions have been observed at lower temperatures and shorter reaction times than those needed under conventional heating. These high conversions are attributed to the formation of microplasmas.

In line with the enhanced performance of heterogeneous reactions under microwave radiation, the upgrading of biogas (CH4 + CO2) into syngas has been studied. The conversion of biogas into hydrogen is currently carried out via steam reforming and after the removal of CO2, which is a highly energy demanding operation. Dry reforming of biogas (reaction between methane and carbon dioxide) is carried out without the need for any conditioning stage. High conversions and production of synthesis gas has been achieved by microwave-assisted dry reforming of biogas, even using low-cost carbon materials as catalysts. Low conversions of CH4 and CO2 have been obtained using similar catalysts and conditions under conventional heating. Moreover microwave-assisted dry reforming of biogas over a mixture of carbon and metal-based catalysts has exhibited similar conversions to those obtained under conventional heating using metal-based catalysts alone. Therefore, the application of microwaves has been proven to allow the replacement of part of metal catalyst by low cost carbonaceous catalysts without affecting conversions.

What’s next?

The enormous potential and applicability of microwave heating in industrial processes is clear. The interest of the bioenergy industry in the microwave technology may increase because of the need of more and more efficient processes. However, there are reservations on the use of this alternative heating source mainly because of the considerable challenges for scaling-up the technology, which are due to the lack of understanding of the interactions of microwaves with biomass and biofuels and the processes trigged. Further research and a multidisciplinary engineering approach are needed to improve the knowledge, understand the scalability options, and contribute to the deployment of the technology in the bioenergy industry and many other sectors.

ABOUT THE AUTHOR

Dr Beatriz Fidalgo is Lecturer in Clean Energy Technology at Cranfield University. She holds a MSc in Chemical Engineering from the University of Santiago de Compostela (Spain) and a PhD in Energy Engineering from the University of Oviedo (Spain). She developed her thesis research at the Spanish National Institute of Coal (CSIC) on the Microwave-assisted CO2 reforming of methane. Her expertise is in thermochemical and thermocatalytic conversion of conventional and renewable carbon-based fuels, and microwave-induced processes involving carbon materials. Her current research interest is in biomass thermochemical conversion, and integrated biorefinery for biofuels, energy and chemicals.

 

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ANUMAKONDA JAGADEESH said

08 September 2015
Excellent. This method we want to adopt.
Biofuel can be produced from Agave. Oxford University study on
agave-to-ethanol: http://pubs.rsc.org/en/content...
“The sustainability of large-scale biofuel production has recently been called into question in view of mounting concerns over the associated impact on land and water resources. As the most predominant biofuel today, ethanol produced from food crops such as corn in the US has been frequently criticised. Ethanol derived from cellulosic feed stocks is likely to overcome some of these drawbacks, but the production technology is yet to be commercialised. Sugarcane ethanol is the most efficient option in the short term, but its success in Brazil is difficult to replicate elsewhere. Agaves are attracting attention as potential
ethanol feed stocks because of their many favourable characteristics such as high productivities and sugar content and their ability to grow in naturally water-limited environments. Here, we present the first life cycle energy and greenhouse gas (GHG) analysis for agave-derived ethanol. The results suggest that ethanol derived from agave is likely to be superior, or at least comparable, to that from corn, switchgrass and sugarcane in terms of energy and
GHG balances, as well as in ethanol output and net GHG offset per unit land area. Our analysis highlights the promising opportunities for bioenergy production from agaves in arid or semi-arid regions with minimum pressure on food production and water resources.
“[...] the emissions of agave-derived fuel are estimated to stand at around 35g of CO2 per megajoule from field-to-wheel, compared to the 85g/MJ emitted when
making corn ethanol.”
Dr Tan and his colleagues found this energy balance is five units to one.
“This compares favourably to the highly efficient sugarcane, and to the less efficient corn as a source of biofuel. It also compares favourably to sugarcane-derived ethanol for its ability to offset greenhouse gas emissions, which we calculated at 7.5 tons of CO2e per hectare per year – taking into account the crop’s complete lifecycle”
The main drawback for wider application of Biofuels is input. There was a big movement for biofuel from Jatropha in India but in reality not much has been achieved. Agave(Americana),Sisal Agave is a multiple use plant which has 10% fermentable sugars and rich in cellulose. The fibre is used in rope making and also for weaving clothes in Philippines under the trade name DIP-DRY. In Brazil a paper factory runs on sisal as input. A Steroid HECOGENIN is extracted from this plant leaves. Since on putrification,it produces methane gas, it can be cut and used as input in
biogas plants. Also in Kenya and Lesotho dried pieces of Agave are mixed with concrete since it has fibres which act as binding.
Here is an excellent analysis on Agave as a biofuel:
Agave shows potential as biofuel feedstock, Checkbiotech, By Anna Austin, February 11, 2010:
“Mounting interest in agave as a biofuel feedstock could jump-start the Mexican biofuels industry, according to agave expert Arturo Valez Jimenez.
Agave thrives in Mexico and is traditionally used to produce liquors such as tequila. It has a rosette of thick fleshy leaves, each of which usually end in a sharp point with a spiny
margin. Commonly mistaken for cacti, the agave plant is actually closely related to the lily and amaryllis families. The plants use water and soil more efficiently than any other plant or tree in the world, Arturo said. “This is a scientific fact—they don’t require watering or fertilizing and they can absorb carbon dioxide during the night,” he said. The plants annually produce up to 500 metric tons of biomass per hectare, he added.
Agave fibers contain 65 percent to 78 percent cellulose, according to Jimenez. “With new technology, it is possible to breakdown over 90 percent of the cellulose and hemi cellulose structures, which will increase ethanol and other liquid biofuels from lingo cellulosic biomass drastically,” he said. “Mascoma is assessing such technology.”
Another plant of great use is OPUNTIA for biogas production.
The cultivation of nopal((OPUNTIA FICUS-INDICA), a type of cactus, is one of the most important in Mexico. According to Rodrigo Morales, Chilean engineer, Wayland biomass, installed on Mexican soil, “allows you to generate inexhaustible clean energy.” Through the production of biogas, it can serve as a raw material more efficiently, by example and by comparison with jatropha.
Wayland Morales, head of Elqui Global Energy argues that “an acre of cactus produces 43 200 m3 of biogas or the equivalent in energy terms to 25,000 liters of diesel.” With the same land planted with jatropha, he says, it will produce 3,000 liters of biodiesel.
Another of the peculiarities of the nopal is biogas which is the same molecule of natural gas, but its production does not require machines or devices of high complexity. Also, unlike natural gas, contains primarily methane (75%), carbon dioxide (24%) and other minor gases (1%), “so it has advantages from the technical point of view since it has the same capacity heat but is cleaner, “he says, and as sum datum its calorific value is 7,000 kcal/m3.
I had been advocating Biofuel from Agave and Opuntia besides Biogas for power production. Unfortunately in India, we are in most cases imitators but not innovators. First Box Type solar cooker was from India. But often we adopt western designs. Unless west recognizes, we don’t recognize.
I submitted a research project on Biofuel from Agave and Biogas from Opuntia to Government of India. If any industrial houses/organisations are interested in promoting this in India I have collaboration with leaders in the field from Mexico,UK.US and Australia.
Here is more important information:
Agave's lower lignin content (down to 2.4%) and higher cellulose content (62%) makes it ideal for production of Biofuel. Agave can be intercropped with Opuntia(Prickly Pear) which will be used to generate biogas for renewable electricity generation. Biogas power generators from KW size to MW size are commercially available from Germany,China,Vietnam etc. The cost of production per Kwh with Opuntia can be as low as US$ 3.00 per million BTU. On an annual basis,one hectare of agave can yield upto ten times the ethanol one hectare of sugarcane in Brazil. Agave to Ethanol's CO2 e emissions are lower than sugarcane and corn.
Water - footprint -- agave does not have any. Agave uses water,light and soil most efficiently amongst plants/trees on earth. Agave is packed with sugars, on an annual basis one hectare of agave yoelds upto 10 thousand gallons of ethanol(from its sap/juice) and 6500 gallons of cellulosic ethanol. No other plant in the World has such potential.
I have a plan: We have SPECIAL ECONOMIC ZONES (SEZ). Just like that we can start YOUTH ECONOMIC ZONES (YEZ). Wastelands can be given to youth on a lease basis(about 10 acres per youth) and 1o such youth can form a co-operative. They can cultivate fast growing multiple use plants like Agave and Opuntia. Power generation plants can be set up at local level. This way there will be decentralised power. This fits in Mahatma Gandhiji's Concept of AGRO INDUSTRIES utilising local resources and resourcefulness. Youth can be given short term training in Agricultural operations. This way we can provide employment to Youth besides bringing waste and vacant land under cultivation.
What is more, large plantations of Agave and Opuntia lead to climate Stability as both are CAM plants. Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but open at night to collect carbon dioxide (CO2). The CO2 is stored as the four-carbon acid malate, and then used during photosynthesis during the day. The pre-collected CO2 is concentrated around the enzyme RuBisCO, increasing photosynthetic efficiency.
Developing countries like ours which have millions of hectares of waste lands can transform rural economy by going in for Agave and Opuntia plantations on a massive scale. As one Exonomist put it, IT IS NOT THE LACK OF RESOURCES BUT RESOURCEFULNESS THAT EXPLAINS WHY PEOPLE PERISH IN THE MIDST OF PLENTY.
Another plant which can be put to many uses is Opuntia:
Opuntia: Biofuel/biogaspower/Biochar
Isolation of pigment from Opuntia Pods for use in Paints
Fruits,Fruit Juice in Tins, Pod Juice in Packed form
SAP after extraction of juice from pods as Mosquito repeller in water logged ponds.
Medical Uses as Diabetics cure.
Dr.A.Jagadeesh Nellore(AP),India

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