לוח שנה משולב

A chemical reaction between two species in porous media can only happen if molecules collide and react. Thus, the level of mixing of the species can become a limiting factor in the onset of reaction. The effect of incomplete mixing upon reaction has important implications on various processes in natural and engineered systems, ranging from mineral precipitation in geological formations to groundwater remediation in aquifers. For example, incomplete mixing that slows down the remediation of a contaminated site can delay site closure, and increase remediation costs.
Numerical models of flow and transport typically fail to describe incomplete mixing effects. Finite difference/element methods usually assume that each of the numerical cells is well-mixed. In order to take into account the incomplete mixing effects, the cells need to be extremely small, leading to impractical computational costs. Thus, we propose a different approach for the modeling of the ADRE (advection-diffusion-reaction equation) by means of a Monte-Carlo particle tracking approach. In this method, each numerical particle represents some reactant mass, and advection and diffusion are modeled by drift and a random walk of the particles (via Langevin equation). The novel part of the approach is the implementation of the reaction term. This is done by annihilating some of the particles in each time step. The probability of the annihilation is proportional to the reaction rate constant and the probability of the particles to become co-located.
To demonstrate the approach we study a relatively simple system with a bimolecular irreversible kinetic reaction A+B→0, where the initial concentrations are given in terms of an average and a perturbation. Such stochastic initial conditions are highly suitable for a particle approach, since noise is inherent to a representation of concentration by discrete particles. An approximate analytical solution for this system exists for one dimension; we extend it to d=2,3. We also derive the relationship between the initial number of particles in the system and the initial concentrations perturbations represented by that number. The numerical results of the particle-tracking simulations demonstrate the well-known phenomena of incomplete mixing (Ovchinnikov-Zeldovich segregation). We compare the results to the approximate analytical solution, and explain the late time discrepancy.

References:
1. Paster A., Bolster D. and Benson D.A., Particle Tracking and the Diffusion-Reaction Equation. Water Resources Research (in press).
2. Ding D., Benson D. A., Paster A., and Bolster D. Modeling bimolecular reactions and transport in porous media via particle tracking. Advances in Water Resources, 53, pp. 56-65, 2013.

Over the past years there has been a dramatic increase in the regulatory requirements for low emissions from both new and existing utility boilers and gas turbines. Traditional methods of reducing NOx emissions, such as: modification of the firing system (to control the fuel and air mixing and reducing flame temperatures); and/or post combustion treatment of the flue gas to remove NOx; are very expensive. Hence, before the implementation of the expensive measures for the emission reduction, it is necessary to evaluate all low cost alternatives. Fuel properties have a major influence on NOx formation during combustion. One of the attractive alternative fuels for combustion in the utility boilers and stationary gas turbines may be methanol. Existing experience has shown that with minor system modifications, methanol is easily fired and is fully feasible as an alternative fuel.
Using methanol has become an important solution for emissions compliance due to their unique constituents and combustion characteristics. Methanol may be referred to as enviro fuel. The considerable advantages of methanol fuel relative to heavy fuel and light fuel oil, methanol can achieve an improved efficiency and lower NOx emissions, due to the lower flame temperature and nitrogen content. Since methanol contains no sulfur and ash, there are no SO2 and very low particulates emissions. The clean burning characteristics of methanol are expected to lead to clean pressure parts, turbine blades and lower maintenance than with fuel oil. Hence, firing methanol alone or as blends with fuel oil is deemed environmentally attractive. Gas Turbine performance on methanol is improved over other fuels due to higher mass flow and lower combustion temperatures resulting from methanol operations.
The present paper discusses the boiler and gas turbine testing in various operation modes during methanol and fuel oil firing. The measurements were accompanied by computer simulations of the combustion process.
Here, we present results of the Israel Electric Corporation (IEC) for specific 140 MWe units consisting of two tangential fired pressurized boilers by Combustion Engineering Inc by using the co-firing of methanol with heavy fuel oil and FT4C TWIN PAC 50 MWe GT provide by Pratt & Whitney by using the methanol firing that show the control of NOx, SO2 and particulate emissions
The experiments performed for gas turbine tested different GT loads during methanol and LFO firing. The results presented here clearly show that with minor low cost fuel system retrofit methanol firing leads to significant NOx, SO2 and particulates emission reduction. NOx emissions were reduced more than 75% and are equal 75 mg/dNm3 at 15%O2. . It is less then required standard even with water injection operation mode (the standard 86 mg/dNm3 at 15%O2 SO2 emissions were reduced from 50 mg/dNm3 at 15%O2 with LFO to zero with methanol firing. Particulate emissions vary from 1.3 to 1.6 mg/dNm3 at 15% O2 with methanol firing, while with LFO this parameter was 13-37 mg/dNm3 at 15% O2.
The experiments performed for the boiler tested different methanol fractions of the total boiler heat capacity (from 33% to 50% heat), at different boiler loads. The results presented here show that NOx emissions were reduced more than 20% and meet the commonly accepted NOx emissions 270-330 mg/dNm3 at 3%O2. SO2 emissions were reduced from 670 mg/dNm3 at 3%O2 with HFO to 430 mg/dNm3 at 3%O2 with methanol co-firing. Particulate emissions vary from 25 to 37 mg/dNm3 at 3% O2 with methanol co-firing, while with HFO this parameter was 40-90 mg/dNm3 at 3%
We believe that the conclusions of the present work are general and can be applied to other boilers and gas turbines as well.