Biogas production from energy crops in northern Greece: economics of electricity generation associated with heat recovery in a greenhouse

Abstract

Herein a techno-economic assessment was performed on an energy-crop-based biogas plant coupled with a greenhouse for utilizing thermal energy produced by cogeneration. Seven energy crops were evaluated: triticale, maize, alfalfa, sunflower, clover, barley and wheat. According to the evaluation, triticale was the most competitive energy crop under selected climate conditions for northern Greece. Although maize displays higher biomass yield and biogas potential than the drought-resistant crop triticale, it has high irrigation demand that contributes significantly to total production costs. For a triticale-based biogas production to become economically feasible, agricultural arable area larger than 500 ha, or biogas plant size larger than 1000 kW_el, is required. However, with public funding, biogas production becomes feasible at smaller area (>250 ha) or biogas plant size (>500 kW_el). The inclusion of a greenhouse into the design of the biogas plant contributes positively to the economic viability of the entire system. Under this scenario, greenhouse financial income accounts for about 17–18% of total income. Results of a sensitivity analysis suggest that the selection of an appropriate energy crop for biogas production should be based principally on both digestibility (specific methane yield) and biomass yield per hectare, these factors being more critical than biomass production costs.

Appendices

Appendix 1

Equation for section “Total land area”

Total land area (S _L) is given from the operational cultivation area (S _C) by the following equation:

$$S_{\text{L}} = S_{\text{C}} \times \frac{1}{{1 - k_{\text{UN}} - k_{\text{FA}} }};\quad S_{\text{L}} = 1.08 \times S_{\text{C}} \;{\text{for}}\;{\text{legumes}};\;S_{\text{L}} = 1.47 \times S_{\text{C}} \; {\text{for}}\;{\text{other}}\;{\text{crops}}$$

(1)

Equations for section “Engine power, number of tractors and fuel consumption for cultivation”

The required size of the ploughing implement (W _IT) attached to the tractor was calculated according to the following equation:

$$W_{IT} = \frac{{10 S_{C} }}{{T_{A} \times v_{T} }}$$

(2)

S _C operational cultivation area (ha), T _A duration available to perform ploughing operations, here: 160 h, v _T tractor velocity (km/h), here 6 km/h for ploughing operation.

Drawbar power (P _D; kW) corresponding to the selected implement size is calculated and converted into power-take-off (P _TO; kW), and engine power for tillage (P _EP; kW) applying the following equation and conversion factors (Mehta et al. 2011; NSW-Farmers 2013):

$$P_{D} = \frac{{R_{S} \times {\text{d}}_{\text{IT}} \times {\text{W}}_{\text{IT}} \times v_{T} }}{3600}$$

(3)

$$P_{TO} = P_{D} \times 1.8$$

(4)

$$P_{EP} = P_{TO} \times 1.18 \times \left( {1 + {\text{k}}_{\text{PM}} } \right)$$

(5)

R _S soil resistance, here: 890 N/(mIW × cmDS), d _IT depth of tillage (plough), here: 20 cm, W _IT width of implement used for tillage (plough), v _T tractor velocity, here: 6 km/h, k _PM additional factor providing a safety margin in tractor sizing, here: 10%

Linear correlation between total engine power (P _EP; kW) and the surface of cultivated area (S _C; ha) is described by the following eq.:

$$P_{\text{EP}} = 0.656 \times S_{\text{C}} + 0.294$$

(6)

Specific power requirements [k _P; (kW/(m_IW × (km/h))] of each implement were calculated with the following equation, with values for the factors R _s and D _I taken from Table 8.

$$k_{\text{P}} = R_{\text{s}} \times D_{\text{I}} \times 1.18 \times 1.8$$

(7)

Time (t _W; h) required to perform each individual field operation is given by the following equations, according to the total distance travelled by the tractors (d _T; km):

$$t_{\text{W}} = \frac{{S_{\text{C}} }}{{d_{\text{T}} \times v_{\text{T}} }}$$

(8)

$$d_{\text{T}} = \frac{{S_{\text{C}} \times \left( {1 + k_{\text{T}} } \right) \times 10}}{{W_{\text{I}} \times v_{\text{T}} }}$$

(9)

k _T factor to include tractor turns on the fields, here: 1%, W _I width (m) of each implement, r _U engine power usage, set at 0.8 (80%) to prevent engine overloading (Mehta et al. 2011).

Hourly diesel fuel consumptions (N _H; L/h) for each field work were estimated using the empirical equation given by Jílek et al. (2008). Subsequently, the total fuel consumption (N _T; L) of tractors was calculated for the work durations (t _W; h) estimated previously for each field work:

$$N_{\text{H}} = 0.036 \times P_{\text{E}}^{0.938} \times r_{\text{U}}^{0.781}$$

(10)

where P _E is for the engine power (kW).

$$N_{\text{T}} = N_{\text{H}} \times t_{\text{W}}$$

(11)

Equations for section “Irrigation and fertilizing of energy crops”

Pump power (P _P; kW) required to drive the sprinklers was calculated from the total manometric water pressure (H _T, m), accounting for pressure losses in the pipes (H _L, m) (Terzidis and Papazafeiriou 1997):

$$P_{\text{P}} = \frac{{H_{\text{T}} \times Q \times 9.81}}{{3600 \times r_{\text{e}} }}$$

(12)

$$H_{\text{T}} = H_{\text{L}} + H_{\text{E}} + H_{\text{W}}$$

(13)

$$H_{\text{L}} = 1.13 \times 10^{11} \times {\left( {\frac{Q}{C}} \right)}^{1.852} \times D^{ - 4.87}$$

(14)

H _E operational manometric pressure of equipment, here: 80 m, H _W depth of the water body from which water is pumped, here: 10 m, Q water flow (m³/h), depends on irrigation requirements, r _e pump efficiency, set at 70% (Buckmaster 2009), C constant depending on material properties, here: 150 (Terzidis and Papazafeiriou 1997), D inner diameter of the tube (mm).

Total fertilization requirements:

$$R_{\text{FERT}} = 100 \times R_{\text{E}} \times \left( {1 - R_{\text{L}} } \right) \times R_{\text{A}} ;R_{\text{FERT}} = 36\%$$

(15)

R _E fertilizing efficiency (here: 80%), R _L nutrient losses during storage of the digestates, here: 35%, R _A direct availability of nutrients in digestates, here: 70%.

Equations for section “Transportation and silaging of energy crops and digestates”

Transport distance of harvested biomass to bunker silos was calculated by:

$$d_{\text{T}} = \frac{2}{3} \times \tau \times \sqrt {\frac{{M_{\text{BF}} }}{{k_{\text{AV}} \times 100 \times Y_{\text{BF}} \times \pi }}}$$

(16)

d _T haul distance (km), τ tortuosity factor, here: 1.8, M _BF fresh mass of crop biomass (t) to be transported, k _AV availability of biomass, here: 0.3, Y _BF fresh biomass yield before ensiling (t/ha).

The total distance (T _D; km) that the tractor(s) need to cover transportations was calculated as follows:

$$T_{\text{D}} = 2 \times \frac{V}{{C_{\text{R}} }} \times d_{\text{T}}$$

(17)

V total volume of biomass to be transported (m³), C _R Carriage capacity of the tractor (m³): here 18 m³.

The number of tractors required for biomass transportation (n_T) was calculated according to:

$$n_{\text{TT}} = 0.44 + 0.0746 \times \left[ {\frac{{1.6 \times P_{TT} \times d_{\text{T}} }}{{M_{\text{TT}} \times v_{\text{TT}} }}} \right] + 1.06$$

(18)

n _T number of tractors required, P _TT engine power of transport tractors (kW), M _TT transportation capacity of the tractors, here: 20 t of dry matter, v _TT average speed of the transport tractors, here: 20 km/h.

The operation time of packing tractors for silaging (t _SIL) was calculated as

$$t_{\text{SIL}} = r_{\text{SIL}} \times M_{\text{BD}}$$

(19)

r _SIL packing rate, here: 0.017 h/t DM, M _BD dry mass of crops to be packed (t DM).

Equations for section “Methane yields of energy crops”

Yields of dry ensiled biomass (Y _SD; t DM/ha) are calculated for each crop:

$$Y_{\text{SD}} = Y_{\text{BF}} \times (1 - k_{\text{LO}} ) \times C_{\text{DM}}$$

(20)

Y _BF yield of fresh biomass (t FM/ha), k _LO factor accounting for losses in harvesting and silaging, here: 13% (Davies 2004), C _DM dry matter content of silage (%FM/100).

Yields of dry ensiled biomass are converted into methane yields per hectare ($Y_{{{\text{CH}}_{4} }}$; m³ CH₄/ha):

$$Y_{{{\text{CH}}_{4} }} = Y_{\text{SD}} \times C_{\text{VS}} \times C_{{{\text{CH}}_{4} }}$$

(21)

C _VS volatile solids content of dry mass (%DM/100), $C_{{{\text{CH}}_{4} }}$ specific methane yield (m³ CH₄/t VS).

Equations for section “Design of the greenhouse system”

Day heating degrees (T _HD; °C) and night heating degrees (T _HN;  °C) are calculated as follows:

$$T_{\text{HD}} = \mathop \sum \limits_{i = 1}^{N} \left( {\left( {T_{\text{d}} - \Delta T_{\text{SUN}} } \right) - \frac{{T_{i\hbox{max} } + T_{{i{\text{av}}}} }}{2}} \right) \times D_{\text{d}}$$

(22)

$$T_{\text{HN}} = \mathop \sum \limits_{i = 1}^{N} \left( {T_{\text{n}} - \frac{{T_{i\hbox{min} } + T_{{i{\text{av}}}} }}{2}} \right) \times D_{\text{n}}$$

(23)

T _imax maximum day temperature in the month (°C), T _iav average day temperature in the month (°C), T _d target day temperature in the month, here: 22 °C, ΔT _SUN indoor/outdoor temperature difference due to greenhouse effect, here: 6 °C, T _n target night temperature in the month, here: 14 °C, D _d day duration (h), D _n night duration (h), N month of the year in the cultivation period (September–July).

Specific investment costs (€/m²) of the greenhouse in relation to the surface area were calculated with the following equation (input parameters for economic evaluation cf. “Appendix 2” section):

$$C_{\text{G}} = 260 \times S_{\text{G}}^{ - 0.155}$$

(24)

S _G greenhouse surface (ha).

Electrical (Y _E;  %) and thermal efficiency (Y _TH;  %) of the CHP unit was calculated with:

$$Y_{\text{E}} = \left( {5.572 \times \log_{10} \left( {P_{\text{EE}} } \right) + 22.43} \right) \times k_{\text{PL}}$$

(25)

$$Y_{\text{TH}} = Y_{\text{E}} \times 1.2$$

(26)

P _EE electrical power of the engine, k _PL partial load operation factor, here: 97%.

Equations for section “Methodology for economic evaluation”

For this purpose, annual cash flow calculations were made using the following equation:

$${\text{NPV}} = \mathop \sum \limits_{i = 1}^{t} \frac{{C_{t} }}{{\left( {1 + r} \right)^{t} }} - C_{0}$$

(27)

C _t net cash inflow during the period t, C ₀ total initial investment costs, r discount rate; here 6%, T number of time periods (years).

The PBP was calculated as follows:

$${\text{PBP}} = 1 + Y_{\text{N}} - \frac{v}{p}$$

(28)

Y _N number of years after the initial investment at which the last negative value of cumulative cash flow occurs, v value of cumulative cash flow at the year with the last negative value of cumulative cash flow, p value of yearly cash flow at the year with the first positive value of cumulative cash flow.

Appendix 2

See Tables 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, and 17.

Table 6 Energy crops: cultivation requirements

Table 7 Energy crops: cost of equipment (data taken from Greek market)

Table 8 Calculation of engine power and specific power requirement with medium-sized equipment for maize cultivation (NSW-Farmers 2013; Summer and Williams 2014)

Table 9 Total mineral fertilizer requirements of the energy crops considered in this study

Table 10 Energy crops and greenhouse: price of fertilizing nutrients (data taken from Greek market)

Table 11 Productivity and methane yields of the energy crops used for biogas production

Table 12 Greenhouse: cost of equipment (data taken from Greek market)

Table 13 Greenhouse: fertilizer requirements

Table 14 Greenhouse: cost of operation

Table 15 Investment and operation costs of the biogas plant

Table 16 Biogas plant: depreciation parameters

Table 17 Indirect input of energy based on primary energy consumption for the production of agro-chemicals