Estimating surface emission rates

To model the surface emission rates of a gas or vapour an analysis of the vertical flow of gas in the ground is required. The most commonly used method to estimate surface emission rates is based on the simple assumption that a 50 mm borehole has a radius of influence of 1.78 m which is equivalent to a surface area of 10 m (Pecksen, 1991). This radius of influence was an arbitrary value chosen to ensure that surface emissions were not underestimated. It is important to understand that the area of 10 m applies to the surface area surrounding the borehole at ground level and is an estimate of the area of emission of gas from a single borehole. It should not be confused with the area of influence over the depth of the borehole in which gas is assumed to migrate and enter the headspace. The area of emission and the area of influence are not necessarily the same but are often misinterpreted by designers of gas protection systems. 

Borehole flow rate recorded as 11/h Methane concentration in borehole = 23% 

Surface emission rate of methane = 1 x 0.23/10 Surface emission rate = 0.023 1/h/m  

There is great uncertainty regarding the radius of influence around boreholes. Historically, recommendations were based on flow velocity readings taken in the open pipe. Most flow readings are now measured via a 4 mm diameter valve. However, flux box testing and comparison of the results with flow readings from both 19 mm standpipes and 50 mm standpipes with gas taps suggests that the relationship should always overestimate the actual surface emissions (as intended by Pecksen), often by a factor of up to 100. 

A more robust method of estimating surface emission rates uses the measured differential pressure in boreholes. Most gas monitoring instruments in use today are capable of recording this parameter. The differential pressure in the well should also be less variable as it should depend less on the radius of influence. It will, however, be influenced by factors such as changing ground-water levels which trap and pressurise air in the top of well. There will also be an effect due to the lag between changing atmospheric pressure and the resultant changes in soil pore pressures. 

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Box 4.7 Example of estimating surface emission rate using Darcy s law... 

In the southern hemisphere the oceans cover about two-thirds of the global surface area. In Section 6.1.2 estimates for emission rates from the ocean were given as 0.3 pg/m2 h for ethane and 0.4 pg/m2 h for propane. This leads to global source strengths of Q(C2H6) = 0.63 Tg/yr and Q(C3H8) = 0.84 Tg/yr for the southern hemisphere. If the ocean provided... 

This can be modelled simply using Darcy s law of fluid flow through porous media. The equation can be used to model both horizontal and vertical gas flow. If can fherefore be used to estimate gas surface emission rates. 

Estimate the surface emission rate for a site with the following parameters ... 

It was relatively recently that heavy cluster emission was observed at a level enormously lower than these estimates. Even so, an additional twist in the process was discovered when the radiation from a 223Ra source was measured directly in a silicon surface barrier telescope. The emission of 14C was observed at the rate of 10-9 times the a-emission rate, and 12C was not observed. Thus, the very large neutron excess of the heavy elements favors the emission of neutron-rich light products. The fact that the emission probability is so much smaller than the simple barrier penetration estimate can be attributed to the very small probability... 

The pavement modelling allows to introduce into the model the temporal evolution of the size distribution of materials at the bed surface. By a progressive decrease of the probability density function of the lift force, this model successfully predicts the temporal decrease in mass flux that occurs with the presence of coarse particles at the surface. The rate of this decrease depends on the flow velocity and the characteristics of the particles. In order to improve the accuracy of the estimation of fugitive particle emissions with a wide size distribution, it is necessary to take into account this temporal decrease. 

Oceans and the marine environment are the major source of biogenic sulfur. The reasons for this are the generally abundant phytoplankton in surface oceans and the areal extent of these waters. A summary of the emission rates of DMS, the major biogenic sulfur species in marine environments, is presented in Table IV. Estimates are included which were determined directly and from model calculations. 

A U. S. national biogenic sulfur emissions inventory with county spatial and monthly temporal scales has been developed using temperature dependent emission algorithms and available biomass, land use and climatic data. Emissions of dimethyl sulfide (DMS), carbonyl sulfide (COS), hydrogen sulfide (H2S), carbon disulfide (CS2), and dimethyl disulfide (DMDS) were estimated for natural sources which include water and soil surfaces, deciduous and coniferous leaf biomass, and agricultural crops. The best estimate of 16100 MT of sulfur per year was predicted with emission algorithms developed from emission rate data reported by Lamb et al. (1) and is a factor of 22 lower than an upper bound estimate based on data reported by Adams et al. 

Detailed material and energy balance represent an alternate method for estimation of liquid pool emission rates. The mass transfer from the liquid pool surface to the surroundings gas phase can be estimated from the following equation ... 

SO2, NOx, and total hydrocarbons. The mass spectrometric gas analysis is on a wet basis, as water vapor is not condensed out of the gas, while the analyzers at the sample port measure a gas stream dried using a permeation tube and refrigeration-type dryers in series. In addition to the measurements described above, surface temperature measurements of the boiler skin are made to estimate radiation losses, using the skin temperature, the room temperature and tabulated heat loss factors based on the temperature difference. Particulate mass emission rate and carbon content are measured for heat and mass balance purposes. At present, material deposited within the boiler during a test is collected but not factored into the heat or mass balances, because this deposition is considered to be negligible. Data taken are used to examine the heat balance for the 20-hp system. 

Table 7-11 summarizes global emission and production rates for particulate matter in the troposphere. The table is based on a review by Bach (1976) augmented by a number of additional data. The emission rates refer to all particles that are not immediately returned to the earth surface by gravitational settling. We shall briefly indicate the methods used in deriving the individual estimates. 

Rate of Photons Reaching the Inner Reactor Surface Application of equation (4-1) requires the determination of the rate of photons reaching the inner reactor surface, Pi t). Consequently, the estimation of Pi t), according to equation (4-2), requires the estimation of the rate of emission of photons by the UV lamp, P/(t), and the rate of absorption of photons in the inner glass tube wall, Pa-wall iO-... 

Burning rate, flame height, flame tilt, surface emissive power, and atmospheric transmissivity are all empirical, but well established, factors. The geometric view factor is soundly based in theory, but simpler equations or summary tables are often employed. The Stefan-Boltzmann equation is frequently used to estimate the flame surface flux and is soundly based in theory. However, it is not easily used, as the flame temperature is rarely known. 

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