2.5.1 Global carbon budget

Carbon on the earth is stored in the atmosphere, oceans, biosphere and lithosphere and carbon components are exchanged among these stores in gas phase (CO2), and in organic or inorganic phase. According to the IPCC 4th Assessment Report, 2007, the global fossil-fuel emission including fossil-fuel use and cement industry is 6.4 GtC/yr in 1990s, and emission by land use changes is 1.6 GtC/yr. On the contrary, the estimated accuracy of net land and ocean uptakes are 2.6 GtC/yr and 2.2 GtC/yr, respectively. The net carbon balance of the atmosphere in 1990s is 3.2 GtC/yr (=8.0-2.6-2.2) as shown in Table 1. These values of carbon balances have been assessed thorough comparative check of many results related carbon cycle. The global fossil-fuel emissions of CO2 and the net carbon balances of the atmosphere are now rather well established, the others in the Table 2.5.1 are less accurate. And also, there are carbon cycles in the land and ocean-biosphere, the net carbon balances are related to the activities of them. The status of carbon cycle in each store is controlled by forest management and climatic changes. The Kyoto Protocol, which accounts for offsetting reduced targets of CO2 emission by carbon sequestration in forests through forest management, became effective in February 2005. The rules for the effective management have focused attention on the role of the terrestrial biosphere in global scale carbon cycles. Therefore, accurate data are required for carbon stocks and cycle over various terrestrial ecosystems. However, uncertainty remains about the change in carbon stock from forest management and the response of the ecosystem CO2 exchange to climate change.

2.14.1 What is sewage sludge?

Sewage sludge is defined in this text as the solid matters discharged from an activated sludge wastewater treatment facility as the result of aerobic wastewater treatment. The sewage sludge, which is usually a mixture of settleable matters in sewage and reproduced microorganisms, is high in organic contents that are possibly considered as reusable biomass.

2.14.2 Types and characteristics of sewage sludge

The standard activated sludge process is depicted in Fig. 2.14.1 as a typical aerobic wastewater treatment process.

In the activated sludge process, settleable matters contained in in-coming sewage are removed in the primary settler and withdrawn as raw sludge. The sewage is then introduced to aeration basin(s) where the sewage is mixed with return activated sludge, aerated and vigorously agitated. The mixture of sewage and return activated sludge in the aeration basin is usually called mixed liquor. Most of the organic pollutants are removed from a liquid phase by bio-adsorption and eventually decomposed by microbial assimilation of the activated sludge. The mixed liquor is then introduced to the final settler where suspended solids (activated sludge) are settled and withdrawn as return activated sludge. A portion of return activated sludge is discharged from the facility as excess sludge and its quantity is approximately equivalent to the amount of microbes reproduced in the aeration basin. The sewage sludge is either a mixture of the raw sludge and the excess sludge, or each of the sludges discharged separately.

The discharged sewage sludge is thickened and dewatered, and is subjected to the final landfill disposal or recycled after an intermediate treatment such as incineration or
gasification-melting. Typical treatment/disposal processes for sewage sludge are shown in Fig. 2.14.2.

©Sewage sludge

©Sewage sludge ©Sewage sludge

NREL, USA, has developed a vortex reactor, in which wood particle slides on the hot reactor wall by the circled hot gas flow. By the revolution, fresh surface is always appeared on the wood particle, and then the flash pyrolysis is realized. BTG, Netherlands, has developed a rotary corn type reactor, in which hot sand is used as the heat transfer medium and the sand fluids by the centrifugal force of the rotated reactor.

Many type of fluidized bed (FB) reactor have been developed by Dynamotive-RTI, Pasquali-ENEL (Italy), Ensyn,(Canada), RedArrow-Ensyn (USA), Union Fenosa-Waterloo (Spain), VTT (Finland), etc. The small particle of wood is introduced in FB with hot gas or hot heat transfer medium, and it is rapidly pyrolyzed for high liquid yield. Since smooth feeding of the wood particle is difficult, tar trouble occurs easily.

AIST, Japan, had developed the pyrolysis using microwave. Microwave can heat wood from inside, and big size wood, log, is directly used. The same result is obtained as the flash pyrolysis of 200°C/s. Bigger size is better for the energy efficiency, and around 1.4 MJ (0.4 kWh) is used for the completed pyrolysis of 1-kg wood. In addition, in this pyrolysis, tar trouble is small.

The product gas evolved from hydrogen and methane fermentation might be utilized to fuel cells which have higher energy conversion efficiency than those of gas turbine and gas engine. Methane from methane fermentation has to be converted to hydrogen for fuel cells.

CH4 + 2H2O -► CO2 + 4H2 AG0’ = 253 kJ (5.4.8)

Since Eq. 5.4.8 is an endothermic reaction, supply of energy is needed to proceed the reaction. Generally, methane gas is converted to hydrogen gas under nickel catalysts at 650-750°C. On the other hand, in hydrogen fermentation, energy yield is higher than that of methane fermentation and the catalytic conversion of methane is not needed to provide hydrogen gas to fuel cells.

Further information

Noike, T.; Mizuno, O., Hydrogen fermentation of organic municipal wastes, Water Sci. Technol., 42, 155-162(2000)

Rachman, M. A.; Nakashimada, Y.; Kakizono, T.; Nishio, N., Hydrogen production with high yield and high evolution rate in a packed-bed reactor, Appl Microbiol. Biotechnol, 49, 450-454(1998)

Tanisho, S.; Tu, H.-R; Wakao, N. Fermentative hydrogen evolution from various substrates by Enterobacter aerogenes, Hakkokogaku, 67, 29-34(1989)

Taguchi, F.; Yamada, K.; Hasegawa, K.; Taki-Saito, K.; Hara, K. Continuous hydrogen production by Clostridium sp. No.2. from cellulose hydrolysate in an aqueous two-phase system, J Ferment. Bioeng, 82, 80-83(1996)

Ueno, Y.; Otsuka, S.; Morimoto, M.; Hydrogen production from industrial waste-water by anaerobic microflora in chemostat culture, J Ferment. Bioeng., 82, 194-197(1996)

The limit of biomass resources human beings use becomes obvious. Therefore, it is expected that competitions between of biomass uses such as food, material, and energy will become severe; consequently land use competitions between forest, arable land, pasture, and other lands will become severe, too.

(a) Competitions between forest, arable land, pasture, and other lands.

Forests have decreased by activity of human beings for long years. The forest area in the world has decreased by 1.2 billion hectares since 1700. Especially, the forest area of North Africa and the Middle East, and China decreased below half. In the same period the arable land in the world increased by 1.2 billion hectares. In the same period, the pasture in the world decreased a little by 70 million hectares. Thus, in the historical view point, the deforestation was caused by the increase of arable land. Conversely, the deforestation area was converted to arable land.

Concerning land use change after 1950, deforestation in developed regions has been stopped and forest areas in Western Europe and North America increase reversely. On the other hand, in developing regions such as tropical regions, China, and Middle East, deforestation continues. The reasons are conversion to arable land and pasture and un-sustainable slash-and-burn farming, etc.

The deforestation in the developing regions causes not only a regional environmental destruction but also global environmental problem of CO2 emission. In addition, in regions in high population density and industries in high speed such as Japan, South Korea, and Taiwan, the arable lands are decreasing. This is because in those regions land demands for housing and factory increase and the agriculture is not competitive in the global agriculture markets. In Japan the arable land decreased by 10% from 5.0 million hectares to 4.6 million hectares between 1976 and 1991. In South Korea, the arable land decreased by 6.6% in the same period.

(b) Competition between food production and energy crop production

On arable land, energy crops are also producible besides food. In countries where surplus arable land is available such as the United States and Brazil, they could use the surplus arable land for energy crop production rather than for fallow field. Especially, in those countries in 2006 to 2007 when the oil price hits the historical record, they produce energy crops and produce biofuel such as bioethanol and bio-diesel (BDF). In the United States they produce bioethanol from corn on a large scale. In Brazil they produce bioethanol from sugarcane on a large scale. In addition, BDF is produced from vegetable oil such as rapeseed oil and palm oil.

However, since arable lands are limited, the production of energy crops reduces the arable land where food crops are producible. It is pointed out that excessive production of energy crop may lead to food shortage. In addition, it is explained in 6.7.5 that the competition between food production and energy crop production is handled in an energy model.

Further information

BIN (Biomass Industry Network) et al., “Bio-nenryo Riyo ni kansuru Kyodo Teigen”, 2007 (in Japanese) The Japan Institute of Energy, “Biomass Yogo Jiten”, Ohmsha, 2004 (in Japanese)

Yamaji, K. et al., “Bioenergy”, Myosin Shuppan, 2000 (in Japanese)

For ethanol, 1-2 million tons of molasses are used as raw material (this is by-product from sugar production which is about 5 million tons from 64 million cane production), another raw material for ethanol is cassava, only 180,000 tons out of 26 million tons of cassava roots is used for ethanol production. For biodiesel, about 100,000 tons of palm oil is used for biodiesel out of

1.5 million tons production in 2007. The use of palm oil for biodiesel in 2008 is expected to reach 300,000 tons levels.

There are two kinds of energy resource: (1)exhaustible resource (=stock type) and (2)renewable resource(flow type; such as solar-,wind-, hydraulic power and biomass). A flow stock is infinitely large, but it should be limitted within a given period of time. Excessive utilization such as a deforestation cannot sustain a renewable production system. Biomass has both types of resources.

(A) Flow type Biomass. The net primary productivity 170 Gt/yr (about 7 times as much as world energy demand)

(B) stock type Biomass. Mainly in forest; 1800 Gt (about 80 times as much as world energy demand/yr)

Biomass consumption (C) has two variations, putrefaction and utilizing consumption. In natural forest, there are nearly equall amounts of growing and the putrefaction, the equilibrium (A)=(C)would be established.

(A) Flow-biomas —(B) Stock-biomas —(C) Putrefaction or useful consumption 170 Gt/y 1800 Gt (variable)

Although we cannot realize (C)>(A), it is capable to get a bigger share of biomass utilization in the distribution among (C), by our policy and technology.

1.2.2 Carbon neutral

Biomass fuel also emits CO2 by the combustion. But people permit a CO2 indulgence for biomass^ because of the CO2 absorption during the growing process.

That is, [CO2 emission] = [CO2 fixation by the growing process]. However, coal also has a biomass-originated history, the carbon is circulating in long term, several hundred million years. Then under the consideration of term of CO2-repayment, CO2 immunity rate should be estimated.

Just after the biomass combustion, none can pay back its CO2. Therefore, it is estimated that a temperate forest (about 25 years for regeneration time) aquires [CO2 immunity rate=1], as a standard. At subpolar forest, the regeneration time =100 years case, the immunity rate becomes [25/100 = 0.25]. In the case of brown coal (the origin was 25 milion years before), this immunity rate is only 1 ppm. So fossil fuel has no hope to get the effective CO2 indulgence.

Cassava (Manihot esculenta) is a woody shrub and widely cultivated in tropical and subtropical regions in the world. In 2006, total production of the world was 226 million tons in

18.6

million ha of harvested area. The biggest cassava producer is Nigeria and followed by Brazil, Thailand and Indonesia (Fig. 2.8.3). The yield of cassava in Thailand is ranked first and about 21 ton/ha averaged over whole 1.1 million ha of harvested area (Fig. 2.8.4).

Fig. 2.8.3. World leading countries of cassava production in 2006.

Cassava is propagated by stem cutting, which is planted on a soil in a vertical and inclined position. It can grow well even on an infertile soil and is tolerant of drought. Starch-rich tuberous root is harvested by hand around 12 months after planting, but it can be harvested as early as 8 months or as late as 24 months. Starch content of tuberous root in Thailand is about 25%. In Thailand, yield of cassava gradually increases mainly due to the consistent development of better varieties (Fig. 2.8.5). Currently, Rayong 9 with up to 30 ton/ha yield has been distributed since 2006.

Tuberous roots of most varieties contain cyanide and it should be removed by soaking cassava flour. Cassava has been used as a staple source of carbohydrate for human consumption in many tropical countries, as an ingredient in animal feeds, and as a source of starch. Bio-ethanol production from cassava starch is developing. Cassava production of Thailand in 2007 was 27 million tons and the flow chart (Fig. 2.8.6) shows the breakdown of cassava applications in food, feed and fuel.

Fig. 2.8.5. History of cassava statistic in Fig. 2.8.6. Uses of cassava in Thailand. Thailand.

Even though corn or maize is not one of the most economic crops in Thailand, there has been an increasing utilization of corn cob in power generation and solid fuel in order to achieve zero-waste philosophy in corn processing industry. The figure below shows the 2006 production of maize around the world, where USA is accounted for almost half of the world
production and also with the highest yield in the world. Although China has a similar amount of land for maize plantation to USA, the lower yield in China makes its production only half of the USA figure.

Fig. 2.17.4. World leading countries in maize
production in 2006.

For the past 10 years in Thailand, the yield has been gradually increasing but not fast enough to compensate the decreasing plantation land, resulting in a decreasing maize production, as shown in Fig. 2.17.6. In 2006, the yield is 411 g/m2 (4.11 t/ha) averaged over whole 9,000 km2 (0.9 million ha) harvested area. Most of maize production in Thailand goes to food and feed industry; both of which corn cob is left as solid waste in the plants. In 2006, 33 Gg (0.33 Mt) of corn cob is reported, where 200 Gg (0.2 Mt) comes from the lower North part in Thailand especially Phetchabun (76 Gg (0.076 Mt)), Tak (35 Gg (0.035 Mt)) and Nakorn-sawan (33 Gg (0.033 Mt)). This corn cob is usually dried before putting in the boiler to generate steam for power generation, e. g. 10 MW power generation unit in the ethanol plant Pornvilai International Group Co.

Another value-added product for corn cob is corn charcoal, where corn cob is first heated to about 900-1000oC before being palletized with proper binder. Then, the final drying at 120oC for 12 h is conducted to obtain charcoal with less than 5% moisture.

Many reports on experiment with laboratory scale reactors exist. There are three pilot plants in operation: Energia Co. plant in Japan; VERENA Plant in Germany; and TEES Process in USA. The capacity ranges from 1 to 2.4 t-wet/d. Feedstocks tested are many including chicken manure, corn silage, and cheese whey. No commercial plants have been built mainly due to the high plant cost at present.

Further information

Antal, M. J., Jr.; Allen, S. G.; Schulman, D.; Xu, X. D.; Divilio, R. J. Biomass gasification in supercritical water, Ind. Eng. Chem. Res., 39, 4040-4053 (2000)

Elliott, D. C.; Hart, T. R.; Neuenschwander, G. G. Chemical Processing in High-Pressure Aqueous Environments. 8. Improved Catalysts for Hydrothermal Gasification, Ind. Eng. Chem. Res., 45, 3776-3781 (2006)

Kruse, A.; Henningsen, T.; Sinag, A.; Pfeiffer, J. Biomass gasification in supercritical water: Influence of the dry matter content and the formation of phenols, Ind. Eng. Chem. Res., 42, 3711-3717(2003) Matsumura, Y.; Minowa, T.; Potic, B.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P M.; van de Beld, B.; Elliott, D. C.; Neuenschwander, G. G.; Kruse, A.; Antal, M. J. Jr. Biomass gasification in near — and super-critical water: Status and prospects, Biomass Bioenergy, 29, 269-292 (2005)

Xu, X.; Matsumura, Y.; Stenberg, J.; Antal, M. J., Jr. Carbon-catalyzed gasification of organic feedstocks in supercritical water, Ind. Eng. Chem. Res., 35, 2522-2530(1996)

Yu, D.; Aihara, M.; Antal, M. J., Jr. Hydrogen production by steam reforming glucose in supercritical water, Energy Fuels, 7, 574-577 (1993)