There are several synonyms that can be used in place of the word "sullens". Some of these synonyms include sulky, moody, glum, surly, and brooding. All of these words have a similar connotation, describing a person who is unhappy or discontented with their situation, and often displays this discontent through their behavior and demeanor. While these words can be used interchangeably, it's important to recognize the slight nuances in their meanings, and choose the word that best fits the context of the situation. Whether you're describing a person's mood or talking about their behavior, there are plenty of synonyms available to help you find the right words to express yourself.