Any balance can be performed dynamically. So is not needed to be performed in equilibrium. Of course, the accumulation term is always 0 in an equilibrium. 

Hussam,  You need to be able to calculate the amount of Carbon in the feed and the amount of Carbon in the product.  You don't necessarily need a balanced reaction equation to perform a Carbon balance but it is usually a good idea to have one.  All you need is the rate and an analysis of your feed that sufficient to calculate C content and the rate and an analysis of the product sufficient to determine C content.  In the oil refining industry that deals with complex mixture of hydrocarbons this is often achieved be analyzing S,N and O in the feed and product (everything that isn't C or H) and measuring C/H ratio of the feed and product by burning the HC feed and product and measuring the amount of CO2 and H20 produced.  More recently H content is often measured via NMR analysis.

Biological test make a balance by measuring C, H, N, O, S, and P. Grams of each going in are compared to grams of each coming out. When losses are accounted for the balances can become very good for each of the atoms all at the same time. Failure to balance is a clue that something is wrong, either in the process, or the laboratory, or in the calculations. So the methods improve with effort. Component balances are used in value audits.

Carbon balance basically refers to stoichiometric considerations. The mass balance has a wider range. This may be based on the results of chemical analysis and stoichiometric equation/equations balance of all the elements, and it may be, for example, the balance of the reactor. It is also worth noting that the mass balance of chemical elements has nothing to do with the chemical equilibrium, though, which is obvious, the equilibrium determines the composition of the reaction mixture.

To balance carbon need only the stoichiometric equation/equations and the results of chemical analysis, including the possible mass of carbon deposited on the catalyst.

Is the balance in soils only? any carbon balance is the result between carbon output and input. So, you need the decomposition and humification coeficients for the crop you are working.

Maribel Gonzalez, the concept can be applied to engine fuel consumption. The hydrocarbons(fuel) + 02 is equated to the output CO2/CO and water.

Estimado investigador:

Yo trabajo el balance del carbono en el suelo, no al nivel de reacción quimica. El carbono en el suelo es el resultado de las entradas (input) y las salidas (output), y su magnitud depende del manejo que reciba el suelo. El procedimiento que yo uso es el de Hening y Dupuis, que se basa en el equilibrio dinámico de las entradas y las salidas de carbono, por lo que, además del carbono determinado en el suelo por cualquier procedimiento quimico, debes tener los coeficientes humico y de descomposicion. Espero haberte ayudado. Saludos