Models for operational and maintenance costs

Energy demand

Energy demand for ammonia recovery includes (1) that required to heat the stripping vessels, and (2) that for recirculating digestate through the vessels. The energy required for heating comprises (a) that to raise the temperature from the 37 °C of digester effluent to 101 °C for the thermal stripping of ammonia (Equation (3)), and (b) that arising from the latent heat of evaporation of water during thermal stripping (Equation (4)). Heat loss through the stripping vessels and the recirculation lines could be offset by increasing the recirculation rate in practical operations. Thus, additional heating needs to offset heat loss were not considered in this study.
3
where Eq = energy required to raise the temperature of manure digestate to boiling inside the stripping vessels, Wh/d; hv = specific heat of digested dairy manure with a total solids concentration of 6.1% = 4.014 kJ/L/°C (Bohnhoff & Converse 1987); and q = recirculation rate, L/d.

4
where Ev = energy required to maintain boiling due to water evaporation, Wh/d; hv = latent heat of water evaporation at 100 °C and 760 torr = 2,258 kJ/L (Haynes 2012); Vw = specific volume of water vaporized, L/L; and q = recirculation rate, L/d. The volume of water evaporated in thermal stripping varied between 0.012 and 0.082 L/L-digestate in laboratory experiments without water reflux (Tao & Ukwuani 2015). With a demister to capture mists and sprays, it is assumed that the thermal stripping – acid absorption system will reduce water vapor loss to 0.018 L/L.
Estimates of the energy demand for digestate recirculation are based on the pumps' power rating. It will take approximately 4 minutes, in total, to feed a single batch and recycle the hot manure. Each batch processing event will last for approximately 3 hours, before hot manure is pumped to the digesters (Tao & Ukwuani 2015), and each stripping vessel will be operated for 8 batches per day. The total energy consumed by the pumps, on this basis, is calculated using Equation (5).
5
where Ep = energy demand for pumping, Wh/d; 368 = wattage of pumps, W; 0.07 = total time for feeding and recycling for each batch, h; 8 = number of batches in a day; and, N = number of stripping vessels used.
The total energy demand of the ammonia recovery system is the sum of the results from the above three models, as given in Equation (6).
6
where Ce = total energy cost, $/d; and Ke = unit energy cost. The unit energy cost was$0.070/kWh on average in 2014 for industrial users in the USA (EIA 2015).

Sulfuric acid

Because of the reaction between sulfuric acid and stripped ammonia, the sulfuric acid solution's normality will decrease over time in a batch. Theoretically, 0.4 L concentrated sulfuric acid is required to form 1 kg ammonium sulfate. The 2013 market value of sulfuric acid was $55/tonne (ICIS 2014), which was equivalent to$0.10/L at a specific gravity of 1.84. The rate of sulfuric acid consumption depends on ammonium sulfate production and the cost model is calculated using Equation (7).
7
where Cs = cost of sulfuric acid, $/d; P = production of ammonium sulfate, kg/d; and Ks = unit cost of concentrated sulfuric acid solution,$/L.

Repairs and maintenance

Maintenance and repairs are considered contingency costs. They may arise due to requirements like vessel and pipe cleaning, unclogging, machine oiling, and parts replacement. To secure efficient performance and avoid unnecessary breakdowns, annual maintenance costs were estimated at 5% of total capital costs (Sinnott & Towler 2009).

With ammonia recovery, the ammonia concentration of the digester effluent will decrease (Figure 1). As a result, a dairy farm may apply more digested manure to its own land with less extra manure to be transported to other farms. In New York State, tankers are used to transport and spread liquid manure. In a survey of dairy farms in the state, Howland & Karszes (2014) reported an average liquid manure application cost of $2.99/m3. The total included (1) operating costs (labor, fuel, utilities, and repairs); (2) ownership costs (depreciation, interest and insurance); and (3) the costs of custom service provided by an off-farm company. Howland & Karszes (2014) did not find a significant relationship between liquid manure application cost and distance to field, between 0.7 and 16 km. Twin Birch Dairy sells the separated solids and stores the separated liquid 2.3 km away for irrigation within a further 5–6 km. The compliance cost used was, therefore, estimated by multiplying the volume of digestate to be transported off-site by this unit manure application cost. The latter is lower than, but in the same order of magnitude as, that in Europe for digestate disposal (Fuchs & Drosg 2013). Based on the ammonia concentrations of digester effluent under the three options (Figure 1), it is estimated that Option 1 will need storage for 30,694 L/d more digestate off-site and Option 3 13,462 L/d more, compared to Option 2. Models for returns from ammonia recovery In addition to the production of ammonium sulfate granules, the thermal stripping of ammonia may enhance anaerobic digestion due to thermal treatment of particulate organic matter in the recirculated digestate and zero risk of ammonia toxicity to methanogens in the digesters (Tao & Ukwuani 2015). The synergistic effects of integrating ammonia recovery to anaerobic digestion have yet to be quantified. The economic analysis in this study does not take the indirect effects of ammonia recovery on anaerobic digestion into account. Production of ammonium sulfate Ammonium sulfate production depends on both the recirculation rate and the digester effluent ammonia concentration (Figure 1). During each 3-hour batch operation, almost all ammonia in the digester effluent could be recovered and it is estimated that ammonium sulfate productivity would be 169.6 kg/d and 95.2 kg/d, respectively, in options 2 and 3. The granular product contains more than 98% ammonium sulfate (Tao & Ukwuani 2015). It could then be sold either as a nitrogen fertilizer or analytical grade chemical. The wholesale price of analytical grade ammonium sulfate is generally above$10/kg (VWR 2015), while the price of ammonium sulfate granules as a chemical fertilizer is approximately $0.2/kg (CRU 2014; VWR 2015). Because of product certification requirements and the limited production scale, the ammonium sulfate recovered from digestate at dairy farms would probably need to be sold through a recognized supplier. Therefore, a manufacturer's price of$6/kg was assumed for this study. Equation (8) is generalized for ammonium sulfate revenue.
8
where Ra = revenue generation from ammonium sulfate recovered, $/d; Pa = unit sale price of ammonium sulfate,$/kg; Ea = digester effluent ammonia concentration, g-N/L; 132/28 = factor converting from ammonia-N to ammonium sulfate; and q = recirculation rate, L/d.

Savings in heating anaerobic digesters

A heat balance shows that the anaerobic digesters will attain the digestion temperature of 37 °C when the ammonia-recovered hot manure is recycled to the inlet of the digesters at the recirculation rate of Option 2, thus saving the cost of energy for digester heating at Ke Eq. The savings can be calculated for Option 3, with its lower recirculation rate, similarly.

Assessment of costs and benefits

As a chemical engineering process, the ammonia recovery system is assumed to have a usable life of 10 years (USDA 2007; Sinnott & Towler 2009). To compare the three options, NPVs and benefit/cost ratios were calculated using a net discount rate of 8% (USDA 2007), as in Equations (9) and (10).
9

10
where NPV = the net present value, $; t = the time of cash flow, year; n = project life span, year; r = discount rate, %; Mt = net cash flow in year t,$/yr; Mo = capital outlay at the beginning of investment, $; Bt = present value of returns in t,$/yr; and Ct = present value of operating cost in t, $/yr. The NPV and benefit/cost ratio are calculated without consideration of a separate inflation rate. The costs and returns are estimated using current unit costs and revenues instead of predicted prices. Instead, sensitivity analysis was carried out to evaluate the uncertainties associated with the key cost and benefit components. RESULTS AND DISCUSSION Capital and operating costs, and revenues were calculated for each ammonia recovery option, using the cost and revenue models above, as well as the design and operating parameters. As summarized in Table 1, options 2 and 3 have similarly high benefit/cost ratios. However, the NPV of Option 2 is 1.85 times that of Option 3, while that for Option 1 is negative. Options 2 and 3 both require small amounts of initial investment so, Option 2 is economically better. In addition, options 2 and 3 will create 3 and 1.5 jobs, respectively, operating the ammonia recovery system 24 hours a day. Labor cost is the largest component of the operating cost, accounting for between 61 and 69% of its total. Table 1 Estimated costs and benefits for three ammonia recovery options from anaerobically digested dairy manure at Twin Birch Dairy Farm Option 1 Option 2 Option 3 Rate of recirculation for ammonia recovery (q), L/d 30,694 15,347 Total capital cost,$ 0 14,226 7,113
Stripping vessels 5,211 2,605
Centrifugal pumps 8,416 4,208
Acid absorption columns 600 300
Total operating cost, $/yr 33,497 226,307 127,995 Energy 64,926 32,460 Sulfuric acid 2,476 1,390 Lime to increase pH 1,040 520 Labor 157,154 78,577 Maintenance and repairs 711 356 Compliance cost 33,497 14,692 Total revenue,$/yr 0 427,362 236,483
Ammonium sulfate production 371,355 208,480
Savings in digester heating 56,007 28,003
Annual cash flow, $/yr −33,497 201,054 108,488 10-year net present value (NPV),$ 224,771 1,334,864 720,852
Benefit/cost ratio 0.0 1.89 1.85
Option 1 Option 2 Option 3
Rate of recirculation for ammonia recovery (q), L/d 30,694 15,347
Total capital cost, $0 14,226 7,113 Stripping vessels 5,211 2,605 Centrifugal pumps 8,416 4,208 Acid absorption columns 600 300 Total operating cost,$/yr 33,497 226,307 127,995
Energy 64,926 32,460
Sulfuric acid 2,476 1,390
Lime to increase pH 1,040 520
Labor 157,154 78,577
Maintenance and repairs 711 356
Compliance cost 33,497 14,692
Total revenue, $/yr 0 427,362 236,483 Ammonium sulfate production 371,355 208,480 Savings in digester heating 56,007 28,003 Annual cash flow,$/yr −33,497 201,054 108,488
10-year net present value (NPV), $224,771 1,334,864 720,852 Benefit/cost ratio 0.0 1.89 1.85 The specific benefit of thermal stripping – acid absorption under Option 2 is$0.018/L-digestate, $3.25/kg-(NH4)2SO4, or$0.50/d/cow. The specific operating cost of the process, excluding labor, under Option 2 is $0.006/L-digestate or$1.12/kg-(NH4)2SO4. This is lower than the optimized values for gas stripping – acid absorption, i.e., $0.014/L-digestate or$1.63/kg-(NH4)2SO4 (Jiang et al. 2014).

The greatest uncertainty in this economic analysis is over the sale price of ammonium sulfate because it can be marketed as either a chemical fertilizer or an analytical grade chemical. The wholesale price of ammonium sulfate granules varies greatly, from $0.2/kg as a chemical fertilizer to between$10 and $30/kg as an analytical grade granular chemical (CRU 2014; VWR 2015). Ammonium sulfate is traded globally and its price, especially as a fertilizer, is affected by transport costs, currency exchange rates, policy decisions and other uncontrollable factors. The variations in NPV and benefit/cost ratio under Option 2 were then determined at different percentage price changes relative to the assumed manufacturer price ($6/kg), i.e., +400% ($30/kg), +233% ($20/kg), +66.7% ($10/kg), and −96.7% ($0.2/kg). The NPV will be 0 at $2.77/kg and the benefit/cost ratio 1 at$2.74/kg (Figure 3).
Figure 3

Strauss plots of NPV and benefit/cost ratio versus changes in the major revenue and cost components at the optimum recirculation rate for ammonia recovery.

Figure 3

Strauss plots of NPV and benefit/cost ratio versus changes in the major revenue and cost components at the optimum recirculation rate for ammonia recovery.

The price of sulfuric acid was between $21 and$141/tonne between 2009 and 2014 (ICIS 2014). This level of price variation relative to current prices ($55/tonne) will not result in any significant changes in NPV or benefit/cost ratio because it only accounts for 1.1% of total operating costs (Table 1). The net energy cost – i.e., total energy cost minus savings in digester heating – is only 5.7% of the labor cost for Option 2, so, the sensitivity analysis on the cost components was carried out only in relation to potential changes in labor cost. The hourly wage for farm equipment mechanics and service technicians in the USA varied in 2013, being$11.18 at 10% percentile, $13.86 at 25%,$17.38 at 50%, $21.55 at 75%, and$25.88 at 90% (BLS 2015b). With these variations, the NPV and benefit/cost ratio responded as shown in Figure 3. The NPV will be zero and the benefit/cost ratio 1 at a wage rate of $40.65/h (Figure 3). As indicated by a greater slope of change, the NPV is more sensitive to the ammonium sulfate price than to wage rates. However, it appears that the benefit/cost ratio is equally sensitive to both factors. The hourly wage will probably not reach$41, so, ammonia recovery via thermal stripping – acid absorption will make a profit when ammonium sulfate granules can be sold for \$2.77/kg-(NH4)2SO4 or more.

CONCLUSIONS

Ammonia recovery via thermal stripping – acid absorption is estimated to be profitable. Although drawing digester effluent at 14 and 28% of the manure loading rate for ammonia recovery yields similar benefit/cost ratios, the higher recirculation rate produces a higher NPV and creates more jobs.

Labor costs account for between 61 and 69% of total operating costs for thermal stripping – acid absorption. Excluding these, the specific operating cost of thermal stripping is substantially lower than that of air-stripping.

The uncertainties in NPV and benefit/cost ratio are mainly associated with the sale price of ammonium sulfate and the hourly wage rate.

ACKNOWLEDGEMENTS

This study was funded by a U.S. EPA grant to Dr. Tao (SU835723). The views expressed are solely those of the authors and do not necessarily reflect those of the Agency. We would like to thank the U.S. Department of State for a Fulbright Foreign Student Program Scholarship to Sohaib Anwar.

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