The simulation results are illustrated in Figure . The results show that the optimal current density of each stack is adjusted to the time-variable electricity prices while at anytime meeting the H 2 /CO ratio of the mixed syngas stream entering the syngas storage. Because of the symmetry-breaking constraints imposed, Stack 1 and Stack 4 always operate at the highest and lowest current densities, respectively. Interestingly, more than two stacks occasionally follow the same trajectory, e.g., Stack 2 and 3 in the case of 2 hr of the maximal ramping duration. This kind of optimal trajectories would facilitate the process control in practice. During the period of low electricity prices, some stacks are operated at relatively high current density, which results in not only the higher syngas production rate but also the higher H 2 /CO ratio than the nominal levels (see Figure ). Other stacks, however, run at below the nominal current density due to the syngas ratio constraint .