Power density

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Proponents of new renewables argue that this challenge can be met by relying, as we had for millennia, on phytomass –- but doing so by cultivating it with much higher yields, using a wider variety of plants and converting them to fuels with higher efficiencies. But producing liquid fuels for transportation is precisely the final energy use with by far the greatest gap between the dominant power densities of our current energy system and those of a new configuration based on the new renewables. In both cases power densities of final processing (refining of crude oil on one hand, and fermentation of feedstocks to produce ethanol or transesterification of plant oils to make biodiesel) and large-scale long-distance transportation (by railcars, tankers and pipelines) would be fairly similar but extraction of crude oil and harvests of phytomass feedstocks differ by at least two, most commonly by three, and in many cases by four orders of magnitude.

Moreover, the existing scale of liquid fuels needed for transportation (roughly 2.5 Gt in 2012) means that, even when assuming no increase, or even a slight decrease in the US demand (USEIA 2013c) and restrained growth of current liquid fuel requirements in Asia, the annual supply of liquid fuels would have to amount to at least 3 Gt (that is 4 TW) by 2040 (ExxonMobil 2013a). Many studies have endeavored to show that such mass-scale biofuel futures are possible. When Berndes, Hoogwijk and van den Broek (2003) reviewed all published long-range forecasts of biomass energy contributions they found that the maxima for the year 2050 ranged from less than 3 TW to nearly 13 TW, the latter total being higher than the world’s TPES (including traditional biofuels) in the year 2000. Three years later Moreira (2006) claimed that phytomass can eventually supply about 32 TW, that is more than twice as much as all fossil fuels produced in 2012.

But the claims of a world whose transportation needs are soon to be completely energized by modern phytomass conversions rest on excessively enthusiastic constructs that assume theoretically high phytomass yields and exceptional conversion efficiency maxima, that posit unrealistic abundance of requisite land and that fail to examine a number of other critical constraints. In reality, outputs of phytomass-based biofuels would be constrained by many factors and consequences of shifting from fossil fuels to biofuels would be profound and I will rely on power densities to explain some of these inescapable outcomes.

I have been always skeptical about any claims of grandiose phytomass futures, and I have always criticized any irresponsible claims. In 1983 I concluded my book on biomass energies by writing that “there are countless better uses for plants than to burn them directly or to use them as feedstocks to make fancier fuels out of them” (Smil 1983, 417) and during the past three decades nothing has taken place to make me change that conclusion: phytomass will continue to be a non-negligible contributor to global energy supply for generations to come, but it cannot provide large fractions (a third, one half, two-thirds) of that rising total. In this respect it is essential to keep in mind huge differences between theoretical appraisals of potentially available phytomass and realistic opportunities for its use. For example, in 2005 a study commissioned by the US Department of Energy conclude that the country has an annual supply of one billion dry tons (that is 900 Mt) of agricultural and forest phytomass (USDOE 2005), theoretically enough to displace about 30% of oil consumption at that time.

But none of the constituent estimates (for crops, wood, residues, wastes) were restricted by the cost of cultivation and harvesting and all referred to phytomass on farms or in forests, excluding all transportation and storage costs, handling losses and quality deterioration, all in all a classical example of a seemingly huge mass that shrinks on closer examination. The study’s update acknowledged those shortcomings and offered a more rigorous assessment (USDOE 2011). Or, as Sinclair (2009, 407) put it, ‘’while biofuels can be a contributor to the energy needs of the future, realistic assessments of the production challenges and costs ahead impose major limits.” I will take a closer look at two critical energy requirements, substitution of liquid fuels in transportation, and replacement of coke by charcoal in iron smelting.

Liquid biofuels As I have documented, we produce crude oil mostly with densities of 102-103 W/m2 and in the largest fields the rate goes up well into 104 W/m2 –- while the dominant feedstocks for the production of liquid biofuels are harvested with power densities of 10-1 W/m2 and the rates are further reduced by their processing. Power density of 0.23 W/m2, typical for the US corn-based ethanol, can be used to demonstrate inherent limits of America’s principal alternative automotive fuel. If all America’s gasoline demand in 2012 (total of 16.96 EJ or 537.87 GW) were to be supplied by corn-based ethanol produced with average power density of 0.23 W/m2 the US would have to be growing corn for ethanol on 234 Mha, and area nearly 75% larger than that of all recently cultivated land and a third larger than the country’s total cropland (USDA 2013a).

Corn ethanol’s low power density also means that there will be extensive qualitative environmental changes associated with the fuel’s production (Howarth and Bringezu 2009; Smil 2010). Corn cultivation has considerable impacts on the environment: soil erosion from the cultivation of a row crop; demand for heavy applications of nitrogen, averaging more than 150 kg/hectare, surpassing 200 kg/hectare in the Corn Belt; ensuing nitrogen leaching causing the eutrophication of coastal waters and an expanding dead zone in the Gulf of Mexico; depletion of aquifers for irrigation; expansion of monocultures as traditional rotations with soybeans and alfalfa; and net increase of greenhouse gas emissions. In addition, ethanol distilleries discharge large volumes of wastewater (10-13 times the volume of the produced fuel) and those discharges increase oxygen demand in streams and water bodies.

Sugar cane is obviously a superior feedstock for crop-based biofuel production: not only it has much higher yields than corn but because of its endophytic bacteria the most productive Brazilian varieties do not require any nitrogen fertilizer. Even so, as I have shown, power densities of cane-derived ethanol are of the same order of magnitude as those of corn-based fuel. The crop’s 2010 averaged nearly 72 t/ha worldwide and 79 t/ha in Brazil. Assuming that the global mean eventually rises to 80 t/ha and that the harvest would be converted to ethanol with Brazilian efficiency (82 L/t) power density of cane-based ethanol production would be almost exactly 0.5 W/m2 (6,560 L x 24 MJ/L/31.536 Ms/104).

Converting the entire global harvest (1.69 Gt in 2010) to ethanol would yield about 105 GW, or merely 3% of global liquid fuel demand in 2010. Conversely, if we were to produce the world’s liquid fuel demand in 2040 (4 TW) from sugar cane we would need about 800 Mha of the crop, slightly more than half of the world’s cropland and about 50% more than all cropped land available in tropics and subtropics where that grass can be grown. And even if aimed to supply only half of all liquid fuel or only to replace all gasoline we would still need 300-400 Mha of cane, and the only way to reach that level of output would require further conversion of tropical grassland and forests to cane fields.

But ethanol is of no use to power jetliners, and aviation has been one of the fastest expanding sectors dependent on high energy density kerosene (jet fuel). In 2010 worldwide demand for kerosene reached about 11.4 EJ or roughly 360 GW (USEIA 2014c), by 2050 it is expected to reach 1 TW (ICAO 2010). Recent power densities of biojet fuel range from just 0.06 W/m2 for soybean-based substitute to 0.65 W/m2 for palm oil (Rossillo-Calle et al. 2012). Even the latter alternative would require about 57 Mha of palm oil plantations, 3.5 times their global 2010 area, a theoretically possible expansion but one that would greatly magnify tropical deforestation that has accompanied the crop’s recent expansion (Kongsager and Reenberg 2012).

Basing the fuel on soybeans would need about 570 Mha of crop dedicated to biojet fuel, 5.5 times the 2010 global total and an obviously impossible extension. Turning to crops grown on marginal, non-arable land, could provide only a partial solution: much touted jatropha (Jatropha curcas, a hardy oilseed-bearing shrub or a small tree able to grow on arid soils) would not produce more than 0.2 W/m2 and hence the world would need 180 Mha of it to satisfy 2010 jet fuel demand, and 500 Mha in 2050, an area equal to slightly more than half of China’s territory. Even if genetically improved cultivars were to double the yield the likely jet fuel demand in 2050 would still call for covering roughly an Argentina- or Kazakhstan-size area with jatropha.

Three important realities should be stressed at this point. First (and setting aside purely theoretical musings about new energy crops genetically engineered from scratch), there is clearly room for increasing jatropha yields as the plant was never systematically cultivated, but no gains in yields or processing efficiencies can fundamentally change low power densities of the tow leading biofuel crops, of grain corn and sugar cane. Global corn yield rose by 65% years between 1980 and 2010 and that of sugar cane increased by 30%, and hence even similar increases between 2010 and 2040 would still leave the power densities of corn-based ethanol lower than the recent densities of the US production (average 2010 yields were 5.2 t/ha worldwide and 9.6 t/ha in the US).

In Asia, where corn yields average only 50% of the US mean, the spatial burden would be nearly twice as large and power densities of corn-based ethanol production would be still below 0.2 W/m2, while those of cane-based ethanol would remain below 0.7 W/m2. Similarly, no likely process gains could boost this rate in any significant way. For example, Novozymes introduced two new enzymes that promise to increase ethanol yield by up to 5% (Novozymes 2013) –- but average American corn yields have recently declined by much more than that: they peaked at 10.3 t/ha in 2009, 9.6 t/ha were harvested in 2010, 9.2 t/ha in 2011 and, with widespread drought, just 7.7 t/ha in 2012 (FAO 2014).

The second reality that should be stressed is that future land claims imposed by high shares of liquid fuels derived from phytomass would be lowered but not massively reduced by producing cellulosic ethanol from crop wastes, from plants grown on marginal land or from surplus phytomass. Annual production of crop residues rivals that of crop themselves: modern cereals have residue:grain ratio of roughly 1:1 which means that the global output of straw and stover is about 2.5 Gt, and when residues from other crops are added that total rises to about 4 Gt (Smil 2013). But crop residues are not wastes waiting to be converted to ethanol: in traditional agricultural societies they are still important sources of cooking fuel and animal feed their recycling is a key ingredient of proper agroecosystemic management as straws, stalks and leaves return to soil the three macronutrients (nitrogen, phosphorus and potassium) as well as many micronutrients, renew soil organic matter, help to retain moisture and prevent wind and water erosion (Smil 1999; 2013).

This means that only a fraction of crop residues (predetermined by crops, harvests and environmental conditions) should be removed, a restriction that further lowers inherently low power densities of their production: these are (with 50% removal rate) less than 0.08 W/m2 for the Great Plains winter wheat, about 0.3 W/m2 for high-yielding German wheat, and (with a nearly complete removal) less than 0.5 W/m2 for sugar cane bagasse. Roughly a third of stover (residue of corn, America’s largest crop) can be removed in conventional cropping, 70% with no-till cultivation, and a weighted mean of 40% translates to an annual harvest of some 80 Mt in terms of dry weight (Kadam and McMillan 2003).

That would produce between 20-25 GL of ethanol or just 3% of the US gasoline supply in 2010, and it would imply final product power density of only 0.06 W/m2. As for any surplus woody phytomass, a review by Smeets and Faaij (2007) offers a corrective quantitative perspective/. While they estimated that the world’s theoretically available surplus wood (after satisfying demand for traditional fuelwood and timber) is about 71 EJ (6.1 Gm3), the technical potential is 64 EJ, economic considerations reduce it to 15 EJ, and inclusion of ecological criteria nearly halves it to 8 EJ, or to only about 250 GW, an equivalent of less than 2 % of the world’s 2010 supply of fossil fuels.

The third, and the most important point is that further adjustments of previously cited power densities are necessary in order to take into account energy costs of crop cultivation and fuel production. Dijkman and Benders (2010) calculated net power densities (they expressed them as GJ/ha/year) for actual bioethanol production from sugar beets and biodiesel production from rapeseed for three specific European cases (central Sweden, Groningen province of the Netherlands and Murcia in southeastern Spain). The lowest net power density, for Spanish bioethanol, was just 0.02 W/m2, the highest rate, for Dutch biodiesel, was 0.08 W/m2. These low rates mean that full replacement of crude oil-derived liquid fuels in Europe would require (even if the mean value rose to 0.1 W/m2) feedstocks grown on an area of more than 600 Mha, six times larger than all arable land in EU 27 (FAO 2014).

Similarly convincing order-of-magnitude calculations are easy to make for replacing fossil fuels by any kind (liquid or solid) of phytomass fuels. The world’s 2012 fossil fuel consumption reached roughly 10.85 Gt of oil equivalent (BP 2013). That is (at 42 GJ/t) 455 EJ or 14.45 TW and if we assume that 2050 demand will be just 40% higher (many forecasts indicating a much larger expansion of demand) then 20 TW are candidates for replacement. Even when assuming that only half of that rate would be supplied by biofuels, and that the total would be split between the combustion of woody phytomass to generate electricity and conversion of phytomass to liquids or gases, then each of these biofuels would have to provide about 5 TW. Even if these conversions could be done with fairly high efficiencies –- 0.5 W/m2 for woody phytomass and 0.3 W/m2 for field crops –- then crops for energy would have to be harvested from nearly 1.7 Gha of arable land and wood for energy would require annual harvests of trees from 1 Gha.

But that would mean that in 2050 the area of energy crops would have to be nearly 10% larger than the total area of arable land and permanent plantations in 2012, and the area of continuously harvested tree plantings would be equal to nearly 30% of all of today’s closed forests (with canopies covering more than 40% of the ground). This would come on top of the higher future demand for food and wood and in the world of diminishing biodiversity: incredibly, even this combination seems to be no obstacle for some uncritical promoters of phytomass energies who keep conjuring large areas of uncultivated land or who assume that more forests and grasslands can be converted to croplands. Read (2008) envisioned having additional 2.4 Gha of rain-fed arable land (compared to the 2013 total of about 1.55 Gha), mostly in the tropics. Marland and Obermeister (2008:335) were polite when they concluded that “it is not now clear if his vision is a dream or a nightmare” –- but I have no problem seeing it as exceptionally nightmarish.

We are already harvesting a significant share of the biosphere’s annual productivity (Smil 2013) and any future massive increase of biofuel production would have to increase this intervention and result not only in competition with food and timber production but also with further weakening of environmental services, particularly those provided by mature forests. And perhaps the greatest irony is that those who would claim to displace fossil fuels by phytomass in order to reduce carbon emissions have been apparently unaware that expansion of the land devoted to biofuels can bring the very opposite outcome.

Fargione et al. (2008) showed that converting rainforests, peatlands and grasslands in order to cultivate food crop-based biofuels in Brazil, Southeast Asia and the US could release 17-420 times (one to two orders of magnitude) more CO2 into the atmosphere than the annual greenhouse gas reductions resulting from the displacement of fossil fuels by these cultivated biofuels. Similarly, Searchinger et al. (2008) demonstrated that corn-based ethanol does not produce, as previously claimed, substantial CO2 savings but that its production nearly doubles CO2 emissions over 30 years and increases greenhouse gases for 167 years, while biofuels from often highly-touted switchgrass increase emissions by 50%.

Many problems (most of them well appreciated and assessed) are bound to accompany any large-scale phytomass harvesting for energy: further destruction of natural ecosystems, demands for nutrients and water, extension of monocultures, vulnerability to pests and diseases, competition with land for food and feed. Expanded deforestation would be among the most likely consequences of any global-scale push for much increased biofuel production and yet even without such pressures the world’s forests have been in retreat: the best high-resolution global mapping showed that between 2000 and 2012 there was a net forest loss of 1.5 M km2 with rising losses in the tropics (Hansen et al. 2013).

As I have demonstrated, inherently low efficiency of phytomass production and subsequent energy losses arising from various fuel conversions (combustion, gasification, chemical and enzymatic processing) limit power densities of final forms of phytomass-based energy use to a fraction of 1 W/m2. This reality remains a key reason why it is most unlikely that modern societies will come complete a full circle and return from fossil fuels to phytomass fuels as the dominant source of their energy, and why any responsibly handled expansion of phytomass production and its least wasteful conversions will be able to make only a limited contribution to the world’s primary energy supply.

Metallurgical charcoal As promised in the introductory chapter, I will now asses the consequences of a surprisingly neglected aspect of energy transition from fossil to renewable sources, the replacement of coke in iron smelting by charcoal from tree plantations, Doing this will require closer looks at modern blast furnaces and at prevailing plantation yields and charcoaling methods. Charcoal supply would have to energize annual smelting of just a bit over 1 Gt (average rate for the years 2008-2012), that is roughly 25-fold increase in global blast furnace iron production between 1900 and 2010. In my calculations will use assumptions based on the best plausible case, the Brazilian charcoal-based iron smelting with recent data on eucalyptus plantation yields, charcoaling efficiencies and blast furnace charges from Sampaio (2005), Swami et al. 2009), Peláez-Samaniego et al. (2008), Piketty et al. (2009), Pereira et al. (2012) and Pfeifer, Sousa and Silva (2012),

Virtually all modern iron is turned into steel by reducing iron’s carbon content from more than 4 % to just 0.1-1 %, and by alloying it with other metals including chromium, manganese and vanadium. Rising share of steel has been coming from recycling the growing mass of accumulated old metal by melting scrap steel in electric arc furnaces. In 2010 this recycled metal accounted for about 25% of the global steel output. The rest comes from primary iron whose production remains dominated by blast furnaces: in 2010 only about 70 Mt came from direct reduction of concentrated ores using natural gas, while blast furnace iron reached 1.035 Gt, and in 2012 1.112 Gt (WSA 2013), while the share of iron smelted in charcoal-fueled blast furnaces declined to only about 0.6% (Sampaio 2005).

Iron is now smelted in imposing columnar structures whose sizes, capacities and efficiencies have been optimized in order to deliver low-cost high productivity. The world’s largest furnaces now have volumes in excess of 5,000 m3: Shougang Jing Tang’s furnace in Caofedian (blown in 2009) has 5,500 m3, ThyssenKrupp’s Schwelgern 2 (since 1993) has 5,513 m3 and Japan’s Japan’s Ōita 2 was enlarged to 5,775 m3 in 2004 (Smil 2008; Hoffmann 2012; ThyssenKrupp 2012). Each of these furnaces requires more than 2.5 Mt of coal equivalent to energize their daily output of more than 10,000 of hot metal. Notice the term coal equivalent rather than coke: modern iron making has been substituting some coke by coal dust, fuel oil or natural gas directly blown into a furnace, and even using peletized plastic waste. Moreover, the entire smelting operation has become much more energy efficient and as a result specific energy requirements for coke (t coke/t of hot metal) have been steadily declining.

As a result, typical consumption of dry coke per tonne of hot metal declined from about 1.3 t in 1900 to 1 t in 1950 and in the year 2000 the world coke:iron output ratio was just 0.6 and the best operations needed only about 450 kg of coke/kg pig iron (de Beer, Worrell and Blok 1998; Smil 2008). At the same time, the global output of blast-furnace output has been steadily increasing –- from less than 50 Mt in 1900 to 580 Mt in 2000 and then, mainly thanks to China’s production surge, to 1.035 Gt in 2010 –- and the demand for coke has reached record levels. In 2010 the global production of coking coal reached 900 Mt and, with 1.37 t of coal needed to produce a tonne of coke, it was converted to about 650 Mt of coke. Hydrocarbons and coal dust directly injected into blast furnaces were equivalent to roughly another 100 Mt of coke, resulting in annual energy input (when assuming 30 GJ/t of coke) on the order of 22 EJ of fossil fuels.

Metallurgical coke energizes the high-temperature melt (1,300-1,6000C) and acts as the reducing agent: its combustion generates CO2 (C+O2 → CO2) whose reduction yields CO (CO2 +C→ 2CO) and rising hot CO-rich gases reduce ore oxides into elemental iron (Fe₂O₃ + 3CO → 2Fe + 3CO₂). But the materials also provides support for heavy charge of iron ore and limestone (fluxing agent added to remove impurities) while being sufficiently permeable to allow for the ascent of reducing gases and the descent of molten slag and metal. In a world run solely by renewable energies we would have to go back and replace all of these fuels (coke and injected fuels) by charcoal made from woody biomass.

With charcoal’s energy density very close to that of coke a straightforward replacement energy used in primary iron smelting in 2010 would then call for approximately 750 Mt of charcoal a year. But the replacement could not be so straightforward because charcoal is a much softer material than coke: depending on the wood species its compressive strength varies between 10-50 kg/cm2 compared to 130-160 kg/cm2 for coke and hence charcoal could not support massive iron ore and limestone charges without getting crushed. Abrasion rate of charcoal is also much lower. That is why Brazilian iron-makers cannot replace all coke by charcoal: economies of scale, competition with foreign producers and basic technical considerations (charging large furnaces with friable charcoal would cause serious equipment damage) make that impossible (NCIB 2012).

As a result, those modern Brazilian blast furnaces that use charcoal are toy-sized with the internal volume an order of magnitude smaller than the coke-fired units deployed around the world: the biggest Brazilian furnace has just 568 m3 (Pfeifer, Sousa and Silva 2012). Moreover, desirable bulk density of metallurgical charcoal is at least 0.4 g/cm3 and while the fuel made from eucalyptus wood is, with 0.53-0.59 g/cm3 well above that rate (Pereira et al. 2012), many wood species yield charcoal with densities just between 0.28-0.4 g/cm3. Even if these material challenges were resolved through construction of larger numbers of smaller furnaces (and the ensuing acceptance of higher metal prices) and the use of specific woods, the necessity to harvest large amounts of woody phytomass would remain. We can calculate the resulting wood demand based not on theoretical assumptions but on commercial practices of Brazil pig iron production, the world’s largest charcoal-based industry.

If the charcoal were to come from operations resembling today’s Brazilian practices the global burden would be huge -– and not only because of inherently low power density of charcoal supply. Most of Brazil’s steel comes from modern enterprises that integrate coke-based smelting of pig iron with the production of continuously cast steel, but about a third comes from enterprises that use charcoal in small blast furnaces concentrated in the states of Pará, Minas Gerais and Mato Grosso do Sul (Uhlig 2011). The wood for charcoal is cut from natural as well as second-growth forests, an increasing share of it has been coming from eucalyptus plantations that covered nearly 5 Mha in 2011 (Pereira et al. 2012) -– but as much as third of the total charge may be much cheaper wood that comes from illegal cutting of natural forests (Monteiro 2006), and Uhlig (2011) estimated that as much as 15% of the Amazon’s deforestation can be ascribed to charcoaling. These shares are uncertain because of significant disparities in nationwide estimates of the country’s charcoal production (Ghilardi and Steierer 2011).

About 80% of the fuel is made overwhelmingly in small brick-and-mud semi-circular kilns commonly called rabo quente, hot-tail. These 2.5 m-high beehive structures, usually grouped in rows by dozens, are stacked with air-dried (about 25% moisture) wood, set alight and let to smoulder for five to seven days; three days after they are extinguished men enter the kilns and unload charcoal. Working conditions in many of Brazil’s small-scale charcoaling operations have been described as akin to slave labor (working off large debts) that is also highly hazardous (Greenpeace 2013). Workers removing charcoal from ovens are exposed to high temperatures, dust and smoke, no less importantly, there is long-term exposure to uncontrolled emissions of nitrogen and sulfur oxides, benzene, methanol, phenols, naphthalene and polycyclic aromatic hydrocarbons (Kato et al. 2005). In mass terms these small kilns convert only between 22-27% of wood into charcoal and the average nationwide rate is no more than 25% (Swami et al. 2009; Bailis et al. 2013).

Average blast furnace charge of 450 kg C/kg of pig iron could be supplied by about 500 kg of coke or 630 kg of charcoal (Sampaio 2005). Minimum theoretical requirement would be thus about 650 Mt of charcoal, which means that the global rate of charcoal production would have to increase at least 14-fold compared to the year 2010 when FAO put its output at 47 Mt (FAO 2014). With average charcoaling efficiency of 25% by weight the global iron smelting at the 2010 rate of 1.035 Gt would consume annually 2.6 Gt of wood. Another (official) Brazilian source gives averages rates of 2.2 m3 of charcoal and 4.4 m3 of wood per tonne of pig iron (Secretaria de Estado de Meio Ambiente do Pará 2008). That would, with mean eucalyptus wood density of 0.55 t/m3, translate into 2.42 t of wood per tonne of pig iron and 2.5 Gt of roundwood would be needed to smelt the world’s 2010 production.

This is an excellent agreement for calculations at this level of aggregation, but the real total would have to be higher due to inevitable transportation and handling losses that would arise during the exports of tropical charcoal to temperate climates, particularly the fuel made from less dense woods than eucalyptus clones. Of course, the alternative is transporting entire logs and setting up massive charcoaling facilities in high-income countries, but that would require unprecedented trade in wood. In 2010 all wood traded globally (industrial roundwood including saw logs, veneer logs and pulp wood, fuelwood and sawnwood) amounted to about 170 Mt (FAO 2014) while exporting most of the wood needed to make charcoal would mean an order of magnitude increase in shipments, a doable task but not one accomplished easily.

And how does the total of 2.5 Gt of charcoaling wood (or, with a 10% mark-up for losses, 2.75 Gt) compare to today’s global roundwood harvest? According to the FAO that total (all wood destined for fuel, lumber and pulp) reached 3.4 Gm3 in 2010 or (assuming 0.65 t/m3) about 2.2 Gt (FAO 2014), and a generous addition of 15% for illegal logging would bring it to 2.5 Gt. Consequently, global charcoal-based iron smelting that would replicate the prevailing Brazilian practices would alone claim as much (or slightly more) wood than the world’s total (legal and illegal) 2010 wood harvest for fuel, lumber and pulp! Another way to look at this is that in 2010 (assuming average 25% conversion efficiency) less than 10% of the world’s wood harvest was converted to charcoal, while charcoal-based iron smelting at the 2010 level would require converting all of the world’s harvest wood into charcoal, leaving nothing for other uses –- and it would require a doubling of recent wood harvests in order to keep the world supplied with timber, veneer, plywood, fuel and paper pulp.

The extent of the harvested area would depend on prevailing yields and those, in turn, vary with wood species, soils, climate and plantation subsidies.

When combining the previously stated assumptions with the annual mean increment at 10 t/ha then the global pig iron output in the year 2010 would have required harvests of at least 250 Mha (2.5 Tm2), an area equal to 60% of Brazil’s Amazon rainforest of 4.1 Tm2, or to nearly half of the total forested area of 5.5 Tm2 in the entire Amazon basin. With 19 MJ/kg of dry wood –- average value for eucalyptus clones according to Pereira et al. (2012) –- that would imply average power density of 0.6 W/m2. This has been, of course, just a theoretical calculation because a large-scale transition to charcoal-based iron smelting would lead to many improvements and hence it is realistic to make more optimistic assumptions about the future of iron production.

For example, Piketty et al. (2009), assuming average (dry matter) wood yield of 16 t/ha, 30% carbonization yield (for 80% C content with 10% handling loss), put the land requirement at 1,290 km2 for 1 Mt of hot metal, and hence the global iron output fully energized by charcoal would require roughly 130 Mha of high-yielding tropical plantations. Even better performance is conceivable. Modern continuous charcoaling methods (retorts) should have average conversion yields of 35%-40% (Rousset et al. 2011) and cultivation of high-yielding clones in eucalyptus plantations should bring annual wood increments as high as 25 t/ha (Pfeiffer, Sousa and Silva 2012). In combination, these two gains would boost the charcoal yields per hectare four-fold when compared to my initial assumptions and the global iron smelting at the 2010 level could be supported by less than 70 Mha of tropical plantations.

But all of these calculations are questionable because even the existing Brazilian plantations have serious environmental impacts, because it is most unlikely that all charcoal would come just from Brazil, and because the global iron smelting will continue to increase in order to supply enormous steel demand for new Asian and African infrastructures, housing and transportation. Extensive areas of tropical eucalyptus (and pine) plantations have been already described by Brazilian environmentalists as deserto verde, destroying biodiversity and maintaining monocultures through intensive applications of herbicides (Reportér Brasil 2011). The standard practice at Brazil’s eucalyptus plantations is to harvest trees in a five-year rotation and coppiced for three cycles: after 15 years of growth glyphosate is applied to kill the remaining rootstock and new seedlings are planted (Bailis et al. 2013).

Monocultural plantations also increase the rate of soil erosion, divert water from nearby farms, contaminate runoff with applications of agrochemicals and their expansion would compete even more extensively for land that would be otherwise used for crops or pasture. In any case, it is extremely unrealistic to expect that all charcoal needed for global pig iron smelting would be made only from harvesting intensive plantations of Brazilian eucalyptus hybrids –- and if a significant share of charcoal were to come from fast-growing clones of temperate species then the average wood yields would be substantially lower. Experimental studies based on short-term cultivation of small plots often cite impressively high yields, but studies that look at productivities in diverse environments to be most likely used for tree cultivation (abandoned farmland, former forest land) show a predictably wide range of outcomes.

For example, Truax et al. (2012) found that for the productivity of hybrid poplar plantations on abandoned farmland in Quebec the site effect (elevation, climate, soil fertility) was far more important than the clone effect, with annual yields as high as 22.4 m3/ha (about 11 t/ha) in bottomlands and as low as 1.1 m3/ha (about 0.5 t/ha) on the poorer soils of hill slopes. Significant yield differences were also found in warmer climates of Italy. In the North, Paris et al. (2011) harvested 15-20 t/ha a year but only with fairly heavy nitrogen applications (300 kg N/ha), with means between 10-14 t/ha elsewhere, while in southern Italy (Latium and Molise) Di Matteo et al. (2012) recorded annual poplar yields of 10-13 t/ha. And a review of 21 studies of poplar and willow plantations in Europe and the US found an expected range of annual wood yields between 5 and 16.8 t/ha (Djomo et al. 2011). Consequently, assuming a future average global wood increment of 15 t/ha would not be too conservative.

Global output of pig iron had doubled between 1990 and 2010, and even if it were to grow only half as fast during the next two decades it would reach about 1.6 Gt in the year 2030. Using the more optimistic assumptions regarding future wood and charcoal productivities it would thus require at least 2.9 Gt of charcoal and annual harvest of wood from more than 190 Mha in 2030. This would be an area of forest or tree plantations only slightly smaller than in the first scenario, and equal to nearly half of Brazil’s Amazon or to more than China’s total forest land. And this massive spatial claim would have enormous environmental repercussions, particularly when most of this wood would have to be grown in high-yielding monocultural plantations that would require large inputs of fertilizer, pesticides, herbicides and, in many drier climates, at least supplementary irrigation.

The world whose dominant metal would be smelted with charcoal produced by annual logging of an area equal to half of the Brazilian Amazon is conceivable but it is hardly desirable, and we simply cannot appreciate all of its eventual consequences. But a fundamental concern is clear: could such massive tree harvests (required to produce a billion tonnes of charcoal a year) offer a truly renewable alternative given their impacts on biodiversity and soil erosion and their constant requirements for water, nutrients and protection against pests? What would be the real cost of this enormous enterprise, and how practical it would be even if the costs were a secondary matter?

And even if it turned out to be more practical than we think today, the industry based on harvesting wood on semi-continental scale would be a very different one compared to our current arrangements with iron production based on carbon-rich coke made from coal that is extracted in just a few thousand large mines in a dozen major coal-producing countries and that is easily distributed to large industrial centers for coking. Of course, our current practice taps a finite energy resource, but given its importance for iron smelting we could accord it a highly preferential status and keep relying on it for many generations to come. That would not difficult to do because we could replace coal’s largest use, for electricity generation, by other energy sources. In any case, these comparisons provide an impressive illustration of how power densities determine and delimit energy use, and its consequences.

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