Keywords

Carbon nanotubes manufacturing costs; Graphene Oxide manufacturing costs; Graphitic Nanofibers manufacturing costs; graphene manufacturing costs; manufacturing costs; synthesizing

5.1 Carbon Nanotubes

Before we go into discussions about applications and their viability, let us arm ourselves with some costing knowledge about the three materials. We are pretty much familiar with the manufacturing and purification processes of graphitic nanofibers (GN), so I will just comment briefly on carbon nanotube (CNT) and graphene oxide (GO). Further down, we will look at the cost of converting graphite flakes to graphene. To keep our focus on practicality, especially for costs, we will skip the many, many references of CNT synthesis by carbon sources other than methane. These exotic gases have served their purpose well for the authors, but would be difficult if not impossible to scale their use as a feedstock for viability on the commercial end. For example, Moreno and Yoshimura [109] discuss hydrothermal production of multiwall carbon nanotubes (MWCNTs) from amorphous carbon at 800°C and 15,000 psig without any catalysts, Magrez et al. [184] talks about C₂H₂ as a feedstock with a low yield, and McCaldin et al. [183] discusses C₂H₄ as a feedstock. As we shall see, even using natural gas as a feedstock makes the products expensive to make. We have discussed the need and complexity of purification of CNTs. These procedures further increase production costs.

5.2 Graphene Oxide

Let me first explain how I have viewed GO in this book: The applications discussed in this book have been chosen with a purpose in mind that GN is an excellent candidate for the given applications. The research and publications cited with “graphene oxide” in their title describe flake graphite as the starting material and oxidation thereof. In my opinion, very few applications I studied would actually require few-layer graphene flakes, except when specifically noted. The materials should really be called graphite oxide (GO). To maintain clarity, I will continue to use the acronym GO and use the word graphene oxide for the oxidized graphite. An overwhelming majority of references cite the procedure for GO synthesis by very harsh oxidation procedures, such as the Hummers and “slightly modified” Hummers (see Figs. 5.1 and 5.2).

The experimental details from the 1958 publication [113] outlining the Hummer’s method are:

Though detailed explanations for the minor modifications to the Hummer’s method are given in the experimental sections, papers claiming facile and cost effective large scale manufacturing pathways neglect to consider the environmental implications of toxic gas generation (NO₂, N₂O₄, ClO₂ (explosive)), waste treatment of the residual acids and oxidants, and the helacious cost of the chemicals required. Underscoring the purpose of the book, I emphasize the need to look at the cost of manufacturing these hyped, exotic materials. To start with, the original process generates oxides of nitrogen, which require treatment before exhaust. Graphite flakes can have many impurities that may vaporize with the NOx vapors. Since the nature of other gases in the exhaust will always be unknown, calculations for costing should be done for Selective Catalytic Reduction (SCR). The reducing agent is usually NH₃ and is injected at a 1:1 molar ratio:

$2\mathrm{NO}+2{\mathrm{NH}}_3={\mathrm{2N}}_2+{\mathrm{3H}}_2\mathrm{O}$

The sodium nitrate used in the reaction (as a catalyst) will be the source of this NO. So let’s see how much NH₃ would be required for the referenced batch of 100 grams of flake graphite. Hummer’s recipe calls for 50 grams or ~0.6 moles of NaNO₃. We would therefore need 0.6 moles of NH₃ or ~10 grams of NH₃ on 100% basis. This cost is incorporated in the calculations to follow.

5.3 Cost Calculations

The itemized costs of production for six products are tabulated in Table 5.1. As a reference point, the pricing of materials were obtained on September 30th, 2016. For commodities, I have taken the world spot prices and added a conservative 50% to arrive at the final price for delivered product, as given in Table 5.1. If a company is large enough to buy at world trading commodity prices, they will also have a much larger overhead factor in the costing. These two should cancel each other out.

The input values and detailed calculations of the tabulated costs in Table 5.1 are presented at the end of this chapter.

Table 5.1 lists the values of individual cost components and final cost per Kg of each material for several feed source options.

5.4 Making Graphene

Let’s look at what it would cost to make graphene, now that we know what GO costs per Kg. Synthesizing large volumes of graphene requires GO or some other form of graphitic nanomaterial. Graphene was first isolated and studied by a mechanical “peeling” of graphite using an adhesive tape. Graphene layers up to a few millimeter in size can be produced by peeling. This technique is currently (and likely to remain) the source for the small demand of basic research. Publications describe new methods of large-area graphite synthesis, including CVD and direct growth on catalysts [185–192]. This method is not practical for large-volume production. The most widely referenced method of synthesizing graphene flakes is via reduction of GO made from flake graphite. A case can be made for GN to be the starting material for synthesizing graphene. Ribbon-type GN would be the best candidates for exfoliation to graphene. The basal planes would stay pristine, and 1 to 10-µm graphene flakes could be synthesized. Exfoliation techniques are worth a look from a cost perspective for ribbon-type GN to graphene. A cursory look at the costs associated with exfoliation to obtain graphene is given below.

5.4.2 Supercritical carbon dioxide

Another method for a non-GO route exfoliation is by using supercritical carbon dioxide [196]. In its critical phase, CO₂ behaves neither like a liquid nor a gas (see Fig. 5.3). ScCO₂ has a surface tension that is an order of magnitude lower than propane, and a very low viscosity. It penetrates tiny pores or cracks and mixes readily with oils. These qualities result in unique solvent-like qualities. ScCO₂ has been used widely for extraction of oils, enhanced oil recovery, etc. Once the material to be extracted is dissolved in the ScCO₂, the pressure and/or temperature conditions are changed, at which point the CO₂ releases as a gas, and the solvate can be recovered. Again, by avoiding the GO route, intercalation and exfoliation by ScCO₂ has the possibility of producing pristine graphene planes for graphite, or consistent sized platelets from ribbon-type GN. Li et al. [196] describe complete depressurization of the ScCO₂ for the rapid exfoliation of the basal planes.

The process equipment for such a route on an industrial scale would be expensive though it has been used by the oil industry for decades now. High-pressure vessels, multistage compressors with heat removal between stages, loss of ScCO₂ due to maintenance of gas seals, special material requirements for O-rings to resist explosive decompression (CO₂ gets trapped in the materials), and last but not least, high-purity CO₂ as a feedstock are some of the reasons. Power consumption on a large scale may be calculated by using atmospheric intake, and a 1800-psig discharge a three-stage compressor, and a power consumption cost at (US$0.10 per KW-h). This gives us an estimated electrical cost of ~$80 per kg. An analysis similar to the one we did on the GN and CNTs, with the proper equipment costs would give more accurate numbers. Further investigation with pilots should be carried out to find the actual numbers.

As is common knowledge, none of the above methods has been tested or implemented on a large-scale commercial exfoliation of graphite/graphene oxide.

5.5 Calculations for Table 5.1

Table 5.6

Prepurification quantity of product required to achieve desired output

Weight of raw product required before purification to achieve rated capacity

$\begin{array}{ll}\mathrm{Weight}\mathrm{of}\mathrm{produced}\mathrm{material}\mathrm{required}\mathrm{to}\mathrm{achieve}1000{\mathrm{kg}}_{\text{GN}-\text{product}}& =\frac{1000{\mathrm{g}}_{\mathrm{total}\mathrm{GN}}\mathrm{required}}{90\%\mathrm{selectivity}}\\ & =1.11{\mathrm{kg}}_{\mathrm{raw}\mathrm{product}}{/\text{kg}}_{\mathrm{GN}}\\ \mathrm{Weight}\mathrm{of}\mathrm{produced}\mathrm{material}\mathrm{requiring}\mathrm{purification}\mathrm{for}1{\mathrm{kg}}_{\text{CNT}-\text{product}}& =\frac{1000{\mathrm{g}}_{\mathrm{total}\mathrm{product}}\mathrm{desired}}{35\%\mathrm{selectivity}}\\ & =3{\mathrm{kg}}_{\mathrm{raw}\mathrm{product}}{/\text{kg}}_{\mathrm{CNT}}\\ \mathrm{Weight}\mathrm{of}\mathrm{flake}\mathrm{graphite}\mathrm{required}\mathrm{to}\mathrm{make}1{\mathrm{kg}}_{\text{GO}-\text{product}}\hfill & =\frac{1{\mathrm{kg}}_{\mathrm{GO}\mathrm{product}}}{180\%\mathrm{due}\mathrm{to}80\%\mathrm{weight}\mathrm{gain}}\\ & =0.56{\mathrm{kg}}_{\mathrm{flake}}{/\text{kg}}_{\mathrm{GO}}\\ \mathrm{Weight}\mathrm{of}\mathrm{flake}\mathrm{graphite}\mathrm{required}\mathrm{to}\mathrm{make}1{\mathrm{kg}}_{\text{GO}-\text{product}}\mathrm{including}\mathrm{waste}& =\begin{array}{l}556{\mathrm{g}}_{\mathrm{flake}\mathrm{graphite}}\times 125\%\mathrm{for}\mathrm{attrition}\mathrm{and}\mathrm{multiple}\mathrm{rinsing}\mathrm{losses}\end{array}\\ & =0.70{\mathrm{kg}}_{\mathrm{flake}}{/\text{kg}}_{\mathrm{GO}}\end{array}$

Table 5.7

Biogas/syngas requirements for GN production

$\begin{array}{ll}\text{Number}\text{of}\text{kg}-\text{moles}\text{of}\text{product}\text{per}\text{day}& =\frac{1000\mathrm{kg}}{12\text{kg}/\text{kg}-\text{mol}}\\ & =83\text{kg}-\text{mol}/\mathrm{day}\\ \mathrm{Yield}& =35\%\mathrm{conversion}\times 90\%\mathrm{selectivity}\\ & =31.50\%\\ \mathrm{Methane}\mathrm{required}(\text{mol}/\text{day})& =\frac{83\text{kg}-\text{mol}}{31.50\%\mathrm{yield}}\\ & =265\text{kg}-\text{mol}/\mathrm{day}\\ \mathrm{Methane}{\text{required}(\text{nM}}^3/\text{day})& =\begin{array}{l}265\text{kg}-\text{mol}\mathrm{methane}\times 22.40{\mathrm{nM}}^3/\text{mol}\\ \mathrm{at}\mathrm{STP}(\text{assuming}\mathrm{Ideal}\mathrm{gas}\text{law})\end{array}\\ & =5926{\mathrm{nM}}^3/\text{day}\\ \mathrm{Biogas}\mathrm{required}& =\frac{5926{\mathrm{nM}}^3/\text{day}\mathrm{pure}\mathrm{methane}}{50\%\mathrm{methane}\mathrm{content}\mathrm{by}\mathrm{volume}}\\ & =11,852{\mathrm{nM}}^3/\mathrm{day}\\ \mathrm{Biogas}\mathrm{required}\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}{(\text{nM}}^3)& =\frac{11,852{\mathrm{nM}}^3\mathrm{biogas}/\mathrm{day}}{1000\mathrm{kg}/\mathrm{day}}\\ & =11.9{\mathrm{nM}}^3/\mathrm{day}\\ \mathrm{Biogas}\mathrm{per}\mathrm{hour}& =\frac{11,852{\mathrm{nM}}^3\mathrm{biogas}/\mathrm{day}}{24\mathrm{h}/\mathrm{day}}\\ & =494{\mathrm{nM}}^3/\mathrm{h}\end{array}$

Table 5.8

Natural gas requirement for GN production

$\begin{array}{ll}\begin{array}{l}\mathrm{Methane}\text{volume}/\text{day}\\ \mathrm{from}(\mathrm{biogas}\mathrm{calculation})\end{array}& =\frac{5926{\mathrm{nM}}^3/\text{day}\mathrm{methane}}{95\%\mathrm{purity}\mathrm{by}\mathrm{volume}}\\ & =6238{\mathrm{nM}}^3/\text{day}\\ \mathrm{NG}\mathrm{volume}\mathrm{per}\mathrm{kilogram}& =\frac{6238{\mathrm{nM}}^3/\text{day}}{1000\text{kg}/\text{day}}\\ & =6.2{\mathrm{nM}}^3{/\text{kg}}_{\mathrm{GN}}\\ \mathrm{NG}\mathrm{volume}\mathrm{per}\mathrm{hour}& =\frac{6238{\mathrm{nM}}^3/\text{day}}{24h/\text{day}}\\ & =260{n\mathrm{{\rm M}}}^3/h\\ \mathrm{Cubic}\mathrm{feet}\mathrm{NG}\mathrm{required}& =6.2{\mathrm{nM}}^3/\mathrm{kg}\times 35.3{\text{CF}/\text{nM}}^3\\ & =219\mathrm{CF}/{\mathrm{kg}}_{\mathrm{GN}}\\ \mathrm{BTUs}\mathrm{value}\mathrm{of}\mathrm{gas}& =219\mathrm{CF}\times 1000\mathrm{BTUs}/\mathrm{CF}\\ & =219,000\mathrm{B}\mathrm{T}\mathrm{Us}\\ \mathrm{Cost}\mathrm{of}\mathrm{gas}\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}& =219,000\mathrm{B}\mathrm{T}\mathrm{Us}/\mathrm{kg}\times \$15\mathrm{per}\mathrm{million}\mathrm{BTUs}\\ & =\$3.39\mathrm{per}{\mathrm{kg}}_{\mathrm{GN}}\end{array}$

Table 5.9

Biogas/syngas required for CNT production

$\begin{array}{ll}\begin{array}{l}\mathrm{Number}\mathrm{of}\text{kg}-\\ \text{moles}\mathrm{of}\mathrm{CNT}\mathrm{required}\mathrm{per}\mathrm{day}\end{array}& =\frac{1000\mathrm{kg}}{12\text{kg}/\text{kg}-\text{mol}}\\ & =83\text{kg}-\text{mol}/\text{day}\\ \mathrm{Yield}& =35\%\mathrm{conversion}\times 35\%\mathrm{selectivity}\\ & =12.25\%\\ \begin{array}{c}\mathrm{Methane}\mathrm{required}(\text{mol}/\text{day})\end{array}& =\frac{83\text{kg}-\text{mol}}{12.25\%\mathrm{yield}}\\ & =678\text{kg}-\text{mol}/\text{day}\\ \mathrm{Methane}\mathrm{required}{(\text{nM}}^3/\text{day})& =\begin{array}{l}678\text{kg}-\text{mol}\mathrm{methane}\times 22.40{\mathrm{nM}}^3/\text{mol}\\ \mathrm{at}\mathrm{STP}(\text{assuming}\mathrm{Ideal}\mathrm{gas}\text{law})\end{array}\\ & =15,187{\mathrm{nM}}^3/\text{day}\\ \mathrm{Biogas}\mathrm{required}& =\frac{15,187{\mathrm{nM}}^3/\text{day}\mathrm{pure}\mathrm{methane}}{50\%\mathrm{methane}\mathrm{content}\mathrm{by}\mathrm{volume}}\\ & =30,374{\mathrm{nM}}^3/\text{day}\\ \begin{array}{c}\mathrm{Biogas}\mathrm{required}\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}{(\text{nM}}^3)\end{array}& =\frac{30,374{\mathrm{nM}}^3\text{biogas}/\text{day}}{1000\text{kg}/\text{day}}\\ & =30.4{\mathrm{nM}}^3{/\text{kg}}_{\mathrm{GN}}\\ \mathrm{Biogas}\mathrm{per}\mathrm{hour}& =\frac{30,374{\mathrm{nM}}^3\text{biogas}/\text{day}}{24h/\text{day}}\\ & =1266{\mathrm{nM}}^3/h\end{array}$

Table 5.10

Natural gas required for CNT manufacturing

$\begin{array}{ll}\mathrm{Methane}\mathrm{volume}\mathrm{per}\mathrm{day}& =\frac{15,187{\mathrm{nM}}^3/\text{day}\mathrm{methane}}{95\%\mathrm{purity}\mathrm{by}\mathrm{volume}}\\ & =15,986{\mathrm{nM}}^3/\text{day}\\ \mathrm{NG}\mathrm{volume}\mathrm{per}\mathrm{kilogram}& =\frac{15,986{\mathrm{nM}}^3/\text{day}}{1000\text{kg}/\text{day}}\\ & =16.0{\mathrm{nM}}^3/\text{kg}-\text{GN}\\ \mathrm{NG}\mathrm{volume}\mathrm{per}\mathrm{hour}& =\frac{15,986{\mathrm{nM}}^3/\text{day}}{24h/\text{day}}\\ & =666{\mathrm{nM}}^3/h\\ \mathrm{Cubic}\mathrm{feet}\mathrm{NG}\mathrm{required}& =16.0{\mathrm{nM}}^3/\text{kg}\times 35.3{\text{CF}/\text{nM}}^3\\ & =565{\text{CF}/\text{kg}}_{\mathrm{GN}}\\ \mathrm{BTUs}\mathrm{value}\mathrm{of}\mathrm{gas}& =565.0\mathrm{CF}\times 1000\text{BTUs}/\text{CF}\\ & =565,000\mathrm{BTUs}\\ \mathrm{Cost}\mathrm{of}\mathrm{gas}\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}& =565,000\text{BTUs}/\text{kg}\times \$\text{15}\text{per}\mathrm{million}\mathrm{BTUs}\\ & =\$8.48\mathrm{per}{\mathrm{kg}}_{\mathrm{GN}}\end{array}$

Table 5.11

Cost of catalyst for GN production

$\begin{array}{ll}\mathrm{Total}\mathrm{carbon}\mathrm{deposited}& =300{g/g}_{\mathrm{catalyst}}\\ \mathrm{Yield}\mathrm{of}\mathrm{GN}{(g/g}_{\mathrm{catalyst}})& =300{\mathrm{g}}_{\mathrm{carbon}}{/g}_{\mathrm{catalyst}}\times 90\%\mathrm{selectivity}\\ & =270{\mathrm{g}}_{\mathrm{GN}}{/g}_{\mathrm{catalyst}}\\ \mathrm{Catalyst}\mathrm{weight}\mathrm{per}{\mathrm{kg}}_{\mathrm{GN}}& =3.33{\mathrm{g}}_{\mathrm{catalyst}}\mathrm{consumed}\\ \mathrm{Catalyst}\mathrm{cost}& =3.33{\mathrm{g}}_{\mathrm{catalyst}}\mathrm{consumed}\times \$1000/1000\mathrm{g}\\ & =\$3.33{/\text{kg}}_{\mathrm{GN}}\end{array}$

Table 5.12

Cost of catalyst for CNT production

$\begin{array}{ll}\mathrm{Total}\mathrm{carbon}\mathrm{deposited}& =300{g/g}_{\mathrm{catalyst}}\\ \mathrm{Yield}\mathrm{of}\mathrm{CNT}{(g}_{\mathrm{CNT}}{/g}_{\mathrm{catalyst}})& =300{g/g}_{\mathrm{catalyst}}\times 35\%\mathrm{selectivity}\\ & =105{g/g}_{\mathrm{catalyst}}\\ \mathrm{Catalyst}\mathrm{weight}\mathrm{consumed}\mathrm{per}\mathrm{kilogram}& =9.5{\mathrm{g}}_{\mathrm{catalyst}}\mathrm{consumed}\\ \mathrm{Catalyst}\mathrm{cost}& =9.5{\mathrm{g}}_{\mathrm{catalyst}}\mathrm{consumed}\times \$1000/1000\mathrm{g}\\ & =\$9.50{/\text{kg}}_{\mathrm{CNT}}\end{array}$

Table 5.14

Cost of purification of GN

Purification–oxidation of GN

$\begin{array}{ll}\mathrm{Weight}\mathrm{of}(31\%)\mathrm{HCl}\mathrm{used}(\text{kg})& =3703{\mathrm{L}}_{\mathrm{HCl}}/\mathrm{day}\times 1.15\mathrm{specific}\mathrm{gravity}\\ & =4\text{kg}/\text{day}\\ \mathrm{Weight}\mathrm{of}\mathrm{HCl}(\text{kg}/{\mathrm{kg}}_{\mathrm{product}})& =\frac{4259{\mathrm{kg}}_{\mathrm{HCl}}/\text{day}}{1000\text{kg}/\text{day}}\\ & =4.26{\text{kg}/\text{kg}}_{\mathrm{GN}}\\ \mathrm{Cost}\mathrm{of}\mathrm{HCl}(31\%)(\$/\text{kg})& =4.26{\mathrm{kg}}_{\mathrm{HCl}}{/\text{kg}}_{\mathrm{product}}\times \$0.31\mathrm{per}{\mathrm{kg}}_{\mathrm{commodity}\mathrm{price}}\\ & =\$1.32\mathrm{per}{\mathrm{kg}}_{\mathrm{GN}}\end{array}$

Table 5.15

Cost of purification of CNTs

Purification of CNTs

$\begin{array}{ll}\begin{array}{l}\mathrm{Weight}\mathrm{of}(15.8\mathrm{M})\\ {\mathrm{HNO}}_3\mathrm{used}{(\text{kg}}_{{\mathrm{HNO}}_3})\end{array}& =9524{\mathrm{L}}_{{\mathrm{HNO}}_3}/\text{day}\times 1.42\mathrm{specific}\mathrm{gravity}\\ & =13,524\text{kg}/\text{day}\\ \begin{array}{l}\mathrm{Weight}\mathrm{of}\\ {\mathrm{HNO}}_3{(\text{kg}/\text{kg}}_{\mathrm{product}})\end{array}& =\frac{13,524{\mathrm{kg}}_{{\mathrm{HNO}}_3}/\text{day}}{1000\text{kg}/\text{day}}\\ & =13.500{\text{kg}/\text{kg}}_{\mathrm{CNT}}\\ \begin{array}{l}\mathrm{Cost}\mathrm{of}\\ {\mathrm{HNO}}_3(15.8\mathrm{M})(\$/\text{kg})\end{array}& =13.500{\mathrm{kg}}_{{\mathrm{HNO}}_3}{/\text{kg}}_{\mathrm{product}}\times \$1{\mathrm{.65\; per\; kg}}_{\mathrm{commodity}\mathrm{price}}\\ & =\$22.31\mathrm{per}{\mathrm{kg}}_{\mathrm{CNT}}\end{array}$

Table 5.16

Cost of chemicals used for GO synthesis

$\begin{array}{ll}\mathrm{Cost}\mathrm{of}{\mathrm{H}}_2{\mathrm{SO}}_4\hfill & \begin{array}{l}=29.62\mathrm{kg}{}_{{\mathrm{H}}_2{\mathrm{SO}}_4}{/\text{kg}}_{\mathrm{product}}{\mathrm{H}}_2{\mathrm{SO}}_4\times \$0.48\mathrm{per}\mathrm{kg}98\%{\mathrm{H}}_2{\mathrm{SO}}_4\\ \times \$0.48\mathrm{per}\mathrm{kg}98\%{\mathrm{H}}_2{\mathrm{SO}}_4\end{array}\hfill \\ \hfill & =\$14.22\hfill \\ \begin{array}{l}\mathrm{Weight}\mathrm{of}\\ {\mathrm{NaNO}}_3{(g/\text{kg}}_{\mathrm{GO}})\end{array}\hfill & =700.000{\mathrm{g}}_{\mathrm{flake}\mathrm{graphite}}\times 0.5(50{\mathrm{g}}_{{\mathrm{NaNO}}_3}/100{\mathrm{g}}_{\mathrm{flake}\mathrm{graphite}})\hfill \\ \hfill & =350\hfill \\ \begin{array}{l}\mathrm{Cost}\mathrm{of}\\ {\mathrm{NaNO}}_3\mathrm{per}{\mathrm{kg}}_{\mathrm{GO}}\end{array}\hfill & =350{\mathrm{g}}_{{\mathrm{NaNO}}_3}\hfill \\ \hfill & =\$0.21\hfill \\ \begin{array}{l}\mathrm{Weight}\mathrm{of}\\ {\mathrm{KMnO}}_4\mathrm{per}({g/\text{kg}}_{\mathrm{GO}})\end{array}\hfill & =700{\mathrm{g}}_{\mathrm{flake}\mathrm{graphite}}\times 2100{\mathrm{g}}_{{\mathrm{KMnO}}_4}\hfill \\ \hfill & =2100\hfill \\ \mathrm{Cost}\mathrm{of}{\mathrm{KMnO}}_4\hfill & =2100{\mathrm{g}}_{{\mathrm{KMnO}}_4}\hfill \\ \hfill & =\$6.30\hfill \end{array}$

Table 5.17

Cost of wastewater treatment for GN purification

Cost of NaOH to neutralize HCl used for GN purification

$\begin{array}{ll}\mathrm{Moles}\mathrm{of}\mathrm{HCl}\mathrm{to}\mathrm{neutralize}(\text{maximum})\mathrm{per}\mathrm{day}& =6238{\mathrm{L}}_{\mathrm{HCl}}/\text{day}\times 9.8{\text{mol}/L}_{\mathrm{HCl}}\\ & =61,132\mathrm{mol}/\mathrm{day}\\ \mathrm{Moles}\mathrm{of}\mathrm{HCl}\mathrm{to}\mathrm{neutralize}(\text{maximum})\mathrm{per}\mathrm{kilogram}& =\frac{61,132\mathrm{mol}/\mathrm{day}}{1000{\mathrm{kg}}_{\mathrm{product}}/\text{day}}\\ & =61.1{\text{mol}/\text{kg}}_{\mathrm{GN}}\\ \mathrm{Moles}\mathrm{of}\mathrm{NaOH}\mathrm{required}\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}& =61{\text{mol}\text{HCl}/\text{kg}}_{\mathrm{product}}\times 1.1\mathrm{mol}\text{NaOH}/\text{mol}\text{HCl}—10\%\mathrm{excess}\\ & =67\mathrm{mol}\\ \mathrm{Weight}\mathrm{of}\mathrm{NaOH}\mathrm{required}\mathrm{as}100\%\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}& =67\mathrm{mol}\text{NaOH}/\text{kg}\times 40g/\text{mol}\mathrm{NaOH}\mathrm{molecular}\mathrm{weight}\\ & =2690{g/\text{kg}}_{\mathrm{GN}}\\ \mathrm{Cost}\mathrm{of}\mathrm{NaOH}\mathrm{to}\mathrm{neutralize}\mathrm{HCl}\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}& =2690{\mathrm{g}}_{\mathrm{NaOH}}\times \$0.75\mathrm{per}\mathrm{kg}100\%\mathrm{NaOH}\\ & =\$2.02{\mathrm{per\; kg}}_{\mathrm{GN}}\end{array}$

Table 5.18

Cost of wastewater treatment for CNT purification

$\begin{array}{ll}\mathrm{Moles}\mathrm{of}{\mathrm{HNO}}_3\mathrm{to}\mathrm{neutralize}(\text{maximum})\mathrm{per}\mathrm{day}& =9524{\mathrm{L}}_{{\mathrm{HNO}}_3}/\text{day}\times 15.8{\text{mol}/L}_{{\mathrm{HNO}}_3}\\ & =150,479\text{mol}/\text{day}\\ \mathrm{Moles}\mathrm{of}{\mathrm{HNO}}_3\mathrm{to}\mathrm{neutralize}(\text{maximum})\mathrm{per}\mathrm{kilogram}& =\frac{150,479\text{mol}/\text{day}}{1000{\mathrm{kg}}_{\mathrm{product}}/\text{day}}\\ & =150{\text{mol}/\text{kg}}_{\mathrm{CNT}}\\ \mathrm{Moles}\mathrm{of}\mathrm{NaOH}\mathrm{required}\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}& =150\mathrm{mol}{\mathrm{HNO}}_3{/\text{kg}}_{\mathrm{product}}\times 1.1\mathrm{mol}\text{NaOH}/\text{mol}{\mathrm{HNO}}_3—10\%\mathrm{excess}\\ & =165\mathrm{mol}\\ \mathrm{Weight}\mathrm{of}\mathrm{NaOH}\mathrm{required}\mathrm{as}100\%\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}& =165\mathrm{mol}{\text{NaOH}/\text{kg}}_{\mathrm{product}}\times 40g/\text{mol}\mathrm{NaOH}\mathrm{molecular}\mathrm{weight}\\ & =6600{g/\text{kg}}_{\mathrm{CNT}}\\ \mathrm{Cost}\mathrm{of}\mathrm{NaOH}\mathrm{to}\mathrm{neutralize}\mathrm{HCl}\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}& =6600{\mathrm{g}}_{\mathrm{NaOH}}\times \$0.75\mathrm{per}\mathrm{kg}100\%\mathrm{NaOH}\\ & =\$4.95\mathrm{per}{\mathrm{kg}}_{\mathrm{CNT}}\end{array}$

Table 5.19

Cost of wastewater treatment from GO synthesis

Cost of NaOH required to neutralize H2SO4 used in GO synthesis

$\begin{array}{ll}\mathrm{Kilograms}\mathrm{of}{\mathrm{H}}_2{\mathrm{SO}}_4\mathrm{to}\mathrm{neutralize}(\text{maximum})\mathrm{per}\mathrm{day}& =16,100{\mathrm{L}}_{{\mathrm{H}}_2{\mathrm{SO}}_4}/\text{day}\times 1.84\mathrm{specific}\mathrm{gravity}\mathrm{of}98\%{\mathrm{H}}_2{\mathrm{SO}}_4\\ & =29,624\\ \mathrm{Kilograms}\mathrm{of}{\mathrm{H}}_2{\mathrm{SO}}_4\mathrm{to}\mathrm{neutralize}(\text{maximum})\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}& =\frac{29,624\text{kg}/\text{day}}{1000{\mathrm{kg}}_{\mathrm{product}}/\text{day}}\\ & =29.62\\ \mathrm{Moles}\mathrm{of}{\mathrm{H}}_2{\mathrm{SO}}_4\mathrm{to}\mathrm{neutralize}\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}& =\frac{29.62{\mathrm{kg}}_{{\mathrm{H}}_2{\mathrm{SO}}_4}{/\text{kg}}_{\mathrm{GO}}}{98{g/\text{mol}\text{H}}_2{\mathrm{SO}}_4}\\ & =302\\ \mathrm{Moles}\mathrm{of}\mathrm{NaOH}\mathrm{required}\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}& =302{\mathrm{mol\; H}}_2{\mathrm{SO}}_4{/\text{kg}}_{\mathrm{GO}}\times 2.2\mathrm{mol}\mathrm{NaOH}\mathrm{per}\mathrm{mole}{\mathrm{H}}_2{\mathrm{SO}}_4—10\%\mathrm{excess}\\ & =664\\ \mathrm{Weight}\mathrm{of}\mathrm{NaOH}\mathrm{required}\mathrm{as}100\%\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}& =665{\text{mol}\text{NaOH}/\text{kg}}_{\mathrm{GO}}\times 40\mathrm{NaOH}\mathrm{molecular}\mathrm{weight}\\ & =26,600\\ \begin{array}{c}\mathrm{Cost}\mathrm{of}\mathrm{NaOH}\mathrm{to}\mathrm{neutralize}{\mathrm{H}}_2{\mathrm{SO}}_4\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}\end{array}& =26,600{\mathrm{g}}_{\mathrm{NaOH}}\times \$0.75\mathrm{per}\mathrm{kg}100\%\mathrm{NaOH}\\ & =\$19.95\end{array}$

Table 5.20

Cost of deionized water for rinsing products after chemical treatment

$\begin{array}{ll}\mathrm{DI}\mathrm{water}\mathrm{required}\mathrm{for}\mathrm{GO}(L)& =700{\mathrm{g}}_{\mathrm{flake}\mathrm{graphite}}{/\text{kg}}_{\mathrm{GO}}\times 0.2(20L/100{\mathrm{g}}_{\mathrm{flake}\mathrm{graphite}})\\ & =140\mathrm{L}\\ \mathrm{DI}\mathrm{water}\mathrm{cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{GO}}& =140{\mathrm{L}}_{\mathrm{DI}\mathrm{water}}{/\text{kg}}_{\mathrm{GO}}\times 0.004{(\text{US}\$\text{4}/m}^3—\mathrm{operating}\mathrm{cost}\mathrm{of}\text{RO}/\text{DI}\text{system})\\ & =\$0{\mathrm{.56\; per\; kg}}_{\mathrm{GO}}\\ \mathrm{DI}\mathrm{water}\mathrm{required}\mathrm{for}\mathrm{CNT}& =10{\mathrm{L}}_{\mathrm{raw}\mathrm{product}}{/\text{kg}}_{\mathrm{CNT}}\times 20\mathrm{bed}\mathrm{volumes}\mathrm{DI}\mathrm{water}\mathrm{to}\mathrm{rinse}\text{acid}+\text{fines}\\ & =190L/\text{kg}\text{CNT}\\ \mathrm{DI}\mathrm{water}\mathrm{cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{CNT}}& =190{L/\text{kg}}_{\mathrm{CNT}}\times 0.004{(\text{US}\$\text{4}/m}^3—\mathrm{operating}\mathrm{cost}\mathrm{of}\text{RO}/\text{DI}\text{system})\\ & =\$0.76{\mathrm{per\; kg}}_{\text{GN}-\text{CNT}}\\ \mathrm{DI}\mathrm{water}\mathrm{required}\mathrm{for}\mathrm{GN}& =3{\mathrm{L}}_{\mathrm{raw}\mathrm{product}}{/\text{kg}}_{\mathrm{GN}}\times 20\mathrm{bed}\mathrm{volumes}\mathrm{of}\mathrm{DI}\mathrm{water}\\ & =60{L/\text{kg}}_{\mathrm{GN}}\\ \mathrm{DI}\mathrm{water}\mathrm{cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{CNT}}& =60{\mathrm{L}}_{\mathrm{raw}\mathrm{product}}{/\text{kg}}_{\mathrm{GN}}\times 0.004({\text{US}\$\text{4}/m}^3—\mathrm{operating}\mathrm{cost}\mathrm{of}\text{RO}/\text{DI}\text{system})\\ & =\$0.24\mathrm{per}{\mathrm{kg}}_{\mathrm{GN}}\end{array}$

Table 5.21

Cost of NO_x abatement from GO synthesis

NOx abatement for GO 2NO+2NH3=2N2+3H2O

$\begin{array}{ll}\mathrm{Anhydrous}{\mathrm{NH}}_3\mathrm{required}\mathrm{per}{\mathrm{kg}}_{\mathrm{GO}}& =700{\mathrm{g}}_{\mathrm{flake}\mathrm{graphite}}\times 0.2(10{\mathrm{g}}_{\mathrm{NH3}}{/\text{50g}}_{{\mathrm{NaNO}}_3})\\ & =140{g/\text{kg}}_{\mathrm{GO}}\\ \mathrm{Cost}\mathrm{of}\mathrm{ammonia}& =140{g/\text{kg}}_{\mathrm{GO}}\times \begin{array}{c}0.00053\end{array}(\text{US}\$0.27/\text{kg})\\ & =\$0.04{\mathrm{per\; kg}}_{\mathrm{GO}}\end{array}$

Table 5.22

Labor costs

$\begin{array}{ll}\mathrm{Number}\mathrm{of}\mathrm{labor}\mathrm{hours}& =2\text{people}\times 24\mathrm{h}\\ & =48\\ \mathrm{Labor}\mathrm{cost}\mathrm{per}\mathrm{day}& =48h/\text{day}\times \$35(\mathrm{hourly}\text{wage})\\ & =\$1680\\ \mathrm{Labor}\mathrm{cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{product}}& =\frac{\$1680\mathrm{per}\mathrm{day}}{1000\text{kg}/\text{day}}\\ & =\$1.68\end{array}$

Table 5.23

Maintenance costs for GN plant configurations

$\begin{array}{ll}\mathrm{Annual}\mathrm{cost}\mathrm{of}\mathrm{GN}\mathrm{biogas}\mathrm{plant}\hfill & =\begin{array}{c}\$6,000,000\end{array}\mathrm{Capex}\times 2\%\mathrm{rule}\mathrm{of}\mathrm{thumb}\mathrm{estimate}\hfill \\ \hfill & =\begin{array}{c}\$120,000\end{array}\hfill \\ \mathrm{Cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{GN}}\mathrm{from}\mathrm{biogas}\hfill & =\frac{\$120,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$0.34\hfill \\ \mathrm{Annual}\mathrm{cost}\mathrm{of}\text{GN}-\text{syngas}\hfill & =\begin{array}{c}\$7,000,000\end{array}\mathrm{Capex}\times 2\%\mathrm{rule}\mathrm{of}\mathrm{thumb}\mathrm{estimate}\hfill \\ \hfill & =\$140,000\hfill \\ \mathrm{Cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{GN}}\mathrm{from}\mathrm{syngas}\hfill & =\frac{\$140,000\mathrm{annual\; cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$0.40\hfill \\ \mathrm{Annual}\mathrm{cost}\mathrm{of}\mathrm{GN}\mathrm{NG}\mathrm{plant}\hfill & =\begin{array}{c}\$5,000,000\end{array}\mathrm{Capex}\times 2\%\mathrm{rule\; of\; thumb\; estimate}\hfill \\ \hfill & =\begin{array}{c}\$100,000\end{array}\hfill \\ \mathrm{Cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{GN}}\mathrm{from}\mathrm{NG}\hfill & =\frac{\$100,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$0.29\hfill \end{array}$

Table 5.24

Maintenance costs for CNT plant configurations

Maintenance costs for respective CNT plants

$\begin{array}{ll}\mathrm{Annual}\mathrm{cost}\mathrm{of}\mathrm{CNT}\mathrm{biogas}\mathrm{plant}\hfill & =\begin{array}{c}\$15,000,000\end{array}\mathrm{Capex}\times 2\%\mathrm{rule}\mathrm{of}\mathrm{thumb}\mathrm{estimate}\hfill \\ \hfill & =\begin{array}{c}\$300,000\end{array}\hfill \\ \mathrm{Cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{CNT}}\mathrm{from}\mathrm{biogas}\hfill & =\frac{\$300,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$0.86\hfill \\ \mathrm{Annual}\mathrm{cost}\mathrm{of}\text{CNT}-\text{syngas}\hfill & =\begin{array}{c}\$16,000,000\end{array}\mathrm{Capex}\times 2\%\mathrm{rule}\mathrm{of}\mathrm{thumb}\mathrm{estimate}\hfill \\ \hfill & =\$320,000\hfill \\ \mathrm{Cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{CNT}}\mathrm{from}\mathrm{syngas}\hfill & =\frac{\$320,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$0.91\hfill \\ \mathrm{Annual}\mathrm{cost}\mathrm{of}\mathrm{CNT}\mathrm{NG}\mathrm{plant}\hfill & =\begin{array}{c}\$12,000,000\end{array}\mathrm{Capex}\times 2\%\mathrm{rule}\mathrm{of}\mathrm{thumb}\mathrm{estimate}\hfill \\ \hfill & =\begin{array}{c}\$240,000\end{array}\hfill \\ \mathrm{Cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{CNT}}\mathrm{from}\mathrm{NG}\hfill & =\frac{\$240,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$0.69\hfill \end{array}$

Table 5.25

Maintenance cost for GO plant

$\begin{array}{ll}\mathrm{Annual}\mathrm{cost}\mathrm{of}\mathrm{GO}\hfill & =\$4,000,000\mathrm{Capex}\times 2\%\mathrm{rule}\mathrm{of}\mathrm{thumb}\hfill \\ \hfill & =\$80,000\hfill \\ \mathrm{Cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{GO}}\hfill & =\frac{\$80,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$0.23\hfill \end{array}$

Table 5.26

Insurance costs

$\begin{array}{ll}\mathrm{Annual}\mathrm{cost}\mathrm{of}\mathrm{GN}\mathrm{biogas}\mathrm{plant}\hfill & =\$6,000,000\mathrm{Capex}\times 3\%\mathrm{rule}\mathrm{of}\mathrm{thumb}\hfill \\ \hfill & =\$120,000\hfill \\ \mathrm{Cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{GN}}\mathrm{from}\mathrm{biogas}\hfill & =\frac{\$120,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$0.34\mathrm{per}{\mathrm{kg}}_{\text{GN}-\text{biogas}}\hfill \\ \mathrm{Annual}\mathrm{cost}\mathrm{of}\text{GN}-\text{syngas}\hfill & =\$7,000,000\mathrm{Capex}\times 3\%\mathrm{rule}\mathrm{of}\mathrm{thumb}\hfill \\ \hfill & =\$140,000\hfill \\ \mathrm{Cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{GN}}\mathrm{from}\mathrm{syngas}\hfill & =\frac{\$140,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$0.40{\mathrm{per\; kg}}_{\text{GN}-\text{syngas}}\hfill \\ \mathrm{Annual}\mathrm{cost}\mathrm{of}\text{GN}-\text{NG}\mathrm{plant}\hfill & =\$5,000,000\mathrm{Capex}\times 3\%\mathrm{rule}\mathrm{of}\mathrm{thumb}\hfill \\ \hfill & =\$100,000\hfill \\ \mathrm{Cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{GN}}\mathrm{from}\mathrm{NG}\hfill & =\frac{\$100,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$0.29\mathrm{per}{\mathrm{kg}}_{\text{GN}-\text{NG}}\hfill \\ \mathrm{Annual}\mathrm{cost}\mathrm{of}\mathrm{CNT}\mathrm{biogas}\mathrm{plant}\hfill & =\$15,000,000\mathrm{Capex}\times 3\%\mathrm{rule}\mathrm{of}\mathrm{thumb}\hfill \\ \hfill & =\$300,000\hfill \\ \begin{array}{c}\mathrm{Cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{CNT}}\mathrm{from}\mathrm{biogas}\end{array}\hfill & =\frac{\$300,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$0.86\mathrm{per}{\mathrm{kg}}_{\text{CNT}-\text{biogas}}\hfill \\ \mathrm{Annual}\mathrm{cost}\mathrm{of}\text{CNT}-\text{syngas}\hfill & =\$16,000,000\mathrm{Capex}\times 3\%\mathrm{rule}\mathrm{of}\mathrm{thumb}\hfill \\ \hfill & =\$320,000\hfill \\ \mathrm{Cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{CNT}}\mathrm{from}\mathrm{syngas}\hfill & =\frac{\$320,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$0.91{\mathrm{per\; kg}}_{\text{CNT}-\text{syngas}}\hfill \\ \mathrm{Annual}\mathrm{cost}\mathrm{of}\mathrm{CNT}\mathrm{NG}\mathrm{plant}\hfill & =\$12,000,000\mathrm{Capex}\times 3\%\mathrm{rule}\mathrm{of}\mathrm{thumb}\hfill \\ \hfill & =\$240,000\hfill \\ \mathrm{Cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{CNT}}\mathrm{from}\mathrm{NG}\hfill & =\frac{\$240,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$0.69\mathrm{per}{\mathrm{kg}}_{\text{CNT}-\text{NG}}\hfill \\ \mathrm{Annual}\mathrm{cost}\mathrm{of}\mathrm{GO}\hfill & =\$4,000,000\mathrm{Capex}\times 3\%\mathrm{rule}\mathrm{of}\mathrm{thumb}\hfill \\ \hfill & =\$80,000\hfill \\ \mathrm{Cost}\mathrm{per}{\mathrm{kg}}_{\mathrm{GO}}\hfill & =\frac{\$80,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$0.23\mathrm{per}{\mathrm{kg}}_{\mathrm{GO}}\hfill \end{array}$

Table 5.27

Depreciation expenses

$\begin{array}{ll}\mathrm{Annual}\mathrm{depreciation}\mathrm{of}\mathrm{GN}\mathrm{biogas}\mathrm{plant}\hfill & =\frac{\$6,000,000\mathrm{Capex}}{10\mathrm{years}}\hfill \\ \hfill & =\$600,000\hfill \\ \mathrm{Depreciation}\mathrm{per}{\mathrm{kg}}_{\mathrm{GN}}\mathrm{from}\mathrm{biogas}\hfill & =\frac{\$600,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$1.71\mathrm{per}{\mathrm{kg}}_{\text{GN}-\text{biogas}}\hfill \\ \mathrm{Annual}\mathrm{depreciation}\mathrm{of}\text{GN}-\text{syngas}\hfill & =\$7,000,000\mathrm{Capex}\times 10\mathrm{years}\hfill \\ \hfill & =\$700,000\hfill \\ \mathrm{Depreciation}\mathrm{per}{\mathrm{kg}}_{\mathrm{GN}}\mathrm{from}\mathrm{syngas}\hfill & =\frac{\$700,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$2.00{\mathrm{per\; kg}}_{\text{GN}-\text{syngas}}\hfill \\ \mathrm{Annual}\mathrm{depreciation}\mathrm{of}\text{GN}-\text{NG}\mathrm{plant}\hfill & =\$5,000,000\mathrm{Capex}\times 10\mathrm{years}\hfill \\ \hfill & =\$500,000\hfill \\ \mathrm{Depreciation}\mathrm{per}{\mathrm{kg}}_{\mathrm{GN}}\mathrm{from}\mathrm{NG}\hfill & =\frac{\$500,000\mathrm{annual\; cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$1.43\mathrm{per}{\mathrm{kg}}_{\text{GN}-\text{NG}}\hfill \\ \mathrm{Annual}\mathrm{depreciation}\mathrm{of}\mathrm{CNT}\mathrm{biogas}\mathrm{plant}\hfill & =\$15,000,000\mathrm{Capex}\times 10\mathrm{years}\hfill \\ \hfill & =\$1,500,000\hfill \\ \begin{array}{c}\mathrm{Depreciation}\mathrm{per}{\mathrm{kg}}_{\mathrm{CNT}}\mathrm{from}\mathrm{biogas}\end{array}\hfill & =\frac{\$1,500,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$4.29\mathrm{per}{\mathrm{kg}}_{\text{CNT}-\text{biogas}}\hfill \\ \mathrm{Annual}\mathrm{depreciation}\mathrm{of}\text{CNT}-\text{syngas}\hfill & =\$16,000,000\mathrm{Capex}\times 10\mathrm{years}\hfill \\ \hfill & =\$1,600,000\hfill \\ \mathrm{Depreciation}\mathrm{per}{\mathrm{kg}}_{\mathrm{CNT}}\mathrm{from}\mathrm{syngas}\hfill & =\frac{\$1,600,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$4.57{\mathrm{per\; kg}}_{\text{CNT}-\text{syngas}}\hfill \\ \mathrm{Annual}\mathrm{depreciation}\mathrm{of}\mathrm{CNT}\mathrm{NG}\mathrm{plant}\hfill & =\$12,000,000\mathrm{Capex}\times 10\mathrm{years}\hfill \\ \hfill & =\$1,200,000\hfill \\ \mathrm{Depreciation}\mathrm{per}{\mathrm{kg}}_{\mathrm{CNT}}\mathrm{from}\mathrm{NG}\hfill & =\frac{\$1,200,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$3.43\mathrm{per}{\mathrm{kg}}_{\text{CNT}-\text{NG}}\hfill \\ \mathrm{Annual}\mathrm{depreciation}\mathrm{of}\mathrm{GO}\hfill & =\$4,000,000\mathrm{Capex}\times 10\mathrm{years}\hfill \\ \hfill & =\$400,000\hfill \\ \mathrm{Depreciation}\mathrm{per}{\mathrm{kg}}_{\mathrm{GO}}\hfill & =\frac{\$400,000\mathrm{annual}\mathrm{cost}}{350,000{\mathrm{kg}}_{\mathrm{GN}}\mathrm{produced}}\hfill \\ \hfill & =\$1.14\mathrm{per}{\mathrm{kg}}_{\mathrm{GO}}\hfill \end{array}$