Methanol from Biomass

Although reserves of methane from natural gas are still very large, they are nevertheless finite and diminishing. The exploitation of unconventional natural gas resources and methane hydrates could significantly increase the amounts of methane available to mankind. Innovative processes continue to be explored for the transformation of methane into easy-to-handle liquid fuels, primarily methanol, but this will not solve the problem of increasing CO2 concentrations in the atmosphere and its detrimental effect on the global climate. Even if methane does release less CO2 than coal or petroleum, its increased utilization to fill the world's growing energy needs will still produce large quantities of CO2. Thus, new ways are needed to fulfill mankind's ever bigger appetite for energy, without adversely affecting our environment. In the long term therefore, methanol will have to be produced from sources that release only minimal quantities of CO2, or even no CO2. In this respect, the use of biomass is one possibility, although natural resources will be unable to cover all our needs.

Biomass is referred to as any type of plant or animal material - that is, materials produced by life forms. This includes wood and wood wastes, agricultural crops and their waste byproducts, municipal solid waste, animal waste, and aquatic plants and algae. Methanol was originally made from wood (hence the name wood alcohol), but due to its inefficiency and the advent of synthesis via syn-gas, this route was rapidly abandoned in the first half of the 20th century. Increased oil and natural gas prices, dependence on foreign countries for energy supply, growing concerns about CO2-induced climate changes as well as the depletion of easily accessible fossil fuel resources, have prompted many to reconsider a wider and more extensive use of biomass to fulfill our energy demands. Since biomass itself is both bulky and heterogeneous, one way to achieve this goal would be to convert it into a convenient liquid fuel, namely methanol. The modern production of methanol from biomass is very different, however, and is much more efficient than the methods used a century ago. Basically, not only wood but also any organic (i.e., carbon-containing) material obtained from living systems can be used in the process. Technologies to transform biomass into methanol are generally similar to those used to produce methanol from coal. They imply the gasification of biomass to syn-gas, followed by the synthesis of methanol with the same processes employed in plants based on fossil fuels.

Before the gasification step, the biomass feedstock is usually dried and pulverized to yield particles of a uniform size and with moisture content no higher than 15-20% for optimal results. Gasification is a thermochemical process that converts biomass at high temperature into a gas mixture containing hydrogen, carbon monoxide, carbon dioxide, and water vapor. The pretreated biomass is sent to the gasifier where it is mixed, generally under pressure, with oxygen and water. There, a part of the biomass is burned with oxygen to generate the heat necessary for gasification. The combustion gases (CO2 and water) react with the rest of the biomass to produce carbon monoxide and hydrogen. With biomass being used as the heating fuel, no external heat source is necessary. The production of syn-gas from biomass in a single-step operation by partial oxidation is very attractive, but it has experienced technical problems. Gasification of biomass feedstocks is therefore generally a two-step process. In the first stage, called "pyroly-sis" or "destructive distillation", the dried biomass is heated to temperatures ranging from 400 to 600 °C in an atmosphere too deficient in oxygen to allow complete combustion. The pyrolysis gas obtained consists of carbon monoxide, hydrogen, methane, volatile tars, carbon dioxide, and water. The residue, which is about 10-25% of the original fuel mass, is charcoal. In the second stage of gasification, called "char conversion", charcoal residue from the pyrolysis step reacts with oxygen at temperatures from 1300 to 1500 °C, producing carbon monoxide. Before being sent to the methanol production unit, the syn-gas obtained must be purified. Compared to most coals, the advantage of biomass is its much lower sulfur content (0.05-0.20 wt.%), whilst heavy metals impurities (mercury, arsenic, etc.) are present only in insignificant quantities.

Whilst the production of methanol is possible on a small scale, large-scale methanol plants are preferable because they induce substantial economies of scale, thereby lowering the cost of production and increasing efficiency. The quantities of biomass needed to feed a world-scale 2500 tonnes per day methanol plant are, however, very large (in the order of 1.5 million tonnes per year). To deliver such amounts, biomass would have to be collected over vast areas of land [209]. The transportation of bulky biomass products with low energy density over long distances is not economical, and consequently its transformation to an easy-to-handle and store liquid intermediate through fast pyrolysis has been proposed. In fast pyrolysis, small biomass particles are heated very rapidly to 400600 °C at atmospheric pressure to yield oxygenated hydrocarbon gases. The generated gases are then rapidly quenched to avoid their decomposition by cracking. The black liquid obtained, called "biocrude" [210,211] (because of its resemblance to crude oil), has a wide range of possible applications. It can be processed into a replacement for fuel oil and used directly in furnaces for heat or electricity production. By altering its chemical composition, biocrude could also be combined and processed alongside petroleum crude in modified refineries. Although further development is required, biocrude holds promise as a domestically available synthetic alternative to petroleum. Besides biocrude, which is obtained in 70-80% yield, fast pyrolysis also produces some combustible gases and char, a fraction of which is used to drive the process. A fraction of the char could also be finely ground and added to the biocrude to form a slurry; this, like biocrude, might be pumped, stored and transported via road, rail, pipelines and tankers in much the same way as crude oil today, facilitating greatly the handling of biomass feedstock. Fast pyrolysis is a relatively simple, low-pressure and moderate-temperature process which is applicable on both large and small scales, though having been introduced only recently, further development is required for its commercial application. In contrast, the production of syn-gas from liquid feedstocks from diverse fossil origins is a well-established process which is used in numerous plants worldwide. The production of methanol in world-scale plants (>2500 tonnes per day) through syn-gas obtained from biocrude generated in delocalized plants is therefore a feasible and a promising option.

All biomass materials can be gasified for methanol production (Fig. 12.7). The efficiency of the process will, however, depend upon the nature and quality of the feedstocks. Low-moisture materials such as wood and its byproducts and herbaceous plants and crop residues are the most suitable. Using wood and forest residues, efficiencies as high as 50-55% have been recorded. Several demonstration projects are under way in different parts of the world. In Japan for example, Mitsubishi Heavy Industries is operating a pilot plant for the production of methanol from biomass [212]. Various feedstocks such as Italian ryegrass, rice straw, rice bran and sorghum have been successfully tested, with the best yields having been obtained with sawdust and rice bran. In the United States, the Hynol process originally developed at the Brookhaven National Laboratory was tested on a pilot scale [213], and was successful in converting materials such as woodchips into methanol. In Europe, the paper industry has turned its attention to black liquor gasification as a possible source for methanol [214]. Black liquor is a pulp-rich slurry obtained as byproduct of the Kraft pulping operation used for paper

Carbon Neutral Cycle Methanol

Methanol synthesis

Figure 12.7 The carbon neutral cycle of biomethanol production and uses.

Methanol synthesis

Figure 12.7 The carbon neutral cycle of biomethanol production and uses.

production, and contains about half of the organic material that was originally in the wood. It has the great advantage of being already partially processed and in a pumpable, liquid form. With the gasification process, 65-75% of the biomass energy contained in black liquor can be converted to methanol. Worldwide, the pulp and paper industry currently produces about 170 million tonnes of black liquor each year. The United States alone, could potentially produce 28 millions tonnes of methanol per year from black liquor. Clearly, since the United States consumption of gasoline and diesel fuels totals the equivalent of more than 1000 million tonnes of methanol, this would replace only a small fraction of the petroleum-derived fuels. In smaller countries with an important paper industry and limited population (e.g., Finland and Sweden), methanol from black liquor could replace a substantial part of the motor fuel demand (50% and 28%, respectively). Municipal solid waste (about 240 million tonnes are produced each year in the United States) is another possible feedstock for methanol - implying that landfills could actually become energy fields of the future.

Although waste products from wood processing, agricultural residues and byproducts, as well as solid municipal waste, represent suitable feedstocks for methanol production in the near term, the quantities which can be generated from these resources are limited. In the long term, the growing demand for bio-methanol will necessitate a larger and reliable source of raw biomass material. Thus, crops selected specifically for energy purposes would have to be cultivated on a large scale if a significant amount of methanol were to be produced from biomass resources. Suitable "energy crops" are being identified, and the most promising - essentially fast-growing grasses and trees - are being field tested. Depending upon the climate, the species giving the best results are logically different. The potential impact of the large-scale culture of energy-devoted plants on ecosystem, soil erosion, water quality and wildlife is also being assessed. With regard to wood, fast-growing species such as poplar, sycamore, willow, eucalyptus and silver maple have been proposed [215] for cultivation in "short rotation" tree farms in the United States and Europe. In conventional silviculture for plywood or paper production, trees are grown for 20 to 50 years, or more. The term "short rotation" means that fast-growing trees planted at close spacing are harvested after only four to 10 years in order to maximize productivity since, after harvesting, new trees can be planted. Most of the trees under consideration, however, have the ability to resprout from cut stumps in a process called "coppicing", thus eliminating the need to replant after each harvest. Such intensive tree culture has shown to produce generally between 4 and 10 tonnes of oven-dry wood per hectare and year under temperate climates. Higher rates can be obtained in tropical climates with appropriate species, such as eucalyptus in Florida, Hawaii and South America. Besides wood, energy crops such as switchgrass, sorghum or sugar cane have been identified as having a high efficiency in converting solar energy into biomass. To contain the cost of planting, growing and harvesting energy crops, highest productivity is expected. In most cases, this means that prime agricultural land would have to be used for their production. In large countries with a low population density such as the United States, Australia or Brazil, a significant portion of idle food-crop lands, pasture range and forest land could be used for energy crops. However, in more densely populated areas such as Western Europe, Japan or China, most of the arable land is already in use for food production. Unless surplus land is available, it is therefore highly likely that the land needed for energy crops will directly compete with food and fiber production.

As with any form of plant life, high and sustainable productivity for energy crops over the years will only be maintained if the necessary nutrients for the plants growth are present in the soil. In intensive agriculture, as practiced in most developed countries, this implies the massive use of fertilizers. Nitrogen fertilizers in particular, obtained from NH3 derived from natural gas, if used for energy crops, could partially offset the benefits expected from the production of fuels from biomass. With nitrogen fertilizers on cultivated soils and animal wastes being responsible for 65 -80% of the total emissions of N2O (a powerful greenhouse gas, 296-fold more damaging than CO2 [216]), a judicious choice of energy crops requiring minimum input of fertilizers is therefore necessary. Crop rotation, which is a widely accepted practice in agriculture to fight soil impoverishment, should also be applied. Although environmental risks must still be carefully assessed, biotechnology and genetic modifications are also expected to provide additional gains in crop yields.

From an industrial viewpoint, the production of energy crops in vast monocultures including only one species would be preferable. Such types of culture are, however, generally more prone to epidemics of pests and diseases, requiring frequent applications of pesticides and fungicides in order to keep them free from potentially disastrous infestations that might wipe out the entire plantation. In order to avoid this situation, several types or species of energy crops could be planted instead of only one; this would also allow a higher degree of biodiversity. At the same time, the introduction of nitrogen-fixing trees such a alder or acacia in fast-growing poplar and eucalyptus plantations could also help to reduce fertilizer requirements [117].

In avoiding the use of prime farmland, energy crops capable of being grown on marginal or degraded lands have also attracted much attention as a means of adding value to vast areas unsuitable for most other agricultural purposes. The mes-quite tree and its sugar-rich pods for example, already thrives in the United States on tens of millions of hectares of semi-desert land which, without proper irrigation, would otherwise be of little value for the production of food crops or as pasture land. Due to its extensive root system, which also helps to maintain the soil in place and to protect the land from erosion, the mesquite tree is able to reach far into the ground to capture water. Also present in the root systems are bacterial nodules that fix the nitrogen contained in air and provide not only the mesquite tree but also its neighboring plants with the nutrients necessary for growth. Although the productivity of plants such as the mesquite trees are is less than for fast-growing poplar or eucalyptus, they have the advantage of growing, without irrigation needs, on vast areas of land that would otherwise be of limited value.

In Australia, the National Science Agency devised a computer model to show that 30 million hectares of fast-growing trees planted over the next 50 years could produce enough methanol to replace 90% of the oil requirements of the country's 25 million people by 2050. It would, by the same time, create numerous jobs in rural areas and help to reduce the impact of salt that otherwise would be brought to the surface by rising water tables.

The European Union, with a population of over 450 million after its expansion to 25 member states, has much less potential for energy crops cultivation. Of the almost 400 million hectares total land area, about 170 million hectares are currently used for food production, while another 160 million hectares are covered by forests. Most of the remaining land, is classified as unsuitable for the production of biomass (mountains, deserts, degraded lands, urban areas, etc.). In order to remain self-sufficient, only a small fraction of the land currently used for food production could be converted to energy crops. Forests are another potential source for large-scale methanol production from biomass in Europe. They could be used directly for wood production or be cleared to enable agriculture, but due to their importance for the environmental balance and biodiversity, transforming a large proportion of the forests into vast, highly standardized and mechanized tree plantations would run contrary to the sustainable use of natural resources. When considering possible feedstocks, biomass is thus not expected to represent more than 10-15% of Europe's total energy demand in the future.

In those developing countries already struggling to produce the necessary food for their growing population, the production capacity for energy crops is clearly limited.

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