Vapour and Gas Emission Control

Vapour and Gas Emission Control

A Calculation Spreadsheet for Voyage Calculations

for ISO 14000 purposes

T.J. Gunner – October 2005

1.0Introduction and Scope

The associated Excel spreadsheet to this guideline contains calculation methodologies for the assessment of Gas and Vapour emissions associated with vessel operation for the following vapour/Gas types:

CO2 emissions
Greenhouse Gas emissions
Sulphur Dioxide emissions
Non Methane Volatile Organic Compound (NMVOC) emissions during transportation

The equations used for the calculations only require data that is readily available to the tanker and are based upon:

The agreed format as per MEPC Circular 471 for Carbon Dioxide Indexing for vessels except that the voyage distance includes any ballast voyage.
A modified format to that of MEPC Circular 471 in order to include the additional Greenhouse Gas for fugitive releases of refrigerant gases
A base equation for emissions of Sulphur Dioxide
A series of developed research models for NMVOC emissions during transportation of crude oil[a]

2.0CO2 Index for Vessels

This index value represents the total grams per tonne – nautical mile of CO2 emission for goods transported by sea in ships.

The single deviation to that proposed in the MEPC Circular 471 for this index is that the ballast voyage is also included in the calculation for the total distance. The logic behind this deviation is to reflect the total environmental cost for the transportation of goods and the necessity for vessels carrying bulk cargoes to travel in ballast so as to re-position for the next cargo. It also allows greater standardisation between ship types where certain ship types are only occasionally in a fully ballast condition (e.g. Chemical tankers). Thus, the total voyage length is considered as an environmental cost for the transportation of a defined quantity of goods together with the bunkers consumed for the complete cycle.

The data required for the completion of the calculation on the enclosed spreadsheet is:

Total HFO consumed - Tonnes
Total MDO or MGO consumed – Tonnes
Total Loaded distance for the transportation of the cargo to the discharge port – nautical miles
Total Ballast voyage distance to the next load port after discharge of the defined cargo – nautical miles
The total Bill of Lading tonnage – tonnes

By use of the following equation as inserted in “column H” of the spreadsheet the CO2voyage index is calculated.

CO2 Index (gr/tonne-mile) = (3.114 x Σ HFO) + (3.206 x Σ MDO/MGO) x 1,000,000

Σ Bill of Lading x (Load Dist. + Ballast Dist.)

Where: Σ denotes total consumption or amount, as appropriate, in tonnes

3.0Greenhouse Gas Index for Vessels

This index value represents the total grams per tonne – nautical mile of CO2 equivalent emission for goods transported by sea in ships.

The index deviates from and adds to alternative fugitive gases that are included in the identified gases of the IPCC that impact global warming. In the case of shipping the additional type of fugitive gas will be the refrigerant gases. The extent or tonnage of the fugitive gas for a voyage is represented in the equation as the quantity of gas used to refill or replenish the system during the voyage. A weighting correction is applied to this type of gas in order for it to be correlated to the base gasCO2. This weighting correction is termed “the Global Warming Potential” (GWP) that, for these gas types in the spreadsheet calculation is 3780. This means that for each tonne of this type of fugitive gas it will have an equivalent emission of 3780 tonnes of CO2.

From the foregoing therefore, in addition to the parameters required for the CO2 Index an additional parameter is required for this calculation

Total Refrigerant Gas refill for the voyage – tonnes

By use of the following equation as inserted in “column I” of the spreadsheet the GHGvoyage index is calculated.

GHG Index = ((3.114 x Σ HFO) + (3.206 x Σ MDO/MGO) + (3780 x Refrig Gas)) x 106

Σ Bill of Lading x (Load Dist. + Ballast Dist.)

4.0Sulphur Dioxide Emissions

The emission of sulphur dioxide (SO2) is a direct function of the sulphur content of the fuel consumed. Unlike NOx emissions it has no engine type or operation function save for the rate of consumption of fuel.

The enclose spreadsheet calculates the total SO2 emission as tonnes of SO2 as a total for the specific voyage and thereafter calculates this emission as a function of grams per tonne-mile. This allows a direct correlation with alternative gaseous emissions. The calculation is undertaken by input of the following parameters into the spreadsheet:

The Sulphur content (%) and quantity of HFO (Tonnes)consumed for the voyage
The sulphur content (%) and quantity of MDO/MGO (Tonnes) consumed for the voyage

Given the requirements of Annex VI to MARPOL and the required content of the Bunker Delivery Note to be found in Appendix V, then the criteria required for this calculation will readily available to the vessel’s command.

The equation used to calculate the tonnes of sulphur dioxide emission for the voyage, transposes the total sulphur within the individual fuels to its equivalent to its sulphur dioxide equivalent as a result of combustion. This is achieved by use of the atomic masses of the two differing components within the compound of sulphur dioxide, namely;

Sulphur – atomic mass 32.1
Oxygen – atomic mass 16.0
Total atomic mass of SO2 molecule – 64.1

From the foregoing the equation used within the spreadsheet is as follows:

Voyage SO2 (Tonnes) = ((Sulphur content x HFO) + (Sulphur Content x MDO)) x 1.9968

100 100

5.0NMVOC Emissions from Crude Oil Tankers

This section within the enclosed spreadsheet only applies directly to crude oil cargoes. The applicability of the equations used in the spreadsheet to product or chemical cargoes is doubtful given the variance of the correlation of the cargoes True Vapour Pressure (TVP) to the equivalent Reid Vapour Pressure (RVP).

The equations used in the spreadsheet and their derivation are as a result of extensive research and reported in a publication entitled “Volatile Hydrocarbon Emissions from Crude Oil carried by Sea in Tankers”. Alternative equations are available to model this emission. A selection of differing publications can be referred to in this regard:

API – Manual of Petroleum Measurement Standards, Chapter 19.4 – Recommended Practice for Speciation of Evaporative Losses – February 28th 2005
API – Atmospheric Hydrocarbon Emissions from Marine Vessel Transfer Operations – API Publication 2514A – reaffirmed 2001.
Concawe – Studies of VOC emissions from External Floating Roof Tanks and Barge Loadings – November 1993
AEA Technology – Measures to Reduce Emissions of VOCs during Loading and Unloading of Ships in the EU – August 2001

The equation used in the enclosed spreadsheet requires the following parameters, which will be known to a tanker’s command, as input data:

The Bill of Lading Tonnage of the cargo – tonnes
The weighted average (in the event of numerous types of cargo being carried simultaneously) RVP of the cargo – psia
The percentage size of the total ullage space onboard after loading - %
The average cargo temperature throughout the carriage of the cargo – the mean of load port and discharge port temperatures – oC.

The resultant calculations will supply:

The Average TVP of the cargo throughout the transit – psia
The NMVOC emission of the cargo – as a % of the Bill of Lading weight – tonnes
The NMVOC emissions – tonnes
The amount of NMVOC per tonne-mile

In order to achieve these resultant values the following equations have been used:

To calculate the Average TVP – psia

Av TVP = (((1.9181 +(6.2106 x LN (Av RVP)) x EXP(0.0253 x Temp)) – 0.31 x (ull % - 2))[b]

2.6022

To Calculate the NMVOC Emission – as a % of cargo (tonnes) carried

% of NMVOC emission of cargo = ( 0.4367 x LN (1/ av. TVP)) + 1.0111[c]

To calculate the NMVOC emission during transit for the voyage – tonnes

NMVOC Emission (tonnes) = (% of NMVOC emission of cargo x Bill of Lading tonnes)

100

The foregoing set of calculations as applied to the spreadsheet enclosed should supply a reasonable indication of the transit emission of NMVOC. The spreadsheet or the foregoing equations do not anticipate the extent of NMVOC emission as a result of loading or gas freeing of cargo spaces. With the anticipation that vapour return requirements of Regulation 15 to Annex VI to MARPOL may now apply the NMVOC loss has been restricted to carriage loss only. However, for guidance purposes, loading losses diverse estimates may be found that vary from approximately 0.1% to over 0.2% of the cargo tonnage loaded dependent upon the temperature and volatility of the cargo at the time of loading.

Note: Clear note should be taken as to the note provided on the spreadsheet. If the calculated TVP is less than 10 psia then the result value of emission becomes positive. Clearly such a circumstance is not real and therefore should be deleted from columns “R” and “S” on the spreadsheet and the value set to “Zero”.

6.0Annual or Period reconciliations

The enclosed spreadsheet provides the necessary summation for an annual or periodic review. The spreadsheet allows 26 “voyage” entries but this can be modified by deleting excessive lines in the spreadsheet format.

The annual or periodic review of both the CO2 and GHG index does not supply an average of the individual voyage calculated indices and due to the form of the equation to be used under the MEPC Circular 471. By use of this equation’s application to the summed values for each variable in the equation a annual index will be provided rather than an average of the individual voyage indices.

7.0Conclusions

It hoped that by use of the enclosed simple spreadsheet that requires only known data to both the Owner’s Operations Department and a Vessel’s Command, operational indications will be provided to assist shipping with decision making towards more effective transportation and to show the industries environmental awareness.

TJG

[a] These equations together with their proof are developed and to be found in “Volatile Hydrocarbon Emissions from Crude Oil carried by Sea in Tankers” – copyright T.J. Gunner