- Email: [email protected]
Energy Balance of Solid Biofuels
J. agric. Engng Res. (1998) 71, 263—272 Article No. ag980325
Energy Balance of Solid Biofuels V. Scholz; W. Berg; P. Kaulfu{ Institute of Agricultural Engineering Bornim, Max-Eyth-Allee 100, D-14469 Potsdam-Bornim, Germany (Received 25 September 1997; accepted in revised form 8 June 1998)
The input and output of energy are two important factors used to determine the energetic and ecological usefulness of a fuel or its production technology. In this paper, a number of different methods for the production of five biofuels which can be produced in agriculture and forestry are analysed and energetic balances are presented. The results show that the energetic input is relatively low compared to the output, especially for by-products and residual substances such as cereal straw and forest pruning timber (thinning). Whenever fuel crops are cultivated, the energetic efficiency is critically determined by the quantity of nitrogen applied. Depending on the crop and technology, each gigajoule of energy input can provide 7—30 GJ or with by-products up to 50 GJ of thermally utilizable energy without any additional CO pollution. 2 ( 1998 Silsoe Research Institute
Notation A S c Q q F K E H M n N P R2 r t ¹ ¼
area, ha or m2 consumable consumption, l/h or kg/ha consumption coefficient cumulative energy demand, MJ/ha yr specific cumulative energy demand, MJ/kg cumulative non-energy demand, MJ/ha yr cumulative process energy consumption, MJ/ha yr energy yield, GJ/ha lower thermal value, MJ/kg mass, kg number natural yield, kg/ha power requirement, kW coefficient of determination availability utilization ratio specific time, h/ha time of a production cycle, yr water content, %
0021-8634/98/110263#10 $30.00/0
Subscripts db dry basis m maintenance h machines and tools s marginal service life span b materials mix mixed g nominal o operation p production d process u use w waste disposal x running index 1. Introduction In an energy balance, the energy inputs and outputs are calculated and compared with one another and conclusions regarding the energetic efficiency of an area, a product or a technology are made from the results. The energy balances for energy sources produced in agriculture and forestry, such as solid biofuels made from stalk materials and wood, reveal some wide disagreement because of different methods of calculation, different production technologies and energy characteristics.1–4 In this paper, five different solid biofuels with various production technologies are considered and energy balances calculated using uniform method developed in the Institute of Agricultural Engineering Bornim (ATB). 2. Method 2.1. Fundamentals The following method applied to determine the energy input is based on the VDI guideline 46005 ‘‘Cumulated energy demand—terms, definitions, methods of calculation’’. According to this procedure, the expenditures
263
( 1998 Silsoe Research Institute
264
V . S CH O LZ E¹ A ¸.
are divided in each process stage into the material and energy input directly related to the process, and the indirect inputs, which can only be assigned by ratio formula, for the provision of the implements, machinery and plant necessary for the process. This means that the cumulative energy demand Q indicates the entire energetic input which comprises the production p, the use u, and the waste disposal w of a supply item or a service: Q"Q #Q #Q (1) p u w As the relevant literature frequently differentiates between operation (employment) o, and maintenance (servicing, repairing, lodgings, etc.) m when discussing energy data for agriculture, it is advisable to use the following specification: Q "Q #Q (2) u m o The individual items are made up of the cumulative process energy consumption K in each case and the cumulative non-energy demand F, such that Q "K #F (3) x x x During the ‘‘life cycle’’ of a supply item used in agricultural production, certain quantities of various final energy sources such as coal, fuel and electricity are used. The ratio of final to primary energy r in each case is taken into account to calculate the cumulative energy requirement (assessed in terms of primary energy) of the supply item and, from this requirement, the cumulative energy input for a process or a whole procedure, which may last for several years. If the energy sources used cannot be distinguished, for reasons of simplification the mean ratio of r "0)651 for Germany in 1993 stated in the national mix energy statistics6 is used for the calculation. Finally, to determine the input/output ratio or the energy gain, the process-related cumulative energy input is compared with the energy output. In order to quantify the energy flows and balance them exactly, the balance scope must be clearly defined and delimited in terms of place, time and technology.7 The boundaries of the energetic system have already been dictated by the aforementioned balancing principle. According to this, the entire cumulated energy input of all the supply items used directly or indirectly in the production process under review are recorded. Human labour and other forms of energy present in the natural surroundings but not used technically are not included, e.g. solar energy used by crops.
specific energy characteristics, often also called energy eqivalents. The literature contains numerous details of the energy expenditure for individual supply items or groups of supplies, e.g. the specific cumulative energy input for the manufacture of tractors, fertilizers and the like.8 A database called ‘‘Energy’’ was designed and set up in the ATB Potsdam-Bornim in 1992 by using a commercial software, the relational database system MS Access, in order to effectively collect and evaluate these data, and so far it has been filled with approximately 6000 record sets.9 With the help of this database, the specific cumulative energy demand q is formed for agricultural machinery, p implement groups and supply items related to their mean mass. This is done by calculating the mean value and/or time-dependent regression function for each state (Fig. 1 and Table 1). 2.2.2. Maintenance The specific energy demand for the maintenance q rem sults mainly from the energy input for servicing and repairing as well as lodging and/or storage. According to figures in the literature, servicing and repairing amount to between 15 and 145% of the manufacturing energy input q , depending on the machine group.10 For the p lodging of machines, standard values were ascertained,10 depending on the type of machine, the space it occupies and the working life of 3—6 MJ/kg. 2.2.3. Operation Energetic expenditures in the form of process energy and consumable (e.g. electricity, heat, fuel, lubricants and twine) are necessary for the operation of machines and equipment. Since for the most part no firm figures are available for this, and furthermore in the case under review comparable, generally valid energy expenditures for the operation of different machines are required, the specific energy input for the operation of agricultural machines and implements q is estimated in a close apo proximation with the help of the following parameters:
2.2 Specific cumulative energy input for agricultural and forestry supplies
(1) nominal power requirement P , i.e. the necessary g engine power of the tractor or combine required to carry out the process under average use conditions; (2) machine utilization time t , i.e. the total working h time of the machine/implement combination necessary with auxiliary process time but without stoppage times; and (3) nominal consumable consumption S , i.e. the og consumption of consumables under nominal conditions.
2.2.1. Production In order to calculate the cumulative energy input of a particular supply item, it is necessary to know its
The mean nominal consumption, S of the main conog sumable in plant production, i.e. fuel, can be determined with the help of the following regression equation which
265
EN E RG Y B AL AN C E O F SO L I D BIO FU EL S
Fig. 1. Determination of the average specific cumulative energy input for the production of nitrogen fertilizer 9; Other Regression (Germany); Regression (USA)
has been derived from the parameters of approximately 180 tractors11 (Fig. 2): S "0)16P #2)55 og g Coefficient of determination R2"0)80
(4)
The actual fuel consumption S depends on the load of o the engine. In the absence of the knowledge of complete
Germany;
USA;
load reversals for fieldwork processes and in the interests of simplification, an average engine power of 65% of the nominal power P is assumed for engine-powered agrig cultural machines. According to the following regression equation derived from various test results of the German Agricultural Society DLG,12 this results in a mean fuel consumption c of 72% of the nominal consumption o
Table 1 Specific cumulative energy demand for the production, maintenance and disposal of selected agricultural operation supplies in Germany (mean values) Specific cumulative energy demand q, MJ/kg Operation supply Four-wheeled tractors Self-propelled harvesters Trailed harvesters Application equipment Trailers Tillage equipment Trucks Nitrogenous fertilizer Phosphatic fertilizer Potash fertilizer Lime Pesticide Rye seed Lubricating oil PE-foil (storage)* Twine Barn (storage)* *MJ/m2.
Production q p
Maintenance q m
Disposal q w
65 70 55 55 50 48 65 59 17 10 3 200 6 54 13 90 3100
27 22 22 15 25 24 62 — — — — — — — — — 800
0)5 0)5 0)5 0)5 0)5 0)5 0)3 — — — — — — — — — 300
266
V . S CH O LZ E¹ A ¸.
materials are generally fully consumed or converted and/or no details are available on this. 2.3. Calculation program
Fig. 2. Nominal fuel consumption of tractors depending on engine power11; Rear wheel tractors Four wheel tractors; Regression
S (Fig. 3): og c "S /S "0)760P/P #0)226 o o og g Coefficient of determination R2"0)99
(5)
Thus, according to Eqn (5) and P"0)65P , the assumed g consumption coefficients are for diesel c "0.72 and for o the other fuels simplified c "1)0. The engine oil cono sumption is assumed as 2)0% of the fuel consumption S .9 o 2.2.4. ¼aste disposal No special characteristic energy values are stated in the relevant literature for the waste disposal of agricultural machinery and implements. The energy input for this results essentially from the input for scrapping the metals and dumping of non-recyclable material parts. The energy expenditures determined so far13,14 for the waste disposal of machine tools, passenger cars and commercial vehicles amount to between 0)2 and 1)8 MJ/kg. For agricultural machinery it is recommended to assume9 an average specific energy input of q " 0.5 MJ/kg. 8 The energy input of the disposal of expendable materials and consumable is ignored because these
A special computer program ‘‘Energy Balance’’ based on the above principles was developed to calculate the cumulative energy input of agricultural and forestry plant production. This program is built on the MS Access relational database system.9 The database consists of the four basic tables ‘‘machine data’’, ‘‘material data’’, ‘‘operational data’’ and ‘‘process data’’. Starting from the ‘‘process data’’ table, which is closely linked to the other three basic tables, a so-called table query of the process to be analysed is compiled. The data for processes, machines or materials which are not contained in the basic tables are determined and entered in the relevant tables. The basic data called up by the process query are linked by means of the following equations Eqns (6)—(8) with the mean specific cumulative energy expenditures from the ‘‘Energy Equivalents’’ table from the ‘‘Energy’’ database and in this way the energy input for the relevant working and expendable materials is calculated. Finally, the cumulative energy input for individual processes and for the entire process is determined by addition (Fig. 4). The production, upkeep or disposal of machines, tools and other equipment is given by Q "q M n n /(t /t #A )/¹ (6) x x h h d h,s h h,s where M is the mass, n is the number, t is the specific time, A is the area and ¹ is the time of a production cycle. The production, upkeep or disposal of expendable materials can be expressed as Q "q S n /¹ (7) x x b d and the operation of machines, tools and other equipment by Q "(q S c #q S c #q S c )t n n /¹ o o1 og1 o1 o2 og2 o2 o3 og3 o3 h h d (8) 2.4. Balanced crops and process chains
Fig. 3. Relative fuel consumption of tractors depending on relative engine power 12 n 20!49 kW; 50!99 kW; ]100!149 kW;#150!199 kW; Regression
The following energy balances relate to four agricultural and/or forest crops, namely, whole crop winter rye, miscanthus sinensis, poplar and pine. Apart from whole plants, by-products (residual substances) are also included such as winter rye straw, pine harvest residual timber and pine thinning. This means that both conventional and unconventional but promising plants and plant residual substances are included which are suitable for burning or gasification. Various processes for the production and conditioning of each of the raw materials named are considered, with new processes currently under development also being included. As far as possible, processes are selected to correspond with general practice in agriculture
EN E RG Y B AL AN C E O F SO L I D BIO FU EL S
267
Fig. 4. Structure of the programmable database **Energy balance++
and forestry. This requires that average conditions, machines and working times are assumed, which can be taken from the relevant tables.15 For new machines which are in some cases still under development, it was necessary to rely on the manufacturers’ statements (Figs 5 and 6).
The boundary conditions and balance borderlines for the different crops are identical, with the exception of some restrictions for pine production. Each process chain starts with soil preparation and ends with the transport of the fuel to the heating station. A
Fig. 5. Process chains for the production and conditioning of whole rye plants and straw
268
V . S CH O LZ E¹ A ¸.
Fig. 6. Process chains for the production and conditioning of poplar
machine/implement combination suitable for the conditions is selected for each process. The machine utilization time includes the transport and turning time according to Ja¨ger.9,16 Altogether 24 process chains were analysed and balanced. Process sequence, inputs and yields were coordinated with agricultural and forestry research institutes in Germany (Table 2).
3. Results 3.1. Energy input Generally, energy sources are judged by the efficiency of their production and/or conversion, i.e. according to the input/output (energy input/energy output). Additionally for energy sources produced by agriculture and
Table 2 Main data for the production of balanced energy crops Parameter Yield, dry matter t/ha yr Harvest cycle, yr Field area, ha Field distance, km Transport distance, km Seeds, kg/ha year Fertilizer CaO, kg/ha yr P O , kg/ha yr 2 5 K O, kg/ha yr 2 N, kg/ha yr Pesticides, kg/ha yr
Energy rye
Miscanthus
9)1* 1 5 2 5 120 400 95 170 120 4)0
* 3)8 t/ha grain#5)3 t/ha straw; s Provisional figures; t ( ) removal line;
10s 1 5 2 5 10 000` 400 70 95 75 0)2 °
Poplar 10s 3 (6)° 5 2 5 12 000` 100 — — — —
( ) Manual line; `plants/ha yr.
Pine 0)7 (1.3)t 140 (10)t — — 10 15 000` — — — — —
269
EN E RG Y B AL AN C E O F SO L I D BIO FU EL S
Fig. 7. Cumulative energy demand for the production of solid biofuels;
forestry, the energy gain (energy output—energy input) per unit area is decisive. Due to the natural limits of the area available, energy crops or production processes should also be assessed according to the input/output ratio as well as the energy gain per hectare of land used. Regarding the energy inputs, there are significant differences between the energies in raw materials of plant
Cultivation;
Harvest#Processing;
Storage#Transport
origin (Fig. 7). Fuels from whole rye plants require the highest cumulative energy input at 19)5—22)2 GJ/ha yr and pine chips at 0)1—1)0 GJ/ha yr have the lowest cumulative energy input. The high values for whole rye plants are a result of the relatively high energy expenditure for annual cultivation. The extremely low values for pine chips are attributable to the long harvest cycle of 140 yr.
Fig. 8. Energy input and yield in the production of solid biofuels;
Input;
Output;
Ratio input: output
270
V . S CH O LZ E¹ A ¸.
Table 3 Structure of the cumualtive energy demand for the production of solid biofuels Cumulative energy demand Q, GJ/ha yr Machines Process chain Energy rye Pellet line Chopping line Round bale line Rectangular ble line Compact roll line Combined bale line Separate grain line Rye straw Round bale line Rectangular bale line Compact roll line Miscanthus Pellet line Chopping line Round bale line Rectangular bale line Compact roll line Combined bale line Poplar Motormanual line 5-Phases sheaf line 4-Phases sheaf line Mechanized chopping line Pine Partly mechan. removal line 4-Phases full harvest line 2-Phases full harvest line Manual removal line
Maintenance Q m
Operation Q o
0)10 2)48 1)38 1)18
0)33 0)32 0)25 0)23
7)81 4)48 5)50 5)12
0)036 0)007 0)005 0)005
13)07 13)07 13)07 13)07
0)84 1)21 1)72
0)21 0)22 0)51
7)20 5)06 3)86
0)005 0)005 0)103
13)07 13)07 13)07
0)82 0)71 0)43
0)09 0)08 0)06
1)94 1)33 2)99
0)002 0)001 0)001
0 0 0
0)76 2)87 1)43 1)12 0)69 1)15
0)23 0)21 0)14 0)12 0)10 0)12
6)32 2)38 3)75 3)32 5)44 3)28
0)032 0)005 0)003 0)002 0)002 0)002
7)85 7)85 7)85 7)85 7)85 7)85
0)49 0)61 0)48 0)21
0)17 0)21 0)16 0)09
5)59 6)23 4.75 2)73
0)004 0)005 0)004 0)001
0)05 0)05 0)05 0)05
0)01 0)04 0)03 0)05
0)004 0)01 0)01 0)03
0)13 0)34 0)43 0)91
0)0001 0)0003 0)0001 0)0003
0 0)01 0)01 0
Miscanthus sinensis, which requires the secondhighest input at 12)4—15)2 GJ/ha yr, also requires a great deal of energy for its cultivation. This share for cultivation results above all from energy-intensive mineral fertilizer. According to the Institute for Fast-Growing Tree Types Hannoversch-Mu¨nden, poplars do not require fertilizer, so here the total energy input is only 3)0—7)1 GJ/ha yr, although the inputs for harvesting and processing poplars are significantly higher than for the other fuels. The process stages harvest#conditioning and storage#transport must be considered together. A higher energy input e.g. input, compacting, can be partly offset by lower expenditures for storage and transport processes. With the assumed transport distance of 5 km, however, pelletizing of the goods does not reduce the
Disposal Q w
Materials Production Q p
Production Q p
total energy input. A similar situation applies to the compact roll method, where the energy input is largely determined by the twine requirement — at least for the current prototype press. The energy requirement of these two process chains is higher than that of the process chain with conventional round bales if the transport distance is less than 100 km. 3.2. Energy yield The energy yield E is directly proportional to the natural yield N and the thermal value H and thus depends on the water content ¼ of the material. It is calculated as follows: E"NH"N[(H (1!¼/100))!2)45¼/100] (9) db
EN E RG Y B AL AN C E O F SO L I D BIO FU EL S
Generally, the lower thermal value H of water-free solid db biomass, which is relevant for technical incineration, is between 17)5 and 19)5 MJ/kg, with the lower range applying to stalk materials and the upper range to wood. 3.3. Energy gain and input*output ratio Looking at the energy gain per unit area and time, the highest values are achieved by poplars with 155—167 GJ/ha yr, Miscanthus with 154—158 GJ/ha yr and rye with 137—140 GJ/ha yr. However, straw, either as a residual substance or a by-product, still achieves 90 GJ/ha yr. By far the lowest energy gain is achieved by pine because of the extensive cultivation of the land. Even for whole tree utilization it amounts to only 24—27 GJ/ha yr. If the stocks are regularly thinned, about 50 GJ/ha yr is generated. The most favourable input/output ratio by far was achieved for pine production at approx. 1 : 50. Straw also achieved very good values of 1 : 25 to 1 : 40. Also similarly, favourable input/output ratios can be achieved in poplar production provided this can be sustained over a long period without the application of nitrogen. Miscanthus and rye still achieve input/output ratios of 1 : 7 to 1 : 14 (Fig. 8). 3.4. Energy structure An analysis of the energy expenditures broken down by supplies and their individual ‘‘life phases’’ makes it clear that for rye and Miscanthus the greater part, namely 52—67% is required for the production of consumable materials, especially for fertilizers. The nitrogenous fertilizer requires more than half of the energy input of the materials. This mineral fertilizer is extremely energy intensive to produce, requiring on average 59 MJ/kg. The other expenditures are caused by machines and other mechanical aid. On average, only 0)07% of the total energy input of a supply item is required for the waste disposal of the item, for upkeep about 3% is required and for production 10—40%. By a wide margin, most of the energy of machines is consumed during the operational phase, namely 60—90%. This results mainly, but not exclusively, from the fuel consumption of the machines. Taking the example of the compact roll line it can be seen that other expendable supplies, such as the twine, can also make quite a difference (Table 3).
of renewable energy sources. The reasons for this are the surplus of agricultural land, the favourable energetic efficiency and the inexpensive, long-term storability of these energy sources. Furthermore, they could secure farmers and forest owners an additional, stable source of income. The selective production of conventional crops such as whole crop cereals can achieve energy yields of more than 150 GJ/ha yr (1 GJ"278 kWh"23)5 kg oil equivalent). The energy input for the production of these biofuels is comparatively low. Depending on the crop and the technology, it is 3—22 GJ/ha yr. The energy yield for residual substances from agriculture and forestry such as cereal straw and timber thinnings is only about 50—90 GJ/ha yr, but at the same time these fuels have a very low-energy input, if the expenditures for cultivation are allocated to the main product. This aspect, along with the relatively low availability costs and the ethical acceptance among the public—in contrast to whole crop cereals, for example—is one of the reasons why these biofuels are most likely to be introduced on a wide scale. One hectare of straw or specially cultivated energyproducing plants can satisfy the heating requirement of one or two single-family houses throughout the year. What is more, the energetic efficiency of the production of these fuels is hardly any less favourable than that for coal briquettes. For each GJ of energy input, energy crops can provide 7—30 GJ or with by-products up to 50 GJ of thermally utilizable energy, and this without any additional CO pollution. 2 References 1
2
3
4
4. Conclusions 5
In the future in those European regions poor in wind and solar radiation, solid fuels produced by agriculture and forestry will certainly make up the largest proportion
271
Born P CO -neutrale Energietra¨ger aus Biomasse? (CO 2 sources from biomass?) BWK 1992, 44(6)2 neutral energy Wintzer D; Fu¨ rniß B; Klein-Vielhauer S; Leible L; Nielke E; Ro¨ sch C; Tangen H Technikfolgenabscha¨tzung zum Thema Nachwachsende Rohstoffe. (Technology assessment to the topic of renewable raw materials) Schrifftenreihe des Bundesministers fu¨r Erna¨hrung, Landwirtschaft und Forsten, (A) Mu¨nster, 1993 Hartmann H; Strehler A Die Stellung der Biomasse im Vergleich zu anderen erneuerbaren Energietra¨gern aus o¨kologischer, o¨konomischer und technischer Sicht (The position of biomass compared to other renewable energy sources refering ecological, economical and technological aspects. Schriftenreihe ‘‘Nachwachsende Rohstoffe’’ Bd. 3, Mu¨nster, 1995 Kaltschmitt M; Reinhard G; Weisser C; Do¨ hler H Ganzheitliche Bilanzierung von nachwachsenden Energietra¨gern unter verschiedenen o¨kologischen Aspekten (Complex balancing of renewable energy sources refering various ecological aspects). Osnabru¨ck, 1995 VDI-Richtlinie 4600 Kumulierter Energieaufwand-Begriffe, Definitionen, Berechnungsmethoden. (Cumulated Energy Demand—Terms, Definitions, Methods of Calculation). Berlin, 1995
272 6
7
8
9
10
11
V . S CH O LZ E¹ A ¸.
Bundesministerium fu¨ r Wirtschaft Energie Daten (Energy Data). Bonn, 1994. Fluck R Energy analysis for agricultural systems. Energy in World Agriculture, 1992, 6, 45—52 Pimentel D Energy Inputs in Production Agriculture. Energy in World Agriculture, 1992, 6, 13—29 Scholz V; Kaulfuß P Energiebilanz fu¨r Biofestbrennstoffe (Energy balance of solid biofuels). Research report 1995/3, Institut fu¨r Agrartechnik Bornim (ATB), Potsdam, 1995 Wunderlich A Beitrag zur Ermittlung des kumulierten Energieaufwandes Iandwirtschaftlicher Maschinen und Gera¨te (Contribution to the determination of the cumulative energy demand of agricultural machines and tools). Diplomarbeit, Humbolt-Universita¨t-Universita¨t zu Berlin und ATB, Potsdam 1995 DLZ Traktoren ’95 (Tractors ’95). dlz-agrarmagazin, 1994, 45(12), 106—203
12
13
14
15
16
DLG Tractor tests. Reports of the German Agricultural Society DLG, Gro{ Umstadt, 1974—1994 Richter K Zur Problematik des Komplexenergieeinsatzes und seiner Bestimmung - ausgewa¨hlte Beispiele von Beeinflussungsmo¨glichkeiten (To the problems of use of complex energy and its determination - selected examples of influencing possibilities). Dissertation 1H Zittau, 1986 Mauch W Kumulierter Energieaufwand von Lastkraftwagen (Cumulative energy demand of trucks). Automobiltecnische Zeitschrift, 1994, 96(2), 116—124 KTBL-Taschenbuch Landwirtschaft 1994/95 (KTBLPocketbook Agriculture 1994/95). 17. Auflage, Mu¨nsterHiltrup, 1994 Ja¨ ger P Berechnung der Teilzeiten fu¨r die Arbeit am Feld. (Calculation of the partial operating times for work in the fields). Landtechnik, 1991, 46(3), 123—128
Energy Balance of Solid Biofuels V. Scholz; W. Berg; P. Kaulfu{ Institute of Agricultural Engineering Bornim, Max-Eyth-Allee 100, D-14469 Potsdam-Bornim, Germany (Received 25 September 1997; accepted in revised form 8 June 1998)
The input and output of energy are two important factors used to determine the energetic and ecological usefulness of a fuel or its production technology. In this paper, a number of different methods for the production of five biofuels which can be produced in agriculture and forestry are analysed and energetic balances are presented. The results show that the energetic input is relatively low compared to the output, especially for by-products and residual substances such as cereal straw and forest pruning timber (thinning). Whenever fuel crops are cultivated, the energetic efficiency is critically determined by the quantity of nitrogen applied. Depending on the crop and technology, each gigajoule of energy input can provide 7—30 GJ or with by-products up to 50 GJ of thermally utilizable energy without any additional CO pollution. 2 ( 1998 Silsoe Research Institute
Notation A S c Q q F K E H M n N P R2 r t ¹ ¼
area, ha or m2 consumable consumption, l/h or kg/ha consumption coefficient cumulative energy demand, MJ/ha yr specific cumulative energy demand, MJ/kg cumulative non-energy demand, MJ/ha yr cumulative process energy consumption, MJ/ha yr energy yield, GJ/ha lower thermal value, MJ/kg mass, kg number natural yield, kg/ha power requirement, kW coefficient of determination availability utilization ratio specific time, h/ha time of a production cycle, yr water content, %
0021-8634/98/110263#10 $30.00/0
Subscripts db dry basis m maintenance h machines and tools s marginal service life span b materials mix mixed g nominal o operation p production d process u use w waste disposal x running index 1. Introduction In an energy balance, the energy inputs and outputs are calculated and compared with one another and conclusions regarding the energetic efficiency of an area, a product or a technology are made from the results. The energy balances for energy sources produced in agriculture and forestry, such as solid biofuels made from stalk materials and wood, reveal some wide disagreement because of different methods of calculation, different production technologies and energy characteristics.1–4 In this paper, five different solid biofuels with various production technologies are considered and energy balances calculated using uniform method developed in the Institute of Agricultural Engineering Bornim (ATB). 2. Method 2.1. Fundamentals The following method applied to determine the energy input is based on the VDI guideline 46005 ‘‘Cumulated energy demand—terms, definitions, methods of calculation’’. According to this procedure, the expenditures
263
( 1998 Silsoe Research Institute
264
V . S CH O LZ E¹ A ¸.
are divided in each process stage into the material and energy input directly related to the process, and the indirect inputs, which can only be assigned by ratio formula, for the provision of the implements, machinery and plant necessary for the process. This means that the cumulative energy demand Q indicates the entire energetic input which comprises the production p, the use u, and the waste disposal w of a supply item or a service: Q"Q #Q #Q (1) p u w As the relevant literature frequently differentiates between operation (employment) o, and maintenance (servicing, repairing, lodgings, etc.) m when discussing energy data for agriculture, it is advisable to use the following specification: Q "Q #Q (2) u m o The individual items are made up of the cumulative process energy consumption K in each case and the cumulative non-energy demand F, such that Q "K #F (3) x x x During the ‘‘life cycle’’ of a supply item used in agricultural production, certain quantities of various final energy sources such as coal, fuel and electricity are used. The ratio of final to primary energy r in each case is taken into account to calculate the cumulative energy requirement (assessed in terms of primary energy) of the supply item and, from this requirement, the cumulative energy input for a process or a whole procedure, which may last for several years. If the energy sources used cannot be distinguished, for reasons of simplification the mean ratio of r "0)651 for Germany in 1993 stated in the national mix energy statistics6 is used for the calculation. Finally, to determine the input/output ratio or the energy gain, the process-related cumulative energy input is compared with the energy output. In order to quantify the energy flows and balance them exactly, the balance scope must be clearly defined and delimited in terms of place, time and technology.7 The boundaries of the energetic system have already been dictated by the aforementioned balancing principle. According to this, the entire cumulated energy input of all the supply items used directly or indirectly in the production process under review are recorded. Human labour and other forms of energy present in the natural surroundings but not used technically are not included, e.g. solar energy used by crops.
specific energy characteristics, often also called energy eqivalents. The literature contains numerous details of the energy expenditure for individual supply items or groups of supplies, e.g. the specific cumulative energy input for the manufacture of tractors, fertilizers and the like.8 A database called ‘‘Energy’’ was designed and set up in the ATB Potsdam-Bornim in 1992 by using a commercial software, the relational database system MS Access, in order to effectively collect and evaluate these data, and so far it has been filled with approximately 6000 record sets.9 With the help of this database, the specific cumulative energy demand q is formed for agricultural machinery, p implement groups and supply items related to their mean mass. This is done by calculating the mean value and/or time-dependent regression function for each state (Fig. 1 and Table 1). 2.2.2. Maintenance The specific energy demand for the maintenance q rem sults mainly from the energy input for servicing and repairing as well as lodging and/or storage. According to figures in the literature, servicing and repairing amount to between 15 and 145% of the manufacturing energy input q , depending on the machine group.10 For the p lodging of machines, standard values were ascertained,10 depending on the type of machine, the space it occupies and the working life of 3—6 MJ/kg. 2.2.3. Operation Energetic expenditures in the form of process energy and consumable (e.g. electricity, heat, fuel, lubricants and twine) are necessary for the operation of machines and equipment. Since for the most part no firm figures are available for this, and furthermore in the case under review comparable, generally valid energy expenditures for the operation of different machines are required, the specific energy input for the operation of agricultural machines and implements q is estimated in a close apo proximation with the help of the following parameters:
2.2 Specific cumulative energy input for agricultural and forestry supplies
(1) nominal power requirement P , i.e. the necessary g engine power of the tractor or combine required to carry out the process under average use conditions; (2) machine utilization time t , i.e. the total working h time of the machine/implement combination necessary with auxiliary process time but without stoppage times; and (3) nominal consumable consumption S , i.e. the og consumption of consumables under nominal conditions.
2.2.1. Production In order to calculate the cumulative energy input of a particular supply item, it is necessary to know its
The mean nominal consumption, S of the main conog sumable in plant production, i.e. fuel, can be determined with the help of the following regression equation which
265
EN E RG Y B AL AN C E O F SO L I D BIO FU EL S
Fig. 1. Determination of the average specific cumulative energy input for the production of nitrogen fertilizer 9; Other Regression (Germany); Regression (USA)
has been derived from the parameters of approximately 180 tractors11 (Fig. 2): S "0)16P #2)55 og g Coefficient of determination R2"0)80
(4)
The actual fuel consumption S depends on the load of o the engine. In the absence of the knowledge of complete
Germany;
USA;
load reversals for fieldwork processes and in the interests of simplification, an average engine power of 65% of the nominal power P is assumed for engine-powered agrig cultural machines. According to the following regression equation derived from various test results of the German Agricultural Society DLG,12 this results in a mean fuel consumption c of 72% of the nominal consumption o
Table 1 Specific cumulative energy demand for the production, maintenance and disposal of selected agricultural operation supplies in Germany (mean values) Specific cumulative energy demand q, MJ/kg Operation supply Four-wheeled tractors Self-propelled harvesters Trailed harvesters Application equipment Trailers Tillage equipment Trucks Nitrogenous fertilizer Phosphatic fertilizer Potash fertilizer Lime Pesticide Rye seed Lubricating oil PE-foil (storage)* Twine Barn (storage)* *MJ/m2.
Production q p
Maintenance q m
Disposal q w
65 70 55 55 50 48 65 59 17 10 3 200 6 54 13 90 3100
27 22 22 15 25 24 62 — — — — — — — — — 800
0)5 0)5 0)5 0)5 0)5 0)5 0)3 — — — — — — — — — 300
266
V . S CH O LZ E¹ A ¸.
materials are generally fully consumed or converted and/or no details are available on this. 2.3. Calculation program
Fig. 2. Nominal fuel consumption of tractors depending on engine power11; Rear wheel tractors Four wheel tractors; Regression
S (Fig. 3): og c "S /S "0)760P/P #0)226 o o og g Coefficient of determination R2"0)99
(5)
Thus, according to Eqn (5) and P"0)65P , the assumed g consumption coefficients are for diesel c "0.72 and for o the other fuels simplified c "1)0. The engine oil cono sumption is assumed as 2)0% of the fuel consumption S .9 o 2.2.4. ¼aste disposal No special characteristic energy values are stated in the relevant literature for the waste disposal of agricultural machinery and implements. The energy input for this results essentially from the input for scrapping the metals and dumping of non-recyclable material parts. The energy expenditures determined so far13,14 for the waste disposal of machine tools, passenger cars and commercial vehicles amount to between 0)2 and 1)8 MJ/kg. For agricultural machinery it is recommended to assume9 an average specific energy input of q " 0.5 MJ/kg. 8 The energy input of the disposal of expendable materials and consumable is ignored because these
A special computer program ‘‘Energy Balance’’ based on the above principles was developed to calculate the cumulative energy input of agricultural and forestry plant production. This program is built on the MS Access relational database system.9 The database consists of the four basic tables ‘‘machine data’’, ‘‘material data’’, ‘‘operational data’’ and ‘‘process data’’. Starting from the ‘‘process data’’ table, which is closely linked to the other three basic tables, a so-called table query of the process to be analysed is compiled. The data for processes, machines or materials which are not contained in the basic tables are determined and entered in the relevant tables. The basic data called up by the process query are linked by means of the following equations Eqns (6)—(8) with the mean specific cumulative energy expenditures from the ‘‘Energy Equivalents’’ table from the ‘‘Energy’’ database and in this way the energy input for the relevant working and expendable materials is calculated. Finally, the cumulative energy input for individual processes and for the entire process is determined by addition (Fig. 4). The production, upkeep or disposal of machines, tools and other equipment is given by Q "q M n n /(t /t #A )/¹ (6) x x h h d h,s h h,s where M is the mass, n is the number, t is the specific time, A is the area and ¹ is the time of a production cycle. The production, upkeep or disposal of expendable materials can be expressed as Q "q S n /¹ (7) x x b d and the operation of machines, tools and other equipment by Q "(q S c #q S c #q S c )t n n /¹ o o1 og1 o1 o2 og2 o2 o3 og3 o3 h h d (8) 2.4. Balanced crops and process chains
Fig. 3. Relative fuel consumption of tractors depending on relative engine power 12 n 20!49 kW; 50!99 kW; ]100!149 kW;#150!199 kW; Regression
The following energy balances relate to four agricultural and/or forest crops, namely, whole crop winter rye, miscanthus sinensis, poplar and pine. Apart from whole plants, by-products (residual substances) are also included such as winter rye straw, pine harvest residual timber and pine thinning. This means that both conventional and unconventional but promising plants and plant residual substances are included which are suitable for burning or gasification. Various processes for the production and conditioning of each of the raw materials named are considered, with new processes currently under development also being included. As far as possible, processes are selected to correspond with general practice in agriculture
EN E RG Y B AL AN C E O F SO L I D BIO FU EL S
267
Fig. 4. Structure of the programmable database **Energy balance++
and forestry. This requires that average conditions, machines and working times are assumed, which can be taken from the relevant tables.15 For new machines which are in some cases still under development, it was necessary to rely on the manufacturers’ statements (Figs 5 and 6).
The boundary conditions and balance borderlines for the different crops are identical, with the exception of some restrictions for pine production. Each process chain starts with soil preparation and ends with the transport of the fuel to the heating station. A
Fig. 5. Process chains for the production and conditioning of whole rye plants and straw
268
V . S CH O LZ E¹ A ¸.
Fig. 6. Process chains for the production and conditioning of poplar
machine/implement combination suitable for the conditions is selected for each process. The machine utilization time includes the transport and turning time according to Ja¨ger.9,16 Altogether 24 process chains were analysed and balanced. Process sequence, inputs and yields were coordinated with agricultural and forestry research institutes in Germany (Table 2).
3. Results 3.1. Energy input Generally, energy sources are judged by the efficiency of their production and/or conversion, i.e. according to the input/output (energy input/energy output). Additionally for energy sources produced by agriculture and
Table 2 Main data for the production of balanced energy crops Parameter Yield, dry matter t/ha yr Harvest cycle, yr Field area, ha Field distance, km Transport distance, km Seeds, kg/ha year Fertilizer CaO, kg/ha yr P O , kg/ha yr 2 5 K O, kg/ha yr 2 N, kg/ha yr Pesticides, kg/ha yr
Energy rye
Miscanthus
9)1* 1 5 2 5 120 400 95 170 120 4)0
* 3)8 t/ha grain#5)3 t/ha straw; s Provisional figures; t ( ) removal line;
10s 1 5 2 5 10 000` 400 70 95 75 0)2 °
Poplar 10s 3 (6)° 5 2 5 12 000` 100 — — — —
( ) Manual line; `plants/ha yr.
Pine 0)7 (1.3)t 140 (10)t — — 10 15 000` — — — — —
269
EN E RG Y B AL AN C E O F SO L I D BIO FU EL S
Fig. 7. Cumulative energy demand for the production of solid biofuels;
forestry, the energy gain (energy output—energy input) per unit area is decisive. Due to the natural limits of the area available, energy crops or production processes should also be assessed according to the input/output ratio as well as the energy gain per hectare of land used. Regarding the energy inputs, there are significant differences between the energies in raw materials of plant
Cultivation;
Harvest#Processing;
Storage#Transport
origin (Fig. 7). Fuels from whole rye plants require the highest cumulative energy input at 19)5—22)2 GJ/ha yr and pine chips at 0)1—1)0 GJ/ha yr have the lowest cumulative energy input. The high values for whole rye plants are a result of the relatively high energy expenditure for annual cultivation. The extremely low values for pine chips are attributable to the long harvest cycle of 140 yr.
Fig. 8. Energy input and yield in the production of solid biofuels;
Input;
Output;
Ratio input: output
270
V . S CH O LZ E¹ A ¸.
Table 3 Structure of the cumualtive energy demand for the production of solid biofuels Cumulative energy demand Q, GJ/ha yr Machines Process chain Energy rye Pellet line Chopping line Round bale line Rectangular ble line Compact roll line Combined bale line Separate grain line Rye straw Round bale line Rectangular bale line Compact roll line Miscanthus Pellet line Chopping line Round bale line Rectangular bale line Compact roll line Combined bale line Poplar Motormanual line 5-Phases sheaf line 4-Phases sheaf line Mechanized chopping line Pine Partly mechan. removal line 4-Phases full harvest line 2-Phases full harvest line Manual removal line
Maintenance Q m
Operation Q o
0)10 2)48 1)38 1)18
0)33 0)32 0)25 0)23
7)81 4)48 5)50 5)12
0)036 0)007 0)005 0)005
13)07 13)07 13)07 13)07
0)84 1)21 1)72
0)21 0)22 0)51
7)20 5)06 3)86
0)005 0)005 0)103
13)07 13)07 13)07
0)82 0)71 0)43
0)09 0)08 0)06
1)94 1)33 2)99
0)002 0)001 0)001
0 0 0
0)76 2)87 1)43 1)12 0)69 1)15
0)23 0)21 0)14 0)12 0)10 0)12
6)32 2)38 3)75 3)32 5)44 3)28
0)032 0)005 0)003 0)002 0)002 0)002
7)85 7)85 7)85 7)85 7)85 7)85
0)49 0)61 0)48 0)21
0)17 0)21 0)16 0)09
5)59 6)23 4.75 2)73
0)004 0)005 0)004 0)001
0)05 0)05 0)05 0)05
0)01 0)04 0)03 0)05
0)004 0)01 0)01 0)03
0)13 0)34 0)43 0)91
0)0001 0)0003 0)0001 0)0003
0 0)01 0)01 0
Miscanthus sinensis, which requires the secondhighest input at 12)4—15)2 GJ/ha yr, also requires a great deal of energy for its cultivation. This share for cultivation results above all from energy-intensive mineral fertilizer. According to the Institute for Fast-Growing Tree Types Hannoversch-Mu¨nden, poplars do not require fertilizer, so here the total energy input is only 3)0—7)1 GJ/ha yr, although the inputs for harvesting and processing poplars are significantly higher than for the other fuels. The process stages harvest#conditioning and storage#transport must be considered together. A higher energy input e.g. input, compacting, can be partly offset by lower expenditures for storage and transport processes. With the assumed transport distance of 5 km, however, pelletizing of the goods does not reduce the
Disposal Q w
Materials Production Q p
Production Q p
total energy input. A similar situation applies to the compact roll method, where the energy input is largely determined by the twine requirement — at least for the current prototype press. The energy requirement of these two process chains is higher than that of the process chain with conventional round bales if the transport distance is less than 100 km. 3.2. Energy yield The energy yield E is directly proportional to the natural yield N and the thermal value H and thus depends on the water content ¼ of the material. It is calculated as follows: E"NH"N[(H (1!¼/100))!2)45¼/100] (9) db
EN E RG Y B AL AN C E O F SO L I D BIO FU EL S
Generally, the lower thermal value H of water-free solid db biomass, which is relevant for technical incineration, is between 17)5 and 19)5 MJ/kg, with the lower range applying to stalk materials and the upper range to wood. 3.3. Energy gain and input*output ratio Looking at the energy gain per unit area and time, the highest values are achieved by poplars with 155—167 GJ/ha yr, Miscanthus with 154—158 GJ/ha yr and rye with 137—140 GJ/ha yr. However, straw, either as a residual substance or a by-product, still achieves 90 GJ/ha yr. By far the lowest energy gain is achieved by pine because of the extensive cultivation of the land. Even for whole tree utilization it amounts to only 24—27 GJ/ha yr. If the stocks are regularly thinned, about 50 GJ/ha yr is generated. The most favourable input/output ratio by far was achieved for pine production at approx. 1 : 50. Straw also achieved very good values of 1 : 25 to 1 : 40. Also similarly, favourable input/output ratios can be achieved in poplar production provided this can be sustained over a long period without the application of nitrogen. Miscanthus and rye still achieve input/output ratios of 1 : 7 to 1 : 14 (Fig. 8). 3.4. Energy structure An analysis of the energy expenditures broken down by supplies and their individual ‘‘life phases’’ makes it clear that for rye and Miscanthus the greater part, namely 52—67% is required for the production of consumable materials, especially for fertilizers. The nitrogenous fertilizer requires more than half of the energy input of the materials. This mineral fertilizer is extremely energy intensive to produce, requiring on average 59 MJ/kg. The other expenditures are caused by machines and other mechanical aid. On average, only 0)07% of the total energy input of a supply item is required for the waste disposal of the item, for upkeep about 3% is required and for production 10—40%. By a wide margin, most of the energy of machines is consumed during the operational phase, namely 60—90%. This results mainly, but not exclusively, from the fuel consumption of the machines. Taking the example of the compact roll line it can be seen that other expendable supplies, such as the twine, can also make quite a difference (Table 3).
of renewable energy sources. The reasons for this are the surplus of agricultural land, the favourable energetic efficiency and the inexpensive, long-term storability of these energy sources. Furthermore, they could secure farmers and forest owners an additional, stable source of income. The selective production of conventional crops such as whole crop cereals can achieve energy yields of more than 150 GJ/ha yr (1 GJ"278 kWh"23)5 kg oil equivalent). The energy input for the production of these biofuels is comparatively low. Depending on the crop and the technology, it is 3—22 GJ/ha yr. The energy yield for residual substances from agriculture and forestry such as cereal straw and timber thinnings is only about 50—90 GJ/ha yr, but at the same time these fuels have a very low-energy input, if the expenditures for cultivation are allocated to the main product. This aspect, along with the relatively low availability costs and the ethical acceptance among the public—in contrast to whole crop cereals, for example—is one of the reasons why these biofuels are most likely to be introduced on a wide scale. One hectare of straw or specially cultivated energyproducing plants can satisfy the heating requirement of one or two single-family houses throughout the year. What is more, the energetic efficiency of the production of these fuels is hardly any less favourable than that for coal briquettes. For each GJ of energy input, energy crops can provide 7—30 GJ or with by-products up to 50 GJ of thermally utilizable energy, and this without any additional CO pollution. 2 References 1
2
3
4
4. Conclusions 5
In the future in those European regions poor in wind and solar radiation, solid fuels produced by agriculture and forestry will certainly make up the largest proportion
271
Born P CO -neutrale Energietra¨ger aus Biomasse? (CO 2 sources from biomass?) BWK 1992, 44(6)2 neutral energy Wintzer D; Fu¨ rniß B; Klein-Vielhauer S; Leible L; Nielke E; Ro¨ sch C; Tangen H Technikfolgenabscha¨tzung zum Thema Nachwachsende Rohstoffe. (Technology assessment to the topic of renewable raw materials) Schrifftenreihe des Bundesministers fu¨r Erna¨hrung, Landwirtschaft und Forsten, (A) Mu¨nster, 1993 Hartmann H; Strehler A Die Stellung der Biomasse im Vergleich zu anderen erneuerbaren Energietra¨gern aus o¨kologischer, o¨konomischer und technischer Sicht (The position of biomass compared to other renewable energy sources refering ecological, economical and technological aspects. Schriftenreihe ‘‘Nachwachsende Rohstoffe’’ Bd. 3, Mu¨nster, 1995 Kaltschmitt M; Reinhard G; Weisser C; Do¨ hler H Ganzheitliche Bilanzierung von nachwachsenden Energietra¨gern unter verschiedenen o¨kologischen Aspekten (Complex balancing of renewable energy sources refering various ecological aspects). Osnabru¨ck, 1995 VDI-Richtlinie 4600 Kumulierter Energieaufwand-Begriffe, Definitionen, Berechnungsmethoden. (Cumulated Energy Demand—Terms, Definitions, Methods of Calculation). Berlin, 1995
272 6
7
8
9
10
11
V . S CH O LZ E¹ A ¸.
Bundesministerium fu¨ r Wirtschaft Energie Daten (Energy Data). Bonn, 1994. Fluck R Energy analysis for agricultural systems. Energy in World Agriculture, 1992, 6, 45—52 Pimentel D Energy Inputs in Production Agriculture. Energy in World Agriculture, 1992, 6, 13—29 Scholz V; Kaulfuß P Energiebilanz fu¨r Biofestbrennstoffe (Energy balance of solid biofuels). Research report 1995/3, Institut fu¨r Agrartechnik Bornim (ATB), Potsdam, 1995 Wunderlich A Beitrag zur Ermittlung des kumulierten Energieaufwandes Iandwirtschaftlicher Maschinen und Gera¨te (Contribution to the determination of the cumulative energy demand of agricultural machines and tools). Diplomarbeit, Humbolt-Universita¨t-Universita¨t zu Berlin und ATB, Potsdam 1995 DLZ Traktoren ’95 (Tractors ’95). dlz-agrarmagazin, 1994, 45(12), 106—203
12
13
14
15
16
DLG Tractor tests. Reports of the German Agricultural Society DLG, Gro{ Umstadt, 1974—1994 Richter K Zur Problematik des Komplexenergieeinsatzes und seiner Bestimmung - ausgewa¨hlte Beispiele von Beeinflussungsmo¨glichkeiten (To the problems of use of complex energy and its determination - selected examples of influencing possibilities). Dissertation 1H Zittau, 1986 Mauch W Kumulierter Energieaufwand von Lastkraftwagen (Cumulative energy demand of trucks). Automobiltecnische Zeitschrift, 1994, 96(2), 116—124 KTBL-Taschenbuch Landwirtschaft 1994/95 (KTBLPocketbook Agriculture 1994/95). 17. Auflage, Mu¨nsterHiltrup, 1994 Ja¨ ger P Berechnung der Teilzeiten fu¨r die Arbeit am Feld. (Calculation of the partial operating times for work in the fields). Landtechnik, 1991, 46(3), 123—128